Comparison of Respiratory Physiologic Features When Infants Are Placed in Car Safety Seats or Car Beds

METHODS

RESULTS

A total of 67 infants were studied, with 34 in the car seat and 33 in the car bed. No infants had abnormalities identified or were receiving any medications. The characteristics of the infants and mothers in the 2 groups were similar (Table 1). Of particular note, the means and ranges of weights for infants in the 2 groups were similar. The ages at the time of study were similar in the 2 groups (car seat: 45.87 hours; car bed: 44.43 hours).

The 2 groups spent similar amounts of time in the devices (car bed: 71.6 minutes; car seat: 74.2 minutes) (Table 2). The mean oxygen saturation values were similar for the groups (car bed: 97.1%; car seat: 97.3%). Two infants (1 in each group) had their studies terminated prematurely (according to the protocol) because of persistent dips of oxygen saturation to <85%. The mean total time spent with saturation of <95% was greater in the car seat group (872 seconds) than in the car bed group (448 seconds). When this time was adjusted for study duration (percentage of study time with saturation of <95%), however, this difference did not reach statistical significance. Each group was assessed for the time spent at a number of oxygen saturation levels (Table 2 and Fig 1). The mean percentage of studies with saturation of <85% was ∼2% in both groups; however, the percentages of studies with saturation values between 85% and 89% and between 90% and 94% were higher in the car seat group than in the car bed group. In both groups, there were a number of infants who spent very high percentages of their study time with oxygen saturation of <95% (Fig 2). It is noteworthy that 6 infants with the greatest amounts of time with oxygen saturation of <95% were all in the car seat group. Six infants in the car seat group spent between 54% and 63% of the study with saturation of <95%, whereas the 6 infants in the car bed group with the most time with saturation of <95% spent between 20% and 42% of the study at that level. Each study was scored for apnea. Very little central apnea was seen in either group. The groups had similar numbers of obstructive and mixed apneic events. End-tidal carbon dioxide levels were low in each group. This was not unexpected, because the normal rapid respiratory rate did not allow plateauing of the end-tidal carbon dioxide trace.

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FIGURE 1

Mean percentages of time recorded for which saturation was >95%, 90% to 94.9%, 85% to 89.9%, or <85%.

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FIGURE 2

Percentages of children with oxygen saturation of <95% for 10%, 20%, 30%, 40%, and 50% of the study time.

DISCUSSION

Car safety seats, which have been recommended by the AAP since 1974, are essential safety devices that have resulted in substantial reductions in motor vehicle-related deaths.¹³ In 1986, it was first demonstrated that premature infants, because of their reduced muscle tone, might develop airway obstruction when placed in these devices.¹ On the basis of that initial study, as well as a subsequent study with a larger group of infants, the AAP recommended a period of monitoring in the car safety seat before discharge for preterm infants (<37 weeks of gestation).³ If the infant demonstrates evidence of respiratory impairment in the car safety seat test, then currently it is recommended that the child be transported in a car bed.⁴ However, no study has compared directly the respiratory physiologic features in car safety seats and car beds. This study focuses on this area for the first time.

Despite the limited study size, significant physiologic changes were observed. Overall, it was surprising how much desaturation occurred in each group (Fig 2). The car seat group seemed to spend more time at lower saturation levels. We had expected that the car bed would perform better, because the airway did not seem to be as vulnerable in that device. This finding raises an important question, namely, why an infant should experience desaturation in a car bed. Partial obstructive apnea was seen in both groups, but the amount was not excessive. Previously, Tomkin et al¹¹ corrected the upper airway vulnerability in car seats with the use of an insert that pushed the shoulders forward while allowing the head to extend. In this study, despite correction of the head position, some desaturation persisted.

It is possible that the desaturation is attributable to a cause other than airway closure. The harness is common to both groups. The tension of this harness was similar for the 2 devices. We speculate that this feature may contribute to the vulnerability to desaturation. The chest wall of infants is more pliable than the chest wall of adults. The rigidity of the chest wall keeps the lungs from collapsing in both adults and children. In infants, however, the more-pliable chest wall is also kept in check by a dynamic balance of the inspiratory and expiratory muscles. With the harness fitting snugly against the chest wall, this finely balanced system is likely to be tipped in favor of collapse. This change is likely to limit respiration by reducing tidal volume and residual volume; therefore, the respiratory reserve is reduced and lower oxygen saturation is expected, particularly when the system is stressed, such as during partial obstruction. However, from the perspective of crash protection, tight adherence of the harness to the chest wall is a critical feature for crash tolerance. Additional studies are needed to address the optimal tightness of the harness. Also common to both groups is the buckle attached to the harness. Because of its hardness and position on the center of the chest, it might increase the vulnerability of the chest wall of the infant. In addition, because infants are abdominal breathers, it is possible that compression on the abdomen is a factor contributing to respiratory compromise. Although this is possible, in our opinion it is less likely, because the harness is not focused in this area.

An interesting paradox might exist here. In the car seat, airway opening may be limited.^1,5,11,14 In contrast, the effects of compression of the chest may be more severe in the car bed. In the car seat, the child is sitting, and diaphragmatic displacement downward is facilitated. This downward displacement might overcome the potential compression of the chest. Therefore, we postulate that the reasons for desaturation might be different for the 2 groups.

This clinical study has a number of limitations. Insufficient power might have made it less likely to identify differences between the groups. We did not anticipate the low oxygen saturation in the car bed group, and this lower value made differences harder to distinguish without larger study groups. In view of the findings for the car bed group, another control group in which the infant was placed flat on a bed without a harness would have been useful. Of interest, when the car bed group is compared with the Collaborative Home Infant Monitoring study group, in which oxygen saturation was monitored among normal infants,^15,16 the car bed group had lower oxygen saturation than those historical control infants. To help exclude the possibility that these desaturations are normal occurrences among infants not placed in devices, we reviewed all of our data again and looked at the times when desaturation occurred. Our data completely parallel the data reported by Merchant et al,⁵ in which there was an increase in desaturation with progression of the study in both groups. This finding suggested that desaturation was device related rather than being related to the baseline characteristics of the child. Other possible informative control positions for future studies might include the placement of subjects in the car safety seat or the car bed with the harness very loose or detached. Another limitation is that we did not stage sleep. Sleep stage could be altered by the device, which might provide an explanation for these results.

Our studies were relatively short, at 70 minutes; a previous study showed that the extent of desaturation while the infant was in the car seat became more obvious with time.⁵ However, the nadir seemed to be at ∼70 minutes, and our study was within that period. In addition, it was not possible to compare each infant in both devices. Such studies would be ideal but, in view of the limited time infants spend in the hospital after birth, are difficult to perform. Also, all of the infants in our study groups were placed by a trained nurse with many years of experience in this area. In real-life conditions, we would expect much more variability in placement, because of both parental practice and differing characteristics of various car safety seat brands. These real-life conditions would need to be tested for generalization of these results to larger populations of infants.

Car safety seats and car beds remain very important transport devices for children. Overall, our study suggests that additional refinements of the car seat and bed are necessary to minimize or to prevent respiratory compromise. Such modifications are possible and need to be addressed seriously by the manufacturers of these devices. In the interim, we suggest caution when considering the use of these devices outside the setting of transportation in the car, for which they were designed.