An investigation of learning during propofol sedation and anesthesia usingthe process dissociation procedure

Background: Many studies have shown that patients may remember words learne d during apparently adequate anesthesia. Performance on memory tests may be influenced by explicit and implicit memory. We used the process dissociati on procedure to estimate implicit and explicit memory for words presented d uring sedation or anesthesia. Methods: We Investigated intraoperative learning in 72 women undergoing per vaginal oocyte collection during propofol and alfentanil infusion. One word list was played once before Infusion, another was played 10 times during s urgery. Venous blood was taken for propofol assay at the end of the intraop erative list. Behavioral measures of anesthetic depth (eyelash reflex, hand squeeze response to command) were recorded and used to adjust the dose of anesthetic where clinically appropriate. On recovery, memory was assessed u sing an auditory word stem completion test with inclusion and exclusion ins tructions. Results: The mean blood propofol concentration was 2.5 mug/ml (median, 2.3 mug/ml; range, 0.7-6.1 mug/ml). Mean alfentanil dose was 2.1 mg (median, 2. 0 mg; range, 1.2-3.4 mg), Comparison of target and distracter hits in the I nclusion condition showed memory for preoperative words only. However, the process dissociation procedure estimates showed explicit (mean, 0.18; P < 0 .001) and implicit (mean, 0.05; P < 0.05) memory for the preoperative words , and a small amount of explicit memory for the intraoperative words (mean, 0.06; 95% confidence interval, 0.01-0.10). Memory performance did not diff er between the 17 patients who consistently responded to command and eyelas h reflex and the 32 patients who remained unresponsive. Blood propofol conc entration and alfentanil dose did not correlate with memory for the intraop erative list. Conclusions: There was no unprompted recall of surgery, but the process dis sociation procedure showed memory for words presented during surgery. This memory was apparently explicit but did not correlate with the measures of d epth of anesthesia used.