3.1. Comparison of the effects of high-dose dobutamine and atropine/low-dose dobutamine on cardiac output as an indicator of global left ventricular function (Study 1)
Introduction
Stress echocardiography recently experienced an increasing interest in equine medicine as a diagnostic tool for the detection of exercise-induced myocardial dysfunction. Echocardiography performed immediately after treadmill exercise has been applied in clinical and research settings, but the rapid drop of HR in the immediate post-exercise period seems to be a limiting factor (Marr et al.,, 1999; Sampson et al., 1999; Durando et al., 2002).
High-dose dobutamine stress echocardiography has recently been tested in horses as an alternative to exercise stress echocardiography (Frye et al., 2003). This study demonstrated that changes in left ventricular M-mode echocardiographic parameters induced by high-dose dobutamine were similar to those measured immediately after maximal treadmill exercise. However, all of the fourteen horses undergoing high-dose dobutamine stress test in this study showed side effects, e.g. signs of restlessness, muscle tremors and sweating. Furthermore, of fourteen horses receiving high-dose dobutamine, three showed ventricular premature complexes and three showed ventricular tachycardia. Lastly, at post-mortem two out of ten horses examined, showed histopathological lesions indicating catecholamine-induced cardiomyotoxicity. Some of the side effects mentioned above were partly attributed to a dobutamine-induced baroreceptor reflex.
The aim of the present study was to investigate whether (1) atropine is able to counteract the dobutamine-induced baroreceptor reflex, (2) the combination of low-dose dobutamine and atropine induces a cardiac stimulation similar to high-dose dobutamine alone, in order to decrease the risk of catecholamine-induced cardiomyotoxicity.
Material and Methods
Thirteen healthy Shetland ponies were selected for this study. They were randomly assigned into two treatment groups. Group 1 (n=7) received a dobutamine infusion at a rate of 2 µg/kg/min for 5 minutes followed by a dobutamine infusion at a rate of 5µg/kg/min for 5 minutes. The infusion rates were then increased in 5-minutes lasting increments of 5 µg/kg/min, from 10 to 40 µg/kg/min.
Ponies of group 2 (n=6) received two intravenous injections of 25 µg/kg of atropine five minutes apart. Five minutes after the second atropine injection, dobutamine infusion was started at a rate of 2 µg/kg/min and then increased in incremental rates of 1 µg/kg/min, every 5 minutes, up to a final rate of 5µg/kg/min. Doppler-echocardiographic recordings of the aortic flow were performed at baseline and between the third and the fifth minutes after the onset of each step of pharmacological stimulation. Heart rate was recorded simultaneously. The velocity-time-integrals of the aortic flows
were obtained by placing the sample volume in the left ventricular outflow tract in order to obtain good quality Doppler spectral profiles. The mean of three velocity-time-integrals was multiplied by the 2D-echocardography-derived aortic diameter to calculate the stroke volume, which was multiplied by the HR to calculate the cardiac output.
Reaching target HR of 170 bpm was considered as diagnostic endpoint of the test. Non-diagnostic endpoints were excessive restlessness and significant cardiac arrhythmias which led to interruption of the test.
Results
Ponies of the high-dose dobutamine group showed considerable side effects during the pharmacological challenge. All seven ponies of this group showed one or more of the following signs at dobutamine rates above 10µg/kg/min: pawing, stomping, stepping back and forth, head shaking, tail twitching, sweating, coughing, trembling, and tachypnoea. The infusion had to be stopped at a rate of 35 µg/min/kg because of excessive restlessness in one pony. Target HR of 170 bpm was reached in one pony at a dobutamine infusion rate of 20 µg/kg/min and in three ponies at a rate of 30 µg/kg/min. The two remaining ponies did not reach target HR at maximal infusion rate.
All the ponies of group 1 developed transient sinus arrhythmia during the pharmacological stimulation, which turned to normal sinus rhythm at a later dose. Moreover, in this group, 1 pony showed isolated ventricular premature complexes and 1 pony showed multiple ventricular premature complexes at an infusion rate of 20µg/kg/min. All cardiac arrhythmias disappeared within 30 seconds after the infusion had been stopped.
In group 2, only a transient slight restlessness was observed in 3 ponies during dobutamine infusion. None of the six ponies showed arrhythmia during the procedure. All ponies received the final dose of 5µg/kg/min, although Doppler echocardiographic measurement of the aortic flow was impossible in 1 pony at the dose of 5µg/kg/min, due to aliasing.
In group 1, mean cardiac output remained statistically unchanged from baseline values until an infusion rate of 20 µg/kg/min. Mean cardiac output at infusion rates from 25 µg/kg/min to 30 µg/kg/min were significantly (p < 0.05) higher than baseline values. In group 2, mean cardiac output was significantly higher at all steps of pharmacological stimulation than controls. Mean cardiac output was significantly higher at each incremental step of the dobutamine infusion than at the previous one. Mean cardiac output obtained in group 2 at dobutamine infusion rates of 2µg/kg/min to 5µg/kg/min were significantly higher than mean cardiac output values obtained in group 1 at dobutamine infusion rates of 2 µg/kg/min to 25 µg/kg/min.
In group 1, mean HR decreased at dobutamine infusion rates from 2µg/kg/min to 10 µg/kg/min, then increased and reached a mean HR (± S.D.) of 123.47 ± 58.27 bpm at an infusion rate of 30 µg/kg/min and 128.19 ± 24.29 bpm at an infusion rate of 40 µg/kg/min. In group 2, mean HR was
higher at each step of cardiac stimulation than at previous steps and reached a maximal mean HR of 168 ± 18 bpm.
In group 1, mean stroke volume obtained at dobutamine infusion rates from 2 to 10µg/kg/min were not significantly different from resting values. Mean stroke volume at infusion rates from 15µg/kg/min to 40µg/kg/min were significantly lower than mean stroke volume at the infusion rate of 5µg/kg/min. In group 2, changes in mean stroke volume were not significantly different from resting values during the pharmacological challenge, even though a downward trend was observable. There was no significant difference in mean stroke volume response between the two groups.
The maximal increase in cardiac output during the pharmacological challenge was 1.7-fold in group 1 and 2.33 fold in group 2. The increase in cardiac output was mediated by a significant increase in HR in both groups, while stroke volume significantly decreased in group 1 and did not change significantly in group 2. Results of HR, stroke volume and cardiac output of the two groups are displayed in fugure 8.
Figure 8: Changes in % in cardiac output (CO), stroke volume (SV), and heart rate (HR) in seven ponies receiving high-dose dobutamine infusion (High DOB) and six ponies receiving low-dose dobutamine/atropine (Low DOB). 0 = baseline, A = atropine, dobutamine in µg/kg/min.
Discussion
Aliasing occurred in one atropinized pony at a rate of 5µg/kg/min and made the measurement of pulsed wave Doppler-derived time-velocity-integral impossible. The same phenomenon has also been described in humans undergoing dobutamine stress test (Pelikka et al., 1995), where peak velocity increased with increasing HRs, while the systolic ejection period decreased, resulting in a reduction in the time-velocity integral.
