Pressure would play little role here, generally it would increas peripheral resistance as well as pressure inside the chest cavity, prompting the heart to work harder. However I doubt at a regular water immersion in a hot tub causes a noticeable effect.
Then heat causes your blood vessels to dilate in an attempt to dump the heat outside, because you are in a hottub this will end up warming your core, which will also dilate more blood vessels inside as well as raising your resting heart rate.
The net effect is slightly faster heart rate (still usually under 100, so not even in light excercise teritory yet, more like walking teritory) with a lower blood pressure.
None of this is close to wrecking havoc, yet the initial drop from getting in quickly can be fast enough, that with a high enough vagal tone (i.e perfect storm of other situations - tired, dehydrated, intoxicated, chronically tired or w/e) you could temporarily pass out. Analogous to someone passing out from getting up too quickly.
Pressure would play little role here, generally it would increas peripheral resistance as well as pressure inside the chest cavity, prompting the heart to work harder. However I doubt at a regular water immersion in a hot tub causes a noticeable effect.
Then heat causes your blood vessels to dilate in an attempt to dump the heat outside, because you are in a hottub this will end up warming your core, which will also dilate more blood vessels inside as well as raising your resting heart rate.
The net effect is slightly faster heart rate (still usually under 100, so not even in light excercise teritory yet, more like walking teritory) with a lower blood pressure.
None of this is close to wrecking havoc, yet the initial drop from getting in quickly can be fast enough, that with a high enough vagal tone (i.e perfect storm of other situations - tired, dehydrated, intoxicated, chronically tired or w/e) you could temporarily pass out. Analogous to someone passing out from getting up too quickly.