Emergency War Surgery NATO Handbook: Part III: General Considerations of Wound Management: Chapter XV: Anesthesia and Analgesia
United States Department of Defense
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Time constraints and the risk of aspiration usually dictate that induction be rapid and controlled. Several intravenous agents are available which are in common use in the trauma setting. These include the rapidly acting barbiturates (such as thiopental), the benzodiazepines, rapidly acting narcotics, etomidate, and ketamine The overriding factors in the decision as to which drug is best relate to the adequacy of blood volume, history of allergic reaction, or recent food intake If the patient is markedly hypovolemic and there is insufficient time for fluid resuscitation, it is best to modify dosages of induction agents or use less cardiovascular depressant drugs, such as narcotics, etomidate, or ketamine. "Normal" dosages become lethal doses in the hypovolemic patient.
Some of the more important induction agents are:
Thiopental. This rapidly-acting barbiturate is quite familiar to most present day anesthetists. It has the advantages of fast onset, short duration, and good patient acceptance However, normal induction dosages may cause disastrous hypotension in the hypovolemic patient. An induction dose is 3-4 mg/kg intravenously, given over one minute in a solution of 2.5% or less in normal saline. This drug can be a potent cardiac depressant and can produce hypotension. It can cause laryngospasm immediately after or during induction, is a poor analgesic, and produces poor muscle relaxation. This drug is best used in combination with a muscle relaxant paralytic drug.
Etomidate. This drug usually preserves cardiovascular stability in the intact elective surgical patient but probably has no advantage over thiopental. in the case of hypovolemia. It also produces localized pain and myoclonic movements on rapid injection.
Ketamine. This agent is also fast acting, has analgesic properties, and supports the blood pressure by sympathetic stimulation. However, one must still be wary in the severely hypovolemic patient in whom sympathetic outflow may already be near maximal intensity. In these cases, the direct depressant effect on the myocardium may produce decreased cardiac output. 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
Narcotics. Rapidly acting narcotics, such as sufentanil, in conjunction with benzodiazepines are an alternative induction technique. If newer agents, such as the narcotic alfentanil, and the benzodiazepine midazolam. prove safe in trauma cases, they should be considered also.
Regardless of the induction agent chosen, the risk of aspiration during induction (and emergence) remains a critical consideration. Drugs such as histamine receptor blockers, metaclopramide, and nonparticulate antacids offer promise as prophylactic measures; however, these agents will not be available on the battlefield, and the rapid and secure control of the airway remains the primary means of preventing this grave complication.
The anesthetist must ensure that an adequately functioning suction apparatus is close at hand and operational prior to induction or emergence.
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