Stimulation induced variability of pulse plethysmography does not discriminate responsiveness to intubation

Background. Hypnotic depth but not haemodynamic response to painful stimulation can be measured with various EEG-based anaesthesia monitors. We evaluated the variation of pulse plethysmography amplitude induced by an electrical tetanic stimulus ( PPG variation) as a potential measure for analgesia and predictor of haemodynamic responsiveness during general anaesthesia. Methods. Ninety-five patients, ASA I or II, were randomly assigned to five groups [ Group 1: bispectral index ( BIS) ( range) 40 - 50, effect site remifentanil concentration 1 ng ml(-1); Group 2: BIS 40 - 50, remifentanil 2 ng ml(-1); Group 3: BIS 40 - 50, remifentanil 4 ng ml(-1); Group 4: BIS 25 - 35, remifentanil 2 ng ml(-1); Group 5: BIS 55 - 65, remifentanil 2 ng ml(-1)]. A 60 mA tetanic stimulus was applied for 5 s on the ulnar nerve. From the digitized pulse oximeter wave recorded on a laptop computer, linear and non-linear parameters of PPG variation during the 60 s period after stimulation were computed. The haemodynamic response to subsequent orotracheal intubation was recorded. The PPG variation was compared between groups and between responders and non-responders to intubation ( ANOVA). Variables independently predicting the response were determined by logistic regression. Results. The probability of a response to tracheal intubation was 0.77, 0.47, 0.05, 0.18 and 0.52 in Groups 1 - 5, respectively ( P < 0.03). The PPG variability was significantly higher in responders than in non- responders but it did not improve the prediction of the response to tracheal intubation based on BIS level and effect site remifentanil concentration. Conclusion. Tetanic stimulation induced PPG variation does not reflect the analgesic state in a wide clinical range of surgical anaesthesia.