Changes in the photoplethysmogram with tracheal intubation and remifentanil concentration.

Autonomic sympathetic activation, for instance following noxious stimuli, decreases the size and flattens the shape of the photoplethysmographic peripheral pulse waveform. We report a simple measure of the waveform shape, the ratio of mean-to-peak wave amplitude, for measuring nociception level during general anaesthesia. Fifty participants, anaesthetised with propofol and remifentanil, were randomly allocated to one of three different remifentanil effect-site concentrations (1, 3 and 5 ng center dot ml-1). Tracheal intubation increased the mean (SD) plethysmographic ratio from 0.38 (0.06) to 0.48 (0.04), p = 2.6 x 10-16. The mean (SD) ratios following intubation at remifentanil effect-site concentrations of 1 ng center dot ml-1, 3 ng center dot ml-1 and 5 ng center dot ml-1, were 0.49 (0.03), 0.48 (0.03) and 0.45 (0.04), respectively. Remifentanil therefore suppressed changes in the mean-to-peak ratio caused by tracheal intubation (p = 0.006). The ratio of the mean-to-peak plethysmographic amplitude may represent a simple measure of the balance of autonomic sympathetic and parasympathetic activity under general anaesthesia, and its performance following intubation was significantly different from peak amplitude (p = 0.046).