Influence of hydrogen ion concentration versus carbon dioxide tension on pulmonary vascular resistance after cardiac operation

Disturbances of respiratory acid-base status are common in patients supported with mechanical ventilation of the lungs after cardiac operations. This study was conducted with two protocols. The purpose was to determine whether respiratory acid-base status influences pulmonary vascular resistance in adults after cardiac operations and whether the influence is mediated by hydrogen ion concentration or carbon dioxide tension. Patients were studied while under general anesthesia immediately after aorta-coronary bypass. In the first protocol, with seven patients, arterial carbon dioxide tension was manipulated by the addition of 5% carbon dioxide to the breathing circuit. Pulmonary vascular resistance index was determined as arterial carbon dioxide tension rose from 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 both as arterial carbon dioxide tension was held constant. We used analysis of variance for statistical data. The results of the first protocol showed that pulmonary vascular resistance index rose by 44% (p < 0.05) as arterial carbon dioxide tension rose from 30 to 50 mm Hg. The results of the second protocol showed that changes in pulmonary vascular resistance index were parallel to changes in hydrogen ion concentration as arterial carbon dioxide tension was held constant (p < 0.05). These data demonstrate that respiratory acid-base status is an important determinant of pulmonary vascular resistance in the adult after cardiac operations. Furthermore, these data suggest the effect is mediated by hydrogen ion concentration, not carbon dioxide tension.