Breath interval as a measure of dynamic opioid effect

We measured breath interval to characterize the time course of opioid effect in anaesthetized patients breathing spontaneously during knee replacement surgery with concurrent regional nerve blockade. Breath interval was recorded before and after a single dose of fentanyl 0.75 μg kg–1 i.v. Breath interval was measured between the start of successive inspirations, identified by a decrease in carbon dioxide concentration, sampled at the laryngeal mask connection. Nineteen patients were admitted to the study, of whom nine were withdrawn (there was a recording failure for one patient, five patients had inadequate block and three were excessively depressed by the fentanyl). Using MKMODEL software, the mean (sd) dynamic elimination half-life and dynamic mean brain residence time of fentanyl were 15.3 (7.8) and 24.1 (8.1) min, respectively. The times to detection of change from baseline, and peak effect of fentanyl on breath interval were 0.9 (0.6) and 5.2 (1.4) min, respectively. Breath interval increased from 2.9 (1.0) s to a maximum of 9.0 (5.7) s. There were no differences between the time course of changes in breath interval and end-tidal carbon dioxide concentrations. End-tidal carbon dioxide concentrations increased from a baseline of 6.6 (0.9)% to a peak of 8.2 (0.8)%. Breath interval was a useful and reproducible method of monitoring the duration of opioid effect in anaesthetized patients breathing spontaneously when surgical stimulation was not affecting the CNS. The data provide information on the duration of action of fentanyl and could guide dosage.