Minimum anesthetic concentration of isoflurane and sevoflurane and cardiorespiratory effects of varying inspired oxygen fractions in Magellanic penguins (Spheniscus magellanicus)

The aim of this study was to determine the minimum anesthetic concentration of isoflurane (MAC_ISO) and sevoflurane (MAC_SEVO) and evaluate the cardiorespiratory changes induced by varying fractions of inspired oxygen (FiO₂) in Magellanic penguins (Spheniscus magellanicus). Twenty adult penguins (3.53 ± 0.44 kg) of undetermined sex were used. Both MAC_ISO (n = 9) and MAC_SEVO (n = 13) were established using an up-and-down design. Next, twelve mechanically ventilated penguins were maintained at 1 MAC_ISO or 1 MAC_SEVO (n = 6 per group) with the FiO₂ initially set at 1.0. Three FiO₂ values (0.6, 0.4 and 0.2) were then held constant during anesthesia for 20 minutes each. Arterial blood samples were collected for gas analysis after the 20-minute period for each FiO₂. Mean ± SD MAC_ISO was 1.93 ± 0.10% and MAC_SEVO was 3.53 ± 0.13%. Other than heart rate at 0.6 FiO₂ (86 ± 11 beats/minute in MAC_ISO and 132 ± 37 beats/minute in MAC_SEVO; p = 0.041), no significant cardiorespiratory differences were detected between groups. In both groups, decreasing the FiO₂ produced increased pH values and reduced partial pressures of carbon dioxide and bicarbonate. Partial pressures of oxygen (PaO₂) gradually lowered from 1.0 FiO₂ through 0.2 FiO₂, though hypoxemia (PaO₂ < 80 mmHg) occurred only with the latter FiO₂. The MAC_ISO and the MAC_SEVO for the Magellanic penguin fell within the upper range of reported avian MAC estimates. To prevent hypoxemia in healthy, mechanically ventilated, either isoflurane- or sevoflurane-anesthetized Magellanic penguins, a minimum FiO₂ of 0.4 should be used.