There are two groups of veins that move blood from the lower extremities back to the heart. The first system is superficial and consists of veins lying close to the surface; the other larger system is the deep venous system which, as its name implies, is deep within the leg. The superficial veins communicate with the deep veins and eventually drain into the deep veins near the groin. From there the blood flows back to the heart.
Have you ever wondered how blood in your lower extremities manages to overcome gravity to return it back to the heart? It’s a complex process that involves the pressure generated by the heart’s contractions, valves in the veins that prevent blood from moving in the wrong direction, and contractions of the muscles of the leg that compress the veins and force blood upward against the opposing force of gravity. Because inactivity such as prolonged sitting results in few, if any, leg muscle contractions, persons who are sedentary may have blood pooling in the veins of their lower extremities because that blood isn’t being pushed up towards the heart efficiently.
Pooled blood is more likely to clot and this can result in serious issues. Clots can form in either the superficial or the deep veins. When clots form in the superficial veins, there is usually pain in the leg as well as swelling. Often one can feel the clot along the course of the vein. These superficial clots are usually associated with inflammation and the condition is called thrombophlebitis – “thrombo” means clot, “phleb” is a prefix for vein and “itis” signifies inflammation. While superficial thrombophlebitis may be uncomfortable and result in significant swelling of the leg, as well as long-term leg problems, it is not potentially as serious as deep vein thrombosis, also known as DVT.
The deep veins are quite large and if a clot forms in them, the clot can also be large. In addition to the potential for leg swelling with a DVT, the real risk is that the clot could become dislodged and travel up to the heart. From there, it would be pumped into the pulmonary artery, which takes blood to the lungs to be oxygenated. The clot can completely block the pulmonary artery thereby preventing blood flow to the lungs. This makes oxygenation impossible and the result is sudden death. A clot that forms in one place and moves to another is called an embolus, therefore this condition is called a pulmonary embolus.
What are some of the risk factors for DVT and why is this condition something pilots should be concerned about? While some risk factors like an inherited blood clotting disorder, various malignancies, recent surgery and an age over 60 are not in our control, other risk factors are. Smoking increases the risk, as does obesity. Pilots taking hormone therapy (including birth control pills) are also at higher risk. Dehydration also increases the chance of a DVT. But most important for pilots, prolonged sitting can increase the risk of DVT significantly. Not only are your leg muscles not contracting while you are sitting, you are often putting pressure on the leg in the lower thigh which can help compress the veins and decrease flow; pilots tend to sit for a long time in one position.
It’s important to try to prevent DVTs. Pilots can lower their risk by staying hydrated and exercising their leg muscles at frequent (every 15-30 minute) intervals. This is done by pushing down first with the toes and then with the ball of the feet to contract the muscles of the calf. Walking every half hour is ideal and while this can be done in an airliner, it’s impossible to do in most light GA aircraft. Additionally, the use of properly fitting compression hose (flight socks) may help. These socks are graduated in their compression and must go up to the knee for maximum effectiveness.
Many pilots take low dose aspirin for prophylaxis against heart attacks and wonder if the anticoagulant effects of aspirin will help with DVT prevention. The short answer is probably not. Aspirin inhibits the action of platelets which are important in arterial clotting but have little to do with clots forming in veins. There are prescription anticoagulants that do decrease the risk of DVT, but these must be taken continuously and there are significant potential side effects. Their use is reserved for persons at high risk such as those with a previous history of DVT, pulmonary embolism, certain cancers and postop of certain surgeries. Pilots using these drugs must have FAA approval via the Special Issuance process and require frequent monitoring. Furthermore, flying is prohibited until at least six weeks after the initiation of therapy, so they are not a reasonable choice for routine DVT prophylaxis.
Finally, be aware of the potential for deep vein thrombosis. If you notice swelling of one leg, or pain in the calf after a flight, consult a physician as soon as you can. Untreated DVT can be fatal.
The items discussed in this column are related to experiences by Dr. Seckler in his many years as an AME, and made hypothetical for the article. Any information given is general in nature and does not constitute medical advice.