0 50 100 150 200 250 300 350 400 450 500
dose
changes in %
0 2 5 10 15 20 25 30 35 40 0 A A 2 3 4 5 HR
HR
CO
CO
SV SV
0 50 100 150 200 250 300 350 400 450 500
dose
changes in %
0 2 5 10 15 20 25 30 35 40 0 A A 2 3 4 5 HR
HR
CO
CO
SV SV
High DOB
Low DOB
In humans, dobutamine is known to be a potent beta-1 and a mild beta-2 and alpha agonist and to have a positive inotropic and chronotropic effect, a rapid onset of action and a short half-life (Tuttle and Mills, 1975). Given the dose-dependent manner and knowing that the equine myocardium is predominantly covered by beta-1-receptors (Horn et al., 2002), we could expect a similar response in horses. However, in our study mean HR did not change until a rate of 15µg/kg/min was reached.
Furthermore, 2 out of 7 ponies in group 1 failed to increase HR to an expected level, even at maximal doses. The cause of this diminished chronotropic effect is unclear, but a parasympathetically mediated high-pressure baroreceptor reflex has been suggested to be the cause in humans (Liang and Hood 1979). The results of a study by Hinchcliff et al. (1991) support a role of the baroreceptor reflex in the inotropic and chronotropic effect of dobutamine in horses. In their study, dobutamine at a dose of 0.5 µg/kg/min increased systemic systolic and diastolic arterial pressure (SAP and DAP, respectively), and MAP in both atropinized and non-atropinized horses, but increased HR only in atropinized horses. Apparently, the chronotropic effect of dobutamine was opposed by a reflex increase in parasympathethic tone in non-atropinized horses. In atropinized horses atropine blocked the reflex bradycardic effect induced by an increased arterial blood pressure, and therefore permitted expression of the positive chronotropic activity of dobutamine (Hinchcliff et al., 1991).
In the present study, individual responses in HR and stroke volume to the dobutamine challenge in non-atropinized ponies were quite variable. Considerable individual variation in stroke volume and HR to dobutamine has also been reported in human medicine (Pelikka et al., 1995). A pharmacokinetic study in humans demonstrated that some individuals who failed to respond to the dobutamine challenge with an increase in HR had high plasma levels of dobutamine and a decreased dobutamine clearance, which might suggest that poor chronotropic response could be due to impaired chronotropic beta receptor responsitivity, rather than to a failure to achieve a sufficiently high plasma dobutamine level. In such patients, the early administration of atropine is advised since they will probably not show sufficient chronotropic response at higher dobutamine doses (Daly et al., 1997).
In our study, there was no significant difference in cardiac response between the low dose of 0.025 mg/kg of atropine and the accumulated dose of 0.05 mg/kg of atropine. An earlier study in conscious horses by Hinchcliff et al. (1991) demonstrated that a single injection of 0.04 mg/kg of atropine increased the HR approximately 2-fold and decreased the stroke volume to half of basal values, the overall cardiac output remaining unchanged. In our study, HR and cardiac output increased significantly while stroke volume showed a non significant trend to decrease. The increase in HR induced by atropine was similar to earlier observations (Hinchcliff et al., 1991;
Slinker et al., 1982) and was thought to be the consequence of the predominantly vagal control of HR in the resting horse (Hamlin et al., 1972; Slinker et al., 1982). Stroke volume is determined by the interaction of four factors: myocardial contractility, preload, afterload and HR (Smith and
Kampine, 1982). Atropine administration in horses is associated with sharp reductions in right ventricular end-diastolic volume and pressure, suggesting a decrease in right ventricular preload, and an increase in MAP, suggesting an increase in left-ventricular afterload (Hinchcliff et al., 1991). These changes, in the absence of increased myocardial contractility, could explain the atropine-induced decrease in stroke volume. The effects of changes in HR on stroke volume are complex since HR influences afterload, preload and myocardial contractility (Berne and Levy, 1988). An increase in HR decreases preload, and increases afterload and myocardial contractility (Berne and Levy, 1988). Since atropine does not affect venous tone (Alexander, 1963), it seems that the atropine-induced increased HR and the reduced preload are the result of a reduction in diastolic filling time and an unchanged venous return.
Dobutamine stress-induced atrioventricular blocks are only rarely observed in humans (Secknus and Marvick, 1997; Hung et al., 1999). In the present study, only ponies that did not receive atropine showed numerous atrioventricular blocks. Obviously, the pre-treatment with atropine abolished the development of dobutamine-induced blocs. This supports the hypothesis of a vagal reflex mechanism.
Not only supra-ventricular, but also ventricular arrythmias have been observed in conscious horses receiving dobutamine. Frye et al. (2003) suggested that two factors might contribute to the occurrence of ventricular ectopy in horses receiving dobutamine: (1) the presence of vagally- mediated rhythms that remove overdrive suppression of an enhanced ventricular escape rhythm, and the (2) inherent ability of dobutamine to induce such arrhythmias. The arrhythmogenic properties of dobutamine might be attributed to beta1-receptor stimulation, reduction in plasma potassium concentration (Goldenberg et al., 1989; Coma-Canella, 1991), enhanced automaticity and alterations in ventricular refractoriness and repolarization (Stumpf et al., 2000). The observation that atropine abolishes the formation of arrythmias, supports the hypothesis that dobutamine-induced ventricular ectopy might have a vagal origin. However, the infusion rate used in atropinized ponies was much lower than in non-atropinized ponies, precluding the comparison of the arrhythmogenic effects of dobutamine in the two groups. Nevertheless, the target HR of 170 bmp was obtained in the atropinized group without any arrhythmia, which makes the atropine low dose dobutamine challenge very attractive for stress echocardiography in horses.
Conclusion
A least some of the side effects observed in horses submitted to high dose dobutamine challenge could be due to activation of a vagal reflex. Atropine premedication reduces this vagal reflex and 1) allows 8-fold reduction of dobutamine dose needed to induce a similar cardiac stimulation, 2) drastically reduces side effects related to higher doses of dobutamine and 3) reduces individual variation in the response of cardiac function to dobutamine challenge. The administration of low-
dose dobutamine in previously atropinized ponies appears to be a safe and efficient pharmacological stress test that can be used to perform stress echocardiography in horses.
3.2. Effect of atropine/low-dose dobutamine stress test on left ventricular echocardiographic B- and M- mode parameters in healthy horses (Study 2)
Introduction
In the first study, an atropine/low-dose dobutamine protocol has been tested as an alternative to cardiac stress induction by high-dose dobutamine in horses. The protocol was well tolerated and induced sufficient cardiac stimulation, which was measured by Doppler echocardiography-derived cardiac output as an indicator of global cardiac function. In human medicine, evaluation of the cardiac function by stress echocardiography is best performed by the use of B- or M-mode echocardiography. Therefore, it seems logical to test these parameters in horses undergoing stress echocardiography.
The aim of the present study is (1) to identify those B- and M-mode stress echocardiographic parameters that are easily measured and (2) to establish reference values for these values in healthy horses.
Material and Methods
Seven healthy untrained warmblood horses ranging from 9 to 22 years old (18.5 ± 6.0 years, mean
± SD) and weighing between 440 and 560 kg (506 ± 51 kg, mean ± SD) were used in this study.
