| Abstrakt: |
In a group of infants with hyaline membrane disease, the level of optimal constant positive airway pressure (CPAP) was assessed by raising CPAP in small steps from an initial low value, and after each change measuring the arterial alveolar difference for CO2(aADCO2) and transmission of airway pressure to the esophagus. Below optimal CPAP there was a progressive increase in mixed alveolar partial pressure of CO2(PACO2) and no change in arterial partial pressure of CO2(PaCO2), so that aADCO2declined and reached a lowest value at optimal CPAP. Correspondingly, transmission of airway pressure increased progressively and reached a highest value at optimal CPAP. Between 1 step below and optimal CPAP, PACO2rose from 30.9 to 34.0 torr, and aADCO2declined from 16.6 to 12.7 torr. Between optimal and 1 step above optimal CPAP, PaCO2increased from 46.7 to 51.0 torr, PACO2rose slightly, and aADCO2increased from 12.7 to 15.6 torr. Thus, the aADCO2was an excellent index of optimal CPAP. In five patients with measurements of PaO2at constant fractional inspired oxygen, calculated values for arterial oxygen saturation changed from 80.8 to 91.5 to 92.2, and calculated values for venous admixture changed from 0.61 to 0.48 to 0.46 as CPAP was raised from 1 step below through optimal to 1 step above optimal CPAP. The results are interpreted to mean a progressive improvement in perfusion of well ventilated lung units as CPAP increased to optimal levels, but a significant reduction of both ventilation and perfusion above optimal CPAP. The values for venous admixture were used to calculate ventilation-perfusion ratio (VA/Qc) in a lung model which includes three compartments, high VA/Qc, open low VA/Qc, and shunt. The calculated VA/Qc values were incorporated in O2-CO2diagrams, and predicted values for PACO2were calculated for conditions 1 step below, at, and 1 step above optimal CPAP. The measured and predicted values agreed closely, validating use of the model. However, a progressive disparity, as CPAP was increased, suggests that cardiac output decreased with higher levels of CPAP. |
