Theory of the determination of systolic time intervals by impedance cardiography.

It is possible to accurately measure the left ventricular ejection time from the dZ/dt signal of impedance cardiography. The pre-ejection period can be measured from simultaneous recordings of ECG and dZ/dt. The thoracic admittance (reciprocal value of thoracic impedance) is the sum of a constant tissue admittance and a varying blood conductance Gb (reciprocal value of the blood resistance). dZ/dt represents the changes in Gb. The notches of the dZ/dt signal, corresponding to opening and closing of the aortic valve, are due to conductivity changes of blood caused by changes in orientation of erythrocytes; near zero-velocity the first derivative with respect to time of these conductivity changes approaches infinity. Therefore, these notches coincide with the actual opening and closing of the valve, although different vessels, including the aorta, contribute to dZ/dt. Comparison of ejection times, simultaneously measured by impedance cardiography and aortic pressure recording, showed excellent agreement for the whole range of measured heart rates (maximum heart rate = 140 beats min-1, n = 70, r = 0.986).