Since the anesthesiologist has control over the partial pressure of anesthetic delivered to the lung, it can be manipulated to control the anesthetic gas concentration in the brain, hence the level of unconsciousness. For this reason, anesthetic dose is usually expressed in terms of the alveolar tension required at equilibrium to produce a defined depth of anesthesia. The dose is determined experimentally as the partial pressure needed to eliminate movement in 50% of patients challenged with a standardized skin incision. The tension required is defined as the minimum alveolar concentration (MAC) and is usually expressed as the percentage of inhaled gases that is represented by anesthetic gas at 1 atm.
Various anesthetic agents require widely different partial pressures to produce the same depth of anesthesia (Table 25.2). Methoxyflurane, for example, with a MAC of 0.16%, is the most potent agent listed in the table. Only 0.16% of the molecules of inspired gas need be methoxyflurane. N2O is the least potent agent, with a MAC that exceeds 100%. Thus, a level of unconsciousness needed to eliminate movement is seldom achieved with N2O.
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