After baseline B- and M-mode echocardiographic recordings of the left ventricle, a single dose of 35 µg/kg of atropine was administered intravenously. Five minutes after the injection of atropine, dobutamine was infused at a rate of 2 µg/kg/min and then increased every 5 minutes in incremental steps of 1µg/kg/min until an infusion rate of 6 µg/kg/min was reached. Echocardiographic images and an ECG were recorded before and 3 minutes after the onset of each step of pharmacological stimulation. Left ventricular internal area (LVIA) and left ventricular external area (LVEA) were measured at end-systole (s) and end-diastole (d) from a B-mode echocardiogram of a standardized right parasternal short axis-view of the left ventricle at the chordal level (Long et al., 1992). From the same view, an M-mode echogram was obtained by intersecting the inter-ventricular septum and the left ventricular free wall in right angles. This M-mode echogram served for the measurement of interventricular septal thickness (IVS), left ventricular internal diameter (LVID), and left ventricular free wall thickness (LVFW). From these measurements, myocardial area (MYA), fractional area change (FAC), FS%, fractional wall thickening of the interventricular septum (FWTS), and fractional wall thickening of the left ventricular free wall (FWTW) were calculated according to formulae previously described (Kriz et al., 2000b). Results were analysed statistically.
Results
All horses tolerated atropine and dobutamine administration without any adverse reactions. None of the horses developed cardiac arrhythmias during or after the pharmacological stress test. Heart
rate increased significantly (p< 0.05) during the pharmacological challenge and a mean (± S.D.) maximal HR of 156.6 ± 12.5 bpm was reached at maximal dobutamine infusion rate. Mean LVEAs, mean LVEAd, mean LVIAs, and mean LVIAd measured after administration of atropine were not significantly different from values at baseline. At all steps of dobutamine infusion, the mean LVEAs, mean LVEAd, mean LVIAs, and mean LVIAd were significantly lower than at baseline and after atropine, but not significantly different between the different steps of dobutamine infusion.
Mean MYAs and mean MYAd showed a tendency to decrease with increasing pharmacological stimulation but these changes were not significant. Mean FAC was highest at a dobutamine infusion rate of 5 µg/kg/min with a mean value of 63.4 ± 6.3 % compared to 56.2 ± 8.6 % obtained at baseline.
-40 -30 -20 -10 0 10 20 30 40 50
0 A 2 3 4 5 6
dose
changes in %
LVIDs LVIDd FS%
IVSs IVSd LVFWs LVFWd
Figure 9: Atropine/low-dose dobutamine-induced changes in left ventricular M-mode parameters in seven healthy horses undergoing stress echocardiography. IVS = interventricular septum thickness, LVID = left ventricular internal diameter, LVFW = left ventricular free wall thickness, FS% = fractional shortening, s = systole, d = diastole, 0 = baseline; A = atropine; 2, 3, 4, 5, 6 = dobutamine infusion rate in µg/kg/min
Mean LVIDd and mean LVIDs decreased significantly during dobutamine infusion. Mean LVIDd reached a minimal value of 8.90 ± 0.43 cm at a dobutamine infusion rate of 3 µg/kg/min (10.80 ±
0.69 at rest). Mean LVIDs reached a minimal value of 4.65 ± 0.15 cm at a dobutamine infusion rate of 6 µg/kg/min (7.02 ± 0.65 at rest). Mean baseline values for IVSs, IVSd, LVFWs, LVFWd, were 4.51 ± 0.27 cm, 3.78 ± 0.10 cm 2.89 ± 0.19cm, and 2.44 ± 0.28, respectively. Mean maximal values of IVSs, IVSd, LVFWs, and LVFWd were 5.65 ± 0.31 cm, 3.72 ± 0.34 cm, 4.77 ± 0.18 cm, 3.11 ± 0.34 cm, respectively, reached at a dobutamine infusion rate 6µg/kg/min. Mean FS%
increased significantly with increasing stimulation, and reached a maximal mean value of 50.56 ± 3.42 %, compared to a resting value of 34.98 ± 3.82 %. Figure 9 displays left ventricular M-mode echocardiographic parameters in changes in % compared to baseline values.
Discussion
As it could be expected from the first study, horses of the present study tolerated the atropine/low- dose dobutamine infusion without adverse reactions or arrhythmias or abdominal discomfort during or after the test. However, in contrast to the previously reported atropine/low-dose dobutamine protocol, atropine was used at a dosage of 35 µg/kg instead of 50 µg/kg. Although the previous study using atropine at a dosage of 50 µg/kg did not result in abdominal discomfort, other studies reported abdominal discomfort after even lower doses of atropine (Ducharme and Fubini, 1983;
Williams et al., 2000). Therefore, it seems that lower doses of atropine could be favourable in order to reduce the risk of abdominal discomfort. The reduced dose of atropine led to a lower maximal mean (± S.D.) HR of 157 ± 7 bpm, copared to the preceding study, where a mean HR of 168 ± 12 bpm was reached. However, in the previous study young Shetland ponies were tested and their response to atropine and dobutamine might be different that of older warmblood horses used in the present study. In horses, as well as in humans, the maximal HR decreases with increasing age (Betros et al., 2002). Given the high mean age of the horses of this study (18.5 years), slightly higher values can be expected in younger performance horses.
The increase in HR and the decrease in LVIDd observed in the horses of this study after the administration of atropine are in accordance with previous studies (Hinchcliff et al., 1991).
Atropine-induced tachycardia is a consequence of the inhibition of the predominantly parasympathetic control of HR in the resting horse (Hamlin et al., 1972; Slinker et al., 1984).
Increases in HR decrease preload (Berne and Levy 1988). In the present study, the administration of atropine led to a decrease in LVIDd, which can be assumed to reflect preload (Feigenbaum, 1985; Braunwald et al., 1988; Guyton, 1991). Preload is mainly defined by venous return and atrial activity (Braunwald et al., 1988). Because atropine does not influence venous return (Alexander, 1963),the observed decrease LVIDd may have resulted from a reduction in diastolic filling time coupled with unchanged venous return (Hinchcliff et al., 1991).
Additionally, reduced venous return could be responsible for the changes observed during the dobutamine infusion. Dobutamine administration resulted in significant increases in IVSs, IVSd, LVFWs and LVFWd, and in significant decreases in LVIDs and LVIDd. These results are similar
to those obtained in a study in humans in whom dobutamine combined with atropine resulted in a gradual decrease in LVIDd and LVIDs, and a in gradual increase in LVFWs, LVFWd, IVSd, and IVSs during increasing pharmacological stimulation (Carstensen et al., 1995). These findings reflect the positive inotropic effect of dobutamine, and are in contrast to exercise stress tests in horses in which the same parameters were not significantly altered immediately after treadmill exercise (Marr et al., 1999, Sampson et al., 1999).During exercise, venous return increases as mean systemic filling pressure rises because of sympathetic stimulation of the veins and because tensing of abdominal and other muscles of the body compresses capacitance vessels (Marr et al., 1999, Poliner et al., 1980 Weiss et al., 1979). In contrast, the muscular pump function that leads to increased venous return is missing during the dobutamine stress test. The finding that the results of dobutamine stress test are related to decreased preload is suggested by studies performed in humans, in whom LVIDd was significantly lower during dobutamine than during physical heart stimulation (Cnota et al., 2003). Furthermore, in humans (Cnota et al., 2003)as well as in horses (Marr et al., 1999), LVIDd is unchanged in the immediate post-exercise period, but decreases thereafter because venous return is no longer maintained by sympathetic stimulation and the compressive effect of muscles while vasodilation of the peripheral vascular beds continues.
The addition of atropine, a cholinergic antagonist, potentiates both the positive chronotropic and positive inotropic effects of dobutamine (Landzberg et al., 1994) but prolongs the relative duration of systolic emptying and shortens the diastolic filling phase of the cardiac cycle (Kelbaek et al., 1991). The progressive decrease in left ventricular chamber diameter and progressive increase in wall thickness during increasing dobutamine infusion rates demonstrated in the study of Carstensen et al., (1995) are in accordance with the properties of the drug and may be explained by a combined effect of increased contractility, decreased afterload, shortening of diastole and a relative reduction in venous return. Similar mechanisms could be responsible for the changes observed in the present study.
