Paul K. Sikka, Shawn T. Beaman and James A. Street (eds.)Basic Clinical Anesthesia10.1007/978-1-4939-1737-2_51

51. Awareness Under Anesthesia

Tiffany Lonchena¹ and Cynthia Wells²

(1)

Department of Anesthesiology, University of Pittsburgh Medical Center, Pittsburgh, PA, USA

(2)

Department of Anesthesiology, University of Pittsburgh School of Medicine, 200 Lothrop Street, Pittsburgh, PA 15213, USA

Cynthia Wells

Email: WellCx2@upmc.edu

Keywords

AwarenessMACTraumaLight anesthesiaBIS

One of the commonly accepted goals of general anesthesia is the achievement of decreased consciousness and the prevention of intraoperative awareness. Intraoperative awareness and recall can have serious psychological effects on the patient as well as legal consequences for the provider. As such, it is imperative to maintain vigilance in the operating room in order to avoid preventable anesthesia awareness, as well as to be prepared to address such events when they arise.

Incidence

The overall incidence of intraoperative awareness has been quoted at 0.18 % with NMBDs and 0.10 % without NMBDs. This correlates with approximately 30,000 surgical patients per year. High-risk surgical cases with a greater incidence of awareness include cardiac surgery (1.0–1.5 %), trauma surgery (11–43 %), and cesarean section under general anesthesia (0.4 %). This increased incidence is attributed to the intentional use of light anesthesia with the desire to minimize the negative hemodynamic effects of anesthetic agents.

Risk Factors

Factors that increase the incidence of intraoperative awareness include the absence of volatile agents or propofol during maintenance of anesthesia, total intravenous anesthesia (TIVA), NMBDs, and prolonged or difficult intubation. Patient-related risk factors include chronic alcohol use, chronic use of neurodepressant drugs (antiepileptic, opiate, and sedative), a history of awareness during general anesthesia, a limited cardiac reserve requiring light anesthesia, and ASA Classes IV–V.

Intraoperative awareness generally occurs secondary to either a decreased dose of anesthesia, often referred to as “light” anesthesia, or to a patient’s decreased response, or resistance, to a seemingly appropriate dose of agent. For inhalation agents, the depth of anesthesia is estimated using minimum alveolar concentration (MAC). While MAC can be used as a general guideline, it is affected by numerous factors, including age, temperature, chronic drug exposure, acute drug exposure, and genetic factors. Furthermore, multimodal anesthesia (the mixture of inhalation and intravenous drugs of varying mechanisms of action) results in the unreliability of MAC as the only method of measuring anesthetic depth.

With the common addition of neuromuscular-blocking drugs (NMBD) to the anesthetic plan, the assessment of intraoperative consciousness becomes increasingly difficult. Movement, which could be indicative of light anesthesia, is now chemically prohibited. Furthermore, NMBDs allow for a decreased required volatile anesthetic dose and thus an at least theoretical greater probability of light anesthesia.

Characteristics

Intraoperative awareness can result in two types of memory formation, explicit and implicit. Explicit memory, or the recall of specific events, is more detrimental to both the practitioner and the patient. Implicit memory, which is characterized by changes in behavior without the recall of specific events, is often still traumatic for the patient but tends to have less legal consequence. The most common memories during cases of intraoperative awareness involve awake paralysis, feeling surgery with or without pain, panic, the process of tracheal intubation, and the recollection of conversations, sounds, or comments concerning body habitus. There have also been incidences of inadvertent paralysis due to residual neuromuscular-blocking drug in the intravenous tubing, both in the operating room as well as in the postoperative care area.

Prevention

Although not all cases of anesthesia awareness can be avoided, it is imperative that the anesthesia provider minimizes the preventable causes of anesthetic awareness. This requires a complete machine check daily, a thorough preoperative evaluation, and vigilant clinical monitoring.

Anesthesia Machine Check:

One of the most preventable causes of intraoperative awareness is the failure to deliver an adequate dose of volatile anesthetic. This can be secondary to equipment malfunction, vaporizer complications, and breathing circuit failures. With a thorough machine check prior to the patient’s arrival into the operating room, most complications can be prevented.

Preoperative Evaluation

Patients at high risk for intraoperative awareness should be identified preoperatively and properly counseled regarding their increased risk. These patients should be asked about previous episodes of awareness and tolerance to sedatives and opioids and whether they have a known difficult airway or are having a surgical procedure with increased risk of awareness. Prior anesthesia records should be checked whether these patients received NMBDs during maintenance of anesthesia, reduced amount of inhalational agents, or total intravenous anesthesia. Patients can be given midazolam preoperatively, as prophylactic administration of midazolam has been shown to decrease the incidence of awareness. In the event of prolonged attempts at intubation after intravenous induction, additional intravenous anesthetic agent should be readily available to maintain unconsciousness.

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