Modern aircraft flying at high altitude are cabin pressurised to an atmospheric partial pressure of up to 8000 feet (2348 metres), equivalent to breathing approximately 15% oxygen. This may expose individuals with cardiorespiratory disease to the risk of developing hypoxia. In 2002 the British Thoracic Society (BTS) issued recommendations for passengers with respiratory diseases who are planning to fly.¹ These recommendations included the use of a hypoxic challenge test in children with a history of respiratory disease too young to undergo conventional lung function tests. While pre-flight hypoxic challenge tests have been evaluated in older children² and adults³ with respiratory disease, there are few data on hypoxic responses in infants and young children with respiratory disease although one study has observed profound desaturation in a small number of healthy infants while asleep.⁴

In the last 6 years we have tested 20 children under 5 years of age with a history of chronic pulmonary disease in early infancy (table 1). At our institution fitness to fly testing using 15% oxygen has been performed as a routine test in older children² and adults³ with respiratory disease for some years, so formal ethical approval was not sought for this study. Children were exposed to a hypoxic challenge with 15% oxygen while sitting on the lap of a carer in a whole body plethysmograph (body box). Oxygen saturation was monitored by pulse oximetry (Spo₂) using a probe attached to the child’s finger. After measuring Spo₂ of the child in air, nitrogen was passed into the body box at approximately 50 l/min to dilute the oxygen content of the air to 15% over a period of 5 minutes. Oxygen and carbon dioxide concentrations were measured via continuous flow sampling using a Centronics 200 MGA mass spectrometer. The Spo₂ could take up to approximately 20 minutes to reach a stable value (constant over 2–3 minutes). In none of the tests did the carbon dioxide concentration in the body box exceed 0.5%. In nine cases oxygen was subsequently administered via nasal cannulae to restore the fall in Spo₂ to the original (air) value so that this flow of oxygen could then be recommended during the flight. However, because of lack of data on the range of the normal desaturation response and the clinical significance, advice was not always consistent (table 1, p 1001). No child was oxygen dependent at the time of the test although four children were receiving nocturnal or intermittent supplementary oxygen. Four children were tested a second time for subsequent flights (cases 1, 3, 4 and 5). Eight of the 20 children desaturated below 90% in 15% oxygen, six of whom had normal (>95%) saturations at rest in air. Outcome information was obtained from all seven families who had been advised to take supplementary oxygen (table 1, p 1001). Case 2 was notable for the profound desaturation episode that occurred during the flight. Information regarding the outcome of flights for children for whom supplementary oxygen was not advised was incomplete. Three cases did not fly and seven were lost to follow up.

We conclude that some children with a history of chronic pulmonary disease in early infancy may have normal oxygen saturations in room air but desaturate significantly below 90% when exposed to a 15% oxygen hypoxic challenge. These children may be at risk of hypoxia when flying at altitude. This uncontrolled observational series suggests that such infants should be advised to take supplemental oxygen during the flight. The hypoxic challenge test is a simple and practical test and may be performed in any lung function laboratory with a whole body plethysmograph, a source of nitrogen, and a means of measuring oxygen. As carbon dioxide concentrations do not reach clinically significant levels, oxygen concentrations in the body box could be measured with a conventional oxygen monitor. Further studies are required to evaluate fully the hypoxic challenge test in young children. Spo₂ measurements during flight on subjects and healthy control children are needed. Measurements should be undertaken both in the awake and sleep states because there is evidence that Spo₂ falls in some older children with cystic fibrosis while asleep during flight² and in normal infants at sea level.⁴