Let me put up a post about my difficult year. I am not sure it is related to unicycling, but I thought people should be aware of it.
I spent 4-5 days in a hospital about a year ago. The problem was a sudden failure in breathing, that was diagnosed as a pulmonary embolism (PE). I got heparin and have been on warfarin for the last year. I have recovered some of my aerobic fitness, but I will probably never get back to where I was a few years ago. I can now walk up two flights of stairs, but it leaves me winded. I can do light unicycle riding, but hills I used to laugh at are out and probably will be for the duration. In my own mind I am working on about one lung. Oh well.
Pulmonary embolisms are usually caused by a a blood clot, often formed in the leg, traveling up to the lungs and blocking blood flow in the lungs. If you block enough blood flow in the lungs, you do not get enough oxygen. More than 10% of PE cases lead to death in the first hour. I was not in that group. 
I was noticing reduced lung capacity almost three years prior to the PE event. I think it was all the same problem, but it was initially diagnosed a exercise-induced asthma. I did not respond significantly to the asthma medication. I suspect it was progressive clots traveling to my lungs and reducing blood flow. I no longer take asthma medication because it has essentially no effect.
What does all this have to do with unicycling? Several years ago I trained for and rode a number of unicycle marathons. One of them was under two hours. Yeah! I spent a lot of time in the saddle, and often after rides of two to three hours ended up with the numb butt effect. One of the primary risk factors for PE is poor circulation in the legs. I wonder if this might have been a factor. Another thing that people cite is dehydration, which is an (avoidable) problem with all endurance exercise. My long-distance unicycle riding is close in time to my early breathing problems that might indicate small PE events. My long distance riding is trivial compared to some of the serious long distance rides people on this forum engage in.
Blood clots in the legs are called Deep Venous Thrombosis (DVT) and are the principle cause of PE. My concern and my doctors concern is that I have no obvious risk factors. Obvious ones are being sedentary and sitting too much, but long-distance athletes are known to be susceptible. I sit at work too much. I do fly around the country. I was not sedentary. I sweat easily, but I was aware of the problem and tried to avoid excess dehydration. DVTs are often accompanied by pain in the legs, but I did not notice anything unusual beyond the normal pain and muscle wear I came to expect from a long unicycle rides. The only indication I had PRIOR to a life-threatening PE was swelling in my left leg. IF YOU NOTICE SWELLING in ONE LEG, GO TO YOUR DOCTOR AND GET THE SONOGRAM TEST FOR A DVT!
After a year on a blood thinner (warfarin), I hope to come off it later this month. I have had two negative sonograms for DVT over the last few months. I am doing moderate exercise and am doing some gentle road riding on my 36 unicycle. I drink water regularly with exercise, and am taking regular yoga classes.
I do wonder if my long unicycle rides (with numb butt and implied poor circulation) contributed to developing DVTs. I do not know if that is a real effect, but these days I will walk for a few minutes after every 30 min in the saddle just to be safe. I am passing this note along to the unicycling community just to make you aware of the dangers of DVTs and PE, and to encourage you to be aware of circulation and swelling issues.
Ride safe!
Scott
