|MadSci Network: Medicine|
Hi Tim, To answer this one correctly, let get some basics down first. The air we breathe is not made up entirely of oxygen as you are aware. What air is composed of is 78.084% Nitrogen(N2), 20.947% Oxygen(O2), 0.934% Argon (Ar), and 0.033% Carbon Dioxide(CO2) which totals about 99.99%. Then there are trace elements in there as well. So, the inspired concentration of O2 is roughly 21%. What makes the difference in how we feel is the arterial partial pressure of oxygen which changes with altitude as based on the alveolar partial pressure of oxygen. This is figured out by the Alveolar Gas Equation which is shown as such. pA02=FiO2(pAtm-pH20)-paCO2/R pAO2 is the Alveolar oxygen pressure. FiO2 is the concentration of inspired oxygen which is .21(21%) considered to be "room air". pAtm is the atmospheric pressure which is based on Altitude at sea level this is 760 mmHg and is less at high altitudes. pH20 is the water vapor pressure which is 47. pCO2 is the arterial pressure of Carbon Dioxide, in a normal individual at sea level is 40 mmHg. R is the "respiratory quotient" which is 0.8 at room air, but is 1.0 when on 100% O2. Finally, paO2, which I'll show you in another equation, is the measured pressure of O2 in the arterial blood after diffusion from the alveoli into the capillaries of the lung. In a normal individual this is roughly 80-100 at sea level on room air. We actually need a blood sample from an artery to measure this. Where we use this equation in the hospital is to assess a person who is in respiratory distress. So what does this mean? Well, if you take a so called normal person at sea level, breathing "room air", and plug in average values for everything above you would find: pAO2= .21(760-47)-40/0.8= 149.73-50= 99.73mmHg Take that same person and put them on 100% O2 and you get: pAO2= 1(760-47)-40/1= 713-40= 673mmHg What does this mean is regards to the arterial O2 pressure(paO2)? Well, at sea level on room air we would expect this to be measured at least at 90(for a nice round value),and this gets higher with greater FiO2. The A-a gradient is the difference between the Alveolar pressure and the arterial (A-a grad= pAO2-paO2), and the expected value is roughly equal to 2.5 + FiO2(age). Remember, the paO2 is measured directly from the blood. So in a 20 year old with a probable paO2 of at least 90 we would expect the A-a gradient to be. room air: 9.73 100%: 22.5 All of a sudden that person is transported to the summit of Mt Everest (Star Trek like I guess). So Mt Everest is at 9000 meters which means the pAtm is 253 torr. pAO2= .21(253-47)-40/0.8= 43.26-50= -6.74, which means you would die of oxygen starvation. You wouldn't even have a gradient! Now you take a 60 yo man who has been smoking 2 packs per day after 40 years and check him on room air at sea level. Let's say for sake of argument his measured paO2 is 57 and he's short of breath. expected A-a for a 60 yo is about 15.1. Calculated A-a is 99.73-57= 42.73.....that is a problem. Means he is hypoxic and at least requires supplemental O2. The reason he needs it would most likely be lung disease from smoking, and/or a few other types of problems that we won't get into. So how do some people, like the Sherpas of Nepal, climb Mt Everest without O2? People who are acclimated, have a higher hemoglobin content, and other factors that allow them to breath at high altitude. They also breath at a higher rate and therefore decrease the paCO2(blow it off rapidly)in the blood stream, getting this value down as far at 7.5. Let's see how that changes things. pAO2= .21(253-47)-7.5/0.8= 43.26-8.375= 33.89 mmHg. Which would allow them to diffuse enough O2 across the membrane into the bloodstream. Of course they are very slow and confused, but it's been done. Now back to your question of 100% O2 dangers. In the hospital we commonly have people on high FiO2 because they have some defecit in their ability to breathe for various reasons. Usually if someone is really in distress we start off at 100% O2 but try to wean this down as quickly as possible. The reasons why include: 1. Increase in free radical formation which can be damaging at the cellular level. At high FiO2 the concentration of free radicals can overwhelm the cells' innate anti-oxidant defense leading to irreparable damage and cell death. 2. Absorptive Atelectasis: Which occurs when high levels of O2 "washout" the Nitrogen in the alveoli, leading to collapse of these sacs (alveolar collapse = atelectasis) and decreased perfusion space leading to something called a shunt. Shunting is basically a mismatch between the sacs that should be filled with oxygen and the flow of blood around them. With collapse of airspace, improper diffusion of oxygen to the blood occurs, lung can become damaged, again irreparably. 3. Seizures: Not a common thing seen in the hospital, but has been known to occur. More common in divers who inspire room air oxygen under pressure. At increased depth Nitrogen actually dissolves in the blood. And when you return to the surface too quickly the dissolved Nitrogen turns to gas again in the blood stream which can lead to the bends, seizures, death. 4. Cardiac: Can lead to constriction of coronary arteries and lead to damage similar to a heart attack. You are right in assuming that inhaled oxygen through a nasal cannula is not 100% because it is mixing with air. The Oxygen coming through the cannula is 100% pure, but the percent that it actually increases the FiO2 is related to how fast the oxygen is flowing, with each liter per minute adding about 4%. So even at 10L per minute, which is about the fastest flow for nasal cannula, the total FiO2 gets to about 60% only. Masks are the next step, of which there are different kinds that restrict the amount of room air so have a higher amount of pure oxygen per room air. The final mask before intubation is the Non rebreather, and if someone fails this mask then they get intubated as quickly as possibly. A ventilator is then used to control respiration and oxygen content in a variety of different ways which is a whole other ballpark. So, does your breathing oxygen at an oxygen bar cause any harm or benefit. Well, I have no idea. I don't know of any studies on how supplemental oxygen at low level in a normal individual might have benefits or risks. To see what the FDA says about oxygen bars visit http://www.fda.gov/fd ac/features/2002/602_air.html. Without any type of scientific investigation I would just be hazarding a guess, which doesn't help you very much. All I know, is that in a healthy individual without any history of lung disease, or other factors that may put them at risk for lung injury such as smoking, room air (21%) should be fine unless you're planning to climb Mt Everest. In that case you're going to need more than an oxygen bar. Hope that explains things without being too much. It's kind of a difficult subject matter even for some Dr.s. Finally, just remember, no smoking in an Oxygen bar, preferably ever. Mark Sullivan References 1. Information on oxygen by nasal cannula for patients. http://patients.uptodate.com/topic.asp? file=copd/14435 2. Gilbert, DL. Oxygen: An overall biological view. In: Oxygen and Living Processes, Gilbert, DL (Ed), Springer-Verlag, New York, 1981, p. 376. 3. Comhair, SA, Erzurum, SC. Antioxidant responses to oxidant-mediated lung diseases. Am J Physiol Lung Cell Mol Physiol 2002; 283:L246. 4. Fridovich, I. Oxygen toxicity: A radical explanation. J Exp Biol 1998; 201:1203. Kind of a cool one from a long time ago: 5. Comroe, JH, Dripps, RD, Dumke, PR, Deming, M. The effect of inhalation of high concentrations of oxygen for 24 hours on normal men at sea level and at a simulated altitude of 18,000. JAMA 1945; 128:710.
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