THE MECHANISM OF CARDIAC SHUNTING IN REPTILES - A NEW SYNTHESIS

The mechanism of cardiac shunting in reptiles is controversial, Recent evidence suggests that a right-to-left shunt in turtles results prima rily from a washout mechanism, The mechanism that accounts for left-to -right (L-R) shunting is unresolved, This study used haemodynamic anal ysis and digital subtraction angiography to determine the mechanism of L-R cardiac shunting in the turtle Trachemys (Pseudemys) scripta, Ani mals were instrumented with ultrasonic blood how probes (Transonic Sys tems, Inc.) for the measurement of total pulmonary blood flow and tota l systemic blood flow, In addition, catheters were inserted into the c ommon pulmonary artery (PA), the systemic arteries, the left atrium an d right atrium, These catheters were used for the measurement of blood pressure or for the infusion of radioopaque material, Haemodynamic co nditions were altered by electrical stimulation of the afferent (VAF) or efferent vagal nerves or by infusion of vasoactive drugs, Under con trol conditions, the peak systolic pressure in the systemic arteries w as slightly higher than that in the PA (30.6 versus 28.3 mmHg; 4.08 ve rsus 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 v ersus 3.25 kPa), During VAF stimulation, the peak systolic pressures i n the PA and aortae almost doubled. Diastolic pressure in the systemic arteries also doubled, but it increased by only 45 % in the PA, Eject ion of blood into the PA preceded that into the left aorta by 53 ms un der control conditions, This difference increased (by as much as 200 m s) as the difference in the diastolic pressures between the two circul ations increased during VAF stimulation, This resulted in the developm ent of a large net L-R shunt, Under these conditions, digital subtract ion angiography showed that the GR shunt resulted from a combination o f both washout and pressure mechanisms.