Experimental study of the relationship between the base impedance and its time derivative in impedance plethysmography.

Transthoracic electrical impedance Z consists of a constant baseline component Z0 and the time variable component delta Z such that Z = Z0 + delta Z. The first time derivative of Z has a negative peak, the magnitude of which is (dZ/dt)min. Measurements of 170 volunteers show that (dZ/dt)min is a function of the constant baseline component Z0. This implies that at least a part of the difference of the magnitudes of (dZ/dt)min on Z0 must be taken into account in order to avoid erroneous conclusions or inaccurate calculations.