Rough year in Colorado. Its a good reminder that people with lung or heart problems should consult a doctor before rafting in cold water. I also advocate a long talk with your crew as its sadly a predictable outcome with people with those health problems. I know of at least one of the best rafters I know who went into semi-retirement for this very reason.
The physiology of cold shock is relatively simple: a person falls into cold water while working hard (normally) and one of the body's first responses is to narrow the arteries (vasoconstriction) which puts an enormous burden on a weakened heart. The heart compensates by pumping harder which can lead to arrest. As I understand it this can also increase the chance of strokes but I am not as familiar with that process or outcome.
Evidently doctors and scientist are studying something known as "Autonomic Conflict
" which may change the way deaths are categorized as they believe many "drownings" are more a result of heart related problems than we previously believed.
More and more it makes me believe that we are often more at risk during the summer when temperatures have risen but water is still cold compared to fringe season boating when air and water temperatures are closer. I actually think summer grand trips are one of the most dangerous in this category when the difference can be as much as 60F.