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To the Editor.—
I read with interest the recent article on cardiorespiratory stability of infants carried in slings by Stening et al.1 This is an important topic for which objective data have not been available. Unfortunately, the article suffers from a number of significant flaws. The most glaring is the definition of desaturation as a decline in oxygen concentration to <88% for at least 10 seconds. There are 10 published studies on the normal oxygen saturation of infants. In a recent review of this issue by Poets,2 it was determined that the baseline saturation of infants is 93% to 100% in term and preterm neonates and 97% to 100% in term and preterm infants; therefore, the current study has not been able to identify the number of abnormal desaturation events that occur in a sling.
It is also troubling that the authors make the assumption that, in the absence of outright respiratory arrest, significant apnea, or bradycardia, chronic and/or recurrent subclinical oxygen desaturation should not be considered a "clinically relevant" event. There are published reports of both cognitive3 and behavioral difficulties4 in infants who are exposed to chronic or recurrent hypoxia. The authors therefore should be more guarded in their conclusions, because the most we can say is that there were no acute events observed.
Another problem in the study is that the authors monitored their patients for a time period of only 20 minutes. This is unfortunate, because previous studies of desaturation associated with car seats5,6 have documented very significant drops in saturation that may not occur until after an hour of study. In addition, studies in preterm infants have demonstrated that baseline hypoxemia may be associated with apparent life-threatening events.7 Considering all of the above, I think it is premature to reassure parents that using a sling is not without clinically relevant risk.
REFERENCES
In Reply.—
We agree with Dr Bass that the 20-minute observation intervals in our study are relatively short. We intentionally chose this design to exclude potential bias, eg from effects of diurnal rhythm or developmental stage.1 Furthermore, we tried to minimize strain on mother and child. To investigate whether the vital parameters changed with longer observation, we subdivided the observation interval into 3 phases of 
7 minutes and compared the measured mean values of oxygen saturation and heart frequency. There was no difference in these parameters between the 3 phases. Of course, this does not exclude the possibility of progressive desaturation after a longer carrying time in the sling. Nevertheless, a comparison with the studies of car seats is only possible to a limited extent. Unlike the studies cited2,3 in which children were observed in a static condition (in a car seat placed in a nursery), our study was intended to examine vital parameters during the more life-like dynamic condition of a normal stroll. Most probably, the children were constantly stimulated by motion, sounds, or their parents’ voices.
With regard to the issue of "normal" oxygen saturation values, we point out again that, in the crossover design of our study, children served as their own controls. Saturation values of babies carried in a sling were compared with those of the same babies in a pram. A comparison with the normal values described in Poets’ review4 is difficult for several reasons. Poets states that his oxygen saturation values are based exclusively on recordings during periods of "regular breathing (corresponding to quiet sleep) and excluding apneic pauses." We did not exclude any measurements on the basis of the prevailing respiratory pattern during the recording period. Poets cites two studies5,6 that describe oxygen saturation in children of similar postconceptional or postnatal age to the premature and term infants in our study. The preterm infants (n = 160) had a median baseline oxygen saturation (Sp
2) of 99.5%; however, the lowest baseline Sp2 was 88.7%. In term infants (n = 60) the median was 98.0%, and the lowest baseline value 86.6%. These so-called normal values in healthy children are lower than in the children of our study. Unfortunately, Poets only cites median values, whereas our study is based on means. Of course, these two different values cannot be compared directly because Sp2 levels are not normally distributed but skewed toward 100%. Furthermore, Poets’ review does not state for what proportion of the time under observation children had Sp2 values of <93% or 97%. Poets does, on the other hand, specify that 355 desaturation episodes were observed in the premature infants and 165 in the term babies (these were defined as Sp2 < 80%). This would equate to 6 to 7 desaturation episodes per hour in the sample described in our study. However, as stated in the article, none of our children desaturated below 80%.
Similarly, our study cannot really be compared to the studies cited describing outcome in infants with presumed postnatal hypoxemia.7,8 In our study, we only included healthy preterm and term infants with stable cardiorespiratory measurements who did not need additional oxygen on the day of observation. One would expect much lower mean Sp2 values in infants with cyanotic heart failure.7 Also, Lou’s8 study on attention-deficit hyperactivity disorder describes infants with recurrent episodes of hypoxemia. Using the definition of hypoxemia given by Poets4,9 (i.e. oxygen saturation <80% for at least 4 seconds), none of our infants suffered from hypoxemia while being carried in the sling. Tin et al10 pointed out in their study that the saturation level in preterm infants is of only little influence on neurological outcome and on the development of retinopathy. Their study showed that the development of cerebral palsy in 296 preterm infants <28 gestational weeks did not correlate with the saturation level (target saturation level between minimum 70%–90% and maximum 88%–98%). Moreover, the percentage of threshold retinopathy in 1-year survivors increased with the saturation level.
As mentioned in our article, we only observed desaturation episodes (<88%) in preterm babies. We advised caution with the use of slings for carrying preterm infants before they reach term postconceptional age, although no desaturation below 80% was seen in our study, and the clinical relevance of desaturation to Sp2 levels between 88% and 80% is unclear. We remain satisfied with our conclusion that term babies are not at risk of desaturation while being carried in the sling.
REFERENCES
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