Minor preload dependence of O2 consumption of unloaded contraction in dog heart.

We studied whether end-diastolic volume (EDV) would affect myocardial oxygen consumption (VO2) of mechanically unloaded contraction in the cross-circulated dog heart, as expected from the concept of the myocardial length-dependent activation. We made preloaded but maximally unloaded contractions from different EDVs by quickly releasing ventricular volume to eliminate systolic pressure development and hence to minimize the VO2 for mechanical load during the contraction. We then studied the relation between VO2 and EDV. The VO2 of the almost unloaded contraction from a relatively large EDV slightly exceeded the VO2 of the isovolumic contraction at V0, where V0 is the volume at which peak isovolumic pressure was zero. However, the excess VO2 could be ascribed to the residual systolic pressure-volume area (PVA) adversely produced from the large EDV, where PVA is a measure of the total mechanical energy generated during contraction. Therefore, we considered that VO2 was practically little dependent on EDV. We interpreted this finding as an indication that an increase, if any, in VO2 due to the length-dependent activation of the excitation-contraction coupling was practically negligible in the whole heart preparation.