Comparative measurements of left ventricular ejection time by mechano-, echo- and electrical impedance cardiography

Several electrical impedance cardiographic and mechanocardiographic methods for assessing left ventricular ejection time (LVET) were compared with the aortic valve LVET echogram to determine whether values from an impedance method allow an exact measurement of LVET. From seven healthy subjects, tracings of signals from the ECG, phonocardiogram, carotid pulse, and electrical impedance cardiography were inscribed simultaneously with M-mode aortic valve echograms which recorded the separation and closure of the aortic valves. LVETs were measured from each cycle according to several established techniques. To yield variations in the LVET, heart rate was increased using injections of atropine 0.25 mg i.v. every 10 min until the rate reached 100 to 110 beats/min. Using biometrical methods with different sensitivities to extreme values, individual regression analyses compared the LVET from carotid and from impedance tracings to the "real" value measured from the echogram. The results showed the carotid pulse values underestimate and the conventional impedance methods over- and underestimate the real LVET. A modified approach identified LVET onset at the crossing of the dZ/dt line from the impedance cardiogram with baseline or, when a presystolic wave appeared at higher heart rates, the first turning point in the dZ/dt curve before this wave, and the LVET termination as the initial signal from the second heart sound. This combination of impedance cardiography and phonocardiography offers a precise estimation of LVET in noninvasive cardiology.