Awareness during anesthesia

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Review the classifications of memory and awareness.

Memory can be classified as implicit (unconscious memory) and explicit (conscious memory). Conscious recollection of events, including intraoperative events, would fall under the category of explicit memory. Awareness with recall (AWR) during general anesthesia is referred to as anesthetic awareness . Although patients may speak of “remembering” certain things under general anesthesia, recall of specific intraoperative events is key to the concept of anesthetic awareness. The 2006 American Society of Anesthesiologists' (ASA) practice advisory for intraoperative awareness excludes dreaming as anesthetic awareness.

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What is the incidence of awareness?

Various estimates of the incidence of adult awareness exist. Overall incidence is likely on the order of one in 19,000, when large cohorts, such as National Audit Project 5 are examined. Similar studies are consistent and have reported ranges of 1:15,000 to 1:23,000. Incidence of awareness can increase, depending on the anesthetic technique or subspecialty setting. Increased incidence occurs in higher risk populations, such as obstetric, urgent/emergent surgery, and cardiothoracic cases, and is often related to depth of anesthesia. Patient characteristics, such as female gender, younger adult age, obesity, and prior awareness under anesthesia are also risk factors.

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Are certain techniques and clinical situations more likely to result in awareness?

Recognized risk factors for AWR include:

• Light levels of anesthesia (common in hypovolemic, obstetric, and trauma patients)

• Prior history of intraoperative awareness

• Cardiac surgery with cardiopulmonary bypass that has historically relied on narcotic-based anesthesia which, while minimizing myocardial depression, produces unreliable amnesia

• Use of muscle relaxants has repeatedly been shown as an independent risk factor

• Malfunction of anesthetic administration: machine failure (e.g., empty vaporizer), intravenous (IV) anesthetic pump failure (e.g., no power, incorrect programming), infiltrated IVs, syringe swaps, and so on.

• Unrecognized increased anesthetic requirements, such as in patients with chronic substance abuse

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