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

Abstract

OBJECTIVESTo 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.BACKGROUNDMwis 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.METHODSPressure-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, kis the ratio of the epicardial shell volumes corresponding to Vwand baseline EDV (EDVB) and LVwallis wall volume.RESULTSIn the first six dogs, kwas found to be essentially constant at 0.7, SBMwestimates were insensitive to wide preload variation, and the relationship between SBMwand multibeat Mwdetermined during caval and aortic constrictions did not differ significantly from the line of identity. When the same constant kvalue was applied to SBMwestimation in a different group of six dogs, SBMwdid not differ significantly from multibeat Mw(83 ± 12 erg·cm−3·103and 77 ± 12 erg·cm−3·103, respectively), neither changed significantly during aortic constriction and both increased significantly with calcium infusion (107 ± 18 erg·cm−3·103and 95 ± 19 erg·cm−3·103, respectively, both p < 0.05). Single-beat Mwwas 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.CONCLUSIONSMwcan 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 Mwestimation should facilitate noninvasive, load-independent assessment of contractile function.