Reduction in minute alveolar ventilation causes hypercapnia in ventilated neonates with respiratory distress

Hypercapnia occurs in ventilated infants even if tidal volume (V(T)) and minute ventilation (V(E)) are maintained. We hypothesised that increased physiological dead space (V(d,phys)) caused decreased minute alveolar ventilation (V(A); alveolar ventilation (V(A)) × respiratory rate) in well-ventilated infants with hypercapnia. We investigated the relationship between dead space and partial pressure of carbon dioxide (PaCO(2)) and assessed V(A). Intubated infants (n = 33; mean birth weight, 2257 ± 641 g; mean gestational age, 35.0 ± 3.3 weeks) were enrolled. We performed volumetric capnography (V(cap)), and calculated V(d,phys) and V(A) when arterial blood sampling was necessary. PaCO(2) was positively correlated with alveolar dead space (V(d,alv)) (r = 0.54, p < 0.001) and V(d,phys) (r = 0.48, p < 0.001), but not Fowler dead space (r = 0.14, p = 0.12). Normocapnia (82 measurements; 35 mmHg ≤ PaCO(2) < 45 mmHg) and hypercapnia groups (57 measurements; 45 mmHg ≤ PaCO(2)) were classified. The hypercapnia group had higher V(d,phys) (median 0.57 (IQR, 0.44–0.67)) than the normocapnia group (median V(d,phys)/V(T) = 0.46 (IQR, 0.37–0.58)], with no difference in V(T). The hypercapnia group had lower V(A) (123 (IQR, 87–166) ml/kg/min) than the normocapnia group (151 (IQR, 115–180) ml/kg/min), with no difference in V(E). Conclusion: Reduction of V(A) in well-ventilated neonates induces hypercapnia, caused by an increase in V(d,phys).