THE EFFECT OF ALTERING PULMONARY BLOOD-FLOW ON PULMONARY GAS-EXCHANGEIN THE TURTLE TRACHEMYS (PSEUDEMYS) SCRIPTA

In resting reptiles, the P-O2 of pulmonary venous return (PLA(O2); lef t atrial blood) may be 20 mmHg (1 mmHg=0.1333 kPa) lower than the P-O2 of gas in the lung, This level of P-O2 is considerably higher than th at observed in resting mammals and birds and results from ventilation- perfusion (V over dot/Q over dot) heterogeneity, pulmonary diffusion l imitation and intrapulmonary shunting. However, the relative contribut ion of each of these factors is unknown, Many reptiles, particularly c helonians, exhibit an intermittent ventilation pattern where pulmonary blood flow (Q over dot L) increases during the ventilatory periods an d, therefore, we hypothesized that V over dot/Q over dot matching woul d improve with increasing Q over dot L. We applied the multiple inert gas elimination technique in anaesthetized turtles at 22 degrees C, Tu rtles were continuously ventilated at a rate of 140 mlkg(-1)min(-1), e quivalent to the rate of ventilation within a ventilatory period, Trac e amounts of six inert gases were infused through the jugular vein, Bl ood samples from the pulmonary artery and the left atrium and mixed ex pired gases were collected for analysis, Q over dot L was reduced by a factor of six (low flow) using a vascular occluder placed around the common pulmonary artery or increased by a factor of two (high flow) th rough bolus injection of adrenaline, V over dot/Q over dot heterogenei ty was significantly reduced with increasing pulmonary blood flow (P<0 .05). Consistent with these changes, the effective lung-pulmonary arte ry P-O2 difference (PL(O2)-PLA(O2)) was reduced (P<0.05) from 58+/-1.6 mmHg to 29+/-5 mmHg (means+/-S.E.M.) and PLA(O2) increased significan tly (P<0.05) from 88+/-17 mmHg (low flow) to 120+/-14 mmHg (high flow) , There was evidence of pulmonary diffusion limitation under all condi tions, which was unchanged with increasing blood flow, These findings suggest that increased pulmonary blood flow during a ventilatory perio d results in both temporal and spatial matching of ventilation and per fusion, without altering pulmonary diffusion limitation.