The pressure-volume loop revisited: Is the search for a cardiac contractility index a futile cycle?

Our previous studies indicate that left ventricular end-systolic pressure-volume relations (ESPVRs) or elastance (Ees) are not reduced in studies where expected reductions of contractility should be found (i.e., heart failure, stunning, and endotoxemia). The present study was done to assess whether this phenomenon is due to a particular load sensitivity of elastance, rendering this index inappropriate as a measurement of contractility in pathologic states in vivo.

Analysis of previously generated data revealed an increased ESPVR in stunned hearts, in pigs made endotoxemic, and in hearts rapidly paced. After inducing acute heart failure by microembolization, the ESPVR was increased when assessed using linear relations but reduced when assessing ESPVR by a curvilinear algorithm. To further evaluate the effect of different load alterations on ESPVR, this relation was generated by (i) inferior vena caval occlusions (VCOs); (ii) gradually occluding the descending aorta (pressure interventions); and (iii) rapidly infusing blood (120 mL) into the left atrium (volume increments). The load protocol was applied in 5 pigs, before and after the left ventricle was stunned by 11 brief left main coronary artery occlusions/reperfusions (accumulated ischemia 20 min affecting 81% of the left ventricle). Correlation coefficients for left ventricular elastance ranged from 0.93 to 0.99 in all the 3 types of loading interventions. Despite significant reductions in stroke volume, stroke work, and dP/dtmax, VCO-calculated linear and curvilinear Ees increased 90 min after stunning (55% +/- 4% and 94% +/- 6%, respectively). Linear Ees during pressure interventions decreased 36% +/- 1%, whereas curvilinear Ees decreased 33% +/- 3%. During volume infusions, linear Ees decreased 27% +/- 2%. We achieved the same results after blocking the baroreceptor reflexes using hexamethonium.

The Ees is particularly load dependent and will reflect load interventions more than the inotropic state of the cardiac muscle. A VCO-generated Ees increase could be an unmasking of a pronounced preload sensitivity in failing myocardium.