Quantification of postoperative vigilance by evoked potentials

Abstract

Patients will be discharged from the postoperative recovery room mostly on subjective clinical assessment. In this study an approach to a more quantitative judgment of postoperative vigilance is made by recording the P300-latency and neuropsychological tests. 22 adult patients undergoing a disc operation were examined. For induction of anesthesia thiopental (4-5 mg/kg), fentanyl (0.1 mg) and a musclerelaxant (atracurium 0.4 mg/kg or succinylcholine 1-2 mg/kg after precurarisation with atracurium 5 mg) were given. Anesthesia was then continued with enflurance (1.0-1.2 MAC) in a mixture of 67% nitrous oxide in oxygen. If postoperative analgesia was needed, piritramid was injected in boli à 3-6 mg. The P300 was acoustically stimulated with an oddball-paradigm and recorded at Fz and Cz. Afterwards the latencies were measured and compared with a vigilance score composed of clinical parameters and neuropsychological tests. Recordings were done preoperatively and every 30 minutes up to 2 hours postoperatively. A correlation between P300-latencies and vigilance score was made with the coefficient of Spearman. Comparison of pre- and postoperative values was managed by using Wilcoxon test for matched pairs with Bonferroni-correction. Immediately after operation, P300 was obtained only in 8 patients (36%). The latencies were delayed (394 +/- 35 ms versus 326 +/- 12 ms preoperatively). During follow-up patients recovered and 2 hours postoperatively only one patient had no P300. At the end of the examination period P300-latencies of most patients had not yet reached the preoperative levels. The vigilance score in parallel showed decreases immediately after the operation and increases later on. However there were discrepancies between P300 latencies and neuropsychological findings, in some cases possibly due to the sedative effects of postoperative analgetics. Recording of P300-latencies showed remarkable cognitive deficits because of subclinical anesthesia hangover even 2 hours after a routine inhalational anesthesia. It is a good quantifiable method for assessment of postoperative vigilance. In some cases P300-latency is a more sensitive parameter for vigilance phenomena than clinical and neuropsychological scores.