New Index for Oxygen Cost of Contractility from Curved End-Systolic Pressure–Volume Relations in Cross-Circulated Rat Hearts

Abstract

We have already reported the linear oxygen consumption per beat (Vo₂)–systolic pressure-volume area (PVA) relation from the curved left ventricular (LV) end-systolic pressure–volume relation (ESPVR) in the cross-circulated rat heart. The Vo₂ intercept (PVA-independent Vo₂) is primarily composed of Vo₂ for Ca²⁺ handling in excitation-contraction (E-C) coupling and basal metabolism. The aim of the present study was to obtain the oxygen cost of LV contractility that indicates Vo₂ for Ca²⁺ handling in E-C coupling per unit LV contractility change in the rat heart. Oxygen cost of LV contractility is obtainable as a slope of a linear relation between PVA-independent Vo₂ and LV contractility. We obtained a composite Vo₂–PVA relation line at a mid-range LV volume (mLVV) under gradually enhanced LV contractility by stepwise increased Ca²⁺ infusion and thus the gradually increased PVA-independent Vo₂ values. As a LV contractility index, we could not use E_max (ESP-V ratio; ESP/ESV) for the linear ESPVR because of the curved ESPVR in the rat LV. A PVA at a mLVV (PVA_mLVV) has been proposed as a good index for assessing rat LV mechanoenergetics. Since the experimentally obtained PVA_mLVV was not triangular due to the curved ESPVR, we propose an equivalent ESP-V ratio at a mLVV, (eESP/ESV)_mLVV, as a LV contractility index. This index was calculated as an ESP-V ratio of the specific virtual triangular PVA_mLVV that is energetically equivalent to the real PVA_mLVV. The present approach enabled us to obtain a linear relation between PVA-independent Vo₂ and (eESP/ESV)_mLVV and the oxygen cost of LV contractility as the slope of this relation.