Summary Six adult ponies were trained calmly to assume and maintain left lateral recumbency without the use of sedative or immobilising agents. During a 30 min recumbent period, pHa, arterial oxygen and carbon dioxide tensions (Pao2 and PacO2) and heart and respiratory rates were monitored at regular intervals to evaluate ventilatory response. Overall, there were no statistically significant differences found between mean control and recumbent or final standing values. When light‐weight ponies were compared to heavyweight ponies, only mean Pao2 at 10 mins recumbency was different. This information supports the hypothesis that ventilation impairment during the first 30 mins of equine general anaesthesia is primarily drug‐mediated, rather than initiated by recumbency per se.

We evaluated cardiac cycle length variability in ponies at rest and during strenuous exercise with and without premedication with atropine. In the absence of premedication, cardiac cycle length at rest was 1,112 +/- 53 ms, the individual cardiac cycle length standard deviation (SDCL) was 75 +/- 23 ms, and the individual cycle length coefficient of variation (CVCL) was 6.32 +/- 1.62. Exercise significantly decreased (P < 0.05) all three indexes (290 +/- 9 ms, 5 +/- 1 ms, and 1.65 +/- 0.20, respectively). Atropine premedication significantly reduced resting cardiac cycle length (685 +/- 46 ms), SDCL (10 +/- 2 ms), and CVCL (1.45 +/- 0.19) compared with nonpremedicated values. Cardiac cycle length was significantly decreased by exercise after atropine premedication, but no statistically significant changes occurred in SDCL or CVCL. Thus, although considerable cardiac cycle length variability exists in nonpremedicated ponies at rest, it is nearly completely abolished by strenuous exercise. The absence of significant differences between the indexes of variability during exercise without premedication, at rest after atropine, and during exercise after atropine indicates that cardiac cycle length variability in the pony is mediated primarily through activity of the parasympathetic system.

Myocardial and haemodynamic responses to strenuous treadmill exercise were monitored with chronically implanted instrumentation in seven physically untrained ponies. In two other ponies, haemodynamics were monitored using a conventional catheter technique. During exercise (mean +/- sem heart rate = 203 +/- 3 beats/min), left ventricular peak systolic ahd end-diastolic blood pressure significantly increased from 125 +/- 2 to 208 +/- 6 mmHg and from 29 +/- 1 to 58 +/- 3 mmHg, respectively (P less than 0.05). Peak positive first derivative of left ventricular pressure (dP/dt) was also increased significantly, (1397 +/- 144 to 6016 +/- 325 mmHg/sec), as were myocardial segment velocity of shortening during systole (dL/dtsys) and stroke work (193 and 109 per cent, respectively). Paradoxically, segment systolic shortening was reduced significantly (P less than 0.05) with respect to resting control values for 10 mins after cessation of exercise, with the maximal reduction of 22 per cent occurring at 2 mins post exercise. In addition, left ventricular segment stroke work decreased significantly to 67 per cent of resting control value at 2 mins post exercise. This study demonstrates that analysis of regional myocardial performance in conjunction with haemodynamic data permits a detailed description of cardiac events during and immediately following exercise, facilitating definition of changes which are not readily reflected by haemodynamics. Based on these findings, the authors conclude that untrained ponies may readily complete short bouts of near-maximal exercise, yet experience significantly diminished left ventricular function in the immediate post exercise period.

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The response of coronary collaterals in nine ponies subjected to repeated reversible occlusions (2 min duration, 30 min interval) of the left anterior descending coronary artery was studied at rest. Each pony was instrumented with a Doppler flowmeter and hydraulic cuff occluder around the left anterior descending coronary artery, left ventricular subendocardial sonomicrometers, and a left ventricular micromanometer. Initial occlusions increased end diastolic myocardial segment length by 3% and decreased segment systolic shortening, stroke work, and velocity of shortening by 103%, 95%, and 79% respectively in the left ventricular apex. Left ventricular systolic and end diastolic pressure, peak positive dP/dt, and heart rate were not significantly changed by occlusion. After 421(70) (mean(SEM)) occlusions no sustained alterations in myocardial segment function occurred in response to occlusion. Thus the presence of a subendocardial plexus did not protect against a severe loss of myocardial segment function when the ponies were initially subjected to occlusions of the left anterior descending coronary artery. However, repeated reversible occlusions enhanced coronary collateral blood flow such that it was adequate to maintain left ventricular function in the absence of left anterior descending coronary artery flow. It is concluded that the pony is highly suitable for use in studies of coronary collateral circulation because of its coronary anatomical similarity to man and its capacity to develop functional collateralisation.