The changes in B-mode parameters obtained in the present study were in agreement with the changes in the M-mode parameters and were most significant for the LVIAs and LVIAd. These parameters also decrease in humans undergoing dobutamine or dobutamine/atropine stress echocardiography (Carstensen 1995). Changes in B-mode parameters were observed after the administration of atropine and at a dobutamine infusion rate of 2 µg/kg/min, whereas no more significant change was observed in the subsequent infusion rates. Consequently, measurements of B-mode parameters at higher infusion rates can be omitted.
Conclusion
The current pharmacological stress test induced the most prominent changes in the M-mode parameters LVID, FS%, IVS, and LVFW in adult healthy horses. The B-mode echocardiographic parameters changed significantly at a dobutamine infusion rate of 2 µg/kg/min, while there was no
significant change in the subsequent steps of dobutamine infusion. Moreover, the measurements of B-mode echocardiographic parameters are more time consuming than the measurements of M- mode parameters. This study confirms that the combination of atropine and low-dose dobutamine is a well tolerated method to induce cardiac stimulation in the horses in order to perform stress echocardiography and that M-mode parameters are the best parameters to be measured during equine stress echocardiography.
3.3. Comparison of exercise to pharmacological stress echocardiography in healthy horses (Study 3)
Introduction
In the first study, the combination of atropine and low-dose dobutamine has been proven to be a well-tolerated method of pharmacological stress induction in horses. It is able to increase cardiac output, a global measure of ventricular performance, 2.3-fold over resting values. In the second study, the responses of left ventricular echocardiographic parameters to this stress test have been described. Parameters that are easy to measure and that show the most important changes have been identified as LVID, LVFW, IVS and FS%.
At this point, it appeared necessary to further evaluate the this pharmacological stress test by direct comparison of echocardiographic parameters obtained during pharmacological stress induction to those obtained after exercise.
Material and Methods
Ten healthy Standardbred or Standard crossbred horses were selected for this study. Horses were assigned into two groups. Five horses (group EXE) were studied immediately after a near-maximal treadmill exercise, which consisted of 5 minutes walk at 1.7 m/sec and 5 minutes trot at a speed of 4 m/sec, followed by 1 minutes each at 8, 9, and 10 m/sec at a slope of 6%. An echocardiographic recording were performed before the test and immediately after the stop of the treadmill until HR was lower than 80 bpm.
Five other horses (group DOB) underwent pharmacological cardiac stimulation consisting of 35µg/kg of atropine, followed by incremental dobutamine infusion at rates from 2 to 6 µg/kg/min.
Each step of the pharmacological stimulation lasted for 5 minutes. Echocardiographic images were recorded before and during the stimulation. In both groups, base–apex ECG was continuously recorded and served for calculation of HR.
Echocardiographic recordings consisted of a right parasternal left ventricular short- axis view at the chordal level. M-mode derived systolic and diastolic IVS, LVFW, and LVID. From these parameters FS% was calculated. All parameters of the two groups were compared HR of 80, 100, 110, 120 130, and 140 bpm.
Results
All the horses tolerated the pharmacological stress test or the treadmill exercise test very well. The mean HR of group EXE during the last step of the treadmill test at 9m/sec was 208 ± 3 bpm (±
S.D.). In two horses of this group, the HR dropped very fast after the treadmill test and therefore echocardiographic recordings were not available at rates of 130 and 140 bpm. Furthermore, horses
of the EXE group showed more restlessness during echocardiographic examination than in group DOB at similar HR. This led to images of poorer quality at HR of 120 to 140 bpm.
Mean values of left ventricular M-mode parameters measured at baseline and at HR of 80, 100, 120, 130, and 140 bpm in group EXE and group DOB are given in table 3. IVSs increased in both groups with increasing HRs. In group EXE, mean IVSs at 130 bpm was significantly higher than at baseline, at 80 and at 110 bpm and mean IVSs at 140 bpm was significantly higher than at baseline, at 80, 100, and 110 bpm. In group DOB, mean IVSs at 110 bpm was significantly higher than at 80 bpm and mean IVSs at 120, 130, and 140 bpm were significantly higher than at baseline and at 80 bpm. IVSd tended to increase with increasing HR in the EXE group, although these changes were not significant. Mean IVSd increased steadily with increasing HR in the DOB group and values obtained at 130 and at 140 bpm were significantly higher than at baseline and at 80 bpm. Mean values of IVSs and IVSd were not significantly different between the two groups at any of the HRs.
In both groups, mean LVIDs decreased with increasing HRs. These changes were significant when comparing mean values at 110, 130 and 140 bpm to mean values at baseline in the EXE group. In the DOB group, mean LVIDs was significantly lower when comparing mean values at HR of 110, 120, 130, 140 bpm to mean values at baseline and at 80 bpm. The mean values of LVIDs obtained at 100, 110, and 120 bpm in the DOB group were significantly lower than corresponding values in the EXE group. Mean values of LVIDd tended to decrease with increasing HR in both groups, although changes were not significant between different HR or between the two groups with the exception of mean values at 100 and 110 bpm that were significantly lower than at baseline.
In the EXE group, mean LVFWs at 120 bpm was significantly higher than at 80 bpm, mean LVFWs at 130 bpm was significantly higher than at all lower HRs, and mean LVFWs at 140 bpm was significantly higher than at baseline, and at 80, 100 and 110 bpm. In the DOB group, mean LVFWs at 130 bpm was significantly higher than at baseline, and at 80, and 120 bpm, and mean LVFWs at 140 bpm was significantly higher than at baseline and at 80 bpm. Mean LVFWs at 130 bpm was significantly lower in the DOB group than in the EXE group. Mean LVFWd at 110 and 140 bpm were significantly higher than at baseline in the EXE group. Mean LVFWd at 140 bpm was significantly higher than at 80 bpm in the DOB group. Mean LVFWd was not significantly different between the two groups.
Mean FS% increased in both groups with increasing HRs. In the EXE group, mean FS% at 130 bpm was significantly higher than at baseline and mean FS% at 140 bpm was significantly higher than at baseline, and at 100 and 120 bpm. In the DOB group, mean FS% at 110, 120, 130 and 140 bpm were significantly higher than at baseline and at 80 bpm. Mean values of FS% were not significantly different between the two groups at any of the HRs.
