Relationship between decrement time and recovery time in pediatric total intravenous anesthesia with propofol and remifentanil

Background: Propofol target-controlled infusion is now used in many countries for the administration of total intravenous anesthesia (TIVA) and uses population pharmacokinetic data incorporated into the target-controlled infusion pump to administer a dose which will achieve a target concentration selected by the practitioner. Modern target-controlled infusion pumps can include an estimate of how long it will take for the plasma propofol concentration to fall to a value programmed into the pump, once the propofol infusion is stopped. This is known as decrement time. Many users consider decrement time to be equivalent to recovery time for the patient, despite the several assumptions that makes. Aim: To determine if the decrement time estimated by the target-controlled infusion pumps at the end of anesthesia could be reliably used to predict recovery time for individual patients. Methods: Target-controlled infusion pumps at Birmingham Children's Hospital are programmed with a propofol decrement concentration of 1.5 mu g.mL(-1). Patients for a variety of surgical procedures were anesthetised with propofol-remifentanil TIVA. Eye opening was used as a surrogate of recovery time. Data were analyzed using median absolute deviation, Mann-Whitney U test, and linear regression to see if there was any correlation with decrement time and recovery time, total dose of propofol administered, duration of anesthesia and patient age. Results: A total of 55 patients were audited. Median age was 7.2 years (range 0.9-15.9), and median weight was 26 kg (range 8.5-77). Median decrement time was 11.8 min (IQR 8.3-21), while median recovery time was 15 min (IQR 9-20). Recovery time was the same as decrement time in 2% of patients, less than in 40% of patients and more in 58% of patients. Two thirds of patients opened eyes at an estimated plasma propofol concentration between 1.2 and 1.8 mu g.mL(-1). The median absolute deviation for the difference between decrement time and recovery time was 5.7 min, from a median of 1 min. Analysis demonstrated there was no statistical correlation between decrement time to 1.5 mu g.mL(-1) and recovery time, age, total propofol dose administered, or duration of anesthesia, for individual patients. Conclusion: There was variation between decrement time to 1.5 mu g.mL(-1) and recovery time of over 5 min in nearly half of patients. Decrement time to 1.5 mu g.mL(-1) must be used with caution when trying to predict recovery time for individual patients following TIVA.