Energetically Optimal Left Ventricular Pressure for the Failing Human Heart

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Abstract
Background An energy-starved failing heart would benefit from more effective transfer of the mechanical energy of ventricular contraction to blood propulsion. However, the energetically optimal loading conditions for the failing heart are difficult to establish. In the present study, we analyzed the optimal left ventricular pressure to achieve maximal mechanical efficiency of the failing heart in humans. Methods and Results We determined the relation between left ventricular pressure-volume area and myocardial oxygen consumption per beat (Vo2), stroke work, and mechanical efficiency (stroke work/Vo2) in 13 patients with different contractile states. We also calculated the optimal end-systolic pressure that would theoretically maximize mechanical efficiency for a given end-diastolic volume and contractility. Left ventricular pressure-volume loops were constructed by plotting the instantaneous left ventricular pressure against the left ventricular volume at baseline and during pressure loading. The contractile properties of the ventricle were defined by the slope of the end-systolic pressure-volume relation. In patients with less compromised ventricular function, the operating end-systolic pressure was close to the optimal pressure, achieving nearly maximal mechanical efficiency. As the heart deteriorated, however, the optimal end-systolic pressure became significantly lower than normal, whereas the actual pressure remained within the normal range. This discrepancy resulted in worsening of ventriculoarterial coupling and decreased mechanical efficiency compared with theoretically maximal efficiency. Conclusions Homeostatic mechanisms to maintain arterial blood pressure within the normal range cause the failing heart to deviate from energetically optimal conditions.