Single-beat determination of preload recruitable stroke work relationship: Derivation and evaluation in conscious dogs

OBJECTIVES To derive and evaluate a method of estimating the slope (M-w) of the preload recruitable stroke work (PRSW) relationship between left ventricular stroke work (SW) and end-diastolic volume (EDV) from a single beat. BACKGROUND M-w is a load-insensitive index of contractile function, but its clinical application has been limited by the need to record multiple beats over a wide volume range. METHODS Pressure-volume loops were recorded over a variable preload and afterload range by vena caval and aortic constrictions in 12 conscious dogs instrumented with epicardial dimension transducers and micromanometers. Single-beat M-w (SBMw) was determined as the ratio SW/(EDV - V-w), where the volume-axis intercept of the PRSW relationship (V-w) (EDV at zero SW) was estimated as k.EDVB + (k - 1)LVwall, k is the ratio of the epicardial shell volumes corresponding to V-w and baseline EDV (EDVB) and LVwall is wall volume. RESULTS In the first six dogs, k was found to be essentially constant at 0.7, SBMw estimates were insensitive to wide preload variation, and the relationship between SBMw and multibeat M-w determined during caval and aortic constrictions did not differ significantly from the line of identity. When the same constant k value was applied to SBMw estimation in a different group of six dogs, SBPMw did not differ significantly from multibeat M-w (83 +/- 12 erg.cm(-3).10(3) and 77 +/- 12 erg.cm(-3).10(3), respectively), neither changed significantly during aortic constriction and both increased significantly with calcium infusion (107 +/- 18 erg.cm(-3).10(3) and 95 +/- 19 erg.cm(-3).10(3), respectively, both p < 0.05). Single-beat M-w was less load-dependent, more reproducible and a more sensitive index of inotropic state than two previously described single-beat indexes, single-beat elastance and maximum power divided by EDV2. CONCLUSIONS M-w can be determined accurately from a single, steady-state beat in the normal canine heart and is sensitive to inotropic alterations while being insensitive to wide variations in preload and afterload. Single-beat M-w estimation should facilitate noninvasive, load-independent assessment of contractile function. CT Am Coil Cardiol 2000;35:502-13) (C) 2000 by the American College of Cardiology.