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 (P0.1) (respiratory drive) and the maximum inspiratory pressure during crying with an occluded airway (Pimax) (respiratory muscle strength) were measured. The P0.1/Pimax ratio at each CO2 level and slope of the P0.1/Pimax response were calculated. Results: The mean P0.1 (p0.05) and P0.1 /Pimax (p0.05) were higher and the functional residual capacity (p=0.031) lower in the supine compared to the prone position. The mean P0.1 and P0.1/Pimax increased independently of position as the percentage CO2increased (p0.001). There was no tendency for the differences in P0.1 and P0.1/Pimax between the prone and supine position to vary by CO 2 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.
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