Table 3: Left ventricular M-mode parameters in horses undergoing either post exercise stress echocardiography (n=5) or atropine/dobutamine stress echocardiography (n=5) at baseline and at HR of 80,100,120,130,140 bpm. Results given as mean ± S.D.
b, 80, 100, 110 ,120, = significantly different from baseline, and from measurements taken at a HR of 80,100,110, 120 bpm, respectively. * = significantly different from other group at the same HR. p < 0.05
Baseline HR 80 bpm HR 100bpm HR 110 bpm HR 120bpm HR 130bpm HR 140 bpm IVSs (mm) EXE 44.5±6.4 46.8±8.3 45.8±5.1 44.8±2.2 37.9±5.9 55.9±5.9b,80,100,110
57.1±4.9b,80,100,110,120
DOB 44.4±2.9 41.6±4.6 49.3±5.8 53.0±3.4b,80 53.3±3.4b,80 53.7±3.9b,80 57.1±3.6b,80,100
IVSd (mm) EXE 30.8±7.3 28.6±4.7 29.4±2.4 31.9±4.4 30.2±43.3 33.0±1.7 35.7±4.0100
DOB 28.5±1.6 30.3±2.4 31.7±4.9 34.6±2.5 b 33.1±3.9 35.5±1.7 b,80 37.7±1.3 b,80 LVIDs (mm) EXE 77.8±18.1 65.5±4.1 b 66.8±11.6b* 62.3±8.8 b* 64.6±8.4 b* 52.2±4.1 b,80,100,120
52.4±6.3 b,80,100,120
DOB 71.2±7.6 66.2±5.7 52.9±6.0b,80 * 47.6±4.6 b,80 * 48.4±4.6 b,80 * 48.5±2.2 b,80 47.1±5.7 b,80 LVIDd (mm) EXE 117.3±17.3 108.9±7.9 109.3±6.9 105.9±7.7 b 105.3±6.4 b 104.0±10.4 b 111.3±3.5 DOB 108.2±8.1 99.6±11.9 91.6±6.1b 88.5±2.4b 94.0±3.8b 96.9±3.2b 96.1±3.9b LVFWs (mm) EXE 37.2±2.3 34.1±2.3 39.9±7.9 40.7±5.8 43.3±9.380 56.2±6.4 b,80,100,110,120*
52.7±5.0 b,80,100,110
DOB 36.7±2.8 37.3±2.3 42.6±4.9 44.9±4.8 b,80 44.3±4.1 b,80 46.0±4.1 b,80,120 *
47.5±1.9 b,80 LVFWd (mm) EXE 22.1±4.4 26.8±3.7 27.0±6.1b 29.6±4.5 b 26.1±5.0 28.1±5.2 b 30.5±2.8 b,80 DOB 24.5±2.1 24.2±1.7 27.8±2.1 30.3±4.0 b 28.4±41.6 28.6±1.8 30.7±3.7 b,80 FS (%) EXE 33.9±8.5 39.6±6.2 38.8±10.0 40.8±9.8 38.3±10.3 49.5±6.2 b,100,120 52.8±1.6 b,80,100,110,120
DOB 34.2±4.0 33.2±4.3 42.0±8.180 46.2±5.2 b,80 48.4±5.4 b,80 49.8±4.0b,80 50.9±2.0 b,80,100
Discussion
In the present study, the combination of atropine and dobutamine produced a chronotropic effect that was comparable to post-exercise HRs. Four out of five horses of the DOB group reached a HR of 140 bpm already at a dobutamine infusion rate of 5 µg/kg/min. In contrast, two out of five horses in the EXE group had HR below 130 bpm in the post-exercise period at the time when the echocardiographic examination was performed. Echocardiographic measurements at 130 bpm at 140 bpm were only available in three horses of grou EXE, which made comparison to the dobutamine group less significant. In a study from Marr et al. (1999), where post-exercise echocardiographic recordings were performed within 2 to 3 minutes of the end of a treadmill exercise test to fatigue, the mean HR was 94.6 ± 8.8 (S.D.) bpm. In another study from Sampson et al. (1999), where horses performed a standardized treadmill test until VO2max, echocardiographic recordings where performed within 2 minutes of run completion and the mean HR was 111.7 ± 2.6 (mean ± S.E.) bpm. In a study of Durando et al. (2002), horses showed mean HR of 214.6 ± 2.9, 138.1 ± 3.3, 122.3 ± 4.2, and 98.1 ± 2.8 (mean ± S.E.) bpm at maximal speed, and 30, 60, and 120 seconds, respectively, after completion of a VO2max test. In this study, post-echocardiographic images were recorded at a mean HR of 99.4 ± 3.3 bpm. Only one study reported post-exercise echocardiographic parameters measured at a mean HR as high as 160 ± 7 bpm (Frye et al., 2003). The fact that HR declines rapidly within 1 to 2 minutes after completion of exercise is a well known phenomenon in horses (Banister and Purvis, 1968; Hall et al., 1976) and probably not only HR but also other cardiac variables return quickly to baseline. In humans undergoing stress echocardiography and in which echocardiography is performed in the post- exercise period, it is recommended to complete the echocardiography within 90 seconds of cessation of exercise, since mildly ischemic myocardial areas can improve within 60 seconds (Maurer and Nanda, 1981). To obviate the rapid drop in HR in the immediate post-exercise period, stress echocardiography is often performed during supine bicycle exercise, which allows recordings during high HR and prolongation of cardiac stimulation at a given level. This is impossible in horses because the echocardiographic window of heart, positioned below the muscles of the shoulder, is not accessible during exercise.
In the present study, echocardiographic images had a better quality in the DOB group than in the EXE at HR of 130 and 140 bpm, which is mainly due to fact that horses of the DOB group were calmer during the dobutamine infusion than horses of group EXE in the immediate post-exercise period. Poor quality images and high coefficients of variation in equine post-exercise echocardiography measurements were also described by Marr et al. (1999). Because of the poorer quality of the images and the lower number of animals available in group EXE at HR of 130 and 140 bpm, the comparison of the results of the present should focus on HR of 120 bpm and lower. Overall changes in the measured parameters from baseline up to a HR of 120 bpm were not significantly different between the groups and are characterized by an increase in wall dimensions and FS%, and a decrease in left ventricular chamber size with increasing HRs. These changes reflect the chronotropic and inotropic
stimulation of the heart during exercise and during stimulation with atropine and dobutamine. The only significant difference between the two groups in the present study was the significantly lower LVIDs in the DOB group at HR of 100, 110 and 120 bpm.
LVIDs, together with left ventricular wall thickness and left ventricular pressure, can be used to evaluate wall stress, which is an approximation of afterload (Cnota et al., 2003). In the study of Cnota et al. (2003), the wall stress was significantly lower in humans during stimulation with dobutamine than during exercise. Although left ventricular and systemic pressures were not measured in the present study, the same mechanism could be responsible for the lower LVIDs obtained in the DOB group when compared to the EXE group.
LVIDd, a non-invasive parameter of ventricular preload estimation, was not significantly different between the two groups. Cnota et al. (2003) compared LVIDd in humans undergoing supine exercise echocardiography and dobutamine echocardiography and found that LVIDd was significantly higher during exercise than during cardiac stimulation with dobutamine. The reason for this is supposed to be the muscular pump function of the legs assisting in venous return and therefore increasing preload during exercise (Weiss et al., 1979; Poliner et al., 1980). After cessation of exercise (and therefore after cessation of muscular pump function), venous return decreases, which is reflected by a decrease in LVIDd. In our study, echocardiography was performed after and not during exercise, and LVIDd was probably already reduced by the time echocardiography was performed. This finding is confirmed by earlier studies of Marr et al. (1999) and Sampson et al. (1999), who both found that there was no significant difference between pre- and post exercise LVIDd. In contrast, Frye et al. (2003) reported a significantly lower LVIDd in the immediate post exercise period when compared to LVIDd after maximal stimulation with dobutamine. The reason for this might be the fact that the authors of this study were able to perform post-exercise echocardiography at a mean HR of 160 bpm, thus probably very quickly after exercise, which is higher than in the present study and in the studies of Marr et al.
(1999) and Sampson et al. (1999). In concordance with this finding, Durando et al. (2001; 2002) described that left and right ventricular maximal systolic and diastolic pressures returned to resting values within 120 seconds of cessation of a VO2max exercise test in horses. This is probably also true for other cardiac variables.
