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Monitoring for Intraoperative Awareness

AORN Journal, Dec, 1998 by Sandra M. Ouellette, Chrysanne Simpson

As the scope of surgical procedures has grown, so has the practice of anesthesia. Today, anesthesia is safer than ever; however, the possibility of awareness in apparently anesthetized patients continues to plague practitioners. Intraoperative awareness (IOA) has been recognized since 1846, when William Morton, MD, demonstrated the first ether anesthetic agent, and the patient later reported that he had been half-awake during the procedure and had experienced pain.(1) Since then, incidences of IOA have continued to be documented even as anesthesia has progressed to modern techniques and as advances in anesthesia pharmacology and technology have brought new and different slants to the phenomenon.

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Bispectral analysis (BIS) is emerging as the most promising prevention of intraoperative awareness and monitor of the hypnotic state. An adaptation of the traditional electroencephalogram (EEG), BIS incorporates both the power spectrum, which reflects the EEG's frequency and amplitude, and the bispectrum, which reflects EEG synchronization. Using values from zero to 100, BIS describes the hypnotic state as it compares to the current level of stimulation. Bispectral analysis appears to be superior to other monitoring methods, including processed EEG variables.(2)

GENERAL ANESTHESIA

General anesthesia is defined as a state of unconsciousness characterized by a concomitant loss of sensation, without interference to vital functions. The condition is incompatible with awareness--a state of perception and consciousness.

According to a popular model of general anesthesia, five components need to be addressed each time an anesthetic agent is administered: anxiolysis, analgesia, hypnosis, muscle relaxation, and suppression of somatic and autonomic responses.(3) Of these, perhaps the most difficult to assess is hypnosis. That is why, in the last 50 years, many avenues have been explored to both describe and detect an adequate hypnotic state--a perplexing task considering that the level of hypnosis is an ever-changing variable. Hypnotic state is a function of the level of sedation versus the level of stimulation; thus, an optimal monitor of hypnosis must provide a real-time, continuous measure in the face of these two factors.

The first attempt to quantify the state of general anesthesia as it compares to wakefulness was made by John Snow in 1858.(4) He described five stages of narcotism that evolved into four classifications: analgesia, light anesthesia, surgical anesthesia, and overdose.

In 1937, Arthur Guedel further identified four planes of anesthesia. Most surgical procedures could be performed in the second or third plane of anesthesia after using inhalation induction techniques.(5)

The signs and stages of general anesthesia became less valuable when IV induction agents and neuromuscular blocking medications were introduced in the 1930s and 1940s. Thiopental eliminated the first two stages of anesthesia, and the skeletal muscle-paralyzing medication, curare, eliminated the need for deep levels of anesthesia.(6) Intraoperative awareness, therefore, has been of the greatest concern in the last 40 years.

INCIDENCE AND ETIOLOGY OF AWARENESS

The reported incidence of patient awareness during anesthesia depends on the type of anesthesia, strength of the stimulus, and the timing and persistence of attempts to elicit recall. Several resources report the overall incidence of IOA is 0.2% to 1% of the 18.6 million anesthetized procedures performed each year.(7)

The risk of IOA seems to vary among types of procedures. Cesarean section procedures pose a 2% to 28% risk of awareness; major trauma procedures, 11% to 43%: cardiopulmonary bypass procedures, 1.14% to 23%; and bronchoscopy procedures, 8%.(8) One study limited to "fast-track" cardiac surgical patients cited an incidence of 3.3%.(9)

There are several reasons patients' awareness occurs while they are under general anesthesia occurs (Table 1). Among them are

* interpatient pharmacokinetic and pharmacodynamic variability,

* failure to maintain adequate medication levels,

* inability to assess depth of anesthesia, and

* selection of inappropriate anesthetic techniques.

Table 1

COMMON CAUSES OF PATIENT AWARENESS

Equipment failure

Inadequate anesthesia

* No premedication

* Decreased use of nitrous oxide

* Increased use of nonamnesic agents

* Suboptimal use of short-acting agents

* Substitution of adrenergic antagonists or vasodilators for anesthetics

* Overuse of neuromuscular blocking medications

Patient-related factors

* Age

* Health status

* History of alcohol or drug abuse

* Obesity

Causative factors also can relate to equipment (eg, improperly calibrated or connected vaporizers, empty vaporizers, general problems with anesthesia machines). At times, leaks in ventilator bellows can cause dilution or loss of anesthetic agents from the system.

Another cause of IOA is disruption in the delivery of ultra short-acting agents such as propofol, desflurane, and sevoflurane. The effect of this is compounded by the reduced use of premedication and nitrous oxide. Overuse of neuromuscular blocking medications also can lead to awareness, along with using adrenergic antagonists or vasodilators to manage tachycardia and hypertension, rather than using adequate levels of anesthesia.

Health Care Industry

Monitoring for Intraoperative Awareness

AORN Journal, Dec, 1998 by Sandra M. Ouellette, Chrysanne Simpson

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