Last November, when I asked my friend Harold whether he’d mentioned our upcoming trip to St. Louis to the orthopedic surgeon who had pinned his broken hip together after a bike crash a few weeks earlier, I got a blank stare. We were, after all, flying, not riding our bikes.

No, he said. Telling his doctor that he was going out of town for a couple of days had never crossed his mind.

But perhaps it should have.

When surgeries are planned, doctors routinely tell patients not to travel for a month or so after the procedure. Mostly it’s an effort to stave off deep vein thrombosis, or blood clots, according to cardiologist Stuart Seides, head of the MedStar Heart Institute at Washington Hospital Center. Usually, blood clots aren’t dangerous in and of themselves. But a clot that forms in the deep veins of the leg can be deadly if it breaks off, travels to the lungs and creates a blockage known as a pulmonary embolism.

“I would certainly say that the highest risk is in the first four weeks” after surgery, Seides said. The risk is elevated for those who have had surgery below the waistline.

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Normally, the risk of developing blood clots during travel is relatively low — about 1 in 6,000 for trips lasting four hours or more, according to a landmark World Health Organization study.

But after surgery, the body’s clotting mechanism goes into overdrive. “It’s a protective mechanism,” Seides said.

I first learned that doctors like to keep recent surgical patients close to home after I blew out my knee in a cross-country ski accident a few years back. Three weeks into my recovery from knee reconstruction, my orthopedist howled in protest when I mentioned in passing that I was about to leave for the West Coast on business.

In the case of Harold, who went straight from the ER to the OR and then was under the care of a busy and unfamiliar medical team, it’s easy to see how a discussion of holiday travel plans might have fallen through the cracks.

Travel increases the risk of clotting and pulmonary embolism in several ways.

First, blood clots are linked to immobility. So the fact that most people recovering from surgery have been relatively sedentary even before they’re scrunched into a tight seat where it’s difficult to move about creates a double whammy.

“We tell people who fly to get up and walk around, but it isn’t always easy,” Seides said.

Dehydration can also promote clotting, so travelers who limit how much they drink — perhaps to limit how often they have to get up to go to the restroom — can further increase their risk. On the other hand, drinking the wrong thing, namely natural diuretics such as caffeinated coffee, beer and other alcoholic beverages — the very drinks folks tend to imbibe on a long flight — can lead to dehydration, Seides said.

The low humidity on planes further increases dehydration risk.

Other factors that raise the risk of blood clots during travel are obesity, being very tall or very short (over 6-foot-2 or shorter than 5-foot-2), use of oral contraceptives and inherited blood disorders, according to the WHO.

Although staying home is the best way for recent surgery patients to minimize their risk of blood clots, several newly developed, fast-acting prescription blood thinners such as Pradaxa, Eliquis and Xarelto may be appropriate and help reduce the danger of clotting in some patients who insist on going on the road before the risk has leveled, Seides said. All blood thinners, however, are powerful medications and should be taken only after you’ve weighed the benefits and risks with your doctor.

Planning a long trip? Here are some ways to reduce the risk of blood clots:

●Avoid travel for at least a month after surgery.

●Move as much as possible during the trip. If you’re traveling by plane, bus or train, stand, stretch and walk down the aisles. During car trips, take a break to walk and stretch at least every three to four hours.

●Promote blood circulation by exercising the calf muscles with up-and-down movements of the feet at the ankles.

●Wear compression stockings, which are designed to increase blood flow in the legs.

●During travel, avoid tight clothing and shoes that can impede circulation.

●Stay hydrated by drinking lots of water and avoid alcohol and caffeinated beverages.

Zeidner is a freelance writer in Arlington.