Validation of the Interaction Model of the Anesthetic Potency of Sevoflurane, Propofol and Remifentanil

Background

In general anesthesia propofol or volatile anesthetic are usually combined with opioids and the synergy between hypnotics and opioids is used to reduce the dose of each component in order to minimize side effects and to allow a rapid recovery. Current pharmacodynamic interaction models allow to estimate the potency of combinations of propofol and opioids, volatile anesthetics and opioids or propofol and sevoflurane respectively. In these interaction models the potency of the drug combinations is expressed as probability that motor response to laryngoscopy is suppressed (= tolerance of laryngoscopy, PTOL). The potency of the hypnotic drugs is represented by the concentration preventing motor response to laryngoscopy in 50% of the population (Ce50 hypnotic). Conversely potency of the opioids is represented as concentration reducing the Ce50 of the hypnotics by 50%.

The data of the three previous studies on propofol-remifentanil, propofol-sevoflurane and sevoflurane-remifentanil interaction were pooled and reanalyzed. The result was a triple interaction model of sevoflurane, propofol and remifentanil where sevoflurane and propofol were additive and either propofol or sevoflurane were synergistic with remifentanil. In contrast to the previous studies the response surface of the propofol-remifentanil and sevoflurane-remifentanil derived from the pooled re-analysis had a similar shape, which is reflected by a common C50 remifentanil and a common slope parameter. This means that remifentanil is equally synergistic to propofol and sevoflurane. The next step is to validate this interaction model with other stimuli than laryngoscopy and with other responses to stimulation that movement.

In clinical practice not motor response but hemodynamic response (heart rate and arterial blood pressure increase) upon surgical stimulation is used to titrate anesthetics and opioids.

In laparoscopic surgery after a small skin incision, carbon dioxide is inflated into the abdominal cavity to maintain an intraabdominal pressure of 14 mmHg. Recently the sevoflurane concentration preventing a heart rate or blood pressure increase greater than 20% upon installation of pneumoperitoneum (MAC BAR pneumoperitoneum) has been determined: The MAC BAR pneumoperitoneum (95% CI) of sevoflurane was 4.6 (4.3-4.9) without opioids and 2.4 (2.2-2.6) and 1.7 (1.4-2.1) vol% with an effect site remifentanil concentration of 1 and 2 ng ml-1.These values all correspond to 90% probability to tolerate laryngoscopy (PTOL) according to our triple interaction model (Hannivoort, BJA 2016), which indirectly supports our model.