Intraoperative awareness - potential causes and decreasing the risk

AIMS is a large, anonymous, multicenter, reporting system of anesthesia incidents that is in widespread use in Australia and parts of Asia.  Bergman et al examined the data base when there were 8372 abstracted reports collected between 1988 and 2001.  The authors queried on the keyword “awareness.”  Using specific definitions of awareness they found 81 reports of which there were 50 cases of definite awareness and 31 with a high probability of awareness.  They could find no obvious cause for awareness in 13 (16%) of the cases. Low inspired volatile agent concentration appeared to be responsible for 36 (44%) of the 81 cases.  In 14 of these cases there was no agent monitor present.  Five of these cases appeared to be due to prolonged attempts at intubation and four cases were due to reducing delivered anesthetic secondary to hypotension or cardiovascular instability.  Thirty two cases were due to inadvertent paralysis of an awake patient.  Case review suggested that inattention or distraction were contributory in 20 cases, haste in 14 and fatigue in 5.

            This study covers a later period than did the closed claims analysis study of awareness (Domino KM et al: Anesthesiology 1999;90:053-61).  In my opinion, at least in the United States, the 1988 – 2001 period is reasonably similar to current practice yet awake paralysis persists and in Bergman et al awake paralysis was the most common cause for awareness.  Picking up the wrong syringe (a paralytic instead of the desired drug) was a recurrent cause of awareness.  Keeping syringes of paralytic drugs away from other drugs might help reduce this problem.  The authors expressed particular concern about the incidents of awareness with no apparent cause.   Two of the 13 cases had agent analyzers and apparently adequate doses of volatile agent.  Two cases occurred during ECT when no volatile agent is typically used.

            Neither this study nor the closed claims study allows calculation of incidence since the total number of cases from which these reports are drawn is not known.  In addition reporting is voluntary and the threshold for reporting most likely varied from practitioner to practitioner.  Finally if patient report was the major source of information then there was most likely underreporting as most patients do not volunteer awareness information unless repeatedly asked.  However, despite these weaknesses, this paper provides an intensive study of a serious of events providing conclusions as to cause and approaches for decreasing the risk of the occurrence of awareness.

            The authors presented eight suggestions based on their data that they feel will help reduce awareness (table 4 modified):  1) Check the anesthesia machine before each use; ensure a correctly mounted and filled vaporizer. 2) When using a volatile agent use an end-tidal agent monitor.  Use a low level alarm set for a sufficient volatile agent concentration to prevent awareness. 3)  Provide further hypnotic doses for repeated intubation attempts. 4) Be aware of the potential for awareness in hypovolemic patients receiving low concentrations of anesthetic.  Restore appropriate anesthetic concentration as soon as possible. 5) Routinely use a peripheral nerve stimulator and ensure sufficient anesthetic concentration until muscle power returns. 6) When using total intravenous anesthetics, ensure a patent intravenous line and periodically check the infusion pump to confirm drug administration. 7) Clearly label all drug syringes immediately when they are drawn up.  Check this label carefully before administration.  Do not rely on recognition of syringe size to confirm its contents.  Consider newer methods of ensuring that the correct drug is given. 8) Consider a depth of anesthesia monitor, if not routinely then for selected cases.

Bergman IJ et al: Awareness during general anesthesia: a review of 81 cases from the Anesthetic incident monitoring study (AIMS).  Anaesthesia 2002;57:549-556

David S. Smith, M.D., Ph.D.