|Fit to Fly with Cystic Fibrosis
The Leeds Method of Management. April, 2008. Fit to Fly with Cystic Fibrosis [online]. Leeds Regional Adult and Paediatric Cystic Fibrosis Units, St James’s University Hospital, Leeds, UK. Available from http://www.cysticfibrosismedicine.com
Aircraft fly at altitudes of between 10,000 to 18,000 metres. Aircraft are therefore pressurised, but not to sea level. Cabin altitude should not exceed 2,450 metres. As a consequence of high altitude, the partial pressure of Oxygen (pO2) is decreased with atmospheric pO2 reaching around 15kPa. This results in passengers receiving less oxygen during their flight. This is not a problem for most people, as the shape of the oxygen dissociation curve does allow for some drop in arterial pO2 without much reduction in blood oxygen. However patients with a reduced arterial pO2 or chronic lung disease are at risk from hypoxia resulting in the requirement of in flight supplemental oxygen. Some patient with CF appear to tolerate low paO2 values probably due to adaptation to chronic hypoxemia and lung function impairment (Kamin et al, 2006).
The fitness to fly test is a sensitive and specific test for identifying those at risk of hypoxaemia during routine commercial airline flights (Oades et al , 1994). While the clinical importance of transient high altitude flight hypoxaemia is unclear and individual tolerance variable, high altitude induced hypoxaemia can induce significant clinical deterioration (Speechly-Dick et al, 1992). Lung function, resting oxygen saturation and resting PO2 measurement do not accurately predict all patients at risk of developing in flight hypoxaemia (Ross et al, 2000, Peckhamet al, 2002). An FEV1 <60% and/or baseline hypoxaemia appear to be the best clinical predictors although a significant number of patients in this group will have negative fitness to fly tests (Ross et al, 2000; Buchdahl et al, 1998, Peckham et al, 2002).
The fitness to fly test aims to determine the level of hypoxia that would occur in a patient when flying. A 100 litre Douglas bag is flushed and then filled with 15% O2, 85% N2 from a certified BOC cylinder. Douglas bag concentrations should be as accurate as possible, ideally O2concentration should be between 14.9 – 15.1%. The Douglas bag is fitted with a three-way tap and attached to a two-way valve box and mouthpiece. The valve box ensures that the patient inspires from the bag and expires into the room. It is essential to ensure the system is free from leaks.
Routine spirometry should be performed to determine the respiratory state of the patient. Resting PaO2 is then ascertained by performing capillary blood gases. If the PaO2 is less than 7kPa the patient will be compromised during a flight and the test should be stopped at this point. After explaining the test to the patient, the patient should be connected to an oximeter and allowed to breathe from the Douglas bag circuit. The patient should breath for sufficient time to equilibrate with the Douglas bag. This can be as much as 20 minutes in a severely obstructed patient. The oximeter reading is used to give an indication of when the patient has stabilised. A second capillary blood gas is then taken to establish the PaO2 the patient has dropped to. If during the test the oximeter reading falls to below 80% a blood gas sample must be taken and the test stopped. If the patient appears to be in distress the test must be stopped. At the end of the test the result form must be correctly completed and sent to the requesting physician for comments. The referring physician will decide if supplemental oxygen is required for a patient.
A fall in PO2 to 6.7 kPa (83% saturation) or below is usually taken as a positive test and is the minimum desirable level of PO2 during a flight. A higher value may be more appropriate for patients on long haul flights.
Individuals with cystic fibrosis appear to tolerate in flight hypoxaemia extremely well. A study by Rose et al suggests that clinically stable adults with cystic fibrosis (without accompanying heart disease) with in flight paO2 of >40 mm Hg can anticipate a safe flight trip. The number of patients studied was small (Rose et al, 2000).