Conclusion
The atropine/low-dose dobutamine stress test used in this study induced changes in left ventricular M- mode parameters that were similar to those obtained after treadmill exercise at HR between 80 and 120 bpm. In both groups, changes were more pronounced during systole than during diastole. Some changes were more pronounced during pharmacological stress test. HR drops very fast in the immediate post-exercise period and echocardiographic imaging is sometimes difficult due to respiration and excessive movement of the horses. Atropine/low-dose dobutamine stress test allows
echocardiographic examination at higher HR and with a better image quality than does exercise stress test. It could therefore be used as an interesting alternative to exercise echocardiography in horses.
3.4. Effect of atropine/low-dose dobutamine stress test on major determinants of myocardial performance in healthy horses (Study 4)
Introduction
The preceding studies have demonstrated that atropine/low-dose dobutamine is a well tolerated protocol for stress induction in equine stress echocardiography. It has been shown to induce a substantial increase in HR and cardiac output. The most convenient echocardiographic parameters to measure are the M-mode-derived LVID, LVFW, and IVS. It has been demonstrated that pharmacologically-induced changes in these parameters are very close to those induced by exercise.
However, these parameters do not necessarily reflect myocardial performance. The main determinants of myocardial performance are HR, preload, afterload and contractility. HR and preload, estimated by LVIDd, have already been investigated in the previous studies. However, other major parameters of myocardial performance, like contractility and afterload, have not been evaluated during atropine/low- dose dobutamine stress test. The prerequisite for stress echocardiography to be a valid tool for the detection of sub-clinical cardiac disease is mimicking exercise as closely as possible.
Therefore, the aim of the present study is (1) to measure STI, as non-invasive indicators of myocardial performance, and (2) to measure SVR and left ventricular walls stress, in order to clarify afterload during pharmacological stress test.
Material and methods
Six healthy horses (4 males and 2 females; age: 11.5 ± 5.5 years, mean ± S.D.; body weight: 421 ± 71 kg, mean ± S.D.) were used in this study. They were equipped with a 20G catheter inserted aseptically into the transverse facial artery. A fluid-filled pressure transducerwas connected to the lumen of the catheter and placed at the level of the scapulohumeral joint to reference all pressure recordings to the level of the right atrium. This catheter served for measurement of SAP, DAP, and MAP, which were continuously displayed on an oscilloscope throughout the whole protocol. Two 8.5 French catheter- introducerswere placed in the left jugular vein in aseptic conditions and under local anaesthesia. A 7.5 French wide and 130 cm long Swan-Ganz thermistorcatheter was inserted into the pulmonary artery via the lower introducer. A multi-opening catheter was passed into the right ventricle via the upper introducer. A fluid-filled extra-vascular pressure transducer was connected to the lumen of the catheters and placed at the level of the scapulohumeral joint to reference all pressure recordings to the level of the right atrium. Correct placement of the catheters was verified by the characteristic waveforms of the various cardiac chambers on an oscilloscope.
Pharmacological stress test protocol consisted of a single intravenous injection of 35 µg/kg of atropine sulphate followed by incremental steps of dobutamine infusion at rates of 2, 4, 6 and 8 µg/kg/min.
Every step of pharmacological challenge was extended to allow sufficient time for good quality echocardiographic recordings. Criteria to interrupt the procedure included excessive restlessness of the
horse, persistent cardiac arrhythmias, or less than 5% increase of HR in comparison to the preceding step.
All echocardiographic recordings were performed with a 2.5 MHz phased array sector transducer and recorded on VHS tape for later analyses. At the beginning of the procedure, the aortic systolic diameter was measured from the right parasternal long axis left ventricular outflow tract view using the leading edge to leading edge method at the aortic valvular annulus. At each step of pharmacological stimulation, a pulsed-wave Doppler tracing with the sample volume placed in the left ventricular outflow tract of a left parasternal view and a right parasternal left ventricular M-mode short axis view were recorded. Simultaneously, the integrated apex-base ECG was recorded.
At each step of the pharmacological stimulation, cardiac output was determined by four injections of 35 ml of ice-cold saline solution of seven seconds duration into the right ventricle and automatically computed by a cardiac output computer connected to the thermistor catheter. The mean of three measurements was calculated to estimate the cardiac output in each of the steps of the pharmacological challenge.
From the recorded echocardiographic images, the aortic flow velocities were measured by tracing the black/white interface of the pulsed wave Doppler signal planimetrically. By using the software of the ultrasound device, the stroke volume was calculated using the following formula: stroke volume = (aortic diameter/2)2 * π * velocity time integral of the aortic flow. Cardiac output was calculated as the product of stroke volume and HR. The HR was calculated from recorded ECG tracings by measurement of the duration of 3 consecutive RR and extrapolation to 60 sec. Stroke volume was divided by body weight to calculate LVFW were measured from the recordings of the right parasternal short axis view of the left ventricle in M-mode. The following STI were measured from the Doppler tracings of the aortic flow. The PEP was measured from the onset of the QRS-complex to the beginning of the spectral wave related to the aortic flow. The LVET was measured from the beginning to the end of the spectral wave related to the aortic flow. The PEP/LVET index was calculated by dividing PEP by ET (Boon, 1997; Lightowler et al., 2002). The Vcirc fibre shortening was calculated according the formula: (LVIDd-LVIDs)/(LVIDd * LVET). Furthermore, Vcirc was corrected for the influence of the HR according to the formula (LVIDd-LVIDs)/(LVIDd * LVET) * √RR.. The SVR was calculated by using the following simplified formula: SVR = 80 x MAP / CO, where 80 is a conversion factor to convert into dyne * s/cm5 (Klabunde 2004). End-systolic left ventricular wall stress (WS) was calculated by the following formula: WS = SAP * (LVIDs / LVFWs * (1+
LVFWs/LVIDs)) * 0.34, where 0.34 is a conversion factor to convert mmHg into g/cm² (Lang et al., 1986).
Each variable was analysed separately with a repeated measures model with an auto-regressive variance-covariance matrix for the repeated measures. Least-square means were computed for each dose. Differences were considered significant at P < 0.05.
Results
Five of the six horses tolerated the procedures of pharmacological stimulation and thermodilution well. One horse showed excessive restlessness during the stimulation with 8 µg/kg/min of dobutamine and the test had to be interrupted before the acquisition of data at that rate. Data of this step were unavailable for later analysis. Four out of six horses showed isolated ventricular premature complexes, one horse at a dobutamine infusion rate of 4 µg/kg/min, the three others at a dobutamine infusion rate of 6 µg/kg/min. In all horses, ventricular premature complexes disappeared after discontinuation of dobutamine infusion. In one horse, systemic arterial blood pressure measurement was unavailable at a dobutamine infusion rate of 8 µg/kg/min due to a technical problem of the arterial catheter. Data of MAP, SVR, and WS were therefore unavailable for this horse at a dobutamine infusion rate of 8 µg/kg/min.
Results of echocardiographic-derived STI are displayed in figure 10. Mean LV and mean ET decreased significantly with increasing stimulation; however the PEP/LVET ration remained unchanged throughout the whole protocol. Mean velocity of circumferential fibre shortening and heart–rate corrected Vcirc increased significantly with increasing stimulation as displayed in figure 11.
