Objectives To test the hypothesis that the ventilatory response to a carbon dioxide (CO₂) challenge would be lower in the prone compared to the supine position in prematurely born infants studied post-term. To determine whether there were postural-related differences in respiratory drive, respiratory muscle strength, thoracoabdominal synchrony and/or lung volume.

Design Prospective cohort study.

Setting Tertiary neonatal unit.

Patients Eighteen infants (median gestational age 31 (range 22–32) weeks) were studied at a median of 5 (range 2–11) weeks post-term.

Interventions The ventilatory responses to three added carbon dioxide (CO₂) levels (0% baseline, 2% and 4%) were assessed in the prone and supine positions.

Main outcome measures The airway pressure change after the first 100 ms of an occluded inspiration (P_0.1) (respiratory drive) and the maximum inspiratory pressure during crying with an occluded airway (Pimax) (respiratory muscle strength) were measured. The P_0.1/Pimax ratio at each CO₂ level and slope of the P_0.1/Pimax response were calculated.

Results The mean P_0.1 (p<0.05) and P_0.1/Pimax (p<0.05) were higher and the functional residual capacity (p=0.031) lower in the supine compared to the prone position. The mean P_0.1 and P_0.1/Pimax increased independently of position as the percentage CO₂ increased (p<0.001). There was no tendency for the differences in P_0.1 and P_0.1/Pimax between the prone and supine position to vary by CO₂ level.

Conclusions Convalescent, prematurely born infants studied post-term have a reduced respiratory drive, but not a lower ventilatory response to a CO₂ challenge, in the prone compared to the supine position.