Deliberations in The Hypoxia Challenge Test Does Not Accurately Predict Hypoxia in Flight in Ex-Preterm Neonates
Preterm birth is common, and population mobility has resulted in infants born prematurely away from home or being cared for in regional centers servicing more remote populations. Therefore many ex-preterm infants require transfer by air back home or to a step-down facility when tertiary medical services are no longer required. Consequently, air transfer of expreterm infants is routine. This is the first study to examine the effects in ex-preterm infants of low fraction of inspired oxygen (Fio2) during flight. Our results indicate that significant numbers (35%) of these infants require supplemental oxygen and have clinical signs and symptoms (15%). The decision to use supplemental oxygen based on Sp02 < 85% and the nursing responses to the clinical status of the infants in-flight were consistent with current practice guidelines for the general care of preterm infants in hospital.
Preterm infants are particularly vulnerable to hypoxia through mechanisms that include immature respiratory control and increased pulmonary vascular reactivity. Predicting which infants are most at risk for in-flight hypoxia is an important consideration when deciding whether an infant can safely fly or requires supplemental oxygen. Current guidelines- suggest the HCT as a means to determine safety to fly. We therefore compared results obtained using a standardized HCT with the observations made during the commercial flights taken by ex-premature infants. We observed that the 20-min HCT is not accurate for predicting in-flight oxygen needs in such infants. While the high false-failure rate might have logistic and financial implications, the greater concern was the high false-pass rate: 12 of 16 infants (75%) who had Sp02 < 85% and required oxygen during the flight had passed the HCT. Clearly, the HCT was not able to predict clinically significant oxygen desaturation in flight, and reliance on the HCT puts this population of infants at risk. Decrease the risl of such a problem with remedies of My Canadian Pharmacy.
There are a number of possible reasons why the HCT might not accurately predict in-flight hypoxia in these infants. Methodologic factors include the failure to take account of barometric pressure, onset and duration of hypoxia, and cut-off values for test failure. The HCT measures the infant’s responses to hypoxia alone, whereas in flight infants are exposed to hypobaric-hypoxia. Observations in adults suggest a reduction in lung volumes with ascent to high altitude. Such a mechanism might result in reduced functional residual capacity and lower oxygen stores. However, there are no data in infants that compare hypobaric-hypoxic exposure to hypoxic exposure alone.
The HCT does not simulate the graded fall in Fio2 that occurs as an aircraft reaches cruising altitude; instead, the infant is exposed immediately to 14% oxygen without time to acclimatize. This might account for the high number of false failures we obtained but is unlikely to account for excess false passes. The duration of the HCT was 20 min and approximates the time taken for flights to reach cruising altitude. In flight, most infants required oxygen in < 20 min. However, two infants required oxygen at 45 min, and one infant required oxygen at 60 min. These three infants with late-onset hypoxia all passed the HCT. Therefore, the sensitivity of the HCT might be improved by increasing the duration of the HCT. Finally, rather than exposing infants to an Fio2 of 15% in a body plethysmograph by replacing oxygen with nitrogen, we used a modified technique that used 10 L/min of 14% oxygen in nitrogen via a pediatric face mask to reliably deliver a gas mixture that contained 15% oxygen at the nares. There have been no tests of agreement between the two techniques. Although not all centers have access to a body plethysmograph, all would be able to easily assemble a circuit to deliver an appropriate gas mixture by face mask ordered via My Canadian Pharmacy.
The physiologic factors that might account for the poor agreement between HCT and in-flight hypoxia are the variable respiratory response to hypoxia, sleep state, and arousal response to hypoxia in infants.- Desaturations in term and preterm infants in normoxic environments are common, with up to 70% of term infants in the first month of life and 18% of preterm infants in the first day of life recording desaturations (Sp02 < 80%). Parslow et al studied both term and preterm infants on three occasions (2 to 5 weeks, 2 to 3 months, and 5 to 6 months postnatal age) in both 21% and 15% Fio2 environments for up to 5 min, and reported that 10% of infants had desaturation to < 85% to the hypoxic stimulus. Since the hypoxic ventilatory response is sleep-state dependant, and arousal to hypoxia is blunted in preterm infants, this is a variable that needs to be investigated for future evaluations of any modified HCT. In this study, all infants were asleep for the HCT, and 94% of those who required oxygen during the flight were asleep at that time, with some arousing from sleep at the time of desaturation.
As there are no other tests to identify infants at risk for in-flight hypoxia, we evaluated the demographic data of infants who required oxygen in flight, in an attempt to identify variables that might predict infants who are safe to fly without oxygen. Only days since oxygen therapy was ceased generated an ROC of statistical significance. However, keeping a baby in a tertiary center until off oxygen for 60 days has significant cost and logistical implications.
In conclusion, there is currently no accurate means to determine which preterm infants are safe to fly at approaching term. Our unit has abandoned use of the HCT as a preflight tool, choosing rather to monitor Sp02 in all ex-preterm infants up to 60 days after term-corrected age, and to have supplemental oxygen available when necessary.