Transfer function analysis of the circulation in patients undergoing sevoflurane anesthesia

Purpose: The effects of sevoflurane anesthesia on the interactions between heart rate, blood pressure and respiration were assessed using transfer function analysis. Methods: Nine ASA 1 or 2 patients undergoing elective surgery were involved. They were paralysed and their lungs were mechanically ventilated during sevoflurane anesthesia. Instantaneous heart rate (IHR) from electrocardiogram, instantaneous lung volume (ILV) by respiratory inductive plethysmography and mean blood pressure (MBP) by arterial tonometry were obtained during conscious state, and 1MAC and 2MAC of sevoflurane anesthesia. Transfer function analysis for the relationships between ILV and IHR, ILV and MBP, MBP and IHR were made for five minute periods during which the respiratory rate was varied in a standardized fashion. Results: In awake patients transfer magnitudes for the relationships between ILV and IHR and between MBP and IHR in the 0.04-0.5 Hz frequency band were 8.9 ± 7.7 bpm·l^-1 and 0.95 ± 0.44 bpm·mmHg^-1 respectively. Sevoflurane 2MAC decreased these values to 1.2 ± 0.7 (P = 0.014) and 0.26 ± 0.14 (P < 0.01) respectively, but phases were not affected. Neither transfer magnitudes nor phases between ILV and MBP were affected during sevoflurane anesthesia. Coherence for the relationships between ILV and IHR and between MBP and IHR were decreased during 1 MAC sevoflurane anesthesia but not affected during 2MAC sevoflurane anesthesia. Conclusions: The interactions between heart rate, blood pressure and respiration were altered by sevoflurane anesthesia. These findings could be explained by the attenuation of autonomic nervous system activity.