Force-time integral does not improve predictability of cardiac O2 consumption from pressure-volume area (PVA) in dog left ventricle

Energy conversion efficiency in human left ventricle

BACKGROUND

Left ventricular mechanical efficiency is one of the most important measures of left ventricular pump performance. Several clinical studies, however, have shown that mechanical efficiency does not fall substantially as the heart fails. To clarify the insensitivity of mechanical efficiency to the change in pump performance, we analyzed human left ventricular mechanical efficiency, applying the concept of left ventricular systolic pressure-volume area (PVA).

METHODS AND RESULTS

PVA correlates linearly with myocardial oxygen consumption per beat (MVO2): MVO2 = a.PVA+b, and represents the total mechanical energy of contraction. We determined MVO2-PVA relation and external work in 11 patients with different contractile states. We also calculated the energy transfer from MVO2 to PVA (PVA/MVO2 efficiency), that from PVA to external work (work efficiency), and mechanical efficiency (external work/MVO2). 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 (Ees). Pressure elevation raised external work by 41.4%, PVA by 71.2%, and MVO2 by 54.5%. These changes were associated with a decrease in work efficiency and an increase in PVA/MVO2 efficiency. The opposite directional changes in these two efficiencies rendered the mechanical efficiency constant. The slope, a, of the relation between MVO2 and PVA was relatively constant (2.46 +/- 0.33) over the range of 0.8-8.8 mm Hg/ml of Ees, but the oxygen axis intercept, b, tended to decrease with the reduction in Ees. PVA/MVO2 efficiency correlated inversely (r = -0.66, p less than 0.05) with Ees, whereas work efficiency correlated linearly with Ees (r = 0.91, p less than 0.01).

CONCLUSIONS

Mechanical efficiency is not appreciably affected by changes in loading and inotropic conditions as long as the left ventricular contractility is not severely depressed.

The energy cost of relaxation in control and hypertrophic rabbit papillary muscles

The energy cost of the onset and relaxation phases of cardiac isometric contractions has been investigated by ergometer controlled-length releases occurring at different times during the contraction cycle, to test the hypothesis that the energy cost of relaxation is normally small. Energy flux has been measured myothermically in 20 or 30 contractions of rabbit papillary muscles. The ergometer releases took place after different delays, starting during the latency period and incrementing in 50 ms steps, until eventually, releases were occurring late into the relaxation phase. The release step was kept constant and of a magnitude sufficient to prevent significant redevelopment of active stress at any release interval. The rate of release was several times greater than the maximum shortening velocity of the papillary muscle preparations. The heat production in each train of contractions was measured, but in order to estimate the total energy output, the elastic energy in the muscle-lever system which was removed by the ergometer release had to be added to the heat. This was estimated by integration of the stress-strain relationship found for each muscle. In normal animals the contraction peak, at 27 degrees C and a 1.0 Hz stimulus rate, was located between the 215 and 265 ms release times, at which point the total energy flux was estimated to be 80%-90% of that measured in a normal isometric contraction. Additional experiments were performed in a group of volume-overloaded hearts and the data were compared with results from sham-operated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractility to minimize oxygen consumption for constant work in dog left ventricle

We studied whether an optimal Emax (contractility index) to minimize myocardial O2 consumption (MVo2) for a constant external work (EW) and, hence, maximize mechanical work efficiency exists as a nadir of MVo2 within the physiological range of Emax. MVo2 and Emax were measured in the left ventricles (LV) of 10 dogs. In each experiment, heart rate, cardiac output, and mean aortic pressure were kept constant by pacing, bypassing the right heart, and adjusting the inflation of an intra-aortic balloon, respectively, to maintain a constant EW. MVo2 was determined as the product of coronary arteriovenous O2 difference, and coronary blood flow drained from the right heart. Emax was obtained by an abrupt aortic occlusion method. We changed Emax over 1.4-16.1 mmHg.ml-1.100 g) by dobutamine, propranolol, and sodium pentobarbital. The obtained MVo2-Emax relationship showed that MVo2 correlated positively with Emax in 9 of the 10 dogs (correlation coefficients = 0.68-0.96). In one of the 9 dogs and the remaining dog, MVo2 slightly increased with decreases in Emax below 7-8 mmHg.ml-1.100 g. We conclude that the optimal Emax to minimize MVo2 for a constant EW does not generally exist as a nadir of MVo2 in the physiological range of Emax in dog hearts.