It’s a sidesplitting parody of the legend of King Arthur and his Knights of the Round Table. In one of the classic scenes, King Arthur, played by Graham Chapman, is seen galloping on an imaginary steed through the woods when he comes upon John Cleese, playing the role of the Dark Knight, similarly riding along on his non-existent horse. Words escalate and they get into a sword fight that the Dark Knight gets much the worse of, having both his arms chopped off by King Arthur. As every good knight would, he refuses to give up the duel, calling Arthur a “chicken,” whereupon Arthur proceeds to cut off both his legs too. But the brave fighter wants to carry on, insisting that it’s “just a flesh wound, ’tis but a scratch!” The fact that the brave knight doesn’t bleed to death but instead manages to clot all those wounds is obviously just a pretend movie thing; it’s also the envy of every surgeon.
The scene reminded me of an email from one of my readers and fellow pilots who asked a great question that I haven’t gotten around to talking about. He asked about blood clots and risks we might face getting one while sitting in our airplanes or on long-haul flights sitting a bit farther back and not right under the pedals. Embarking on a recent 18-hour flight my wife and I just took, I had the same question so I looked into it: what, if any, is the risk of getting a blood clot while sitting for endless hours in an airplane? It certainly happens, and living in a popular vacation spot here in South Florida I’ve seen plenty of patients who drove or flew long distances to get here and present with a blood clot and complications from it.
Abnormal clotting from inactivity usually happens in veins, since veins have more sluggish flow; it’s known as venous thromboembolism (VTE). It’s an unfortunate occurrence, but it’s common and sometimes lethal. Estimates of abnormal clotting vary in the United States and run as high as 900,000 people a year. It results in 60,000 to 100,000 annual deaths. Let’s take a look at what blood clots are all about, and just as importantly, why blood doesn’t normally clot as it zips through our circulation. It’s an important issue for all travelers, and especially for us pilots since we spend so much time in the sky.
Our earliest ancestors obviously knew there was some kind of red stuff in the body that spilled out in plain sight when they got injured. Just ask the Dark Knight. The old Greeks called it “hema” and thought that it was (correctly) synonymous with life. It comes from an ancient Greek word that means “to make red-hot,” “to roast,” or “to warm and heat.” Appropriately named, it’s the root of all the vocabulary we use to talk about blood like hemoglobin, hematologist, hematocrit, etc. Getting gored by a woolly mammoth or stabbed with a sword by King Arthur caused that warm red stuff to pour out, but what made it stop, if indeed it did, was not figured out for a long time. In the early 1800s Francis Buchanan, a Scottish surgeon and botanist, first studied the mechanism of clotting. He figured out there is a component in our blood essential for clot formation that he called “thrombin,” and that name gave rise to the medical word for clot, thrombus. Only a few years later, in the 1890s, thrombin was isolated and began to be used by the odd combination of barbers and boxers to stop bleeding from shaving cuts (probably some pretty deep ones in those days) and slashes and gashes suffered in fights.
Another key factor in clot formation is the tiny cellular elements in the bloodstream called platelets. The role of these little miracles was also studied in the 1800s by the most imminent surgeon in history, William Osler (1849–1919), the acknowledged father of modern medicine and medical education. He was the first to describe platelets, the key component in the formation of blood clots, and even (correctly) surmised that they were made in the bone marrow. The real complexity of thrombus formation, i.e., clotting, started to become clear in the late 1940s with the discovery of another key component of clot, fibrin. Around this time multiple other clotting factors were also discovered.
Whether you’re the Dark Knight after a duel or just shaving after too much morning coffee, your life depends on blood clotting after an injury or surgical procedure. Having any one of several hereditary or acquired abnormalities in your blood-clotting mechanism (hemophilia) can be life threatening. Simply making a little clot on your chin, or on your knee if you shave your legs, is an incredibly complicated cascade of events that involves 25 different factors; proteins, cell components, vessel wall components, and vitamin co-factors and pre-factors that all contribute to the end result—hemostasis—stopping bleeding. Just as important as clotting off an injured vessel is not forming clots inside your arteries or veins when you’re not injured—like when you’re sitting in a plane seat for hours. The balance between blood flowing or clotting is an incredibly fascinating and intricately choreographed biological ballet played out between clotting factors (pro-coagulants) and clot-blocking factors (anti-coagulants or fibrinolytic factors). Any imbalance in this process is life threatening and can cause blood to clot anywhere inside the body or, alternatively, not clot when it should.
