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Noninvasive estimation of the end systolic pressure-volume relationship using impedance cardiography.

PURPOSE
Traditional measures of cardiac contractility such as dp/dt and ejection fraction has been noted to be sensitive to preload and afterload conditions. The end systolic pressure-volume relationship of the left ventricle (ESPVR, Suga Index or Emax) has been found to be the best load independent measure of the cardiac contractile state. However, determination of the ESPVR requires very highly invasive procedures. Impedance cardiography (IC) is a reliable noninvasive method for calculating stroke volume and may also be useful for estimating the end systolic volume.


METHODS
An equation was derived using the systolic time intervals (PEP = pre-ejection period, LVET = left ventricular ejection time) and determined stroke volume (SV) as calculated from the impedance cardiograph to estimate the end systolic volume of the left ventricle. Likewise, and systolic pressure (ESP) was estimated from brachial cuff pressures using a previously published method. The resulting ESPVR was then calculated from tracings recorded in healthy normal subjects and compared to those obtained from patients in decompensated congestive heart failure (ejection fraction < 30% by echocardiogram) using the standard t test (p < 0.05).


RESULTS
Using the derived equation (ESPVR = ESP/(SV/(1.125-1.25(PEP/LVET)) - SV), the ESPVR for the normal group of 6 averaged 2.72 +/- 0.71 and was significantly different from the 1.04 +/- 0.45 found in 6 patients with known systolic dysfunction. In a further test of the method, 15 patients who received concurrent echocardiographic and IC evaluations were found to have calculated ESPVR values that significantly correlated with determined ejection fractions (r = 0.83, p < 0.01).


CONCLUSIONS
A noninvasive method for estimating the ESPVR that differentiates the myocardial contractile state in the clinically setting was derived using parameters obtained from IC. While further studies are needed to correlate this new equation with invasive measurements, this method has the potential for easily estimating load independent contractility in patients with cardiac dysfunction.