Stability of cardiac impedance measures: aortic opening (B-point) detection and scoring.

The accurate assessment of cardiac stroke volume measured by impedance cardiography depends on the reliable measurement of dZ/dtmax and the accurate detection of the relevant points on the impedance cardiogram. In one study, impedance cardiograms were collected for 75 subjects during resting conditions and during four psychologically challenging tasks, in two identical testing sessions. The reliability of stroke volumes calculated from dZ/dtmax measured from the electrical baseline and from the dZ/dt B-point on the waveforms were assessed for the two testing sessions. Consistently higher (but not statistically different) test-retest correlations were observed for the measures calculated using values of dZ/dtmax measured from the B-point. In a second study, dZ/dt waveforms with no B-point reversal from 21 subjects were scored using two alternative B-point detection algorithms: the reversal of the second derivative of delta Z and the peak of the third derivative of delta Z. Both algorithms demonstrated high reliability compared with an expert human operator. The reversal on the second derivative may better locate an inflection point on the dZ/dt signal and the third derivative may provide a good B-point estimation in the absence of any inflection point.