▪ RESIDUAL EFFECTS OF ANESTHETICS
Residual sedation from anesthetic agents often contributes to prolonged unconsciousness after surgery. Generally, an unconscious state related to residual anesthesia is time-limited and characterized by a rapid and progressive lessening of depth. The rate of emergence varies with the type of anesthetics used and the specific characteristics of the individual patient. Also, prolonged unconsciousness from residual anesthesia almost always reflects the combined effects of several agents, each of which exhibits a different rate of resolution.
Opioids
Opioids are frequently implicated in producing a prolonged state of unconsciousness. The degree and duration of postoperative sedation when opioids are administered intraoperatively is related to the timing, route, and total dosage of agents administered. When long- and intermediate-acting opioids are used, the resolution of sedation is slower than that caused by residual inhalational anesthetics because opioids require hepatic metabolism and/or renal excretion for clearance. Prolonged depression is especially common after the intraoperative administration of longer-acting opioids such as morphine, meperidine, or hydromorphone. However, other opioids such as fentanyl that usually have a shorter duration of clinical action, secondary to redistribution, can also exhibit a long “tail” of sedation when high dosages or continuous infusions are administered.
8 It is less likely that the shortest-acting opioids, such as alfentanil, sufentanil, and particularly remifentanil, will significantly contribute to postoperative unconscious states unless very high dosages are given over extensive periods.
Intramuscular opioid administration leads to slower uptake and prolonged action, especially in surgical patients who are hypothermic or hypovolemic. The administration of intrathecal or epidural opioids can result in the rostral spread of opioid into the cerebral ventricles, thereby resulting in unconsciousness and ventilatory depression.
Several interesting aspects of opioid pharmacology can increase their impact on prolonged unconsciousness. Some opioids are metabolized to active metabolites that prolong and add to central depression. Sedation induced by opioids is usually accompanied by a decrease in spontaneous minute ventilation that slows the washout of residual inhalational anesthetics, thereby prolonging sedation. Opioids exert a synergistic effect on the depressant properties of sedatives, leading to a greater degree of sedation than the sedative would have caused by itself. Also, when compared to other agents, the intense analgesic influence of opioids minimizes the arousal generated by postoperative pain; this effect also blunts the response to tactile stimuli and accentuates the depressant effects of sedatives or antiemetics.
The administration of additional opioids in the PACU adds to the residual depression from medications that were provided intraoperatively. While determining the reason for prolonged unresponsiveness, it is appropriate to assume that the patient does not perceive significant levels of pain, and therefore, the initiation of analgesic and sedative regimens, such as loading for patient-controlled analgesia, should be delayed until the source of unconsciousness is determined. Assessing pain levels should be avoided at this time, as well as administering analgesic medications based solely on signs indicating increased sympathetic nervous system activity. Generally, a patient who generates significant tachycardia and hypertension in response to postoperative pain will also exhibit some degree of consciousness. In an unresponsive patient, these physical signs can reflect critical abnormalities of oxygenation, ventilation, systemic perfusion, or intracranial pressure. Administering opioid analgesics under these circumstances could result in the patient’s death.
To assess whether prolonged unconsciousness is related to residual opioids, small, incremental, titrated
doses of intravenous naloxone (40 µg increments) can be administered. Careful titration can reverse both ventilatory depression and sedation without precipitating the dangerous reversal of analgesia and excess sympathetic nervous system activity that can result. Unless a patient has received a massive opioid overdose, the ventilatory rate and level of consciousness will increase with a total dosage of 200 µg or less of intravenous naloxone; if unconsciousness persists, it is most likely
not related to the depressant effects of residual opioids on the CNS.
Sedatives and Antiemetics
The administration of sedative premedication to achieve anxiolysis or amnesia can contribute to prolonged unconsciousness, particularly if long-acting sedatives (e.g., pentobarbital, hydroxyzine, promethazine, lorazepam) are administered orally, rectally, or intramuscularly. The possibility of unacknowledged “self-premedication” by patients with long-acting oral sedatives or other psychotropic medications should also always be considered. Even the judicious use of intravenous midazolam before induction can, in some patients, affect the level of consciousness in the PACU. The administration of sedatives or antiemetics as part of the anesthetic regimen likewise adds even more profound depression in the PACU, especially if given toward the end of surgery.
Parenteral medications such as propofol, short-acting barbiturates, or etomidate by frequent bolus or continuous infusion can generate high circulating serum levels and resultant redistribution of high concentrations of drug into the tissues. The delayed excretion of medication can cause or accentuate delayed awakening after discontinuation.
