What happens: Sitting for 6–12 hours reduces venous return in the legs, thickens the blood due to dehydration, and promotes clots that can travel to the lungs (embolism).
Warning signs: pain or heaviness in the calves, unilateral swelling, localized heat.
How to reduce the risk: Stay hydrated, walk every 60–90 minutes, wear graduated compression stockings, choose an aisle seat, and ask if you need prophylaxis if you are taking anticoagulants or have a history of heart disease.
2) Cardiopulmonary stress due to altitude, cabin pressure, and extreme climates
What happens: Pressurized cabins are equivalent to an altitude of 1,800–2,400 meters; there is less oxygen per breath. Heat/humidity or extreme cold require rapid adjustments from the heart and lungs.
Warning signs: Unusual shortness of breath, chest tightness, dizziness, palpitations, swollen ankles.
How to reduce the risk: Choose destinations with temperate climates, take shorter trips, use a portable pulse oximeter if you have COPD/heart failure, and plan with your cardiologist or pulmonologist before long flights.
