Differentiating systolic from diastolic heart failure using impedance cardiography

Differentiating systolic from diastolic congestive heart failure (CHF) is often difficult in the ED. Impedance cardiography (IC) allows for the noninvasive evaluation of systolic function and measurement of diastolic time intervals. This study was designed to assess the ability of IC to accurately measure isovolumic relaxation time (IVRT) and determine relative cardiac contractility, thereby differentiating systolic from diastolic mechanisms in acute CHF.

In an evaluation of the technique, the average differences in the diastolic time intervals measured in normal subjects by both IC tracings and phonocardiography were compared. Likewise, the average Heather index (HI) of patients with known systolic dysfunction (ejection fraction < 30% by echo-cardiography) was compared with the mean HI measured in the normal subjects. In a retrospective analysis, the clinical performance of the method was examined by extracting the values of IVRT and HI from IC tracings of patients presenting with CHF. The determined IVRT and HI values were then correlated to clinical markers for diastolic and systolic dysfunction.

Analysis of 280 IC tracings in eight healthy volunteers revealed an average difference of 0.0075 seconds (95% CI = -0.0067 to 0.0217) when IVRT intervals measured by phonocardiography and IC are compared. The HI in this normal group averaged 14.2 (95% CI = 9.4 to 19.0), contrasting to the much lower value of 2.8 (95% CI = 1.98 to 3.62) seen in eight subjects with documented systolic dysfunction. In 26 patients with decompensated CHF, there was a close correlation (r = 0.81) of the measured IVRT to left ventricular hypertrophy by voltage criteria and while a fall in the HI was correlated with intravascular volume expansion. Though there was considerable overlap (46%) in mechanisms of CHF, 35% of the patients were found to have only systolic dysfunction (HI < 5 and IVRT < 0.125), while 19% had a predominantly diastolic etiology (IVRT > 0.125 and HI > 5) for their failure.

IC measures of contractility and diastolic time intervals are a potentially effective method for differentiating the dominant mechanisms of CHF in the emergent setting and categorizing CHF patients into different subsets.