Cars, trains and blood clots

It is not just flying that puts us at risk of deep vein thrombosis (DVT), a new study claims, but any journey that lasts more than four hours (1).

This puts an average car trip from London to Liverpool within the danger zone. Will we soon see, in petrol stations across the nation, families pacing up and down, taking deep breaths, and slugging from bottles of Evian?

Dr Emile Ferrari, a heart expert at the Pasteur Hospital in Nice, France, has compared the travel history of 160 patients with 160 heart patients with another type of blood problem. He found that a quarter of the clot patients had been on a journey of more than four hours in the previous month; only one sixteenth of the non-clot patients had done so. This appears to show an association between travelling and DVT.

But does this mean we should all watch our travelling?

In other words, does Ferrari’s finding have relevance for the general population, as the headline ‘Blood clots risk “for any journey over four hours”‘, suggests? Or could travelling for long periods trigger DVT among the (very small) proportion of the population who are medically or genetically predisposed to develop blood clots?

According to the House of Lords select committee on DVT, predisposing factors include (2):

— Pregnancy

— Former or current malignant disease

— Blood disorders leading to increased clotting tendency

— Inherited or acquired impairment of blood-clotting mechanisms

— Some types of cardiovascular disease or insufficiency

— Personal or family history of DVT

— Recent major surgery or injury, especially to lower limbs or abdomen.

Because Ferrari only surveyed groups with circulatory problems, a highly unrepresentative sample, it would be unwise to generalise to the vast majority of us who have no such problems, nor have any significant likelihood of developing them for many years. In March 2001 a World Health Organisation (WHO) meeting of scientists, aviation experts and airlines launched a full-scale survey to look at hundreds of thousands of passengers before and after flying, to see which ones develop blood clots (3). The results from this will suggest whether most of us should take steps to reduce our risk of getting DVT on long journeys.

But on the basis of present evidence we can rest easy. According to Dr John Scurr, consultant surgeon at University College London, the risk from air travel is small and mainly affects passengers already at risk from DVT (4). The same is likely to be the case for other forms of travel.

Yet this fact does not prevent medical or political authorities from warning millions of travellers that they could be at risk, and should take ‘precautionary’ measures when travelling.

Dr Michel Clerel, chief of the medical department for Paris Airports, recently published a study which found 109 cases of pulmonary embolism between 1990 and 2000 among passengers arriving at Paris. The biggest risk factor was the length of travel, with 95 percent of the patients coming off flights that had lasted at least six hours (5). These 109 people represent a tiny proportion of the millions who have passed through Paris over the last 10 years. And those affected were hardly a random sample – their average age was 57, compared to 38 for the average traveller.

But Clerel sees fit to warn everybody: ‘We insist on safe and easy physical measures such as movement during flights and regular deep breathing, ample hydration, avoidance of smoking and alcohol.’

Indeed, even after the House of Lords Select Committee on Science and Technology had concluded in its report on DVT that ‘for healthy individuals, the risk of getting a clinically significant deep vein thrombosis solely because they are taking a flight seems to be exceedingly small’, it still recommended that passengers without predisposing factors should:

— Move around in seat and in cabin as much as practicable

— Exercise calf muscles while seated

— Avoid excess of alcohol and drinks containing caffeine (6).

Until there is sound evidence that travelling puts the general population at risk from DVT, perhaps we could be left alone to enjoy our in-flight movies and free G&Ts?

And perhaps the relevant authorities could take a few deep breaths of their own before publishing scaremongering stories.

Read on:

Jet planes and blood clots

Are you the one in four?

(1) Daily Telegraph, 9 April 2001

(2) Report by the house of lords select committee

(3) Financial Times, 13 March 2001

(4) Financial Times, 13 March 2001

(5) Daily Telegraph, 9 April 2001

(6) Reported in the British Medical Journal 25 November 2000