Sakurako and Ruffian, you guys both have it half right.
I didn’t mean to imply that hot water immersion is not a problem for individuals with pre-existing heart problems. I just disagreed with the idea that you need the water to come in contact with the area overlying the heart to cause problems.
The truth is that anybody that has problems increasing their cardiac output or problems constricting their capacitance vessels (mainly big leg veins) and other peripheral vessels, will get into hot water if they get into hot water. The general effect of hot water is to drastically increase blood circulation to the skin. Of course this means that there is less circulatory reserve. In a healthy individual, various mechanisms compensate for the huge diversion of blood to the skin. For one thing the heart must ppick up the pace and pump ma greater volume of blood per unit time. If it can’t, myocardial ischemia or high output heart failure may result.
The reason why people pass out when standing up to exit a hot tub is that they transiently fail to properly perfuse their brains. When you stand-up, to get out of the hot tub, a reflex constriction of your capacitance vessels and reflex increase in cardiac output must occur to maintain your blood pressure and cerebral circulation. If these reflexes fail (as in individuals on B blockers, pregnancy, neuropathy due to diabetes, congestive heart failure), you pass out as you try to stand.