Systemic vascular resistance measured by Doppler echocardiography and thermodilution, decreased significantly during pharmacological stimulation. However, afterload estimated as left ventricular wall stress did not change significantly during the stress test as displayed in figure 12.
Figure 10: Systolic time intervals in 6 healthy horses undergoing atropine/low-dose dobutamine stress echocardiography. LVET = ejection time, PEP = pre-ejection period, PEP/LVET = pre-ejection period/ejection time ratio. B = baseline, A = atropine at 35µg/kg; 2,4,6,8, = dobutamine infusion rate at 2, 4, 6, 8 µg/kg/min, respectively. * = significantly different from baseline and from atropine (p <
0.05).
0 0,1 0,2 0,3 0,4 0,5 0,6
B A 2 4 6 8
Dose
seconds PEP
LVET PEP/LVET
* * * *
*
* * *
0 0,1 0,2 0,3 0,4 0,5 0,6
B A 2 4 6 8
Dobutamine rate (µg/kg/min)
STI (seconds)
PEP LVET PEP/LVET
* * * *
*
* * *
Figure 11: Mean velocity and HR corrected mean velocity of circumferential fibre shortening in 6 healthy horses undergoing atropine/low-dose dobutamine stress echocardiography. 0 = baseline, A = atropine at 35 µg/kg; 2, 4, 6, 8 = dobutamine infusion rate at 2, 4, 6, 8 µg/kg/min, respectively. * = significantly different from atropine and baseline, ** = significantly different from baseline, (p <
0.05).
Figure 12: Doppler echocardiographic- and thermodilution-derived SVR in 6 healthy horses undergoing atropine/low-dose dobutamine stress echocardiography stress echocardiography. 0 baseline, A = atropine at 35 µg/kg; 2, 4, 6, 8, = dobutamine infusion rate at 2, 4, 6, 8 µg/kg/min, respectively. * = significantly different from baseline (p < 0.05).
0 0,5 1 1,5 2 2,5 3 3,5
0 A 2 4 6 8
Dobutamine rate (µg/kg/min)
Vcirc (circ/sec)
rate-corrected Vcirc Vcirc
*
*
*
*
*
*
*
**
*
0 50 100 150 200 250 300 350 400 450 500
0 A 2 4 6 8
Dobutamine rate (µg/kg/min) SVR (dyne*sec*cm-5 )
0 20 40 60 80 100 120
WS (g/cm²) SVR doppler SVR Thermo WS
Discussion
Systolic time intervals are influenced by HR, contractility and loading conditions. Therefore, STI should not be considered as indicators of myocardial contractility alone, but rather as non-specific indicators of cardiac performance.
Mean PEP after atropine administration was not significantly different from baseline, which confirms studies in humans and dogs, where pacing and atropine-induced increase in HR did not alter PEP (Harris et al., 1967; Pipers et al., 1978; Hassan and Turner, 1983). However, stimulation with dobutamine induced a significant reduction of PEP.
In the present study there was an obvious negative correlation between HR and LVET. This finding confirms a study of Lightowler et al. (2003), who found a high correlation between ET and HR in horses. To eliminate the effect of HR on ET, different formulas to calculate rate-corrected ejection time index (ETI) has been proposed for humans: ETI = ET + (1.7*HR) (Weissler et al., 1968), in dogs:
ETI = ET + (0.55* HR) (Pipers et al., 1978) and in ponies: ETI = ET + (0.0036 * HR) (Amend et al., 1972). The present study revealed a linear regression analysis of ET to HR of y = -421.27x + 235.7, which could be used to calculate a heart-rate corrected ETI for the horses of the present study by application of the formula ETI = ET + (HR * slope of the regression analysis), as proposed by Atkins and Snyder (1992).
Several studies proposed the use of PEP/LVET ratio as a rate independent-measure of systolic function in humans (Weissler et al., 1968; Cokkinos et al., 1976). Howver, other studies, demonstrated a significant correlation between PEP/LVET and HR (Pipers et al., 1978; Spodick et al., 1984).
During pharmacological stimulation in the present study PEP/LVET ratio did not change, which is probably due to the summation of different opposing factors. In general, increases in preload and reductions in afterload mimic STI changes similar to enhanced myocardial work and include a reduced PEP and PEP/LVET ratio (Hassan and Turner, 1983; Weissler, 1983; Lewis et al., 1977). Conversely, reduction in preload and increases in afterload mimic diminished left ventricular performance and is reflected by an increased PEP, and PEP/LVET ratio and a reduced LVET (Hassan and Turner, 1983;
Weissler, 1983; Lewis et al., 1977). The effects of afterload and inotropic agents are more complex because both an increased and a decreased afterload will lead to prolonged LVET and positive as well as negative inotropes will reduce LVET.
Cardiac disease affects the pattern of STI. With aortic stenosis, the increase in blood flow resistance will decrease PEP and prolong LVET, while failure of the pump function (cardiomyopathy) or counter-current blood flow (interventricular communication without right hypertension, mitral regurgitation) increases PEP decreases LVET (Atkins and Snyder, 1992).
Mean velocity of circumferential fibre shortening is considered as a preload-independent measure of contractility, but it is affected by afterload (Atkins and Snyder, 1992). To overcome the influence of HR on Vcirc, a rate-correcting formula developped by Colan et al., (1984) was used in the present
study. Both, rate corrected and uncorrected Vcirc significantly increased with dobutamine stimulation, which is similar to observations made in dogs (Lang et al., 1986) and humans (Colan et al., 1984) and demonstrates the positive inotropic effect of dobutamine.
In clinical settings, SVR is commonly used as a measurement of ventricular afterload. It is generally calculated by MAP minus the mean right atrial pressure time the conversion factor 80 dyne *cm-2/ mmHg divided by the thermodilution-derived cardiac output. This formula has been validated angiographically (Grossmann and McLaurin, 1980). However, Klabunde (2004) suggested that mean right artrial pressure can be neglected in order to facilitate measurements in clinical settings. In horses during submaximal exercise, cardiac output increases approximately 4-fold (Thomas et al.,, 1983), while MAP increases 1.7-fold (Hornicke et al, 1977). Using the formulas mentioned above, the calculated SVR value during sub-maximal would beapproximately 40% of the resting value.
Dobutamine is known to reduce SVR (Leier et al., 1977). In the present study, dobutamine reduced thermodilution-derived SVR from 324 to 137 dyne * s/cm5, which represents changes observed during submaximal exercise. However, SVR may not adequately assess left ventricular afterload (i.e.
ventricular internal fibre load during systole) since it reflects only peripheral vasomotor tone (Lang et al., 1986).
Left ventricular afterload is defined as the force opposing ventricular fibre shortening during left ventricular ejection (Weber et al., 1982; Nichols and Pepine, 1982). It is not synonymous with systemic arterial pressure, peripheral vasomotor tone, or SVR. Left ventricular afterload it can be more appropriately measured on the basis of left ventricular wall stress during ejection. It includes factors both internal and external to the myocardium. According to the La Place’s principle, left ventricular wall stress is directly related to chamber dimension and pressure and inversely related to wall thickness (Gould et al., 1974; Grossmann et al., 1975; Weber and Janicki, 1980). In the present study, wall stress did not change significantly during the pharmacological stimulation which is probably due to the sum of effects of the decrease in left ventricular dimension and increasing SAP and LVFW.
Conclusion
This study demonstrated that PEP, LVET, Vcirc and rate-corrected changed significantly with increasing stimulation and that the observed changes suggest an increased contractility during the stress test.
