Anesthetic Classes

The use of anesthetic agents is dictated in large part by the nature of the surgical procedure. In some cases there is a requirement for the patient to be fully 'asleep' in order to avoid discomfort and interference with the procedure, whereas in other cases, local or regional anesthesia (e.g., pudenal block in pregnancy, dental surgery) may suffice and the patient may remain awake during the procedure.

6.15.2.1 Inhalation or General Anesthetics

Inhalation or general anesthetics are administered via the lungs and allow the depth of anesthesia to be rapidly altered by controlling the amount of inhaled anesthetic. This can be advantageous over the use of intravenous anesthetics that require the agent to be gradually removed from the body via urinary or fecal excretion. The rapid elimination of inhalation anesthetics also provides for a shorter duration of postoperative respiratory depression; however, inhalational anesthetics provide no postoperative analgesia. Inhalation anesthetics used in surgical procedures require the use of a general anesthesia machine for administration. This comprises a ventilator to maintain breathing during deeper stages of anesthesia, vaporizers to deliver the anesthetic into the breathing circuit, as well as monitors and alarms to monitor patient safety. The use of inhalation anesthetics is driven by several considerations1: (1) minimizing the side effects of the anesthetic, e.g., renal and hepatic toxicity, undesirable cardiovascular side effects, e.g., cardiac depression, cerebral vasodilation, and the techniques used for its administration; (2) optimizing pharmacokinetics, e.g., rapid onset and offset; (3) maintaining physiological homeostasis; and (4) improving postoperative outcomes.

6.15.2.1.1 General anesthetic potency

The potency of volatile anesthetics is measured as the minimum alveolar concentration (MAC) - the alveolar concentration (measured as end-tidal concentration) of the anesthetic (at one atmosphere) that prevents movement in 50% of patients in response to pain like a surgical skin incision. Measuring the MAC for skin incision does not predict the concentration of inhalation anesthetics necessary to avoid the motor responses to other painful stimuli, e.g., endotracheal intubation. Modifications of MAC include MACawake, the ability to respond to verbal commands. Following a brief equilibration period, the alveolar concentration of the anesthetic equals the blood concentration and subsequently reflects the brain concentration, e.g., the partial pressure of the anesthetic in the CNS. The MAC is age-dependent, being highest in infants and decreasing as a function of age. MAC values for inhaled anesthetics are additive and can be modulated by opioid treatment. When given alone, inhalation anesthetics frequently do not suppress hemodynamic responses to painful stimuli.

6.15.2.2 Parenteral Anesthetics

Parenteral anesthetics are small-molecular-weight compounds, the hydrophobicity of which dictates their pharmaco-kinetics and efficacy.1 These agents preferentially partition into the lipophilic tissues in the central and peripheral nervous systems within a single circulation cycle. As blood levels fall, the anesthetic depot in nervous tissue re-equilibrates, resulting in a distribution of the agent to other lipophilic tissues. Patients treated with a single bolus of thiopental emerge from anesthesia in 10min while those receiving a prolonged infusion may take a day or more to recover.1

6.15.2.3 Local Anesthetics

Local anesthetics are used to reduce sensation in a part of the body, e.g., skin, oral cavity, without the loss of either consciousness or control of vital functions.14 In contrast to general anesthesia, local anesthesia avoids the major physiological consequences of the former, e.g., amnesia and impairment of cardiac or respiratory function, and can be used to involve the patient in assessing pain responses.

6.15.2.3.1 Infiltration anesthesia

Infiltration anesthesia involves the induction of local anesthesia by injecting a local anesthetic, e.g., lidocaine, procaine, or bupivacaine (Figure 3), directly into tissue to avoid impacting other body functions. Epinephrine can be used to enhance the duration of infiltration anesthesia.

6.15.2.3.2 Nerve block

Nerve block involves the injection of a local anesthetic proximal to discrete peripheral nerves to produce large areas of anesthesia. Injection directly into the nerve is avoided to prevent pain. Examples of nerve block include: (1) trigeminal for dental surgery; (2) brachial for surgical procedures in the shoulder; (3) cervical plexus for neck surgery; and (4) sciatic and femoral for surgery distal to the knee. Local anesthetics used to produce nerve block can be divided into three classes based on duration of action: (1) short (20-30min: procaine); (2) intermediate (1-2h; lidocaine, mepivicane); and (3) long (6-7 h; bupivicaine, ropivicaine).

6.15.2.3.3 Field block anesthesia

Field block anesthesia involves the subcutaneous injection of local anesthetic to produce anesthesia distal to the injection site and involves similar paradigms to those used for infiltration anesthesia.

6.15.2.3.4 Topical anesthesia

Topical anesthesia involves the application of aqueous solutions of a local anesthetic, such as cocaine or lidocaine, to mucous membranes in the nose, throat, mouth, esophagus, tracheobrachial tree, and the urogenital system. Peak anesthetic effect occurs within 2-10 min as local anesthetics are rapidly absorbed into the mucous membrane. Topical anesthesia can be used as a prelude to nerve block, especially in the dental setting.

6.15.2.3.5 Intravenous regional anesthesia

Intravenous regional anesthesia, also known as Bier's block, uses the vascular system to introduce local anesthetics to the nerve trunks and endings. A double tourniquet is used in the affected area, typically a limb, and local anesthetic, such as lidocaine (0.5% solution; 40-50 mL), is injected into a peripheral vein in the same area, resulting in anesthesia of the area from the level of the tourniquet. The tourniquet prevents toxic amounts of anesthesic from entering the general circulation.

6.15.2.3.6 Spinal or subarachnoid anesthesia

Spinal or subarachnoid anesthesia involves the injection of local anesthetic, lidocaine or bupivicaine, into the cerebrospinal fluid of the lumbar space to block the fibers in the spinal nerve roots. This type of anesthesia is typically used in hernia repair, gynecological or urological surgery, and lower-extremity orthopedic procedures. It is widely used in older patients and individuals with chronic respiratory disease, hepatic and renal impairment, and diabetes. The duration of the anesthesia can be increased by the addition of epinephrine.

6.15.2.3.7 Epidural anesthesia

Epidural anesthesia involves the introduction of local anesthetic into the epidural space, usually via catheterization, and can provide repeat administration or infusion of the anesthetic. The key difference between spinal and epidural anesthesia is that a much larger amount of anesthetic is required to produce epidural block because the anesthetic must diffuse into the spinal cord, nerve roots, and cerebrospinal fluid from the epidural space. Consequently, the onset of anesthesia is significantly slower with epidural as compared to spinal block. Blood levels of local anesthetic will be significantly higher with epidural administration, although this is seldom a clinically important problem. The ease of placing a catheter in the epidural space allows for continuous drug administration, which is advantageous for control of pain during labor and postoperatively. Altering the concentration of local anesthetic in the epidural space can be used to advantage to produce a differential block of sympathetic, sensory, and motor pathways, while obtaining these gradations of effect with subarachnoid administration is more difficult. Opioids can also be used to provide spinal and epidural analgesia.15

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