blood pressure and on/off gassing

[eternaljonah]

I have been reading some materiel on decompression dives and they were talking about tissue saturation and the speed of that. this happens as the blood moves trough the body, right. now i was wondering if some one with low blood pressure would on/ off gas slower then some one with high blood pressure. does blood pressure have any thing to do with how long before your tissue is saturated till lets say x%.

 

[flots am]

I have been reading some materiel on decompression dives and they were talking about tissue saturation and the speed of that. this happens as the blood moves trough the body, right. now i was wondering if some one with low blood pressure would on/ off gas slower then some one with high blood pressure. does blood pressure have any thing to do with how long before your tissue is saturated till lets say x%.

No idea about your actual question, but if you have hypertension, I'd suggest getting it fixed before going diving.

flots

 

[eternaljonah]

i'm healthy and have no issues. people whom have lower blood pressure(not super low and hypertension) normally are in good physical condition, i wonder if having lower blood pressure would affect on/off gassing and reaching tissue saturation limits slower than some one with higher blood pressure?

 

[Bubbletrubble]

I'm not an expert on this topic, but here's how I'd attempt to answer your question...

Most experts would say that bloodflow to a particular tissue is one of the important factors in determining the rates of on-gassing and off-gassing. Those rates affect how quickly the given tissue reaches saturation.

With respect to a specific tissue, variance in systemic blood pressure (measured pre-dive to be within the "normal" range) probably plays a less important role in altering the rate of tissue nitrogen saturation than actual blood flow to and from the tissue in question. Or put another way, due to hemodynamic changes, there can exist a constant systemic blood pressure while blood flow to a given tissue is higher or lower than baseline. For instance, we know that peripheral-to-central blood shifts occur when a diver is immersed in water. We also know that exercise can induce an increase in bloodflow (vascular recruitment) to the activated muscles. That being said, not all organs/tissues possess the ability to induce vascular recruitment.

Please understand that I'm comparing people who have blood pressures in the low and high ends of the normal range -- not individuals who would be classified as hypotensive or hypertensive.

 

[TSandM]

I don't think anyone would argue against the idea that changing the amount of blood flow through a tissue will affect the amount of nitrogen on- or off-gassing. For example, studies show that gentle exercise, or being warmer, (both of which increase circulation to various parts of the body) increase the rate of nitrogen offgassing.

The problem is that blood pressure is a very poor surrogate for blood FLOW. Blood pressure is created by the interaction of volume of blood flow with resistance to that flow. Think of it this way -- if you are going to pour a gallon of water down a 4" pipe, gravity alone is enough to make it go. If you want to get the same gallon of water through a straw in the same amount of time, you're going to have to have a pump to get it done. The pressure in the straw is going to be higher than the pressure in the big pipe, but the flow rate is going to be lower.

Thus, it is often true that, as long as you are talking about people who are basically healthy (not infected or bleeding or dehydrated), the person with a low blood pressure may even have a HIGHER cardiac output than the person with mild high blood pressure.

So the answer to your question is that blood flow definitely affects on and off-gassing, but blood pressure cannot be used as an equivalent metric.

 

[Bombay High]

Jonah,
Good blood circulation will improve offgassing, but high blood pressure will likely create problems, and not speed up off gassing.
High blood pressure is a sign usually of stress or unhealthy blood vessels. Not conducive to good offgassing.

 

[lowviz]

....//...now i was wondering if some one with low blood pressure would on/ off gas slower then some one with high blood pressure. ....//...

I think that I get your question. Normal systolic pressure is capped at 119 mm Hg, crisis is 180. Blood pressure - Wikipedia, the free encyclopedia

So we are talking about 60 mm Hg max difference.

760 mm Hg is one atmosphere. So 60/760 is about 0.08 atmosphere. Thirty three feet of seawater is equivalent to one atmosphere. So we are talking about a depth change of about 2.5 feet (0.08 atm) for a blood pressure change that spans the range of normal to outright crisis.

In my non-medical, non-professional opinion, absolute BP doesn’t count for anything with regard to on/off gassing. Circulation certainly does.

 

[TSandM]

BTW, before we have a huge influx of terrified people into ERs because their blood pressure is at "crisis" levels -- let me tell you that, in the vast majority of cases, high blood pressure is a chronic disease, and the damage it does is done over time. Unless you know you have ALWAYS had normal blood pressure, and you suddenly have a very high reading (with no obvious reason -- if you're upset, scared, or in pain, your blood pressure can go WAY up, and it isn't pathological at all) there is no reason to go shooting off to the emergency room because of your blood pressure.

If you are having symptoms RELATED to very high blood pressure, like severe headache, chest pain, or shortness of breath, that's a different story. Those are symptoms of end-organ damage, and that needs to be treated more urgently.

I see people in the ER all the time who took their blood pressure at home and thought it was a little high. So they sit around for half an hour worrying about it, and check it again -- and you know what? It's HIGHER. So then they get really worried, and ten minutes later, check it again, and it's higher still. So they come in to see me. No symptoms, just high blood pressure. I don't do anything about it. Usually some reassurance, a glass of water and 20 minutes of sitting will bring it back down.

If you check your blood pressure at the same time of day on multiple days, and you are running greater than 130 for the upper number, or 80 for the lower, you definitely need to see your doctor, but you have time to make an appointment.

 

[eternaljonah]

thanks guys i am starting to get it. the whole process of on/off gassing and tissue saturation is slowly, very slowly starting to make sense to me thanks again..

 

[lowviz]

...//....the whole process of on/off gassing and tissue saturation is slowly, very slowly starting to make sense to me .....//....

It is complicated. Here is my version of a "fast and loose" walk-through of the process starting with your intro:

One’s “blood pressure” is measured on the discharge side of the heart, the arteries. It varies greatly around the circulatory system. Blood only takes about a minute for a round-trip.

In the lung, the capillary walls are only about one cell thick so the blood pressure in the capillary bed can’t be too much different from the gas pressure in the lungs.

Gas exchange the lungs is extremely fast and efficient due to the huge area of capillaries in the lung.

The concentration of gaseous nitrogen in what you are breathing vs. the concentration of dissolved nitrogen already in your blood determines whether your blood is on-gassing, in equilibrium, or off-gassing.

So in a minute or so, blood is in equilibrium with what you are breathing.

Gas is compressible. Think of pressure as being able to change the effective concentration of your breathing gas. Air is about 20% oxygen and 78% nitrogen. Pressure won’t change those percentages, but pressure certainly will change how much of either gas is in direct contact with your blood.

Now use the same thinking for other tissues in the body. No more lung, the other tissues get their nitrogen from the blood. If a tissue’s dissolved nitrogen (say in the muscle) is lower than the dissolved nitrogen in the blood, that tissue is on-gassing.

So a tissue has to off-gas to the blood then the blood has to off-gas to the lung to finally get rid of the stuff. It is all driven by the level of nitrogen in the lungs.

Last point, the dissolved nitrogen in your entire body is being held in the dissolved state by the total pressure acting on you (one atmosphere plus depth converted to an equivalent atmospheric pressure).
So the instant we start to ascend, the pressure drops and all the nitrogen starts to bubble out of us? Luckily, no. We can become super-saturated with nitrogen, but only to a point. We can withstand a reduced pressure of about half a standard atm before bubbling starts.

This is why the US Navy Dive Table #3 says that we have unlimited bottom time down to 20 feet. That would be 1+(20/33) = 1.6 atm. at the bottom and 1.0 atm upon return.

Hope this helps…