The clotting system is designed to be triggered by a vessel injury that releases chemicals into the local area and attracts those sticky little platelets that Dr. Osler identified. This starts the process of literally plugging up the holes and preventing us from losing all our blood volume (exsanguinating in medical speak). But sluggish blood flow, as when we’re sitting around at altitude or any long period of inactivity, can cause an imbalance between factors that promote clotting. Several medical conditions are known to make spontaneous clot formation more likely. Among the more common risks are recent surgery or injury (high levels of activated clotting factors), age > 65, hormones (estrogen), cancer (a “hyper-coagulable” state), obesity, inactivity, smoking, varicose veins, and cardiac disease (sluggish blood flow). Add any of these conditions on top of long periods of blood stasis due to inactivity and the risk of abnormal clotting goes up.
There are a lot of places in the body where clot can occur, and as I said at the start, it most commonly is in the legs that hang down as we sit and blood flow gets sluggish. Clot formation there can propagate (embolize) to the lungs and block pulmonary blood flow, causing catastrophic and sometimes instantly fatal disruptions in circulation through the heart and lungs. This is a dreaded occurrence known as a pulmonary embolism or PE and can lead to sudden death in nearly 25% of patients who get blood clots in the lungs. It’s so common and so worrisome that we treat virtually all hospitalized patients who are inactive or bedridden with blood thinners to prevent it.
An association between extended travel and blood clotting was first documented in the 1950s. The link was considered so strong that travel-associated venous blood clots were nicknamed “economy class syndrome” in the 1970s. Further studies showed that it’s not just air travel, so it’s not altitude that’s the cause, but it can also happen with any long inactive period. It would seem that if there’s a big risk of getting a blood clot we should take something to prevent clot formation like a blood thinner before we fly, like we give hospital patients. The big question is how big is the risk to us, how can we avoid it, and do we need to take those blood thinners for a long trip?
There was a good “meta-analysis” (a review of multiple other studies) published last year that compiles data from 18 peer-reviewed papers on the risk of travel-associated venous thrombosis. Based on the data presented, the authors concluded that there is a definite association between VTE and travel and it’s related to the length of time you sit in the airplane, train, or car. After about 4 hours of idleness, the risk of forming an abnormal blood clot goes up by about 25% for every 2 hours of air travel. In the GA world that works out pretty well, since most of our planes don’t have a range of much more than 4 hours. Compiling all the study data, the authors concluded that the risks were real, but not as high as thought in the 1970s. They also said that the risk wasn’t high enough to justify taking blood thinners for a trip unless the traveler has one of the conditions mentioned above that makes clotting more likely. That’s because blood thinners can have nasty side effects too, like excessive bleeding.
The authors concluded that low side effect precautions such as graduated compression stockings, which you can get at the corner drug store, were effective, low risk, and worthwhile for clot prevention. Other things you should do to avoid VTE are keep your feet and legs elevated as much as possible, don’t cross your legs or your feet, avoid drinking alcohol (that’s dehydrating) or anything with caffeine like soda and coffee—that’s dehydrating too. Do drink plenty of water to stay well hydrated, move your legs around, and get up every few hours to do a few laps around the plane.
The risk of getting a clot is real but not excessive, so take that dopamine holiday, take a trip like Jimmy Buffett sang about in the post last month, and enjoy it. Watch a few Monty Python reruns and get a good laugh; just get up and walk around between each episode. It’s not necessary to take blood thinners because of the risk of excessive bleeding; unless you have any of those other conditions, it’s not justified in the risk-reward balance. Then you won’t have to worry about King Arthur’s final quip as he left the Dark Knight in the woods, “What are you going to do, bleed all over me?”
