What is emergence in anesthesia?

What is Emergence in Anesthesia

Emergence from anesthesia is the critical transition phase from unconsciousness to complete wakefulness following general anesthesia, during which consciousness is restored and protective airway reflexes return. 1

Definition and Key Characteristics

Emergence represents the ending stage of anesthesia, featuring the transition from unconsciousness to complete wakefulness and recovery of consciousness. 1 This phase is neurobiologically distinct from anesthetic induction and should not be considered simply as the reverse of the induction process—a phenomenon known as "hysteresis." 1

The critical distinction is that while anesthetics produce loss of consciousness primarily by affecting cortical connectivity, arousal processes during emergence are triggered by structures deep in the brain rather than being induced within the neocortex. 1

Timeline and Clinical Phases

Emergence is confined specifically to the period as consciousness is restored, which sharply distinguishes it from other postoperative delirium states that may occur later. 2 The process involves several key transitions:

• Discontinuation of anesthetic agents while maintaining ventilation and monitoring 3
• Progressive restoration of brain activity through distinct electroencephalographic patterns, typically transitioning from slow-wave anesthesia (characterized by delta 0.5-4 Hz and alpha/spindle 8-14 Hz power) to non-slow-wave states before awakening 4
• Return of protective airway reflexes and adequate spontaneous ventilation 5
• Recovery of consciousness with ability to follow commands 5

Critical Monitoring During Emergence

Respiratory Monitoring

Uninterrupted capnography monitoring must continue throughout emergence until the tracheal tube or supraglottic airway is removed. 5 This includes monitoring for:

• Adequate respiratory rate and pattern 5
• Absence of periods of hypopnea or apnea for at least one hour 5
• Return of oxygen saturation to pre-operative values 5

Neuromuscular Function

Reversal of neuromuscular blockade should be guided by quantitative nerve stimulation, with the goal of restoring a train-of-four ratio >0.9 before awakening the patient. 5, 6 Residual neuromuscular blockade (ToF ratio <0.9) carries significant risks including:

• Generalized muscle weakness and delayed recovery 5
• Reduced chemoreceptor-mediated responsiveness to hypoxia 5
• Increased risk of aspiration 5
• Postoperative pulmonary complications 5
• Risk of accidental awareness during general anesthesia 5

Depth of Anesthesia

Processed EEG-guided anesthesia care may facilitate rapid emergence and recovery, particularly in older patients where avoiding burst suppression reduces the risk of postoperative delirium. 5

Safe Extubation Criteria

Patients should have return of their airway reflexes and be breathing with good tidal volumes before tracheal extubation, which should be performed with the patient awake and in the sitting position. 5 Specific criteria include:

• Routine discharge criteria are met 5
• Respiratory rate is normal 5
• No periods of hypopnea or apnea for at least one hour 5
• Arterial oxygen saturation returns to pre-operative values with or without oxygen supplementation 5
• Train-of-four ratio >0.9 documented 5, 6

Common Complications During Emergence

Emergence Agitation

Emergence agitation is a self-limited state of psychomotor excitement during awakening from general anesthesia, with an incidence of approximately 19% in adult noncardiac surgery. 2 Young adults undergoing otolaryngology operations with volatile anesthetic maintenance may be at highest risk. 2

Delayed Emergence

The principal factors responsible for delayed awakening are anesthetic agents and medications used in the perioperative period. 7 However, nonpharmacological causes requiring recognition include:

• Metabolic disorders (hypoglycemia, severe hyperglycemia, electrolyte imbalance) 7
• Intraoperative cerebral hypoxia, hemorrhage, embolism, or thrombosis 7
• Residual effects of sedatives, analgesics, or neuromuscular blocking agents 5, 7

Respiratory Complications

Both NAP4 and NAP5 showed a high incidence of problems during extubation in obese patients, requiring an extubation plan in accordance with Difficult Airway Society guidelines. 5

Special Considerations for High-Risk Patients

Obese Patients

• Insert a nasopharyngeal airway before waking to mitigate partial airway obstruction commonly seen during emergence 5
• Maintain head-up position throughout recovery 5
• Reinstate home CPAP therapy on return to ward or in PACU if oxygen saturation cannot be maintained 5, 6
• Monitor oxygen saturations until mobile postoperatively 5

Patients with Sleep-Disordered Breathing

All obese patients should be assumed to have some degree of sleep-disordered breathing and managed accordingly during emergence. 5 This includes:

• Observing for signs of hypoventilation while unstimulated 5
• Watching for episodes of apnea or hypopnea with associated oxygen desaturation 5
• Extended monitoring in PACU if concerning patterns emerge 5

Pharmacological Management During Emergence

Antagonism of Anesthetic Effects

Specific antagonists should be available whenever benzodiazepines or opioids are administered, but should not be used routinely. 5

• Flumazenil may be administered to antagonize respiratory depression and sedation in selected patients after benzodiazepine use 5
• Naloxone may be administered to antagonize respiratory depression in selected patients after opioid use, though acute antagonism may result in pain, hypertension, tachycardia, or pulmonary edema 5
• After pharmacologic antagonism, patients should be observed long enough to ensure cardiorespiratory depression does not recur 5

Temperature Management

Normothermia should be a goal during emergence and recovery, with forced air warming systems used for treating hypothermia when available. 5 Meperidine should be used for treatment of patient shivering when clinically indicated, though hypothermia as the underlying cause should be treated by rewarming. 5

Key Safety Principles

Primary management during any emergence complication is to maintain airway, breathing, and circulation. 7 An extubation plan must be in place before discontinuing anesthesia, particularly in patients with anticipated difficult airways. 5

The heterogeneity of emergence patterns observed in surgical patients—with 31% transitioning abruptly from slow-wave anesthesia to waking—suggests that these transitions may correlate with post-operative outcomes such as pain. 4 Patients who transition abruptly are more likely to express pain in the post-operative period. 4