During the stress test, afterload appeared to decrease, when estimated on the basis of SVR. However, afterload did not appear to change significantly during the stress test, when estimated on the basis of left ventricular wall stress.
3.5. Cardiac power output measurement using a dobutamine stress test in healthy horses (Study 5)
Introduction
In the preceding studies the value of atropine/low-dose dobutamine as a diagnostic tool for the detection of exercise-induced myocardial dysfunction has been described. The effects of the pharmacological stimulation on HR, loading conditions and contractility have been described. The most convenient parameters to be measured during the test were demonstrated to be M-mode parameters of the left ventricle. Pharmacologically-induced changes in these parameters are similar to these induced by exercise, which makes the test an interesting for the diagnosis of exercise-related pathologies. However, in human stress echocardiography is used as a prognostic tool as well, in the form of cardiac power output (CPO) measurement. It has been suggested that CPO, the product of cardiac output and MAP, is an excellent parameter to represent cardiac performance and has been shown to be closely related to long-term outcome in low- and mid-grade chronic heart failure. Cardiac power output can be measured either during exercise or during pharmacological stimulation, and consists of MAP and cardiac output measurement during maximal cardiac stimulation.
The aim of the present study was (1) to establish reference values of CPO for healthy horses undergoing pharmacological stress testing and (2) to compare CPO measured by thermodilution to CPO measured by Doppler echocardiography, in order to establish a non-invasive method of CPO measurement in horses.
Material and methods
Six healthy horses aging from 3 to 19 years and weighing between 330 and 470 kg were used in this study. They were equipped with two 8.5 French catheter-introducersplaced in the left jugular vein in aseptic conditions and under local anaesthesia. A 7.5 French wide and 130cm long Schwan-Ganz thermistor catheterwas inserted into the pulmonary artery via the lower introducer. A multi-opening catheter was passed into the right ventricle via the upper introducer. A fluid-filled extra-vascular pressure transducer was connected to the lumen of the catheters and placed at the level of the scapulohumeral joint to reference all pressure recordings to the level of the right atrium. Correct placement of the catheters was verified by the characteristic waveforms of the various cardiac chambers on an oscilloscope.
A 20G catheter was inserted aseptically into the transverse facial artery. A fluid-filled pressure transducerwas connected to the lumen of the catheter and placed at the level of the scapulohumeral joint to reference all pressure recordings to the level of the right atrium. This catheter served for the measurement of MAP throughout the whole protocol, which consisted of a single intravenous injection of 35 µg/kg of atropine followed by incremental steps of dobutamine infusion, with rates of 2, 4, 6 and 8 µg/kg/min. Each step lasted for five minutes. Criteria to interrupt the procedure included excessive
restlessness of the horse, persistent cardiac arrhythmias, or less than 5% increase of HR in comparison to the preceding step.
Doppler echocardiographic images of the aortic flow were recorded from a left parasternal left outflow tract view according to Long et al., (1992). In the same way as in the preceeding study, these images served for the measurement the flow velocity integral and subsequently for the calculation of the stroke volume. Simultaneously, an apex-base ECG was recorded for subsequent calculation of HR.
The same recordings were repeated three minutes after the onset of each step. During the recordings of the aortic flow, cardiac output was determined by four injections of 35 ml of ice-cold saline solution of seven seconds duration into the right ventricle and automatically computed by a cardiac output computer connected to the thermistor catheter. The mean of three measurements was calculated to estimate the cardiac output in each of the steps of the pharmacological challenge.
Cardiac output was calculated as the product of stoke volume and HR. stroke volume was divided by body weight to calculate stroke index. Cardiac power output in watts was calculated for each step and for the two methods of cardiac output measurements according to the following formula: CPO = (CO
* MAP) * k, where k is a conversion factor (2.22*10-3) to convert into watts (Chantler et al., 2005).
Results
All horses tolerated the procedures of pharmacological stimulation and thermodilution well. However, in horses the test had to be interrupted prematurely at a dobutamine infusion rate of 8 µg/kg/min of dobutamine because of obvious discomfort of the horse. Four horses showed isolated ventricular premature complexes at dobutamine infusion rate of 4 to 8 µg/kg/min, but all horses retruned to normal sinus rhythm after the end of the pharmacological stimulation. The presence of premature ventricular complexes did not necissitate a premature interruption of the test. Data of MAP and CPO were unavailable in two horses at a maximal dobutamine infusion rate of 8 µg/kg/min.
Cardiac output measured by thermodilution and by Doppler echocardiography increased significantly with increasing pharmacological stimulation. Mean cardiac output measured by Doppler echocardiography (CODo) after the stimulation with atropine and at a dobutamine infusion rate of 2µg/kg/min was significantly higher than at baseline. Mean CODo at infusion rates of 4, 6, and 8 µg/kg/min were significantly higher than at an infusion rate of 2 µg/kg/min. Mean CO measured by thermodilution (COTD) at an infusion rate of 2 µg/kg/min was significantly higher than at baseline and after atropine administration. Mean COTD at each of the infusion rates from 2 to 8 µg/kg/min were significantly higher than the preceding step of the pharmacological stimulation.
Linear regression analysis between COTD et CODo measurements demonstrated a correlation coefficient (R²) of 0,7356 and the equitation: COTD = 1,0242 * CODo - 11,937. Bland-Altman analysis demonstrated a mean bias between the two methods (COTD vs. CODo) of 10.6 L/min. The limits of agreement were -25.88 and 47.12 L/min. The 95% confidence interval for the upper limit of agreement
was 34.01 to 60.29 L/min, the 95% confidence interval for the lower limit of agreement was -18.68 to -33.07 L/min, and the 95% confidence interval for the mean bias was 5.54 to 15.69 L/min.
Mean HR increased constantly during the pharmacological stimulation and was significantly higher at each step of the challenge than at the preceding one, except of mean HR at an infusion rate of 8 µg/kg/min, which was not significantly different from the preceding step. Maximal mean HR was 146
± 5.5 bpm at a dobutamine infusion rate of 8 µg/kg/min.
Mean MAP after the administration of atropine and during dobutamine infusion was significantly higher than at baseline. Mean HR at an infusion rate of 4 µg/kg/min was higher than at 2 µg/kg/min and mean HR at infusion rates of 6 and 8 µg/kg/min were not significantly different from each other, but significantly higher than at 4 µg/kg/min.
Figure 13: Cardiac power output in six healthy horses undergoing atropine/low-dose dobutamine stress test measured by thermodilution and by Doppler echocardiography. 0 = baseline, A = atropine at 35 µg/kg/min; 2, 4, 6, 8 = rates of dobutamine infusion in µg/kg/min. * = significantly different from preceding dose
Results of CPO measurement are displayed in figure 13. Mean Doppler-derived CPO (CPODo) at
dobutamine infusion rate of 2 µg/kg/min was significantly higher than at baseline and than after the administration of atropine. Mean CPODo at infusion rates of 4, 6, and 8 µg/kg/min were higher than at an infusion rate of 2µg/kg/min. Maximal mean CPODo was 60.4 ± 5.1 watts at a maximal dobutamine infusion rate of 8 µf/kg/min. Mean thermodilution-derived CPO (CPOTD) at an infusion rate of 2 µg/kg/min was significantly higher than at baseline and than after administration of atropine. Mean
0 10 20 30 40 50 60 70 80
0 A 2 4 6 8
Dobutamine rate (µg/kg/min)
CPO in Watts
CPOdo CPOtd
