Care of the Patient
EMERGENCE FROM GENERAL ANESTHESIA
Recovery from general or regional anesthesia is a time of great physiological stress. Emergence from general anesthesia should ideally be characterized by a smooth and gradual awakening in a controlled environment. However, problems such as airway obstruction, shivering, agitation, delirium, pain, nausea and vomiting, hypothermia, and autonomic lability are frequently encountered. Patients receiv-ing spinal or epidural anesthesia may experience decreases in blood pressure during transport or recovery; the sympatholytic effects of major con-duction blocks prevent compensatory reflex vaso-constriction when patients are moved or when they sit up.
Following an inhalational-based anesthetic, the speed of emergence is directly proportional to alveolar ventilation, but inversely proportional to the agent’s blood solubility . As the duration of anesthesia increases, emergence also becomes increasingly dependent on total tissue uptake, which is a function of agent solubility, the average concentration used, and the duration of exposure to the anesthetic. Hypoventilation delays emergence from inhalational anesthesia.
Emergence from an intravenous anesthetic isfunction of its pharmacokinetics. Recovery from most intravenous anesthetic agents is dependent primarily on redistribution rather than metabolism and elimination. As the total administered dose increases, however, cumulative effects become clini-cally apparent in the form of prolonged emergence; the termination of action becomes increasingly dependent on the metabolism or elimination. This is the basis for the concept of a context-sensitive half-time. Advanced age or renal or hepatic disease can prolong emergence. Short and ultrashort-acting anesthetic agents, such as propofol and remifentanil, significantly shorten emergence, time to awakening, and dis-charge. Some studies show that the use of a Bispec-tral Index Scale (BIS) monitor may reduce total drug dosage and shorten recovery and time to discharge. LMA (rather than an endotra-cheal tube) use may also allow lighter levels of anes-thesia that could speed emergence.
The speed of emergence can also be influenced by preoperative medications. Premedication with agents that outlast the procedure (eg, lorazepam) may be expected to prolong emergence. The short duration of action of midazolam makes it a suitable premedication agent for short procedures. The effects of preoperative sleep deprivation or drug ingestion (alcohol, sedatives) can also be additive to those of anesthetic agents and can prolong emergence.
The most frequent cause of delayed emergence (when the patient fails to regain consciousness 30–60 min after general anesthesia) is residual anes-thetic, sedative, and analgesic drug effect. Delayed emergence may occur as a result of absolute or relative drug overdose or potentiation of anesthetic agents by prior drug or alcohol ingestion. Naloxone (in 80 mcg increments in adults) and flumazenil (in 0.2 mg increments in adults) will readily reverse the effects of an opioid and benzodiazepine, respectively. Physostigmine (1–2 mg) may partially reverse the effect of other agents. A nerve stimulator can be used to exclude persisting neuromuscular blockade in poorly responsive patients on a mechanical ventila-tor who have inadequate spontaneous tidal volumes.
Less common causes of delayed emergence include hypothermia, marked metabolic distur-bances, and perioperative stroke. A core tempera-ture of less than 33°C has an anesthetic effect and greatly potentiates the actions of central nervous system depressants. Forced-air warming devices are most effective in raising body temperature. Hypox-emia and hypercarbia are readily excluded by pulse oximetry, capnography, and/or blood gas analysis. Hypercalcemia, hypermagnesemia, hyponatremia, and hypoglycemia and hyperglycemia are rare causes of delayed emergence that require laboratory measurements for diagnosis. Perioperative stroke is rare, except after neurological, cardiac, and cerebro-vascular surgery ; diagnosis is facili-tated by neurological evaluation and radiological imaging.
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