Fatal Pulmonary Embolism Immediately after Transatlantic Air Travel to the United States: Less than One in a Million

Jobs encompassing prolonged sitting in cramped positions and risk of venous thromboembolism: cohort study

OBJECTIVES

There is mounting evidence that prolonged cramped sitting in connection with long-lasting air travel increases the risk of deep vein thrombosis of the legs and pulmonary embolism, i.e. venous thromboembolism (VTE). Prolonged cramped sitting may occur even in various jobs unrelated to air travel, and, theoretically, be associated with an increased risk of VTE. The issue is unsettled.

DESIGN

A dynamic cohort study (open for both entry and departure) in Danish national registers 1997-2006.

PARTICIPANTS

20-59-year-old manual workers. The cohort consisted of an exposed group with job functions basically characterised as sedentary (n = 105,564) and a control group with more dynamic work (n = 283,966).

SETTING

Denmark.

MAIN OUTCOME MEASURES

Deaths and hospital contacts due to pulmonary embolism and deep vein thrombosis.

RESULTS

Compared with occupations characterized by dynamic physical activity, the group encompassing those who would potentially be exposed to prolonged sitting in ergonomically adverse positions, had a significantly higher risk of pulmonary embolism (n = 77, relative risk 1.28; 95% CI 0.97-1.67, P = 0.04 in one-sided test). With respect to deep vein thrombosis they had a slightly but not significantly higher risk (n = 256, relative risk 1.09; 95% CI 0.94-1.26, P = 0.12).

CONCLUSION

Despite the crude method used allowing for no control of individual risk factors, occupations encompassing longer and shorter periods of sitting in potentially adverse ergonomic positions, i.e. cramped sitting, had a higher risk of VTE, than a socioeconomically comparable group with more dynamic physical work demands.

Guidelines on travel-related venous thrombosis

Long duration travel is a weak risk factor for the development of venous thromboembolism (VTE). The incidence of VTE after flights of >4 h is 1 in 4656 and for flights of more than 8 h in low and intermediate risk flyers is around 0.5%. Severe symptomatic pulmonary embolism in the period immediately after travel is extremely rare after flights of <8 h. In flights over 12 h the rate is 5 per million. VTE may be attributable to travel if it occurs up to 8 weeks following the journey. The risk of travel-related thrombosis is higher in individuals with pre-existing risk factors for the development of VTE. There is no evidence for an association between dehydration and travel-associated VTE and so whilst maintaining good hydration is unlikely to be harmful it cannot be strongly recommended for prevention of thrombosis (recommendation grade 2, level of evidence, B). There is indirect evidence that maintaining mobility may prevent VTE and, in view of the likely pathogenesis of travel-related VTE, maintaining mobility is a reasonable precaution for all travellers on journeys over 3 h (2B). Global use of compression stockings and anticoagulants for long distance travel is not indicated (1C). Assessment of risk should be made on an individual basis but it is likely that recent major surgery (within 1 month), active malignancy, previous unprovoked VTE, previous travel-related VTE with no associated temporary risk factor or presence of more than one risk factor identifies those travellers at highest thrombosis risk (1C). Travellers at the highest risk of travel-related thrombosis undertaking journeys of >3 h should wear well fitted below knee compression hosiery (2B). Where pharmacological prophylaxis is considered appropriate, anticoagulants as opposed to anti-platelet drugs are recommended based on the observation that, in other clinical scenarios, they provide more effective thromboprophylaxis. Usual contraindications to any form of thromboprophylaxis need to be borne in mind (2C).

Travel and venous thrombosis: a systematic review

In the past decade, numerous publications on the association between venous thrombosis (VT) and travel have been published. Relative and absolute risks of VT after travel, and particularly after travel by air, have been studied in case-control and observational follow-up studies, whereas the effect of prophylaxis has been studied through intervention trials of asymptomatic clots. The mechanism responsible for the association between travel and VT was addressed in pathophysiologic studies. Here, we systematically reviewed the epidemiologic and pathophysiologic studies about the association between travel and VT. We conclude that long-distance travel increases the risk of VT approximately two to fourfold. The absolute risk of a symptomatic event within 4 weeks of flights longer than 4 h is 1/4600 flights. The risk of severe pulmonary embolism (PE) occurring immediately after air travel increases with duration of travel, up to 4.8 per million in flights longer than 12 h. The mechanism responsible for the increased risk of VT after (air) travel has insufficiently been studied to draw solid conclusions, but one controlled-study showed evidence for an additional mechanism to immobilization that could lead to coagulation activation after air travel.

[The risk of thromboembolism in airline passengers]

Long carrier flights favor thromboembolism. The risk is well known to the general public but is probably really very low. The risk is however pertinent due to the volume of air travel in the world. Sitting in the same position for a long time induced venous stasis which is the principal factor favoring thrombosis in travelers, irrespective of the means of transportation. Long carrier flights also expose passengers to specific factors related to air travel: coagulation disorders and alterations of the endothelium related to hypobarism, hypoxia and low hygrometry in the cabin. Presence of pathogenic thrombophilia is not a constant factor. The prevention of venous thrombosis in air travelers calls upon simple measures: abundant hydration, avoiding use of alcohol and tobacco, walking and dorsal flexion of the feet. The only preventive option with proven efficacy is the use of elastic contention which reduces the risk of thromboembolic events. There is no consensus concerning the use of drugs proposed by certain authors; prudence should be the rule. All passengers should become aware of the risk of thromboembolism related to sitting in the same position for a long time.

Travellers' thrombosis and economy class syndrome: incidence, aetiology and prevention

In this article we review economy class syndrome or travellers' thrombosis as it should be more appropriately described. We review the evidence for and against the existence of this condition. Much of the evidence included in this article has been justifiably criticised but it is the best evidence available to date. We conclude that there is a probable link between thrombosis and long distance travel. However this association is confined to individuals with additional risk factors and fatal pulmonary embolus is very rare. The probable aetiology of this association is prolonged immobilisation in cramped conditions. Other factors are likely to play a minor role. When considering preventative measures, for the majority of individuals the risk is very small. Simple advise can be given with no requirement for additional intervention. Compression stockings should be considered for high-risk patients. In individuals planning prolonged travel with multiple risk factors for thrombosis low dose low molecular weight heparin should be considered. The criteria for defining risk groups remains unclear and we outline our recommendations. There is conflicting evidence with regard to the use of aspirin but considering the latest evidence we do not recommend it.

The airline passenger: current medical issues

It is widely accepted that travel by air is unlikely to be a hazard to the vast majority of passengers. However, there are potentially adverse effects of cabin air of poor quality and of the reduced oxygen tension of the cabin environment. There is also the possibility of thrombosis related, at least in part, to the relative inactivity of a long journey. It may well be that the toxicity of the oil additives that are used in aircraft engines should be revisited, and that research should be carried out on the relative importance and potential interactions of the many risk factors for thrombosis that could be enhanced during a long flight. Formal guidelines need to be developed for passengers.