ANESTHETIC CONSIDERATIONS IN FINDING THE CORRECT DOSE
When selecting an anesthetic, anesthetists often consider, among others, the questions presented in Table 1–1. To answer these questions, anesthetists turn to textbooks, journal articles, and drug package inserts. These resources provide dosing recommendations (ie, bolus doses and infusion rates) and important features of anesthetic drugs but often fall short of providing useful answers. Anesthetists therefore rely on years of training and experience to formulate the correct dose and safely administer it. Experienced anesthetists develop a sense of how individual drugs behave and can easily tailor them to meet the needs of their patients. For example, anesthetists have a good “feel” for what a 3-mL (150 mcg) intravenous bolus of fentanyl will accomplish in a healthy adult and can accurately predict the onset and duration of analgesic effect.
Common questions when formulating an anesthetic.
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Table 1–1 Common questions when formulating an anesthetic.
|What drug or drug combination will work best for the patient? |
|What are the adverse effects? |
|After giving a dose, when will it start to have an effect and how long will it last? |
|When using a combined technique, how do different anesthetics interact with one another to prolong various drug effects? |
|How do age, body habitus, blood loss, gender, organ function, medications, health supplements, and so on, influence the onset and duration of anesthetic effects? |
|Once turned off, how long will it take for the patient to emerge from anesthesia? |
|In procedures associated with moderate to severe postoperative pain, what dose of analgesic will be safe but still provide adequate pain control? |
Most anesthetics, however, are neither single agents nor are they consistently administered to healthy patients. Suppose an anesthetist has to answer the same questions posed in Table 1–1 for that 3-mL bolus of fentanyl in the presence of 2% sevoflurane. How are the onset and duration changed? For a morbidly obese patient, how does the difference in body habitus influence the onset and duration of effect? With unanticipated severe blood loss, how will fentanyl behave?
The most widely used predictor of anesthetic effect is the minimum alveolar concentration (MAC), the concentration of inhalation agent in the alveoli necessary to keep 50% patients from moving when exposed to a noxious stimulus. Originally used in laboratory investigations to differentiate the potency of inhalation agents from one another, MAC has become a well-known clinical descriptor of drug effect. In fact, modern physiologic monitors display estimates of anesthetic effect as percentages of MAC based on expired concentrations of inhalation agents.
MAC and its derivatives (the concentration of inhalation agent in the alveoli necessary to block an autonomic response in 50% patients when exposed a noxious stimulus [MACbar] and the concentration of inhalation agent in the alveoli in which 50% patients are awake [MACawake]) are by design ...