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Antiemetics, such as droperidol, prochlorperazine, or scopolamine, have sedative side effects that can augment the residual sedation from anesthetics. Other antiemetic agents such as dexamethasone and serotonin-blocking agents (e.g., ondansetron, dolasetron) do not exhibit significant sedative side effects, and therefore do not contribute to postoperative unconsciousness.
Evaluating the contribution of sedatives or antiemetics to prolonged unconsciousness is not quite as straightforward as evaluating the influence of opioids. If a patient has received benzodiazepines, intravenous flumazenil, a competitive benzodiazepine antagonist, can be given in titrated, incremental dosages of 0.1 mg every 2 minutes. In the perioperative setting, <1 mg is typically needed to reverse residual benzodiazepine effect. Flumazenil directly reverses the sedation caused by midazolam, diazepam, lorazepam, and other benzodiazepines, although its duration of action is relatively short. If unconsciousness due to benzodiazepines is reversed by intravenous flumazenil, it is theoretically possible that benzodiazepine’s duration could exceed that of flumazenil reversal, leading to the return of unconsciousness an hour or so after reversal. However, the dosages of benzodiazepines used in contemporary anesthesia care are low enough that the serum concentration decreases significantly during the effective duration of flumazenil. In addition, the sedative effects of other medications, such as opioids and inhalational anesthetics, also wane during this interval. Therefore, the likelihood of “re-sedation” after flumazenil reversal is insignificant, unless a benzodiazepine overdose has occurred.
Reversal Agents
There are no specific reversal agents available to counteract the depressant effects of barbiturates, propofol, phenothiazines, and butyrophenones. The administration of intravenous physostigmine (1.25 mg) generates a degree of central arousal that can counteract, but not reverse, depression from sedatives, antiemetics, and other depressant medications such as baclofen.
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11 Application of this modality is usually not warranted unless the etiology of sedation is unclear and immediate resolution is important.
Inhalational Anesthetics
High alveolar partial pressures of residual volatile anesthetic agents can sometimes leave a patient deeply sedated early in the postoperative course. This phenomenon occurs predominantly after extended exposure to high concentrations of a more soluble agent such as isoflurane.
12 During long surgical procedures, significant amounts of soluble anesthetic agents build up in tissues that have lower perfusion levels, consequently leading to a more gradual washout after discontinuation. Obese patients may be at particular risk of prolonged sedation after long procedures, given their relatively high proportion of body fat. Also, if high inspired concentrations are continued through the end of surgery to maintain bronchodilation or to facilitate a “deep” extubation, alveolar partial pressures and level of sedation will naturally be higher during the initial recovery period.
It is unlikely that low solubility agents, such as sevoflurane and desflurane, are the primary cause of persistent unconsciousness because they are eliminated very rapidly, soon after their discontinuation in the operating room. However, these agents may contribute to sedation when combined with other, longer-acting parenteral medications such as opioids. Nitrous oxide is seldom implicated, because of its low solubility and relatively weak anesthetic properties.
Considering the inevitable washout of anesthetic vapor during normal breathing, it would be unusual that the residual effects from any volatile inhalational anesthetics would be the primary cause of an unconscious state that lasts over 30 minutes after discontinuation of inhalational anesthesia. However, if the residual volatile agent should, indeed, significantly contribute to prolonged unconsciousness, then the culprit can be easily detected from breath odor or through a quantitative analysis of expired gas using standard intraoperative monitors. There are no specific agents available that will reverse residual sedation from volatile anesthetics.
If it is essential to assess whether residual inhalational anesthesia is causing prolonged unconsciousness, the administration of intravenous physostigmine can be tried to counteract the sedative effects. However, simply
allowing adequate time for the inhalational agent to wash out through alveolar ventilation will provide sufficient differentiation. If the administration of appropriate doses of naloxone, flumazenil, and physostigmine does not elicit a response, unconsciousness is not likely related to sedation from residual anesthetic medications.
Neuromuscular Relaxants
Neuromuscular agents do not have any significant sedative or analgesic properties and therefore would not exacerbate postoperative unconsciousness. Rarely, profound residual neuromuscular blockade can mimic unconsciousness during recovery by completely eliminating any voluntary motor response to verbal or tactile stimuli in a conscious but completely paralyzed patient. Although unlikely, this degree of neuromuscular blockade can occur after gross overdose with neuromuscular blocking agents or if reversal agents are omitted. Similarly, complete postoperative paralysis is conceivable in patients with unrecognized neuromuscular disease or in those exhibiting phase II blockade caused by excessive succinylcholine administration or pseudocholinesterase deficiency.
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