HYPOTHESIS that the physical work performed during ventricular systole is largely determined hy ventricular diastolic volume has had currency for the last 45 years. 1 " 4 The oxygen cost of the heart's contraction has likewise been related to diastolic volume, and also to stroke work, mean arterial blood pressure times the heart rate, the integral of systolic pressure in time, and ventricular wall tension. 2 -3 ' r>~9 Many have attempted to identify the factors that determine the oxygen needs of the heart. Their conclusions have not been without controversy. As early as 1915, it was noted that "an increase of [cardiac] output was more economically performed than a comparable increase of 'aortic pressure'." 10 In 1927 Starling and Visscher, 2 and Hemingway and Fee, 5 indicated that myocardial oxygen consumption was dependent on diastolic volume even when distention of the ventricle had progressed to the point of diminished stroke work. Later studies relating myocardial oxygen consumption to the area under the systolic pressure tracing were interpreted to show that at a constant heart rate systolic pressure was the preponderant, if not the sole, determinant of myocardial oxygen consumption, and strongly implied that the metabolic needs of the heart were not determined by diastolic volume or diastolic fiber length. 7 ' 8 Eecent work in which left ventricular circumference was measured is consistent with this concept. 11 In these studies stroke volumes were known, but absolute volumes were not measured. The experiments described in this paper were dpsigned to control absolute ventricular volume accurately.The intact left ventricle of the dog has a fairly discrete blood supply and lends itself to metabolic determinations. 12 By means of a previously described preparation, 13 a combined pressure-volume and metabolic study was performed on an isovolumetrically contracting left ventricle. The preparation was also arranged to allow volume changes during systole and to permit the measurement of absolute volume. The isovolumetric pressurevolume studies, studies involving changes, and the results of concomitant metabolic determinations are reported in this paper. The intent was to relate myocardial oxygen consumption to ventricular pressure and volume, and to test specifically whether or not myocardial oxygen consumption is determined by diastolic volume. Methods Isovolumetric StudiesThe isolated heart preparation has been described previously in full 13 and will be described briefly herein. A schematic diagram of the preparation used for isovolumetric studies is seen in figure 1.Hearts from healthy mongrel dogs were rapidly excised under ehloralose-urethane anesthesia. A lucite button was ligated in the vicinity of the aortic cusps with a suture passed under the left coronary artery. By this procedure the left ventricular cavity was completely isolated from the coronary circulation. The right coronary artery was ligated, as were the bundle of His and the inferior and superior venae eavae. A catheter was placed in the coro...
• As early as 1915 it was known that an increase in systolic pressure is accompanied by a greater myocardial oxygen cost than a proportionate increase in stroke volume.1 Recent investigators have related myocardial oxygen consumption to the product of the mean systolic pressure and the heart rate, 2 or to the integral of systolic pressure in time. 3 In the isovolumetrically contracting ventricle, myocardial oxygen consumption could be equally well correlated with peak developed pressure, the integral of systolic pressure in time, or the tension developed by the myocardial fibers, as approximated by the Laplace relationship between wall tension and volume of a sphere. 4 Inasmuch as intraventricular pressure is the predominant hemodynamic factor in the oxygen cost of the heart's contraction, it was decided to investigate this further by repeatedly releasing the intraventricular pressure at specific points in the pressure cycle and comparing the amount of oxygen used by the ventricle when the pressure cycle was so interrupted to that used by the ventricle with a full pressure cycle. In view of the difficulties in measuring directly the tension developed by the myocardial fibers it was assumed that at a constant end diastolic pressure, volume, and loading, systolic intraventricular pressure would be a reasonable reflection of the tension developed by these fibers. 294Methods The isolated heart preparation has been described previously in full 4 "' and will be outlined briefly herein. Schematic diagrams of the preparation used are seen in figures 1 and 2.Hearts from healthy mongrel dogs, averaging 18 kg in weight, were rapidly excised under chloralose-urethane anesthesia. A Incite button was placed in the aorta and secured at the level of the valve cusps with a suture placed under the left coronary artery. By this procedure the left ventricular cavity was isolated from the coronary circulation. The bundle of His was ligated, thereby interrupting atrioventricular conduction. The right coronary artery was occluded by a ligature. The right atrium was closed by appropriate ties. A catheter was placed in the coronary sinus through a perforation in the right atrium. Cannulas were placed in the pulmonary artery, the aorta, and the mitral orifice and secured by ligatures.The isolated heart was perfused continuously with blood from an anesthetized donor dog according to the perfusion circuit shown in figure 1. The reservoirs and filters were filled prior to the perfusion with blood obtained from the animal sacrificed for the heart. Arterial blood was pumped from the donor dog to a reservoir of adjustable height, from which it descended through a filter, a Fischer-Porter rotameter, warming coils, and entered the left coronary artery
Left ventricular performance in the isolated heart of a dog was observed at normal temperatures (37.7 C) and under hypothermia (32.2 C) at comparable heart rates. The peak pressure of isovolumic contractions at the same ventricular end-diastolic pressures averaged 40% higher at the lower temperature. Diastolic pressure-volume relationships were similar at both temperatures. In studies in which the ventricle ejected fluid and performed work the hypothermic ventricle was capable of performing greater work at comparable heart rates, left ventricular end-diastolic pressures, and loading. When the ventricle was allowed to perform work by compressing air into a chamber of constant volume left ventricular oxygen consumption (Vo2) increased with the peak systolic pressure as the temperature was lowered. If the peak systolic pressure was maintained constant by increasing the volume of the air chamber as the temperature was lowered no consistent relationship could be shown between left ventricular Vo2 and the integral of systolic pressure in time which invariably increased with hypothermia.
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