An airplane checklist - Blood clot prevention
Part 2 - Blood clot prevention
Welcome to part two of a nuanced (and somewhat neurotic) discussion of health safety on airplanes. While there are many considerations here, we are focusing on just three that came up in the examining room: infections, blood clots, and radiation exposure.
In the first post I made the case for wearing a good mask. Coincidentally, JAMA published a game-changing, record-straight-setting report about masks shortly thereafter. Of course they help. So now that we have more validation for part one, here’s what the evidence says about blood clots while flying, and how we can best avoid sustaining one.
I’m pulling mostly from two trusted sources for this post. The first is a chapter from the clinical reference UpToDate entitled: “Prevention of venous thromboembolism in adult travelers.” Access to this comprehensive resource costs doctors $600/year. The second is from the American Family Physician article entitled: “Medical Advice for Commercial Air Travel.” Membership in the AAFP costs me $775 a year, and so I get this journal.
First, some oversimplified terminology. Blood clot = thrombus = thrombosis. DVT stands for deep vein thrombosis, and indicates a blood clot located in a larger, deeper vein, which generally confers a higher risk than a superficial blood clot. Thrombophlebitis encompasses the reactive inflammation, swelling, and pain that accompany a blood clot.
Traveling for long distances increases our risks of forming blood clots, especially in the legs. Gravity makes blood pool in the lower extremities. Muscular contractions help squeeze that blood back to the heart. With prolonged immobilization or sitting, circulation becomes sluggish and a thrombus can form. Small blood clots can be asymptomatic. They occur more often than we might think. When they are small and confined to superficial veins they can be clinically unimportant, but when larger and found in deeper veins blood clots can be life threatening. The primary complication we fear is a thrombus detaching and traveling to the heart and then to the lungs to cause a pulmonary embolus.
How common are blood clots with traveling?
There are a lot of different studies with different answers. Risk is greatest in the two weeks following travel, and does not return to normal until eight weeks later! Here are some estimates from the two major sources I consulted:
The incidence of developing clinically important/symptomatic DVTs after air travel is relatively rare:
Some estimate serious blood clots occur in <0.05% of travelers. That’s 1 out of every 2,000 for those playing at home.
This is still 2-3 times the risk compared to non-travelers.
Less severe blood clots are much more common:
DVTs after >4 hours of travel have been found in 2.8% of travelers.
DVTs after >12 hours of travel have been found in 3.6% of travelers, and in up to 5.4% of high-risk groups.
most of these DVTs are small, involve the distal calf veins, and are not life-threatening or clinically important. Most would go undiagnosed without looking for them outside of research studies.
Which factors present the highest risk for developing a blood clot while traveling?
Recent major surgery (including hip or knee replacements) within six weeks. A prior history of DVT, including travel-induced. Active malignancy. Pregnancy. Advanced age. Use of estrogen-containing birth control or other estrogen preparations. Obesity.
Who needs to worry about preventing a blood clot while traveling?
With the incidence of serious blood clots in low risk travelers being so low, the studies suggest that most people don’t need to worry. But I think that most of the following measures make sense anyway, as they are not that burdensome - and who wants to get any kind of DVT if we can prevent it? At the end of the post I’ll share a table that stratifies recommendations based on low, moderate, and high risk traveler categories.
Can we please have that checklist now? Sure! I was hoping you might ask!
☑️ Move around on the plane
Don’t worry about pissing people off. The airlines did not worry about cramming you into an uncomfortable, dehumanizing, sardine can environment. The inconsiderate guy next to you is hogging the armrest (see illustration below). He’s inconsiderate at best, and a hostile sociopath at worst. All of this is not your fault. So get up and walk the aisle for 10-15 minutes every one to two hours. You deserve that more than your complimentary little snack bag which contains three micro-pretzels. Perform frequent flexion and extension movements of the ankles (calf muscle stretching) and knees (thigh muscle stretching). This keeps blood flowing and prevents clots.
☑️ Stay hydrated
We hold this truth to be self evident, as pretty much everyone tells us to drink more water for everything from kidney stone prevention to fighting wrinkles. There is no definitive proof that dehydration on a plane increases the risk of DVT, but a plausible mechanism exists in that it can interfere with the proper regulation of coagulation pathways.
☑️ Avoid alcohol on the plane
This has not been proven to reduce the risk of blood clots, but alcohol consumption reduces mobility and makes us less likely to move around, so experts advise avoiding it. Alcohol can also contribute to dehydration.
☑️ Consider wearing compression stockings
A high quality Cochrane meta-analysis of 12 randomized trials reported that the use of properly fitted, below-the-knee stockings on flights lasting >4 hours reduced the rates of asymptomatic blood clots by 90%. Leg swelling and superficial phlebitis were also reduced.
The analyzed studies typically used graduated compression stockings that provide 15 to 30 mmHg of pressure. But unless you need real medical grade compression stockings, I like the more casual and comfortable ones made by brands like Smartwool and Bombas.
☑️ If you are higher risk, consider a blood thinner with your doctor
While pharmacologic measures are not routinely indicated, it is an option for people at particularly high risk of blood clots. Risks of bleeding must be weighed against benefits of clots prevented.
For the average traveler there is a paucity of data examining the safety and efficacy of medications that thin the blood for preventing travel-associated DVTs. Data from small or retrospective studies provide insufficient evidence to support routine pharmacologic prophylaxis options like aspirin, or heavier duty blood thinners.
What do they tell family doctors to do?
Here is a sneak peak at a table from the AAFP article I linked to at the top of this post. It summarizes a lot of what I have already presented. The high risk category gets a bit technical and should be reviewed with a doctor:
Blood clots with long distance travel are fairly common when we consider studies looking for the asymptomatic ones. Fortunately most serious blood clots cause symptoms like pain and occur rarely, although still occuring 2-3 times more commonly than when we are not traveling. Simple measures to reduce our risks of forming blood clots are harmless and include moving around frequently, staying hydrated, avoiding alcohol, and wearing compression socks/stockings. There is high quality evidence for those stockings actually. For people with higher risk conditions, higher risk prophylaxis with medications that thin the blood should be considered with the help of a doctor. And jockeying for an equitable arrangement to share the armrest with the stranger next to you is a study in why humans will never have world peace.
And so concludes part 2 of this travel health safety miniseries. Stay tuned for the next installment about radiation risks and whether we should wrap ourselves in lead aprons before each flight.
Spoiler: I don’t think we should do that… although I do favor blueberries.