Measurement of pulmonary venous and arterial pH oscillations in dogs using catheter tip pH electrodes

Respiratory pH oscillations were studied in situ in the aorta and superior vena cava of rabbits, and in the pulmonary vein and aorta of open-chested dogs, using catheter tip pH electrodes. Preliminary experiments on two spontaneously breathing rabbits showed pH oscillations of 0.006-0.010 pH units in the ascending aorta at a respiratory rate of 40/min. No oscillations were apparent in the superior vena cava. In anaesthetized, ventilated dogs, the delay from the upstroke in airway pressure to the onset of the alkaline limb of the pH oscillation was 1.15 +/- 0.9 s in the pulmonary vein and 4.1 +/- 0.1 s in the descending aorta (mean + S.E. of mean, n = 28 tracings in four dogs). Pulmonary venous oscillations were equal to or larger in amplitude than those in the aorta. Aortic oscillations became proportionately smaller at shorter respiratory cycle lengths, although both were related to tidal volume. Stop flow experiments were performed using a pH electrode placed in situ in the left atrium inside a wide-bore tube. A small fall in pH occurred when flow was stopped (-0.0036 +/- 0.0012, mean +/- S.E. of mean, n = 22 recordings in four dogs). The findings were not dependent upon the phase of the oscillation at which flow was stopped. Mean plasma free haemoglobin was very low (1.6 mg/dl), indicating minimal haemolysis of the blood withdrawn past the electrode. It is concluded that the plasma CO2 hydration reaction is virtually at equilibrium by the time blood reaches the pulmonary vein.