PART 7: SUMMARY

PART 7: SUMMARY

PART 7: SUMMARY

Two major reasons demonstrate the need for the development of new diagnostic techniques in equine cardiology: first, there is no evidence that the incidence of poor performance of cardiac origin is declining and second no new technique has been described for the use in horses since the development of Doppler echocardiography in the late 80ies and early 90ies (Reef et al., 1989, Reef, 1990, Long et al., 1992). Stress echocardiography is certainly the only technique for the diagnosis of exercise-

induced myocardial dysfunction, which occurs in about 5 to 8 % of horses examined for poor performance and which is characterized by lack of significant lesions at resting examination (Reef 1997, Martin et al., 2000). Furthermore, stress echocardiography might be useful in clinical settings for the assessment of the behaviour of valvular insufficiencies during exercise, in order to refine their prognosis (Gehlen 2005b). In research settings, stress echocardiography might become a tool for the evaluation of the relationship between valvular disease and ventricular dysfunction.

Stress echocardiography in the form of post-exercise stress echocardiography has already been described in the context of exercise-induced myocardial dysfunction. Unfortunately, post-exercise stress echocardiography only partly mimics the conditions during exercise (Durando et al., 2002).

Maximal or near-maximal exercise is needed to induce measurable changes in echocardiographic parameters in the immediate post-exercise period. It is a technically demanding procedure that has to cope with bad quality images due to intense respiration after strenuous exercise, and still a rapid drop in heart rate, which decreases the sensibility of the test. Pharmacological stress induction, in the same manner as used in human medicine for the diagnosis and prognosis of coronary artery disease has been described to overcome the limitations of post-exercise stress echocardiography but proved to be cardiomyotoxic (Frye et al., 2003).

The aim of our studies was to develop a pharmacological stress protocol that is well tolerated, easy to perform and that induces changes that mimic exercise as closely as possible. In the first study we compared the effect of a conventional high-dose dobutamine to a low-dose dobutamine protocol in previously atropinized ponies. We suspected that the reason for the side effects of the high-dose dobutamine, notably the adverse reactions of the horses and the lack of increase of HR are due to a dobutamine-induced high-pressure reflex of the vagal nerve and that these side effects can be abolished by premedication with the vagolytic agent atropine. Indeed, atropine-premedicated ponies need dobutamine doses that were eight times lower that in the conventional group. Cardiac stimulation, measured by Doppler echocardiography-derived cardiac output as a global indicator of cardiac performance, was slightly higher, side effects were less commonly observed and response in HR and stroke volume were showed less inter-individual variability in the atropine-premedicated group. This study clearly demonstrated that atropine/low-dose dobutamine protocol is superior over other forms of stress induction.

The second study served for the identification of echocardiographic parameters most conveniently measured during stress echocardiography. Doppler echocardiographic-derived cardiac output is

subject to measurement errors and measurable changes in cardiac output will only occur during advanced states of cardiac disease. Therefore cardiac output does not seem to be an ideal parameter to be measured during stress echocardiography. Six healthy horses underwent the pharmacological stress protocol described in the first study and at each step of pharmacological stimulation B- and M-mode echocardiographic parameters were measured. Most important changes were induced in the M-mode parameters: systolic left ventricular internal diameter, systolic and diastolic left ventricular free wall thickness, systolic and diastolic interventricular septum thickness and fractional shortening. Changes in B-mode parameters, although less pronounced, reflected changes observed in the M-mode parameters but measurements were more time consuming and showed a higher variability due to measurement errors. Furthermore, changes between neighboured doses of the incremental dobutamine infusion were not always significant and the omission of every second dose would considerably reduce test time.

After the first two studies, the ideal protocol of atropine/low-dose dobutamine was determined to consist of atropine premedication of 35 µg/kg bodyweight intravenously, followed by dobutamine in incremental infusion rates of 2, 4, 6, and 8 µg/kg/min bodyweight. The aim of the third study was to compare changes in M-mode echocardiographic parameters induced by this protocol to those changes induced by exercise. Healthy standard or standard crossbred horses underwent either near-maximal treadmill exercise or pharmacological stimulation, and M-mode parameters were compared at resting HR and at HR of 80, 100, 110, 120, 130, and 140 bpm. We experienced the same difficulties as authors of other studies: HR drops very fast in some of the horses and echocardiographic recordings were not available in all horses at high heart rates. In the other horses image quality is sometimes disappointing because of movement of the horses and respiration. M-mode echocardiographic parameters were not significantly different between the two groups with the exception of LVIDs at 100, 110 and 120 bpm, which was lower in the pharmacological stimulation group than in the exercise group. In the exercise group, most of the significant changes were limited to HR of 130 and 140 bpm.

This underlines the importance of intensive exercise and the need for immediate image acquisition.

Test sensibility of post-exercise stress echocardiography still might be questionable.

