Effects of bigeminies and paired-pulse stimulation on oxygen consumption in dog left ventricle

In 10 excised, cross-circulated, isovolumically beating dog left ventricles, we examined the effects of bigeminy, including paired-pulse stimulation, on cardiac oxygen consumption (VO2) and evaluated whether the VO2 versus pressure-volume area (PVA) relation obtained from regularly beating hearts in our previous studies could account for the changes in VO2 during bigeminies with various coupling intervals. The extrasystolic interval (ESI) was decreased in four steps from 300 msec (regular rhythm) to paired-pulse stimulation (less than or equal to 210 msec). The sum of ESI and the postextrasystolic interval was always held constant (600 msec); therefore, the number of excitations was 200/min at any ESI. Both VO2 and PVA were measured in four runs: a small volume run at a ventricular volume of 13 ml, a large volume of 12 ml, a calcium run in which contractility was enhanced by CaCl2 at a small volume of 12 ml, and a verapamil run in which contractility was depressed by verapamil at a large volume of 20 ml. In any run, both VO2 and PVA were unchanged at long ESIs (greater than or equal to 250 msec) from each control value at regular rhythm, but VO2 markedly increased at short ESIs (less than or equal to 230 msec). VO2s during bigeminies with various ESIs were comparable with the theoretical VO2 values that were calculated by the VO2-PVA relation at regular rhythm. However, the theoretical VO2 values underestimated the measured VO2 values at the shortest ESIs under a high volume loading and in a high contractile state. This indicates that the PVA concept can be expanded to various arrhythmias unless ESI becomes very short.