Isoflurane Potency in the Dog and Cat

Cardiopulmonary effects of isoflurane, a new inhalation anesthetic, were investigated in healthy unpremedicated dogs and cats under conditions of spontaneous and controlled (dogs only) ventilation. Measurements were made at minimal alveolar concentration (MAC) multiples of 1.0, 1.5, 2.0, and 3.0 in dogs and 1.0, 1.5, 2.0, and 2.4 in cats. The isoflurane MAC was previously determined in these animals and was 1.28 +/- 0.06% for dogs and 1.63 +/- 0.02% for cats. We found that as anesthetic dose increased, mean arterial pressure consistently and significantly (P less than 0.05) decreased. Cardiac output, measured only in dogs, was sustained only during light-moderate levels (1.0 to 2.0 MAC) of anesthesia because the heart rate significantly increased. Stroke volume, total peripheral resistance, and left ventricular work tended to decrease as anesthesia deepened. We found no significant difference in cardiovascular measurements in dogs between spontaneous and controlled ventilation at equal MAC multiples. That isoflurane is a profound respiratory depressant in dogs and cats is supported by our findings of a dose-dependent increase in PaCO2. In addition, the alveolar isoflurane concentration required to produce at least 60 seconds of apnea divided by MAC (i.e., the anesthetic index) averaged 2.5 for dogs and 2.4 for cats. The anesthetic index which we determined for isoflurane in dogs equals or is less than the index reported for other inhaled anesthetics in this species.