The phase relationship indicates the time delay between the input signal (systolic blood pressure, SBP) and output signal (R-R interval, RRI). In contrast to the awake state, little is known about the effects of general anesthesia on the phase shift. In the present study, we tested the hypothesis that sevoflurane anesthesia causes a phase change between the two signals.

We assessed changes in phase, coherence, and baroreflex sensitivity between SBP and RRI by the use of transfer function analysis in 50 ASA 1 patients during the awake state and during end-tidal 2% sevoflurane-50% N₂O anesthesia.

SBP and RRI decreased significantly during sevoflurane anesthesia (P < 0.001). The phase in the low frequency (LF) region remained unchanged, but the phase in the high frequency (HF) region changed significantly from -29.52 ± 50.70 to 27.28 ± 80.22° during sevoflurane anesthesia (P < 0.001). Coherence and baroreflex sensitivity between the two signals in the LF and HF regions decreased significantly during sevoflurane anesthesia (P < 0.001, respectively).

We found that in the HF region and not in the LF region, sevoflurane anesthesia provokes the shift of the SBP-RRI phase relationship, suggesting that this change is inconsistent with a vagally mediated response.