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 (Mw) of the preload recruitable stroke work (PRSW) relationship between left ventricular stroke work (SW) and end-diastolic volume (EDV) from a single beat. BACKGROUND Mw 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 Mw (SBMw) was determined as the ratio SW/(EDV − Vw), where the volume-axis intercept of the PRSW relationship (Vw) (EDV at zero SW) was estimated as k·EDVB + (k − 1)LVwall, k is the ratio of the epicardial shell volumes corresponding to Vw 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 Mw 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, SBMw did not differ significantly from multibeat Mw (83 ± 12 erg·cm−3·103 and 77 ± 12 erg·cm−3·103, respectively), neither changed significantly during aortic constriction and both increased significantly with calcium infusion (107 ± 18 erg·cm−3·103 and 95 ± 19 erg·cm−3·103, respectively, both p < 0.05). Single-beat Mw 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 Mw 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 Mw estimation should facilitate noninvasive, load-independent assessment of contractile function.