Systolic Pressurevolume Area Predicts The Oxygen Consumption Of The Heart

Virtually all of the heart's energy is derived from oxidative metabolism. Substrate is primarily fatty acids and to a much lesser extent carbohydrates. As a result, a linear relationship exists between the heart's energy production and its oxygen consumption. Because most of the heart's energy expenditure goes into mechanical work, it is not surprising that there is a close correlation between the oxygen consumption of the heart and its mechanical activity. Figure 13 reveals this relationship. The external physical work done by the heart (pressure times volume) is exactly given by the area within the ejection loop. That area, however, correlates poorly with the heart's oxygen requirements. If, however, an internal work component as shown in Fig. 13 is added to the external work, that sum correlates almost perfectly with the heart's energy demands. This systolic pressure-volume area index leads to some interesting and surprising predictions. For example, notice that raising the aortic pressure with no change in venous pressure decreases stroke volume and may even decrease external work but always increases oxygen demand. The preceding analysis explains why high blood pressure is so stressful to the

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