Permissive hypercapnia and oxygenation impairment in premature ventilated infants

In permissive hypercapnia high levels of carbon dioxide (CO₂) are tolerated in ventilated preterm infants to minimise lung injury, but hypercapnia could directly impair oxygenation. We aimed to quantify the association of elevated CO₂ with oxygenation impairment in preterm infants by measuring the right-to-left shunt and the ventilation/perfusion (V_A/Q) ratio.

Pre-existing datasets from preterm infants during the acute phase of respiratory distress syndrome or with evolving or established bronchopulmonary dysplasia were analysed. Non-invasive paired measurements of the fraction of inspired oxygen (F_IO₂) and transcutaneous oxygen saturation (SpO₂) were used to calculate the degree of right-to-left shunt, right shift of the F_IO₂ versus SpO₂ curve and the V_A/Q.

A total of 75 infants (43 male) with a median (IQR) gestational age of 26.4 (24.7-27.7) weeks were studied at 7 (2-31) days. Thirty-six infants (48 %) had an arterial partial pressure of CO₂ (PaCO₂) above 6 kPa. The PaCO₂ was independently associated with the right shift of the curve [adjusted p < 0.001, unstandardised coefficient; 2.26, 95 % CI: 1.51-2.95] and the right-to-left shunt [adjusted p = 0.016, unstandardised coefficient; 1.86, 95 % CI: 0.36-3.36] after adjusting for confounders. An increase of the PaCO₂ from 5 to 8 kPa, corresponded to a right shift of the curve of 20.2 kPa or a decrease in the V_A/Q from 0.66 to 0.24.

Increased carbon dioxide levels were significantly associated with impaired oxygenation in preterm infants with respiratory distress syndrome or bronchopulmonary dysplasia.