Norepinephrine increases the economy of pressure development in isolated canine hearts

To test for oxygen wasting by norepinephrine (NE) without relying on normalization by measures of performance such as the pressure-volume area, myocardial oxygen consumption (MV̇O₂) was determined for isovolumic beats at five different left ventricular (LV) end-diastolic volumes (EDV) in nine isolated cross-perfused canine hearts in each of three states: a basal anesthetic state (B); after depression with halothane (H); and after adding NE to increase contractility back to the B state (H + NE). The end-diastolic and peak systolic pressure-volume lines were identical for B and H + NE. The R² for a linear regression of MV̇O₂ per beat for B vs. H + NE for beats originating at the same EDV and developing similar (within 10%) peak isovolumic pressures for all hearts was 0.85. The slope and intercept were 0.83 and 0.01, which are significantly less than one (P < 0.001) and greater than zero (P < 0.001), respectively. These data suggest that NE increases both the economy of pressure development as well as activation energy of an isovolumically contracting LV.