Intracardiac Shunting Affects Minimal Alveolar Concentration of Isoflurane in the Red-Footed Tortoise (Chelonoidis carbonaria)
Abstract
The incompletely divided chelonian ventricle provides the opportunity for mixing blood between pulmonary and systemic circuits, referred to as intracardiac shunting.3 Isoflurane anesthesia increases vagal tone4 and decreases systemic vascular resistance in mammals,1 both circumstances which likely promote a right-to-left (R-L) intracardiac shunt in tortoises where blood bypasses the lungs. In contrast, anticholinergic drugs are known to reverse intracardiac shunting in reptiles.2 In a randomized, cross-over study, six red-footed tortoises (Chelonoidis carbonaria) were anesthetized with isoflurane. Following intubation and injection with atropine (1 mg/kg) or a similar volume of saline, the minimal alveolar concentrations (MAC) was determined using classic bracketing. MAC was significantly lower (p<0.002) with atropine (mean MAC=2.2±SD 0.3%) than with saline (3.2±0.4%). Using a similar anesthetic set-up, the arterial flows of the left pulmonary artery and left aorta were measured using ECG-gated Magnetic Resonance Imaging (MRI). Mean flow in the left pulmonary artery was significantly higher (p<0.008) upon atropine (1.45 and 0.27 mL/sec, respectively), while left aorta flow did not differ. The study demonstrates that the red-footed tortoise exhibit significant R-L shunting during isoflurane anesthesia, which can be reversed with atropine. The reduction in R-L shunt and the rise in pulmonary blood flow enabled a much faster distribution of the anesthetic gas within the systemic circulation, providing a significant and sizable reduction in MAC. Gated MRI served as a useful tool to study arterial flow and shunt physiology.
Acknowledgments
The authors wish to thank Heidi Meldgaard Jensen and Claus Wandborg for the care of the animals. Eva Maria Greunz is supported by a grant from Annie and Ottos Johs. Detlefs’ Foundations.
Literature Cited
1. Eger El 2nd. Isoflurane: a review. Anesthesiology. 1981;55(5):559–576.
2. Hicks JW. Adrenergic and cholinergic regulation of intracardiac shunting. Physiol Zool. 1994;67(6):1325–1346.
3. Hicks JW. The physiological and evolutionary significance of cardiovascular shunting patterns in reptiles. News Physiol Sci. 2002;17:241–245.
4. Seagard JL, Elegbe EO, Hopp FA, Bosnjak ZJ, von Colditz JH, Kalbfleisch JH, Kampine JP. Effects of isoflurane on the baroreceptor reflex. Anesthesiology. 1983;59(6):511–520.