The preceding studies showed that atropine/low-dose dobutamine is well tolerated and that changes in echocardiographic M-mode parameters with the exception of LVIDs are similar to those observed in the post-exercise period. The reason for this might be explained by a lower afterload during pharmacological stimulation. However precise measurements are needed to determine loading conditions of the heart. Preload and afterload, together with HR and contractility are the major determinants of cardiac performance. HR response to atropine/low-dose dobutamine has been extensively studied in the first studies. Therefore, the evaluation of the response of preload, afterload and contractility to the pharmacological stress test is the aim of the fourth study. Systolic time intervals are influenced by heart rate, contractility and loading conditions. Therefore systolic time

intervals should not be considered as direct indicators of myocardial contractility, but rather non- specific indicators of cardiac performance.

Mean resting PEP of the present study were at the upper end of the range of values reported for healthy horses at rest ranging from 0.07 ± 0.01 sec to 0.21 ± 0.02 sec (mean ± s.d.) (Lightowler 2003, Lescure and Tamzali 1984). Mean PEP decreased with increasing pharmacological stimulation. 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 (Hassan and Turner 1983, Pipers et al., 1978, Harris et al 1967). Mean resting ET of the present study was within the range of 0.39 ± 0.03 to 0.53 ± 0.1 sec (mean ± s.d.) normally observed in horses (Lescure and Tamzali, 1984; Lightowler 2003). In the present study there was an obvious negative correlation between HR and ET (Figure 3). This finding confirms a study of Lightowler et al (2003), who found a high negative correlation between ET and HR in horses.

Several studies proposed the use of PEP/ET ratio as a rate independent-measure of systolic function in humans (Weissler et al., 1968, Cokkinos et al 1976). Other studies however, demonstrated significant correlation of PEP/ET and HR (Pipers et al, 1978; Spodick et al 1984). In the present study, the mean resting PEP/ET ratio 0.36 ± 0.06 (± s.e.) was higher than that reported by Lightowler (2003), but similar to that reported by Lescure and Tamzali (1984). During pharmacological stimulation of the present study PEP/ET 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 reduced PEP and PEP/ET ratio (Hassan and Turner, 1983, Weissler 1977, Lewis et al 1977). Conversely, reduction in preload and increases in afterload mimic diminished left ventricular performance and is reflected by increased PEP, PEP/ET ratio and reduced ET (Hassan and Turner, 1983, Weissler 1977, Lewis et al 1977). The effects of afterload and inotropic agents are more complex because both increased as well as decreased afterload will lead to prolonged ET and positive as well as negative inotropes will reduce ET.

Mean velocity of circumferential fibre shortening (Vcirc) 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 of 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 demonstrating the positive inotropic effect of dobutamine.

In clinical settings, systemic vascular resistance is commonly used as a measure of ventricular afterload. It is calculated by as the mean aortic 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). Klabunde (2004) suggested that right atrial pressure can be neglected in order to facilitate measurements in clinical settings. SVR has been measured extensively in horses undergoing general anaesthesia and is reported

to be in the range of approximately 200 to 400 dyne * s/cm⁵ (Yamanaka et al., 2001, McDonell et al., 2006, Teixeira Neto et al., 2004) which is similar to baseline values of the present study. Similar values of 260 dyne * s/cm⁵ have also been reported for standing sedated horses (Cruz et al, 2004).

SVR is a product of cardiac output and mean arterial pressure and cardiac output increases approximately 4-fold with sub-maximal exercise (Thomas et al.,, 1983), while MAP increases 1.7-fold with sub-maximal exercise (Hornicke et al, 1977), which would result in a hypothetical decrease of 68%. Dobutamine is known to reduce systemic vascular resistance (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 peripheral arterial pressure, peripheral vasomotor tone, or systemic vascular resistance. Rather it can be more appropriately thought as 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, Weber and Janicki 1980, Grossmann et al., 1975). 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 decreasing left ventricular dimension and increasing mean arterial pressure. Left ventricular free wall thickness slightly increased.

In conclusion, this study demonstrated that the systolic time intervals, PEP, ET, Vcirc and rate - corrected Vcirc are reliable indicators of ventricular performance in horses undergoing dobutamine stress test. All these parameters changed significantly with increasing stimulation.

Afterload, decreases significantly during the test when measured as SVR but decreases only slightly when measured as left ventricular wall stress.

Finally, the objective of the last study was to investigate the prognostic value of stress echocardiography in the form of cardiac power output (CPO) measurement. Therefore CPO, the product of cardiac output and mean arterial pressure was measured in six healthy horses by Doppler echocardiography and by thermodilution. CPO at rest measured by thermodilution and by Doppler echocardiography was 10.7 ± 3.3 and 13.7 ± 4.5 watts, respectively. CPO increased significantly with pharmacological stimulation and reached maximal CPO at a dobutamine infusion rate of 8 µg/kg/min.

Maximal CPO measured by thermodilution and by Doppler echocardiography was 70.4 ± 3.6 and 60.4

± 5.1 watts, respectively. Correlation between the two different methods of CPO measurement was R²

= 0.82. This study is the first report that describes the estimation of CPO in horses.