Increased arterial pH, not decreased PaCO2, attenuates hypoxia-induced pulmonary vasoconstriction in newborn lambs.

Mechanically induced hyperventilation is used in the treatment of newborn infants with persistent pulmonary hypertension syndrome to induce respiratory alkalosis, which may attenuate their pulmonary vasoconstriction. Whether this treatment is effective because of the increase in arterial pH or the decrease in Paco2 was investigated in nine sedated, mechanically ventilated newborn lambs with hypoxia-induced pulmonary vasoconstriction. We found that respiratory alkalosis and metabolic alkalosis were equally effective in attenuating hypoxia-induced pulmonary vasoconstriction, but that hypocapnia (low Paco2 with a normal arterial pH) was ineffective. These results indicate that increased arterial pH, not decreased Paco2, attenuates hypoxia-induced pulmonary vasoconstriction in newborn lambs and possibly the pulmonary vasoconstriction in newborn infants with persistent pulmonary hypertension syndrome.