Anesthetic Agents Used in Induction and Maintenance

Intravenous Agents

Most induction is performed with one or a combination of intravenous agents:

• Barbituate (thiopental)

• Benzodiazepine (midazolam)

Thiopental is a common barbituate used for induction, having a rapid onset of action and short duration. Thiopental decreases intracranial pressure and the cerebral metabolic rate making it useful for initiating a barbituate coma in severe head trauma. Thiopental depresses the myocardium, causes peripheral vascular pooling, and airway irritability leading to laryngospasm and bronchospasm thus limiting usefulness in trauma and emergency intubations.

Midazolam and diazepam are useful for induction in trauma and with cardiac surgery patients since they have modest cardiovascular side effects and additionally reduce cerebral metabolic rate and blood flow. Midazolam is more frequently used than diazepam because of its faster hepatic extraction and lack of active metabolites causing little residual sedative effect after surgery. Reversal of benzodiazepines can be accomplished with flumezanil which acts as a competitive antagonist at GABA receptors.

Propofol is commonly used as an induction agent because of its rapid onset of action, antiemetic properties and extrahepatic metabolism. Propofol has a greater cardiac depressive effect than the benzodiazepines and barbiturates but does not cause bronchospasm making it the agent of choice in patients requiring airway instrumentation.

Etomidate has minimal adverse effects on the respiratory and circulatory systems making it useful for patients with hypovolemia, cardiac disease, and asthma.

Side effects of etomidate include myoclonic jerks, hiccups and postoperative nausea and emesis limit its use.

Ketamine is useful in hypovolemic patients through its stimulatory effects on the cardiovascular system and its ability to increase cerebral blood flow. Ketamine produces a dissociative state associated with hallucinations on emergence and is the only intravenous induction agent with analgesic activity.

Inhalational Agents

Inhalational agents can be used for both induction and maintenance of general anesthesia as they produce unconsciousness, amnesia, analgesia and a small degree of muscle relaxation. Dosing and titration of these agents are affected by the solubility of the inhalational agent in blood, the patient's cardiac output and the minimum alveolar concentration of the anesthetic sufficient to prevent movement in 50% of patients. Agents with low solubility in blood have faster onset and recovery from the effect of the anesthetic. The inhalational anesthetics used in clinical practice include nitrous oxide, halothane, isoflourane, enflurane, and sevoflurane.

Nitrous oxide is a weak anesthetic and is administered in combination with other anesthetic gasses or during conscious sedation for its analgesic effect.

Halothane is one of the most commonly used anesthetic gasses but should be avoided in patients with increased intracranial pressure, cardiac disease, hypovolemia and liver disease secondary to its effects in causing cerebral vasodilation, nodal rhythm, ventricular irritability, hypotension and halothane hepatitis.

Isoflourane is only a mild cardiac depressant and does not induce ventricular irritability but has major side effects of increasing intracerebral blood flow and increasing the incidence of postpartum hemorrhage.

Enflurane is a more potent muscle relaxant, respiratory depressant and circulatory depressant than halothane and is metabolized to produce nephrotoxic fluorides.

Sevoflurane is the most effective agent for the cardiac patient with no significant effect on cardiac output, blood pressure, or peripheral vascular resistance. Sevoflourane is also useful in patients with increased intracerebral pressure.

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