Effect of hypotensive anaesthesia on cochlear function

ABSTRACT

During surgery of the small structures of the middle ear, even minor bleeding can impair the surgeon's ability to operate under an optical microscope. Hypotensive anaesthesia is effective in keeping bleeding to a minimum. However, preliminary results showed that cochlear function was worsened by systemic hypotension which could play a role in the pathogenesis of sudden hearing loss. By recording transient evoked otoacoustic emissions [TEOAEs] in the external auditory canal, the present study was designed to a- Evaluate the possible auditory functional consequences of cochlear blood flow changes during hypotensive anaesthesia. b- Determine the safe mean arterial blood pressure [MAP] below which cochlear function is deleteriously affected. 20 patients scheduled for middle ear surgery received general anaesthesia with isoflurane. After 15min. of haemodynamic stability, controlled hypotension was induced by an i.v. infusion of glyceryl trinitrate. TEOAEs were recorded in the contralateral ear on the day of surgery prior to admission to the theatre [control value], and then intraoperatively during controlled hypotension at MAP of 65, 60, and 55mmHg, respectively, as well as on the first postoperative day. Statistical analysis included the arithmetic mean, standard deviation, standard error, hypothesis Student's "t", and Pearson's correlation tests. During controlled hypotensive anaesthesia, the intraoperative TEOAE amplitude decreased significantly [p<0.01] and was completely abolished at a MAP controlled hypotension and cochlear blood flow probably has a reduced ability for autoregulation compared to the cerebral blood flow. We recommend keeping the MAP above 60mmHg during middle ear surgery. The use of TEOAEs to monitor cochlear function during middle ear surgery can be of great help for both the anaesthetist and the surgeon