Elimination of Intracardiac Shunting Provides Stable Gas Anesthesia in Tortoises

Inhalant anesthesia is challenging in chelonians due to a great capacity for breath-holding and an incomplete separation of the cardiac ventricle. Deoxygenated blood can recirculate back into systemic circulation by bypassing the lung in a process referred to as intracardiac right to left (R-L) shunting. Via electrocardiogram gated magnetic resonance imaging, a novel modality to investigate arterial flows in reptiles, intracardiac shunting and its elimination via atropine during gas anesthesia in tortoises (Chelonoidis carbonaria) was demonstrated. The great vessels of the heart were visualized confirming that after shunt-elimination, the flow (mean ± sd) in the pulmonary arteries increased significantly (54.6 ± 9.5 mL min(−1) kg(−1) vs 10.8 ± 3.4 mL min(−1) kg(−1); P < 0.008). Consequently, animals required significantly lower concentrations of inhaled anesthetics to maintain a stable anesthesia. To that end, the minimum anesthetic concentration (MAC) of isoflurane needed to maintain surgical anesthesia was measured. A significantly lower MAC was found after administration of atropine (mean MAC ± sd 2.2 ± 0.3% vs 3.2 ± 0.4%; P < 0.002). Previously, MAC has been indeterminable in chelonians likely due to intracardiac shunting, so this report constitutes the first MAC study performed in a tortoise.