Determinants of pulmonary dead space in ventilated newborn infants

Background: Pulmonary dead space (V_D) is an index of ventilation inhomogeneity and one of the determinants of the magnitude of tidal volume to maintain optimal blood gases.

Aims: To identify the determinants of V_D in ventilated newborns and to investigate differences in V_D between prematurely born and term infants and those prematurely born infants who did or did not develop bronchopulmonary dysplasia (BPD).

Methods: Sixty-one mechanically ventilated infants (15 term, 46 preterm) were studied at a median age of 8 (IQR 2-31) days; 32 of the preterm infants developed BPD. V_D was determined from the difference between arterial and end tidal carbon dioxide (CO₂) using a low dead space CO₂ detector using the Bohr/Enghoff equation and was related to body weight (V_D/kg) at the time of study. The time to peak tidal expiratory flow/expiratory time (T_PTEF/T_E) was measured during spontaneous breathing using a fixed orifice pneumotachograph.

Results: V_D/kg was related to gestational age (r=-0.285, p=0.001), birth weight (r=-0.356, p<0.001), weight (r=-0.316, p<0.001) and postmenstrual age (r=-0.205, p=0.020) at measurement, days of ventilation (r=0.322, p<0.001) and T_PTEF/T_E (r=-0.397, p=0.003). The median V_D/kg was higher in prematurely born infants [2.3 (IQR: 1.7-3.0) ml/kg] compared to term infants [1.5 (1.3-2.1) ml/kg, (p=0.003)] and in premature infants that developed BPD [2.6 (IQR 1.8-3.4) ml/kg] compared to those who did not [1.7 (IQR 1.1-1.9) ml/kg], (p<0.001).

Conclusions: Numerous factors influence pulmonary dead space and thus an optimum tidal volume will differ according to the underlying demographics and respiratory status.