Emergency War Surgery NATO Handbook: Part III: General Considerations of Wound Management: Chapter XV: Anesthesia and Analgesia

Maintenance of Anesthesia

United States Department of Defense

Inhalation Agents. These agents have the advantage of being relatively easy to titrate, thereby facilitating changes in anesthetic depth. There is a considerable body of experience in the use of inhalation anesthetics in trauma surgery. Halothane was used to a great extent during the Vietnam conflict. However, one must remember that all potent halogenated agents depress the respiratory and cardiovascular systems and that such effects are even more pronounced in the presence of hypovolemia. Often sub-minimal alveolar concentration (MAC) dosages are adequate to provide analgesia and amnesia in the critically injured casualty. Of the potent halogenated agents in current usage, isoflurane appears to offer the advantages of very limited metabolism and a lesser degree of cardiac depression than halothane or enflurane. These three halogenated drugs are potent bronchodilators and therefore are useful in the asthmatic patient.

Isoflurane has a MAC of 1.15 in oxygen; 0.5 in 70% nitrous oxide. Inspired levels must be maintained at 40% higher than MAC. For induction, one should use inspired gas concentrations 3-4 times the maintenance. This drug is a potent respiratory depressant, decreases peripheral vascular resistance, produces hypotension with little cardiovascular depression, can cause malignant hyperthermia, has good muscle relaxant properties, and allows rapid recovery due to low solubility. Nitrous oxide is usually safe, provided adequate oxygen is administered. Therefore, an in-line oxygen analyzer should be used in the circuit when nitrous oxide is given. The tendency for nitrous oxide to expand in any closed space in the body (eg., pneumothorax, pneumocephalus, bowel obstruction) should also be kept in mind. Although diethyl ether has had widespread use as a battlefield anesthetic in the past, its flammability and lack of familiarity to most recently-trained anesthetists make it a less attractive choice. Cyclopropane, in addition to being highly explosive, is, like diethyl ether, no longer in common use and neither one should be used in modern combat anesthesia.

Intravenous Agents. Narcotics, such as fentanyl and morphine, are good analgesics and in adequate dosage are effective in blocking autonomic reflexes generated by noxious stimuli during operations. The combined use of nitrous oxide, muscle relaxants, and amnestics in a balanced anesthetic regime produces a "complete" anesthetic. Although considerably less depressant than the potent inhalation agents, narcotics should be carefully titrated in the unstable patient.

Fentanyl is a short-acting narcotic. One hundred mcg (2 ml) is equivalent to 10 mg of morphine. This drug may be given as intermittent bolus injection of 1-2 mcg/kg or as a continuous infusion of 2-50 mcg/min as titrated against blood pressure, pulse, or evidence of reaction to pain. Side effects of fentanyl include respiratory depression, bradycardia, bronchoconstriction, vomiting, and muscle stiffness. The muscle stiffness may need to be treated by intravenous muscle relaxant& Fentanyl has minimal effect on blood pressure.

Newer agents, such as sufentanil and alfentanil, in conjunction with short-acting muscle relaxants, may provide effective shortduration anesthesia and be successfully used in outpatient surgical procedures. These can be especially useful when recovery ward or ICU staffing is limited. The newer agonist-antagonist type of opiate drugs, such as nalbuphine. and butorphanol, have also proved to be effective trauma anesthetics.

Ketamine is an effective analgesic and dissociative maintenance agent which can be used either in incremental doses or as a continuous infusion. To prevent recall in a lightly anesthetized patient, anterograde amnesia can be induced with scopolamine or small doses of a benzodiazepine.

After an intravenous dose of 1-2 mg/kg rapid induction of anesthesia (within 30 seconds), an intense analgesia is produced. An endotracheal tube may not be required, but one must remember that respiratory depression, apnea, coughing, and laryngospasm are possible at any time. Ketamine produces increased salivation and tracheal secretions, and can cause unwanted tachycardia, hypertension, increase in intraocular and intracranial pressures, and eye movements.

This is an effective drug for treating bronchospasm that. is resistant to commonly used bronchodilators. It is an excellent agent for induction of anesthesia in the asthmatic.

Ketamine's intense analgesia makes it useful in the treatment of the bum patient for repeated debridement and dressing changes.

Postoperative excitement and dysphoria, which is produced at times by this drug, may be minimized by using lower dosages or giving small amounts of benzodiazepines in combination with ketamine. A continuous infusion can be used to reduce the total dose required for an anesthetic. A solution of 1 mg/ml can be infused at a rate of 1-25 ml/min. A loading dose of 50 mg should be used in the adult.

Midazalam, a new short-acting benzodiazepine, is probably the intravenous agent of choice to reduce ketamine emergence' reactions.

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Operational Medicine 2001

Health Care in Military Settings

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