[The effect of acute hypocarbia on human auditory brainstem responses]

The effect of acute hypocarbia on auditory brainstem responses (ABRs) was studied in nine patients scheduled for cranioplasty under N2O/O2/fentanyl anesthesia. PaCO2 was allowed to stabilize for 15 min before ABR recordings were obtained during normocarbia [N] (PaCO2 = 40.8 +/- 1.9 mmHg), hypocarbia [H] (PaCO2 = 25.9 +/- 0.8 mmHg), and after return to normocarbia [NR]. No significant change in the absolute or interpeak latencies of waves I, III, and V was observed between during [N] and [H]. On return to normocarbia, the absolute latencies of waves I, III, and V increased slightly but significantly when compared with [N] or [H], although the interpeak latencies were unchanged. It is concluded that the latencies of ABR are unaffected directly by acute hypocarbia to a PaCO2 of 25 mmHg. The increase of absolute latencies on return to normocarbia may be explained by increased middle ear pressure with N2O and/or increased intracranial pressure.