Physiological dead space and alveolar ventilation in ventilated infants

BACKGROUND: Dead space is the volume not taking part in gas exchange and, if increased, could affect alveolar ventilation if there is too low a delivered volume. We determined if there were differences in dead space and alveolar ventilation in ventilated infants with pulmonary disease or no respiratory morbidity. METHODS: A prospective study of mechanically ventilated infants was undertaken. Expiratory tidal volume and carbon dioxide levels were measured. Volumetric capnograms were constructed to calculate the dead space using the modified Bohr–Enghoff equation. Alveolar ventilation (V(A)) was also calculated. RESULTS: Eighty-one infants with a median (range) gestational age of 28.7 (22.4–41.9) weeks were recruited. The dead space [median (IQR)] was higher in 35 infants with respiratory distress syndrome (RDS) [5.7 (5.1–7.0) ml/kg] and in 26 infants with bronchopulmonary dysplasia (BPD) [6.4 (5.1–7.5) ml/kg] than in 20 term controls with no respiratory disease [3.5 (2.8–4.2) ml/kg, p < 0.001]. Minute ventilation was higher in both infants with RDS or BPD compared to the controls. V(A) in infants with RDS or BPD was similar to that of the controls [p = 0.54]. CONCLUSION: Prematurely born infants with pulmonary disease have a higher dead space than term controls, which may influence the optimum level during volume-targeted ventilation. IMPACT: Measurement of the dead space was feasible in ventilated newborn infants. The physiological dead space was a significant proportion of the delivered volume in ventilated infants. The dead space (per kilogram) was higher in ventilated infants with respiratory distress syndrome or evolving bronchopulmonary dysplasia compared to term controls without respiratory disease. The dead space volume should be considered when calculating the most appropriate volume during volume-targeted ventilation.