Awareness results partly from the inability to accurately measure the depth of anesthesia. There is no monitor that can guarantee no intraoperative awareness/recall. It is primarily subjectively judged by observing trends in heart rate, blood pressure, lacrimation, and movement. Level of consciousness monitors (depth of anesthesia monitors) can also be helpful, especially in certain patients and when using certain anesthetic techniques (i.e. TIVA).
In general, there are four causes:
- Individual variability in dose requirements for anesthetic drugs (genetics, drug habits: benzodiazepines, cocaine, EtOH)
- Patient inability to tolerate full anesthesia due to low physiologic reserves (heart failure, hypovolemia)
- Iatrogenic masking of physiologic responses that would indicate the need for an anesthetic dose change (use of beta-blockers or the presence of pacemaker)
- Compromised delivery of anesthetic due to equipment malfunction or misuse (broken vaporizer, disconnected circuit/IV line/leak)
Patient risk factors contributing to increased likelihood of intraoperative awareness include age, limited cardiac reserve, drug resistance or substance abuse, history difficult intubation or previous episodes of intraoperative awareness.
Surgical procedures related to increased risk of intraoperative awareness: C-section, cardiac surgery, trauma surgery, and procedures for which muscle relaxants are used
3 categories of depth of anesthesia devices for awareness: measure EEG (BIS), auditory evoked potentials (BAEPs), scalp EMG activity. There is no empiric data to suggest that these monitors are superior to preventing awareness than simply keeping patient >0.7 MAC of end-tidal inhaled anesthetics during routine general anesthesia cases. Various styles of anesthetics or physiologic changes (cerebral ischemia, hypoperfusion, NMBDs, ephedrine, elderly with low amplitude EEG) can also affect the sensitivity/specificity of readings on these monitors. These monitors probably have the most utility in cases where patients are at increased risk for intraoperative awareness.
Defined by: Ashley Kang, MD