Paired pulse pacing increases cardiac O2 consumption for activation without changing efficiency of contractile machinery in canine left ventricle
The relation between cardiac O2 consumption (Vo2) and the total mechanical energy (TME) generated by contraction was studied under paired-pulse (PP) pacing and compared with that under single-pulse pacing at the same basic rate as PP pacing and at the double-pacing rate in ten excised cross-circulated canine left ventricles (LV). TME was assessed by the systolic pressure-volume (P–V) area (PVA) defined as the area bounded by the end-systolic and end-diastolic P–V curves and the systolic P–V trajectory. The Vo2-PVA relation was linear under PP pacing as well as at control and double heart rates. PP pacing increased LV contractility index Emax from 6.3±3.3 (SD) to 18.0±8.6 mmHg/(ml/100 g) and elevated markedly the Vo2-PVA relation by increasing the Vo2-axis intercept (or PVA-independent Vo2) from 0.62±0.11 to 1.13±0.35 J · beat−1 · 100 g−1. However, PP pacing did not change the slope of the Vo2-PVA relation at 2.24±0.53 (dimensionless). The efficiency from PVA-dependent Vo2 (total Vo2-PVA-independent Vo2) to PVA (= TME), calculated as the reciprocal of the slope of the Vo2-PVA relation, was also constant at 47±11% regardless of PP pacing. These results are similar to previous results obtained by positive inotropic interventions with catecholamines and Ca2+. We conclude that PP pacing augments the PVA-independent Vo2 for activation without affecting the efficiency of the contractile machinery to generate TME from the PVA-dependent Vo2.
Key wordsHeart Cardiac energetics Pressure-volume area Excitation-contraction coupling Efficiency
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