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Ventilatory response to added deadspace in newborns of substance abusing mothers

Presented at the Neonatal Society 2007 Summer Meeting (programme).

Kassim Z, Khetriwal B, Bhat R, Hannam S, Rafferty GF, Milner AD, Greenough A

Division of Asthma, Allergy and Lung Biology, King’s College London, King’s College Hospital, Denmark Hill, London, UK

Background: Infants of mothers who substance abuse during pregnancy are at increased risk of sudden infant death syndrome. A possible explanation for the association is that such infants have neurodevelopmental abnormalities of the control of ventilation. If that explanation was correct then the infants of substance abusing mothers might be expected to have a reduced ventilatory response to hypercarbia. This can be assessed by examining ventilation before and following the imposition of an added dead space (tube breathing) (1). We have already shown that newborns of smoking mothers have a dampened ventilatory response to added dead space (2).

Objective: To test the hypothesis that infants of substance abusing mothers (SA infants) compared to infants of non substance abusing, non smoking mothers (NSA infants) and infants of smoking mothers (SM infants) would have a poorer ventilatory response to added dead space, and to determine if there was an additive effect of substance abuse to that of maternal smoking.

Methods: Breath by breath minute volume was measured at baseline and when a deadspace of 4.4 ml/kg was incorporated into the breathing circuit. The maximum minute ventilation during tube breathing was determined and the time constant of the response calculated. The study was undertaken in accordance with UK ethical guidelines.

Patients: 10 SA, 18 SM and 28 NSA infants were assessed, median gestational age 39 (range 35-42) weeks. All SA mothers also smoked between 1-20 cigarettes per day.

Results: The time constant or response to tube breathing of the SA infants was significantly longer than that of the NSA infants (p=0.017), but not the SM infants, indicating the SA infants had a delayed response to tube breathing. Unexpectedly, the SA infants had a higher respiratory rate which may reflect irritability due to withdrawal.

Conclusion: These results are consistent with infants of substance abusing mothers having a dampened ventilatory response to hypercarbia and compatible with them having chemoreceptor insensitivity. Whether this is due to in utero exposure to substance abuse, smoking or both merits further investigation.

1. Upton CJ, Milner AD, Stokes GM et al. Paediatr Pulmonol 1990;9:72-9
2. Bhat R, Broughton S, Khetriwal B, Rafferty GF, Hannam S, Milner AD, Greenough A. Arch Dis Child 2005; 90(4): F316-319.

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