Article

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

Department of Emergency Medicine, University of Mississippi Medical Center, Jackson 39216, USA.
Journal of the Mississippi State Medical Association 05/2000; 41(5):575-8.
Source: PubMed

ABSTRACT 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.
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).
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).
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.

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    ABSTRACT: Whereas end-systolic and end-diastolic pressure-volume relations (ESPVR, EDPVR) characterize left ventricular (LV) pump properties, clinical utility of these relations has been hampered by the need for invasive measurements over a range of pressure and volumes. We propose a single-beat approach to estimate the whole EDPVR from one measured volume-pressure (Vm and Pm) point. Ex vivo EDPVRs were measured from 80 human hearts of different etiologies (normal, congestive heart failure, left ventricular assist device support). Independent of etiology, when EDPVRs were normalized (EDPVRn) by appropriate scaling of LV volumes, EDPVRns were nearly identical and were optimally described by the relation EDP = An.EDV (Bn), with An = 28.2 mmHg and Bn = 2.79. V0 (the volume at the pressure of approximately 0 mmHg) was predicted by using the relation V0 = Vm.(0.6 - 0.006.Pm) and V30 by V30 = V0 + (Vm,n - V0)/(Pm/An) (1/Bn). The entire EDPVR of an individual heart was then predicted by forcing the curve through Vm, Pm, and the predicted V0 and V30. This technique was applied prospectively to the ex vivo human EDPVRs not used in determining optimal An and Bn values and to 36 in vivo human, 12 acute and 14 chronic canine, and 80 in vivo and ex vivo rat studies. The root-mean-square error (RMSE) in pressure between measured and predicted EDPVRs over the range of 0-40 mmHg was < 3 mmHg of measured EDPVR in all settings, indicating a good predictive value of this approach. Volume-normalized EDPVRs have a common shape, despite different etiology and species. This allows the entire curve to be predicted by a new method with a potential for noninvasive application. The results are most accurate when applied to groups of hearts rather than to individual hearts.
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