Phosphorescence lifetime microscopy and contrast-enhanced ultrasound imaging revealed strong left-to-right intraventricular shunt in anesthetized Xenopus laevis.

Amphibians exhibit intracardiac shunting, by which oxygen from multiple respiratory surfaces is variably distributed between systemic and pulmonary circulations. Although shunting has been extensively studied in terrestrial species that rely heavily on pulmonary respiration, much less is known about more aquatic, cutaneous-respiring species, in which cutaneously oxygenated blood returns to the right atrium, so the right-left oxygen gradient should vary with context. Moreover, the proposed role of ventricular trabeculation in limiting intraventricular mixing has not been directly demonstrated. We combined phosphorescence lifetime microscopy for in vivo pO2 measurement with microbubble-based contrast-enhanced ultrasound for high-resolution tracer-tracking in aquatic Xenopus laevis. In anesthetized animals, blood pO2 was uniformly low (25-40 mmHg), with little arteriovenous difference. Right-to-left shunting was minimal (8%), whereas a strong intraventricular left-to-right shunt (47%) maintained threefold greater pulmonary than systemic flow. Although its significance remains to be elucidated, these approaches provide a foundation for advancing shunt research.