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Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois, Urbana 61801.

1. Left ventricular (LV) myocardial O2 extraction was studied in five healthy ponies which had catheters implanted in the great cardiac vein and main pulmonary artery 15-30 days before the study. The abdominal aorta was percutaneously catheterized to sample arterial blood. 2. In addition, phasic LV and aortic pressures, LV dP/dtmax and rate-pressure product were also studied; dP/dtmax is the maximal rate of rise of the left ventricular pressure during the isovolumic phase, and is considered an index of myocardial contractility. Measurements were made at rest (control) and during adenosine infusion (3 mumol kg-1 min-1) at rest, moderate exercise (heart rate 169 +/- 10 beats min-1), heavy exercise (heart rate 198 +/- 7 beats min-1), maximal exercise (heart rate 232 +/- 7 beats min-1), and adenosine infusion (3 mumol kg-1 min-1) during maximal exercise (heart rate 230 +/- 6 beats min-1). 3. In resting ponies, LV arterial to coronary venous O2 content difference (delta LVa-v O2) was 8.9 +/- 0.5 ml dl-1 and O2 extraction was 59.9 +/- 2.2%. Adenosine infusion at rest decreased delta LVa-v O2 and O2 extraction precipitously (2.6 ml dl-1 and 14.3 +/- 1.7%, respectively), thereby indicating superfluous LV myocardial perfusion. 4. Moderate, heavy and maximal exercise increased delta LVa-v O2 to 185, 194 and 218% of its control value and O2 extraction rose to 71 +/- 2, 75 +/- 1.5 and 78 +/- 0.9%, respectively. The widening of the delta LVa-v O2 gradient was due to the increased arterial O2 content during exercise. 5. Combining these observations with equine myocardial perfusion, the LV O2 consumption was calculated to be 7.8, 47.9 and 103.6 ml min-1 100 g-1 at rest, moderate and maximal exercise. In order to achieve the 13.4-fold increase in LV O2 consumption, the LV perfusion rose only 6-fold; the rest being met by widening the delta LVa-v O2. 6. Adenosine infusion during maximal exercise decreased delta LVa-v O2 and O2 extraction (10.7 +/- 1 ml dl-1 and 45%, respectively; P less than 0.0001). This indicated that coronary vasodilator capacity was not being completely expended in maximally exercising ponies. It is concluded that coronary circulation is unlikely to be a limiting factor to further exertion in ponies. Organ/tissue perfusion studies in exercising ponies have demonstrated that of all working muscles, the left ventricular (LV) myocardium received the highest level of blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)

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