The mechanism of cardiac shunting in reptiles is controversial. Recent evidence suggests that a right-to-left shunt in turtles results primarily from a washout mechanism. The mechanism that accounts for left-to-right (L-R) shunting is unresolved. This study used haemodynamic analysis and digital subtraction angiography to determine the mechanism of L-R cardiac shunting in the turtle Trachemys (Pseudemys) scripta. Animals were instrumented with ultrasonic blood flow probes (Transonic Systems, Inc.) for the measurement of total pulmonary blood flow and total systemic blood flow. In addition, catheters were inserted into the common pulmonary artery (PA), the systemic arteries, the left atrium and right atrium. These catheters were used for the measurement of blood pressure or for the infusion of radio-opaque material. Haemodynamic conditions were altered by electrical stimulation of the afferent (VAF) or efferent vagal nerves or by infusion of vasoactive drugs. Under control conditions, the peak systolic pressure in the systemic arteries was slightly higher than that in the PA (30.6 versus 28.3 mmHg; 4.08 versus 3.77 kPa), whereas diastolic pressure in the PA was significantly less than that in the systemic arteries (9.8 versus 24.4 mmHg; 1.31 versus 3.25 kPa). During VAF stimulation, the peak systolic pressures in the PA and aortae almost doubled. Diastolic pressure in the systemic arteries also doubled, but it increased by only 45% in the PA. Ejection of blood into the PA preceded that into the left aorta by 53 ms under control conditions. This difference increased (by as much as 200 ms) as the difference in the diastolic pressures between the two circulations increased during VAF stimulation. This resulted in the development of a large net L-R shunt. Under these conditions, digital subtraction angiography showed that the L-R shunt resulted from a combination of both washout and pressure mechanisms.

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