Correlation between maximumdZ/dt and parameters of left ventricular performance

The maximum rate of change of the thoracic impedance waveform (max dZ/dt) was compared with left-ventricular stroke volume (s.v.), maximum aortic blood flow (m.a.f.), enddiastolic pressure (e.d.p.) and the maximum rate of rise of ventricular pressure (max dP/dt) in 27 experiments performed on chloralose-anaesthetised mongrel dogs. In this study, cardiac performance was altered by acute hypoxia or drugs. Although when max dZ/dt was compared with s.v. and m.a.f., positive correlations (P<0·01) were found in 80% of the experiments, the correlation coefficients varied greatly (0·324 to 0·986) and negative correlations were observed in two experiments. Positive correlations (P<0·05) were found in 77% of the experiments where max dZ/dt was compared with max dP/dt, and no consistent correlation was found when max dZ/dt was compared with e.d.p. This study suggests that left-ventricular pump performance may not be the sole factor influencing max dZ/dt, and that max dZ/dt may be a source of error in the presently accepted empirical formula for calculating cardiac output by impedance cardiography.