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

Hypercapnia occurs in ventilated infants even if tidal volume (VT) and minute ventilation (VE) are maintained. We hypothesised that increased physiological dead space (Vd,phys) caused decreased minute alveolar ventilation (VA; alveolar ventilation (VA) × respiratory rate) in well-ventilated infants with hypercapnia. We investigated the relationship between dead space and partial pressure of carbon dioxide (PaCO2) and assessed VA. Intubated infants (n = 33; mean birth weight, 2257 ± 641 g; mean gestational age, 35.0 ± 3.3 weeks) were enrolled. We performed volumetric capnography (Vcap), and calculated Vd,phys and VA when arterial blood sampling was necessary. PaCO2 was positively correlated with alveolar dead space (Vd,alv) (r = 0.54, p < 0.001) and Vd,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 Vd,phys (median 0.57 (IQR, 0.44-0.67)) than the normocapnia group (median Vd,phys/VT = 0.46 (IQR, 0.37-0.58)], with no difference in VT. The hypercapnia group had lower VA (123 (IQR, 87-166) ml/kg/min) than the normocapnia group (151 (IQR, 115-180) ml/kg/min), with no difference in VE.Conclusion: Reduction of VA in well-ventilated neonates induces hypercapnia, caused by an increase in Vd,phys. What is Known: • Volumetric capnography based on ventilator graphics and capnograms is a useful tool in determining physiological dead space of ventilated infants and investigating the cause of hypercapnia. What is New: • This study adds evidence that reduction in minute alveolar ventilation causes hypercapnia in ventilated neonates.