Presented at the Neonatal Society 2007 Summer Meeting.

Adeniji T, Landolfo F, Rao H, Hannam S, Milner A, Rafferty G, Greenough A

King’s College Hospital, London, UK

Background: Prone position has been associated with increased risk of sudden infant death syndrome (SIDS) especially in preterm infant with an odds ratio of 48.8. We have demonstrated that convalescent prematurely born infants have a poorer response to added dead space in the prone position suggesting a reduced response to hypercarbia. This could reflect poorer respiratory control or respiratory muscle strength.

Aim: To determine whether the poorer response to added dead space in the prone position reflects poorer respiratory drive and/or respiratory muscle strength.

Patients: Twenty-five infants (7 oxygen dependent), median gestational age 30 weeks (25- 32weeks) and birth weight 1.148kg (0.74–1.816 kg) were studied at a post conceptional age of 36 weeks (range 35 – 39).

Method: Each infant was studied after being in the prone and supine position, each position maintained for at least two hours. The ventilatory response to added dead space was assessed by measuring the breath by breath minute volume at baseline and when a deadspace of 4.4mls/kg was incorporated into the circuit. The maximum minute ventilation following addition of the dead space was determined and the change in minute volume and the time constant (t0.63) of the response calculated. In addition, respiratory control was assessed by measuring the pressure generated in the first 100milliseconds following airway occlusion (p0.1) and respiratory muscle strength by measuring the maximum inspiratory pressure (Pimax) following airway occlusion. Informed written consent was obtained from the parents of all infants who participated in the study and the King’s College Hospital Ethics Committee approved the study.

Result: The change in minute ventilation was higher (p=0.026) and the t0.63 was lower (p=0.036) in the supine compared to prone position. Both p0.1 (p=0.036) and Pimax (p=0.042) were significantly higher in the supine compared to prone position. There were significant correlations between the change in minute ventilation and p0.1 both in the supine (r = 0.513, p = 0.009) and in the prone position (r = 0.561, p = 0.004). There was, however, no significant correlation of the change in minute ventilation and Pimax in either the supine or prone position.

Conclusion: These results suggest that the impaired response to added dead space in the prone position in convalescent prematurely born infants reflects reduced respiratory drive rather than respiratory muscle strength. These data support the hypothesis that reduced respiratory drive in the prone position may contribute to SIDS in prematurely born infants.

References
1. Bhat RY, Broughton S, Khetriwal B, Rafferty GF, Hannam S, Milner AD, Greenough A. Dampened ventilatory response to added dead space in newborns of smoking mothers. Arch Dis Child Fetal Neonatal Ed. 2005 90(4):F316-9.
2. Leipala JA, Bhat RY, Rafferty GF, Hannam S, Greenough A. Effect of posture on respiratory function and drive in preterm infants prior to discharge. Pediatr Pulmonol. 2003: 36(4):295-3008.