Myocardial Oxygen Consumption During Ventricular Contraction and Relaxation

Abstract

• 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

Increased Myocardial Oxygen Consumption and Contractile State Associated with Increased Heart Rate in Dogs

“…Since both wall stress and the contractile state of the myocardium have been postulated to be important determinants of myocardial oxygen consumption (6,9), comparisons of oxygen consumption at different heart rates were made at matched levels of peak wall stress to minimize the effects of wall stress on oxygen consumption. Since Monroe (26) and others (27,28) have found that peak tension rather than the integrated tension is most closely related to MVo 2 , peak wall stress levels were matched in this study. This resulted in a marked reduction in integrated wall stress per beat at the higher heart rates, and it is possible that this reduction may have minimized the augmentation of oxygen consumption per beat observed at higher heart rates.…”

Control of myocardial oxygen consumption: relative influence of contractile state and tension development

“…6). The work of Monroe (50), demonstrating that over 90%o of the MVo2 was accounted for by the time peak tension was-reached, serves to emphasize the major energetic importance of tension development in contrast to the total developed tension (TDT), i.e., the area under the tension-time curve. Accordingly, in the present study, peak developed tension rather than total developed tension was held constant when Vmax was altered.…”

Experimental Subendocardial Ischemia in Dogs with Normal Coronary Arteries

“…Monroe (34) observed that myocardial oxygen uptake was almost maximal by the time the peak systolic pressure had been reached and that peak systolic pressure could also be used to predict myocardial oxygen uptake. The increased coronary flow following distal aortic constriction was more closely associated with change in TTI than with this pressure in our studies so that we used TTI to estimate myocardial oxygen needs.…”