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CLINICAL STUDIES

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

^a Victor Chang Cardiac Research Institute and Cardiology Department, St. Vincent’s Hospital, Sydney, Australia

Manuscript received September 28, 1998; revised manuscript received September 20, 1999, accepted October 25, 1999.

Reprint requests and correspondence: Dr. Michael P. Feneley, Cardiology Department, St. Vincent’s Hospital, Victoria Street, Darlinghurst NSW 2010, Australia
M.Karunanithi{at}unsw.edu.au

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 (SBM_w) 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·EDV_B + (k – 1)LV_wall, k is the ratio of the epicardial shell volumes corresponding to V_w and baseline EDV (EDV_B) and LV_wall is wall volume.

RESULTS

In the first six dogs, k was found to be essentially constant at 0.7, SBM_w estimates were insensitive to wide preload variation, and the relationship between SBM_w 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 SBM_w estimation in a different group of six dogs, SBM_w did not differ significantly from multibeat M_w (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 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.

Abbreviations and Acronyms   ANOVA = analysis of variance   dP/dtmax = maximum of the time derivative of ventricular pressure   EDV = end-diastolic volume   EDVB = the baseline, steady-state EDV   EDVB,epi = the volume within the epicardial shell corresponding to EDVB   Ees = slope of the ESPVR   ESPVR = end-systolic pressure-volume relationship   LV = left ventricle or ventricular   LVwall = left ventricular wall volume   LVV = left ventricle chamber volume   Mw = slope of the PRSW relationship   PRSW = preload recruitable stroke work   PWRmax/EDV2 = maximum left ventricular power divided by the square of end-diastolic volume   SBMw = single-beat Mw   SD = standard deviation   SW = stroke work   Vw = volume-axis intercept of the PRSW relationship   Vw,epi = the volume within the epicardial shell corresponding to VW

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