General anesthetics depress the central nervous system to a sufficient degree to permit the performance of surgery and unpleasant procedures. General anesthetics have low therapeutic indices and thus require great care in administration. The selection of specific drugs and routes of administration to produce general anesthesia is based on their pharmacokinetic properties and on the secondary effects of the various drugs, in the context of the proposed diagnostic or surgical procedure and with the consideration of the individual patient's age, and associated medical condition.
GENERAL PRINCIPLES OF SURGICAL ANESTHESIA
The administration of general anesthesia is driven by 3 general objectives:
Minimizing the potentially deleterious direct and indirect effects of anesthetic agents and techniques
Sustaining physiologic homeostasis during surgical procedures that may involve major blood loss, tissue ischemia, reperfusion of ischemic tissue, fluid shifts, exposure to a cold environment, and impaired coagulation
Improving postoperative outcomes by choosing techniques that block or treat components of the surgical stress response, which may lead to short- or long-term sequelae
HEMODYNAMIC EFFECTS OF GENERAL ANESTHESIA. The most prominent physiological effect of anesthesia induction is a decrease in systemic arterial blood pressure. The causes include direct vasodilation, myocardial depression, or both; a blunting of baroreceptor control; and a generalized decrease in central sympathetic tone. Agents vary in the magnitude of their specific effects, but in all cases the hypotensive response is enhanced by underlying volume depletion or preexisting myocardial dysfunction.
RESPIRATORY EFFECTS OF GENERAL ANESTHESIA. Nearly all general anesthetics reduce or eliminate both ventilatory drive and the reflexes that maintain airway patency. Therefore, ventilation generally must be assisted or controlled for at least some period during surgery. The gag reflex is lost, and the stimulus to cough is blunted. Lower esophageal sphincter tone also is reduced, so both passive and active regurgitation may occur. Endotracheal intubation has been a major reason for a decline in the number of aspiration deaths during general anesthesia. Muscle relaxation is valuable during the induction of general anesthesia where it facilitates management of the airway, including endotracheal intubation. Neuromuscular blocking agents commonly are used to effect such relaxation (see Chapter 11). Alternatives to an endotracheal tube include a face mask and a laryngeal mask, an inflatable mask placed in the oropharynx forming a seal around the glottis.
HYPOTHERMIA. Patients commonly develop hypothermia (body temperature <36°C) during surgery. The reasons include low ambient temperature, exposed body cavities, cold intravenous (IV) fluids, altered thermoregulatory control, and reduced metabolic rate. Metabolic rate and total body oxygen consumption decrease with general anesthesia by ~30%, reducing heat generation. Hypothermia may lead to an increase in perioperative morbidity. Prevention of hypothermia is a major goal of anesthetic care.
NAUSEA AND VOMITING. Nausea and vomiting continue to be significant problems following general anesthesia and are caused by an action of anesthetics on the chemoreceptor trigger zone and the brainstem vomiting center, which are modulated by serotonin (5HT), histamine, ACh, and dopamine (DA). The 5HT3 receptor antagonists, ondansetron and dolasetron (see Chapters 13 and 46), are very effective in suppressing nausea and vomiting. Common treatments also include droperidol, metoclopramide, dexamethasone, and avoidance of N2O. The use of propofol as an induction agent and ...