Abstract
Summary: The effects of 100% oxygen breathing and constant positive pressure breathing (CPPB) on venous admixture (Qva/Qt) and arterial-alveolar difference for PCO2 (aADCO2) were examined in seven infants with hyaline membrane disease (HMD). Increasing FIO2 from 0.63-0.99 with CPPB constant at 2 cm H2O resulted in significant decrease in Qva/Qt from 0.67-0.47, but produced no change in aADCO2 (13.0 torr vs. 15.0 torr). Increasing CPPB to 8 cm H2O with FIO2 returned to 0.63 also resulted in decreased Qva/Qt (0.50), but in addition aADCO2 decreased significantly to 7.0 torr. The reduction in Qva/Qt with oxygen breathing and with CPPB is interpreted as a reduction in true right-to-left-shunt and a corresponding increase of effective blood flow through the lung. In 100% oxygen the increase in effective pulmonary perfusion occurred in a poorly ventilated compartment and as such was not reflected in the aADCO2. On the other hand, with CPPB, the increase in perfusion was accompanied by an increase in ventilation and, hence, the aADCO2 decreased. To illustrate these effects we constructed a three compartment model for the lung in HMD, calculated the VA/Qc for the well-ventilated compartment in each circumstance, constructed O2-CO2 diagrams and arrived at predicted values for the aADCO2 for each of the three clinical conditions. These predicted values agree well with those measured, considering the possible errors in our methods and assumptions and considering the absolute changes that may occur with CPPB, namely, increased cardiac output and decreased ventilation. This, in turn, provides strong support for the proposed three compartment model and for the existence of an open, but severely underventilated compartment in HMD.
Speculation: In HMD, the aADCO2 seems to be responsive to the effects of CPPB on both ventilation and perfusion, and as such would be valuable clinically in determining optimal levels of CPPB.
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Hansen, T., Corbet, A., Kenny, J. et al. Effects of Oxygen and Constant Positive Pressure Breathing on aADCO2 in Hyaline Membrane Disease. Pediatr Res 13, 1167–1171 (1979). https://doi.org/10.1203/00006450-197910000-00018
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DOI: https://doi.org/10.1203/00006450-197910000-00018