underwater photographer and fish

 Increases in Barometric Pressure: 
The physiological effects of diving

Just as the barometric (atmospheric) pressure decreased as we increased our altitude above sea level, it increases as we move below sea level.    Although we no longer find that getting oxygen into or carbon dioxide out of our systems to be problematic, there are special considerations that need to be taken into account whenever the human body is subjected to increased barometric pressure.  Although I will use the example of diving as the prototype for an increase in the barometric pressure, the same issues are confronted in any hyperbaric situation (hyperbaric chambers). 

Quantifying the increase in barometric pressure:  Luckily for us, it is quite easy to calculate the barometric pressure at any given depth of a dive (this was not the case for what happens at altitude - if you'll notice on the graph presented in that section, the total atmospheric pressure does not follow a simple linear relationship - it is curvilinear).  The important fact to know:  For every 10 m below the water surface (sea water) you go, the barometric pressure increases by 1 atm. Let's say a diver descends to a depth of 40 m: 

swimmer with fish
 40 m x (1 atm/10 m)
 40 m x (1 atm/10 m)
40 x (1 atm/10)
= 4 atm

    BUT WAIT!!!!  The 4 atm listed here is the additional pressure resulting from the column of water on top of us.  We have neglected to account for the air (atmosphere) that is sitting "on top of" the column of water sitting on us.  Therefore our calculation is not quite complete: 

swimmer with fish
Total barometric pressure = (Pressure due to water) + (Pressure due to air) 
= 40 m x (1 atm/10 m) + (1 atm)
= 40 m x (1 atm/10 m) + (1 atm)
 = 40 x (1 atm/10)+ (1 atm)
= 4 atm + (1 atm)
= 5 atm

Remember:  Always add on the 1 atm of pressure produced by the atmosphere on top of the water!  (hint - forgetting to add on that 1 atm. is a common way to get a test question wrong!)

(Just to be complete - fresh water is different than sea water and the pressure resulting from submersion in fresh water is 1 atm for every 10.4 m below the surface.  I do not expect you to know this number, but you should know the sea water calculation).

Effects of Hyperbaric conditions:

As we noted above, the problems associated with an increase in barometric pressure are very different from those produced at altitude (where our difficulties are the result of the hypoxia and eventual respiratory alkalosis produced by the decreased atmospheric pressure).  Not surprisingly, the problems we encounter under hyperbaric conditions are the result of getting too much of the gases in our systems. 

Effects of hyperbaric pressure 1:  Oxygen

Oxygen is a double-edged sword - on the one hand, it is absolutely required for our survival.  In excess, however, it has toxic effects and can damage multiple organ systems.  Oxygen toxicity results in irritation of the tracheobronchial tree, nasal congestion, sore throat, coughing, muscle twitching, tinnitus (ringing in the ears), dizziness, convulsions, and death.  These effects are due to the formation of large amounts of the superoxide anion (O2-) and peroxide (H2O2).  Both of these are highly reactive species (especially the superoxide anion, which is a free radical) and are toxic to cells (in fact, we have multiple enzymes to protect ourselves from these species).  Under hyperbaric conditions, the production of these species exceeds the ability of the cell to protect itself. 

The development of these symptoms is directly related to the FIO2 and the total pressure to which you are exposed.  If given 100% oxygen at 4 atm of pressure, half the people exposed will develop serious symptoms of oxygen toxicity within 30 minute.  At 6 atm of pressure, convulsions develop within minutes.  At 1 atm pressure (sea level) 80 - 100% oxygen produces respiratory tract irritation within 8 - 10 hours (but the more severe side effects are limited). 

Despite these concerns, hyperbaric oxygen treatment is useful in treating certain conditions (carbon monoxide poisoning, injuries resulting in or related to decreased perfusion etc...).  Exposure to 100% oxygen at 2 - 3 atm for less than 5 hours can greatly increase the PaO2 (~ 2000 mm Hg) and increase tissue PO2 without toxic side effects (or minimal side effects). 

If given for prolonged periods, supplemental oxygen at 1 atm of pressure can have severe side-effects, particularly in infants.  These include the development of bronchopulmonary dysplasia (abnormal lung growth, particularly the presence of lung cysts and densities) and retrolentil fibroplasia (retinopathy of prematurity).  It appears that the pulmonary effects are the result of oxygen toxicity, while the retinopathy is related to disordered capillary growth because the normal oxygen gradient is disrupted by the increased oxygen.  Retinal development proceeds from the center of the retina to the periphery under normal conditions.  Since the maturing rods and cones have a much greater oxygen consumption than the immature receptors, there is an oxygen gradient in the retina (low oxygen in the areas of mature/maturing  receptors, more oxygen to the periphery).  This gradient triggers the growth of new capillaries.  With oxygen supplementation, we lose the gradient and the growth of the capillaries is disordered. 


Effects of hyperbaric pressure 2:  Nitrogen

Although we've not really discussed it, the air we breathe is 79% nitrogen.   Just like oxygen, nitrogen in our alveoli come to equilbrium with the arterial blood and we can measure a PaN2.  We've ignored it up until now because, at sea level, it is an inert gas in our systems.  

At increased barometric pressures, nitrogen becomes problematic for us.  More of it dissolves in our plasma and it does start to exert effects on these.  The first of these is nitrogen narcosis (also known as the "rapture of the deep").  At higher concentrations, nitrogen exerts an effect very similar to alcohol on the neurons of the CNS.  This can cause serious problems when you are submerged - there are reports of divers dying because they removed their SCUBA apparatus while underwater.  

Nitrogen also becomes problematic for us during the ascent from a dive.  As I noted above, more nitrogen is dissolved in our plasma (and all our bodily fluids) while we are submerged.  As we ascend, the barometric pressure decreases and so the extra nitrogen in our bodies must leave our bodily fluids.  If the ascent is slow, the excess nitrogen has plenty of time to get to the lungs and convert to the gas state there.  If, however, ascent is rapid, the nitrogen leaves the dissolved state and becomes a gas while still in the bodily fluids/blood vessels.  The resulting bubbles block circulation and are very painful, producing the bends or decompression sickness.  The symptoms include severe pain (resulting from bubble formation in the tissue, particularly the joints) and neurologic symptoms such as parasthesias, itching, and in extreme cases, paralysis (which may or may not be temporary).  These symptoms usually appear within 30 minutes of surfacing.  Treatment in a hyperbaric chamber can be used to force the gas back into solution and then control the rate of decompression to allow the dissolved gas to be exhaled.  As a note:  decompression sickness can result from any rapid decompression - so losing cabin pressure in a plane at very high altitude will have the same effect.  

Effects of hyperbaric pressure 3:  Air Embolism

alveolar capillaries in cross section

During the ascent from diving, the glottis must remain open so that the expanding air can leave the lungs.  If the air is not allowed to leave the lungs during the ascent it will find another place to go.  Unfortunately, the only other place it can go is into the blood.  If a diver panics and rapidly ascends with a closed glottis, the pulmonary veins may rupture and air can enter the blood stream, creating an air embolus.  This can be a fatal event and has been reported with ascents from as little as 5 m below the surface. 

This is the end of the online lecture. There is a practice quiz but it is not required.