INFLUENCE OF HYDROGEN-ION CONCENTRATION VERSUS CARBON-DIOXIDE TENSIONON PULMONARY VASCULAR-RESISTANCE AFTER CARDIAC OPERATION

Disturbances of respiratory acid-base status are common in patients su pported with mechanical ventilation of the lungs after cardiac operati ons. This study was conducted with two protocols. The purpose was to d etermine whether respiratory acid-base status influences pulmonary vas cular resistance in adults after cardiac operations and whether the in fluence is mediated by hydrogen ion concentration or carbon dioxide te nsion. Patients were studied while under general anesthesia immediatel y after aorta-coronary bypass. In the first protocol, with seven patie nts, arterial carbon dioxide tension was manipulated by the addition o f 5 % carbon dioxide to the breathing circuit. Pulmonary vascular resi stance index was determined as arterial carbon dioxide tension rose fr om 30 mm Hg to 50 mm Hg and back to 30 mm Hg. In the second protocol, with 10 different patients, hydrogen ion concentration was manipulated by the addition of 0.2N hydrochloric acid, sodium bicarbonate, or bot h as arterial carbon dioxide tension was held constant. We used analys is of variance for statistical data. The results of the first protocol showed that pulmonary vascular resistance index rose by 44 % (p < 0.0 5) as arterial carbon dioxide tension rose from 30 to 50 mm Hg. The re sults of the second protocol showed that changes in pulmonary vascular resistance index were parallel to changes in hydrogen ion concentrati on as arterial carbon dioxide tension was held constant (p < 0.05). Th ese data demonstrate that respiratory acid-base status is an important determinant of pulmonary vascular resistance in the adult after cardi ac operations. Furthermore, these data suggest the effect is mediated by hydrogen ion concentration, not carbon dioxide tension.