Mid-latency auditory evoked potentials and circulatory response to loud sounds

We investigated in 60 patients scheduled for elective aorto-coronary bypass grafting if loud sounds by themselves can induce cardiovascular responses and if these could be related to mid-latency auditory evoked potentials (MLAEP). Anaesthesia was induced in group l (n = 20) with flunitrazepam-fentanyl 0.01 mg kg −1 and maintained with flunitrazepam-fentanyl 1.2 mg h −1 . Patients in groups II (n = 20) and III (n = 20) received etomidate 0.25 mg kg −1 and fentanyl 0.005 mg kg −1 for induction and 0.6–1.2 vol% isoflurane and fentanyl 1.2mg h −1 , or propofol 4–8 mg kg −1 h −1 and fentanyl 1.2 mg h −1 for maintenance of general anaesthesia. After preparation of the sternum the operation was stopped for several minutes. Then, as a loud auditory stimulus, the sound of the running sternotomy saw was presented to the patients by putting the saw inverted on the sternum for several seconds. Heart rate (HR), arterial pressure (SAP), pulmonary capillary wedge pressure (PCWP), cardiac index, systemic vascular resistance and MLAEP were measured in the awake state, before and after presentation of the sound. Latencies of the peak V, Na, Pa, Nb and P1 were measured. In group I there were statistically significant increases in HR (63.5–70.2 beat min −1 ), SAP (123.9–146.5 mm Hg) and PCWP (9.2–11.7 mm Hg) after presentation of the sound. These haemodynamic changes were not observed in patients in groups II and III. In the awake state, AEP had high peak-to-peak amplitudes and a periodic waveform. During general anaesthesia brainstem auditory evoked potentials (BAEP) remained stable. In contrast, MLAEP in groups II and III showed a marked increase in latency and decrease in amplitude or were suppressed completely. In contrast, before and after the presentation of the sound, there was only a slight increase in latency or decrease in amplitude of the MLAEP in patients in group I. Latencies of the early cortical potentials Na and Pa correlated negatively with circulatory responses after presentation of the sound. Loud sounds by themselves may induce circulatory responses during general anaesthesia, when the electrophysiological conditions of primary cortical stimulus processing is preserved. (Br. J. Anaesth. 1994; 72: 307–314)