Variability of the maximal amplitudes of impedance cardiogram and of its first derivative during supine, standing, paced breathing, and exercise maneuvers

Beat-to-beat variability of the maximal amplitudes of thoracic impedance (ΔZ_max) and of its first derivative (dZ/dt_max) remain unexplored. We examined the effects of four maneuvers eliciting different sympathetic activity levels on ΔZ_max and dZ/dt_max time-frequency spectral measures. Seventeen healthy subjects performed 5-min maneuvers: lying, controlled breathing, standing (S) and exercise (E). Time-frequency spectra of RR intervals (RR), ΔZ_max, dZ/dt_max and respiration (Res) time series were estimated to compute their low-frequency (LFΔZ_max, LFdZ/dt_max) and high-frequency powers (HF_RR, HFΔZ_max, HFdZ/dt_max, HF_Res) and the time-frequency coherences of HF_Res with HFΔZ_max and HFdZ/dt_max. HF_RR and RR level decreased in S and E. LFΔZ_max was negligible. HFΔZ_max was maximal in E and showed significant coherences with HF_Res. LFdZ/dt_max was maximal in E and decreased in S. HFdZ/dt_max was much greater than LFdZ/dt_max and showed significant coherences with HF_Res. Because LFdZ/dt_max power is cardiac sympathetic activity and stroke volume dependent, it can be considered a noninvasive index of global left ventricular systolic function. HFdZ/dt_max power is originated by thoracic volume changes. The mechanical respiratory effect is the main source of ΔZ_max and dZ/dt_max time series variability.