The effect of cooling the heart on the characteristics of reactive hyperemia (RH) of the coronary vessels has not been investigated, although such studies have been conducted in the intact and sympathectomized limbs of man, 1 ' 2 and in the blood-perfused hindlimbs of cats (unpublished studies of Pappenheimer, Rapela and Badeer). The purpose of the present study was to investigate the quantitative aspects of the RH of the coronary vessels at different hypothermic temperatures of the working

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... ardium. The dog heart-lung preparation (HLP) served as the experimental preparation. Methods Mongrel dogs of both sexes weighing 11 to 20 kg were anesthetized with pentobarbital sodium, (30 mgAg •?). Bilateral vagotomy and midsternal thoracotomy were performed. The phrenic nerves were excised and the left coronary artery was dissected at its origin. Subsequently, the conventional HLP was established using defibrinated blood from donor dogs anesthetized with chloroform. The lungs were ventilated with 100% oxygen at a tidal volume of 150 to 200 ml and a frequency of 14 per minute. A Gregg coronary cannula was connected to the arterial side of the circuit with a recording Shipley-Wilson rotameter of 200 ml capacity interposed between the two. The left coronary artery was cannulated by way of the left subclavian artery. Aortic pressure was monitored with a mercury manometer connected to a Sanborn transducer, model 267A. Coronary flow and arterial pressure were recorded by means of a Sanborn polyviso recorder. Cardiac output was set between 700 and 900 ml/min and the mean arterial pressure at 100 mm Hg. In a few pilot experiments, the temper-From the ature of the blood in the venous inflow cannula was compared with that of the left ventricular myocardium registered by means of a needle electrode of a Yellow Springs tele-thermometer. The values checked so well that in all the experiments to be reported the temperature of the blood in the venous cannula was recorded and taken to represent the myocardial temperature. Under normothermic conditions, RH was induced by clamping the left coronary inflow for a period of 10 seconds. Subsequently, the temperature of the heart was lowered by cooling the water bath. At about 30°C, RH was again recorded in response to 10-second occlusion. The procedure was repeated at 25°C, which is the lowest temperature compatible with regular heart beats in the HLP. The experiments were concluded not later than 90 minutes after the completion of the HLP. During the cooling period, blood was taken for hematocrit determinations. At the termination of each experiment the perfusing blood was used for the calibration of the rotameter. A Dale-Schuster pump served to deliver various flow rates. Calibrations were done at 37, 30, and 25°C blood temperatures. The wet weight of the whole heart and that of the left ventricle (including the septum) were determined. Results In a few preliminary experiments the influence of the duration of the heart-lung preparation (after completion) on the magnitude of RH was investigated. Figure 1 shows the results of such an experiment. The rectangular area between the occlusion (O) and release (R) of the coronary artery represents the "blood flow debt" incurred during the period of circulatory arrest and the area above the level of control flow represents the excess blood flow during the hyperemic period (RH) or the "repayment" of the blood flow debt. The hyperemic response was quantified by measuring the area of excess flow with a planimeter and comparing it with