Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: drkev on 02/05/2005 17:22:23
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Hi all, haven't posted in a while been busy. I have noticed that whenever I get out of the bath I have a hypotensive event and nearly pass out. I have asked my professors and mentors at University but not one of them can explain it.
My own explanation would be postural hypotension due to post immersion stress due to hydrostatic squeeze but I have never encountered this outside of the pre-Hospital environment and even then I have only ever seen it in the severly hypothermic casualty following immersion for long periods in cold water.
Can anyone suggest why this happens to me every time I have a bath?
Live long and Love life
Kevin Fisher
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Hi Kevin
You are spot on about postural hypotension, and I suspect hydrostatic squeeze. Also there is the heat exchange from the water to your body to take into account. This can raise blood pressure significantly. Furthermore, there is a very high humidity level in the bathroom, which also poses problems for the lungs to expel fluid into the atmosphere as you are breathing in as much water as your lungs are trying to expell.
Following the Death Of John Smith, former Labour MP, who collapsed and died following a hot bath, I personally never have the water hot and never stay in the bath longer than it takes me to get a de-coke. I could explain in a lot more detail, but the controversy here would be too much.
"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
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Hi Kev
good to see you back.
Just so that other people know what we're talking about, I'll assume that by 'hypotensive event' when you jump up from the hot bath you are experiencing a momentary loss of vision and lights in front of your eyes and a sensation of being on the verge of fainting ? This quickly passes and then you feel right as rain ?
The reason for this is two-fold. Sitting in a hot bath raises body temperature (because heat is transferred from the water to you, and your own heat-loss mechanisms - like sweating - stop working (because you're in the bath !)). The body responds to a rise in body temperature by triggering peripheral vaso-dilation; in other words, it opens up blood vessels supplying the superficial tissues, in effect turning the skin into a giant radiator to help you cool down. This causes a drop in the resistance of blood vessels to blood flow. As you'll know from your physiology lectures, mean arterial pressure (Pa) = cardiac output (CO) x TPR (total peripheral resistance). So if your TPR drops, your Pa drops. When you are lying down in the bath blood can return to the heart along the veins in your legs very easily (because it doesn't have to climb a hill) and so the heart can compensate for the drop in vessel resistance by beating slightly faster (that's why a hot bath puts your heart rate up).
But when you stand up, suddenly the blood flowing back along your legs now has to climb 1.5 metres or so back to the heart. This causes a momentary drop in venous return, which causes a momentary drop in cardiac output, which causes a momentary drop in arterial pressure (hypotension). Blood pressure dips triggered by changes in position are referred to as postural hypotension.
The consequence of postural hypotension is that it causes a momentary drop in perfusion of the retina and brain. Since the retina has the highest metabolic rate of all tissues in the body, followed by the rest of the brain, it's not suprising that the symptoms of postural hypotension are most marked in these organs and you feel transiently woozy and experience strange visual effects.
Under normal circumstances your body anticipates possible postural changes and vaso / veno-constricts different vascular beds to maintain steady perfusion. But when it is already at the limit of compensation dealing with the effects of the hot bath, the sudden change in posture can be the straw that breaks the camel's back.
Postural hypotension is a serious problem, especially for older people, and leads to thousands of falls, fractures, and other injuries every year. It is a frequent side effect of certain drugs, especially anti-depressant agents, and people with a history of postural hypotension should always pre-empt the problem and ensure that they get up slowly, or in stages, pausing to make sure that they are ok before moving on.
Hope that helps
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Thanks Chris. That's what I was thinking why are some of my lecturers so thick? We had a lecture on the sodium potassium pump and as a former Pharmacology student I knew the diagram was wrong. When I said to the lecturer I think the picture is wrong she replied "How am I supposed to know?" And this is from one of the best Universities in the country?!?!?!
Live long and Love life
Kevin Fisher
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I've lived with postural hypotension all my life and I simply make allowance for it. I can tell you now that it is never going to keep me from a long soak in the tub. I do, however, have a habit of staying in till the water is tepid.(Topping up with hot water is a no-no because I'm on tank water.) I also make sure that I have plenty of ventilation in the bathroom.