Electrical impedance cardiogram in derivation of systolic time intervals.

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Abstract
The reliability and use of the differentiated electrical impedance cardiogram (dZ/dt) in calculating systolic time intervals were assessed in 185 subjects under varying conditions. In the 1st phase, simultaneous recording of carotid pulse, dZ/dt, electrocardiogram and phonocardiogram (PCG) was obtained in 50 normal volunteers under controlled conditions. In the 2nd phase dZ/dt, electrocardiogram and phonocardiogram were obtained initially, and carotid pulses, electrocardiograms, and phonocardiograms were recorded immediately afterwards. Serial recordings were obtained in 35 subjects at sea-level, and at 3658 m altitude after 1, 2, 3, 4, 5, and 10 days of induction. The subjects returned to sea-level and daily studies were carried out days 2 and 3. The impedance ventricular ejection time (LVETz) and the ventricular ejection time derived from the carotid pulse (LVETc) were within 10 ms of each other in all recordings and a close correlation was obtained between the 2 (R = 0.976). The impedance pre-ejection period (PEPz) obtained by direct measurement (from the onset of the Q wave of the electrocardiogram to the onset of dZ/dt wave form) also had an excellent correlation with carotid pre-ejection period (PEPc) obtained by the formula QS2 - LVETc (R = 0.91), where QS2 represents electromechanical systole. In 50 subjects LVETc and LVETz were compared after isometric handgrip exercise. There was no significant difference between them. Thirty patients with ischemic heart disease and 20 normal subjects participated in dynamic bicycle ergometer exercise. In this group technical difficulties prevented recording of satisfactory dZ/dt tracings beyond 50 watts load during exercise. Tracings immediately after exercise were, however, satisfactory for analysis, and the parameters calculated yielded a good correlation with those obtained from carotid pulse recordings.