Analysis of intracardiac shunting in the lizard, Varanus niloticus: a new model based on blood oxygen levels and microsphere distribution.

The central sites of the cardiovascular system (right and left aortic arches, RAo and LAo, pulmonary artery, PA, and right and left atria, RAt and LAt) were chronically and non-occlusively cannulated for an analysis of intracardiac shunting in Varanus niloticus. Oxygen partial pressure (PO2) and oxygen concentration (CO2) were significantly higher in right aortic blood than values determined in left aortic blood. The difference was larger in animals acclimated to 25 degrees C (RAo CO2 = 4.5 +/- 1.00 vol %, LAo CO2 = 3.8 +/- 1.14, X +/- SD, n = 19) than at 35 degrees C (RAo CO2 = 5.8 +/- 1.24, LAo CO2 = 5.4 +/- 1.35, n = 18) (P less than 0.001 for both temperatures, paired t-test). These data are explained by a new model describing the differential shunting patterns of the two aortae in addition to the conventional overall right-to-left and left-to-right shunt fractions. This model was solved on the basis of blood gas data collected by simultaneous multiple-site gas analysis, together with data on the differential blood flow in the central vascular system, collected by application of the microsphere method. At 35 degrees C both right-to-left and left-to-right shunts were relatively small (about 9%), with the right-to-left shunt fraction directed exclusively into the left aorta. Thus right aortic blood represented left atrial blood, whereas left aortic blood was composed of 80% left atrial and 20% right atrial blood. Ninety percent of the pulmonary arterial blood was derived from the right atrium and 10% from the left atrium. At 25 degrees C the composition pattern of effluent blood for each vessel was similar, the absolute flow distribution, however, was different from that at 35 degrees C. These findings are discussed with respect to their significance and compatibility with the wash-out shunt model.