The functional significance of many features of the reptilian cardiopulmonary system remains unknown; particularly the importance of cardiac shunts. One hypothesis for a physiological function for shunts is that they play a role in myocardial oxygenation and are therefore important when cardiac work is elevated. In this study we examined cardiac function by monitoring electrocardiograms in red-eared slider turtles (Trachemys scripta) with a reduced myocardial oxygen supply. Exposing the animals to a hypoxic gas mixture reduced oxygen levels in the pulmonary venous return. When cardiac work was elevated during hypoxia, the electrocardiogram changed in a manner consistent with myocardial hypoxia, suggesting enrichment of the luminal blood with oxygen by the intracardiac shunt facilitates cardiac performance.