Take a doctor's advice on clotting risks

A young Australian woman recently died from a clot in the leg after long-distance travel. DAVID POWELL* assesses the risks.

Deep vein thrombosis is not new, nor is its association with prolonged immobility. The connection was observed in air raid shelters during the Second World War. With the advent of long-haul air transport, it was observed that DVT was more common after long flights. In the 1970s and 1980s the appealing term "economy class syndrome" appeared, perhaps misleadingly since this condition appears no less likely to occur in business class.

It is important to distinguish between what we know as a result of medical and scientific evidence, and what we suspect. We know that immobility is a risk factor.

A study undertaken by Air New Zealand in 1991 showed that DVT was more common in patients who had recently completed long-distance travel but this was common to air, road and rail travel. A French study in 1998 confirmed these findings.

We also know that there are other risk factors, including pregnancy, heart failure and certain cancers. Injuries and surgery to the lower limbs are strong risk factors, some oral contraceptives increase the risk, as do smoking and obesity.

We do not know what we can do to reduce risk, but many things make sense. Airlines encourage passengers to do lower-limb exercises in their seats and to keep well-hydrated, but no studies show these to be effective in preventing DVT.

Surgical stockings have reduced DVT risk post-surgery, so these may be effective for airline passengers, but we don't know. For those who can take it, aspirin would be expected to reduce risk and is advocated by many doctors, but the evidence is limited.

Getting up and walking around seems to make sense but in practice it can be difficult and, again, we don't know how effective it is.

Those passengers who are at high risk should consult their doctors or a travel-medicine specialist about using aspirin or other blood-thinners and any other measures they may be able to take. Breaking a long journey half-way for an overnight stop is wise, particularly for older or more frail travellers.

Much has been made of dehydration as a risk factor. The major cause of dehydration in airline passengers is excess of alcohol and caffeine. The dry atmosphere (typically 5-20 per cent humidity) causes the eyes, nose and mouth to dehydrate but several studies have shown that fluid loss to the atmosphere is relatively minor in most passengers.

The simple advice is to drink enough non-alcoholic fluid to produce urine of a light-straw colour.

Claims about reduced airflow or stale air are misguided. The reduced pressure in airline cabins causes reduced oxygen levels in the blood. This can cause symptoms in some people but this is not part of the cause of DVT.

The air quality in planes is good. The fresh air flow is 10 cubic feet a minute a passenger, producing between 20 and 30 air changes an hour - well in excess of any public building - and the measured air purity is almost as good as in operating theatres.

It has been shown that the air-conditioning systems do not encourage infections to spread and bacteria and viruses are spread directly from one person to another.

So what should airlines do to inform passengers? Major airlines already provide advice in their in-flight magazines about keeping well-hydrated, occasionally walking around and doing leg-exercises.

Some airlines are putting health information on their websites. Air New Zealand is also looking at introducing some in-flight video information about common-sense measures which may help to prevent DVT.

Since many of the risk factors relate to individuals, the advice is not the same for all travellers, so the passenger's doctor may be the best person to advise on the steps to take.

* Dr David Powell is Air New Zealand's chief medical officer.