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.