A couple of weeks ago, I had a swim lesson at Aquatic Park, which is a swimming area in the San Francisco Bay. It was a warm sunny day, but the water was still cold. After warming up for a few minutes, we started with some intervals to practice open water pacing. After a fast interval lasting several minutes, I brought the intensity down to an easy level. Normally, I would start to recover right away after backing off on the pace; this time, I was struggling to get my breath back. I swam for several minutes more and did not improve. By the time the set was over, I could not swim another stroke.
Once I got out of the water, I started coughing profusely and spitting up slightly pinkish froth (sorry, a little gross). I definitely had fluid in my lungs, and felt severely short of breath. I could still walk and talk, and didn't feel like I was in any imminent danger, but the coughing did not let up; nor did the frothy spit. Weird! I had to go back to work after my lesson and continued to cough and feel very tired.
After doing some research, I identified the event as an episode of swimming-induced pulmonary edema, or SIPE, which has been infrequently reported in medical journals. My symptoms fit exactly with other described cases. Conditions which lead to SIPE have been described as cold, open water swimming or diving at high intensities. The thought is that the cold water in addition to the position of the body during swimming causes central pooling of blood; during intense swimming the pressure in the pulmonary capillaries becomes too great and fluid and some red blood cells (along with some other large proteins) break through to enter the alveolar sacs in the lung. Where there is water, no air will be exchanged so this is not a good state to be in!
I actually contacted the authors of one of the papers and we may work together to collect some more data on this condition, especially in triathletes. If it becomes severe enough, it can be fatal through drowning. I was surprised that no one in the triathlon community (with whom I have spoken) has ever heard of this, especially considering that we all swim in very cold water frequently, and may often try to swim fast. (Cold water and intense swimming are typical contributing factors.)
Fortunately, the condition seems to resolve within a week, and some careful physiological tests done in one study show that within a month, it is impossible to distinguish those who had it from those who didn't. I felt mostly better the next morning and completely better by the day after. I've been able to complete further swim workouts without any problems like that; although I still need to get back in the Bay to see how that goes. Fortunately, I won't be swimming as hard as I was in Coeur d'Alene, and the lake water temperature there is warmer than at Aquatic Park.