Letters

Air travel and risk of venous thromboembolism

Passengers should reduce consumption of alcohol on flights

Pulmonary embolism after air travel may occur by chance alone

Passengers should reduce consumption of alcohol on flights

EDITORGeroulakos,¹ like previous reviewers of the relation between air travel and venous thromboembolism,² did not mention the theoretical and experimental evidence of thrombogenesis in venous valve pockets that colleagues and I have published.³ Modelled on one of the six possible permutations of Virchow's triad, our experiments produced experimental thrombi in venous valve pockets for the first time since Virchow described them in 1858.⁴ The specific triad model was (1) interrupted circulation in venous valve pockets causing (2) hypoxaemic metabolic endothelial injury and leading to (3) ectopic haemostatic plug formation (blood metamorphosis) in valve pockets.

Merely to move blood clotting from position 1 to position 3 in the triad sequence gives a new explanation for thrombogenesis. This suggests that thrombogenesis during long haul flights is attributable to individual passengers' behaviourspecifically, taking an excess of drugs that suppress the central nervous system (alcohol, long acting tranquillisers, or other sedative drugs which, alone or in combination, may induce quasi-anaesthetic muscle paresis or paralysis).

During deep sleep, muscle areflexia may mean that muscles stop pumping blood towards the head and underperfuse deep venous valve pockets. The problem starts when non-pulsatile circulation into or within venous valves stops.⁵ We did not establish the time for which valve pockets must be underperfused before their intima is suffocated and ectopic haemostatic thrombogenesis begins: our objective was to cause experimental thrombi, not prevent them. More than two hours' paralysis of limb muscles harmed valve pocket intima, but less than 90 minutes' paralysis produced no thrombi.

Geroulakos points out that airlines disclaim responsibility for thrombotic events and that no strict scientific basis exists for the standard medical advice given to passengers. Airlines may certainly disclaim responsibility if the lesions are caused by passengers' self injuring behaviour.

The standard advice to sober passengers is to move their legs, drink water, walk the aisle, be aware that there is a mild lack of oxygen, and seek more knee room if possible.

The advice to reduce consumption of alcohol and drugs that suppress the central nervous system is probably so pertinent that a legal limit on consumption on long haul flights might be introduced as prophylaxis against thrombotic disasters.

P C Malone, retired general practitioner. 
129 Viceroy Close, Birmingham B5 7UY pcmalone{at}Doctors.org.uk

Pulmonary embolism after air travel may occur by chance alone

EDITORIn his editorial Geroulakos states that "there is only circumstantial, but no epidemiological, evidence connecting air travel with venous thrombosis."¹ He adds that the incidence of venous thrombosis associated with air travel is "much less than the impression given by the recent publicity" surrounding the death of a 27 year old woman from a pulmonary embolism after she disembarked from a flight from Australia to London.

This episode reminds me of a letter published 15 years ago in the New England Journal of Medicine.² Its authors reported a pulmonary embolism in a 40 year old man the day after he watched three consecutive football games on television on New Year's Day. The correspondentspresumably with tongue in cheektermed this condition "bowl-game pulmonary embolism," in reference to the college football bowl games played in the United States in December and January.

I enjoy creative humour, but as an epidemiologist I felt compelled to point out that many pulmonary emboli will occur by chance alone among people viewing football games on television. In a letter (which the journal did not publish but which is on bmj.com³) I presented calculations estimating that pulmonary embolism would be expected to occur by chance alone in 34 "hard-core" viewers of bowl games during the 24 hours after the games.

One could do a similar calculation to estimate the expected occurrence of pulmonary embolism among the millions of people who have travelled by air during the past 24 hours. Some of those pulmonary emboli may be caused by the conditions of air travel that favour venous thromboembolism,¹ but many are probably related to air travel by mere coincidence.

I am not arguing against the sensible preventive measures that Geroulakos recommends at the end of his editorial. Rather, I am reinforcing his call for research to determine whether air travel is a genuine risk factor for venous thromboembolism, and to identify those at risk and the factors that correlate with risk.

Ronald M Davis, North American editor, BMJ. 
Center for Health Promotion and Disease Prevention, Henry Ford Health System, Detroit, MI 48202-3450, USA rdavis1{at}hfhs.org