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 (PaCO₂) 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. PaCO₂ 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 ≤ PaCO₂ < 45 mmHg) and hypercapnia groups (57 measurements; 45 mmHg ≤ PaCO₂) 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.