Correlation Between Arterial Blood Pressure Levels and (dZ/dt)min in Impedance Plethysmography

(dZ/dt)min, which is the magnitude of the negative peak of the first time derivative of transthoracic electrical impedance Z, plays a key role in impedance plethysmography because it reflects the pumping action of the heart. Its current applications for measuring stroke volume and quantification of myocardial contractility, however, ignore the possibility that (dZ/dt)min may be strongly modified by factors that are independent on the heart. Measurements of (dZ/dt)min on 146 volunteers are statistically correlated with the systolic, diastolic, and mean arterial blood pressure and heart rate. Statistically significant correlations are obtained between the (dZ/dt)min and blood pressures, but not with heart rate. The correlations indicate that (dZ/dt)min is expected to decrease as the arterial blood pressure level increases. This relationship is elucidated with the help of a theoretical model which combines the parallel conductor theory with a mechanical model of an elastic artery. The analysis of the model is in-agreement with statistical predictions based on measured data, indicating that (dZ/dt)min explicitly depends on blood pressure level in an inverse manner. It is concluded that the functional dependence of dZ/dt signals on Z and blood pressure levels should be taken into account if valid conclusions from applications of dZ/dt signals are to be drawn. Suggestions are made to use the measured correlations as a means to eliminate Z and blood pressure levels as factors in applications of dZ/dt signals.