Suppose I'd better weigh in on this one...
Kind of a funny story - I got a DVT after I tore my calf muscle, from slipping on an oil slick at a petrol station (a few days after returning from Brisbane, but its probably not related to the flight). I couldn't walk very well on it, and the doctor said to stay off the leg so it could heal. Given that I was 23 at the time, I suppose he thought the risk of DVT was low. Trust me, you will know if you get one. It starts off as a dull ache which gets progressively more painful, so it is unbearable to stand on it,or use the muscles. I continued working, and by Friday, I started feeling short of breath, but didn't have any other respiratory / cold like symptoms. I even joked to a friend that I was dying of a DVT, and remember drawing a grave stone with my name on it on my whiteboard! haha..
Went to the doctor that night, and he said I should get to emergency, though he didn't call me an ambulance, mind you. I guess he wanted to save me the money and thought I'd make it. I made it to St Mary's Cathedral, opposite Royal Perth, before I collapsed. A hospital orderly came past, and I asked him if he could take me to emergency, but all he said was, 'It's right over there!' and walked off. I closed my eyes and was feeling quite relaxed, when a lady came past and called over some ambulance orderlies, who put me on a stretcher and got me in to emergency.
I woke up some hours later, still in emergency, with an ECG on and heparin drip in. There were 3 guys having heart attacks next to me, and they wheeled in a motorbike accident victim, with his limbs facing the wrong way (all broken) and I watched as they unsuccessfully tried to revive him and pronounced him dead. The doctor told me I was lucky to be alive, as I had PEs (pulmonary embolisms) in my lungs. The DVT had broken up and travelled through my blood stream, and eventually wound up in my lungs.
A few days later, when I was strong enough, they gave me a test (I can't remember the name of it) which involved injecting radioactive isotopes into my blood stream, while I inhaled radioactive gas, which enabled them to see the movement of the blood, and the air into my lungs at the same time, as the isotopes glowed. I watched my lungs work on the big screen. There were several dark spots, where neither air nor blood was reaching. The radiologist explained that I had several PEs throughout both lungs.
Meanwhile, I was still on the heparin drip, and had to wear TED compression stockings 24/7. I remember walking up and down the hospital hallways, several times a day, as I was encouraged to move to help it heal. Still, my breathing was terrible, so I could only manage one lap at a time.
After 10 days in hospital, the doctor said I could change to Clexane injections, which have to be injected twice daily, as they last for 12 hours. I was thoroughly sick of being in hospital by then, so said I could learn to inject if they would allow me to go home. A few days later, I was home.
This is only the beginning of a story which lasted two years. I had to be on warfarin, a blood thinning drug for two years, and was not allowed to drink alcohol. Warfarin is notoriously hard to control, as everyone reacts differently to it. I had to go for daily blood tests, every morning before work for two months, then twice a week, then finally weekly. One time I couldn't stop my arm bleeding for half an hour - that was a sign to reduce the dose.
Now it's over, but I still need to inject every time I fly for longer than three hours, wear compression stockings, and avoid alcohol 24 hours either side of a flight. It is believed that my risk of DVT was elevated due to the Pill I was taking, which I can never take again (any kind of Pill). Many women are completely unaware of this risk, as I was (isn't everyone on the Pill?!). Since then I have heard of 2 women I know on the Pill (both young and otherwise healthy) getting flight related DVT. One of my staff had a blood clot travel to her eye, and now she has a permanent blank spot on her vision, and will be on aspirin for the rest of her life. I also know one young guy who got a DVT from flying after a hockey injury to his leg. He is one of the fittest and healthiest guys I know, and it was only a domestic (5 hour) flight!
I've read many an article on the topic, and compression stockings (though they look lovely on) are one of the best defences. If you intend to sleep on the plane, wear them under your pjs - they stimulate the blood flow even when you're not moving. I have sourced some sexy black ones, that look almost cute with a skirt or shorts (think Japanese schoolgirl) but guys could easily hide them under pants. Also try not to get dehydrated etc.
I used to think DVT was a big joke, and what are the chances? As if it would ever happen to me! Only old fogies get that, right?! Admittedly I didn't get mine while flying, but now I have had one (and more importantly the more serious flow on effects) I always take measures to prevent it. Take extra care of your SWMBO, especially if she's on the Pill. Get her some travel socks as a gift. =D Sure, it's a slim chance, but it does happen... sorry for long post!