Validation of the MAC technique in dogs and rabbits

Minimum alveolar concentrations (MAC) are determined using one of the different noxious stimuli (clamping, electrical stimulation, or surgical incision), based on a study conducted in the 1960s with three dogs. This study compares different noxious stimuli applied in a randomized order in dogs (n = 10) anesthetized with isoflurane (I) and halothane (H), and in rabbits (n = 10) anesthetized with I. Anesthesia was induced with the anesthetic in oxygen and maintained with mechanical ventilation. End‐tidal anesthetic (Fe ′A) and CO₂ (Pe ′CO₂) concentrations were monitored with a calibrated infrared gas analyzer. Pe ′CO₂ and body temperature were maintained within normal limits. Noxious stimuli included skin incision on the lateral chest (SI), clamping of the tail (TC), fore‐ (FC) and hindlimb paw (HC), and electrical current (50 V at 50 cycles second for 10 msecond pulses) applied to the fore‐ (FE) and hindlimb (HE), and oral mucosa (OE) (except rabbits). SI was applied first and only for the first two consecutive measurements using the up–down method for sequential sampling of quantal‐response data. After an initial equilibration period of at least 20 minutes at an Fe ′A of 1.4% (H) or 1.7% (I), the Fe ′A was decreased in the first animal to 0.85% of H (dog) or 1% of I (dog and rabbit) and maintained for at least 20 minutes before the noxious stimuli. If the animal responded or did not respond, the stimuli were then tested at an Fe ′A 0.1% higher or lower, respectively. The new Fe ′A was kept constant for at least 20 minutes and the noxious stimuli repeated until purposeful movement ceased or returned, respectively. MAC was defined as the Fe ′A mid‐way between the value permitting and preventing purposeful movement. Data were analyzed using an anova . MAC for I in dogs was 1.27 ± 0.047 (mean ± SEM) for TC, FC, and HC; 1.36 ± 0.035 for OE; 1.35 ± 0.040 for FE and HE; and 0.99 for SI. MAC for H in dogs was 0.97 ± 0.028 for TC; 0.96 ± 0.032 for FC and HC; 1.04 ± 0.033 for OE, FE, and HE; and 0.73 for SI. MAC for I in rabbits was 2.08 ± 0.021 for TC, FC, and HC; 2.04 ± 0.023 for FE and HE; and 0.90 for SI. MAC for SI was significantly lower than the other methods. In conclusion, electrical current and clamping resulted in similar MAC values.