Variability of left ventricular ejection and diastolic times obtained from impedance cardiography: A comparison with heart rate variability

Our aim was to compare the spectral measures of heart rate variability (HRV) with those of the variability of: impedance heart periods (IHPV), ejection time (ETV) and diastolic time (DTV), computed from the impedance cardiography (ICG) traces obtained from 17 healthy volunteers during four 5-min steady-state conditions that elicit different vagal activity levels. From the time-frequency spectra of RR intervals (RR), impedance heart period (IHP), IHP-RR difference, ejection time (ET) and diastolic time (DT) series, their corresponding high frequency (HF_RR, HF_IHP-RR) and low frequency powers (LF_RR, LF_IHP-RR) were computed. Time-frequency coherences of HF_RR and LF_RR with the respective components of the other series were obtained. Coherences and correlations of HRV spectral measures with those of IHPV and DTV were very strong and much greater (p<;0.01) than with those of ETV. HF_IHP-RR power was much greater (p<;0.005) than LF_IHP-RR in all maneuvers. Although HRV determines most IHPV, the latter contains additional respiratory modulation that limits their interchangeability. Intra-beat variability is documented by DTV, a surrogate of HRV and therefore susceptible to autonomic modulation, and by ETV, which is much reduced and less responsive to modulation.