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_wis 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, kis the ratio of the epicardial shell volumes corresponding to V_wand baseline EDV (EDV_B) and LV_wallis wall volume.

RESULTS

In the first six dogs, kwas found to be essentially constant at 0.7, SBM_westimates were insensitive to wide preload variation, and the relationship between SBM_wand multibeat M_wdetermined during caval and aortic constrictions did not differ significantly from the line of identity. When the same constant kvalue was applied to SBM_westimation in a different group of six dogs, SBM_wdid not differ significantly from multibeat M_w(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 M_wwas 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_wcan 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_westimation should facilitate noninvasive, load-independent assessment of contractile function.