EI has recently spent a few days living at 1400 metres above sea level. During that time we also ascended to above 3450 metres (actually to 3482m - Ed.)in the space of a day.
Whilst not as extreme as some of the experiences of mountain climbers flying from Heathrow via China into Lhasa (at an altitude of approximately 3650m), it was still pretty hard going, and we have to confess to having suffered from mild AMS, with severe headaches and breathlessness.
This is hardly surprising given that as you reach go from sea level with a barometric pressure of 101kPa (760mmHg) to 3842m with a barometric pressure of 66 kPa (507 mmHg), the number of oxygen molecules in the atmosphere decreases by about 35%. Oxygen concentration remains at approximately 21% (actually it’s nearer 20.93% - Ed.) of atmospheric, but this means that there is now a partial pressure of oxygen of approximately 14 kPa as opposed to approximately 20kPa (these numbers are just calculating aloud, aren’t they, I mean, they’re not precisely, exact….? – Ed.) (Author’s response - No, they’re just calculating aloud…can we carry on please?)
This set us to thinking about altitude and physiological changes in response to altitude, which in turn set us thinking about other aspects of altitude and anaesthesia, specifically the vaporiser question, which will be covered in another article very soon.
This one is about the physiological changes in response to altitude, which is a question that has come up before.
So:
There are a number of changes which occur.
Most obviously and the first is the changes in respiratory system and function. As the partial pressure of oxygen decreases (remember Dalton’s Law (see at the end)), chemoreceptors sensing the decreased arterial oxygen tension (partial pressure) cause you to increase your respiratory rate and depth. In other words, your minute volume and alveolar ventilation increase. As a result you feel dyspnoeic. Apparently, as you rise from sea level to 4200m you increase your ventilation by 15% (once acclimatised at rest), and as you hit 5800m, that hits 70%!
At the same time you are increasing your CO2 losses, which has a two fold effect. Firstly, according to the alveolar gas equation (see here) you push up the concentration of oxygen in the alveoli, and secondly, you develop a respiratory alkalosis, which we’ll come back to in a minute. After a brief time, the increased ventilatory rate stops increasing as the CSF pH equilibrates with the blood. A couple of days later, bicarbonate levels in the CSF have been actively reduced, and pH is restored to normal, and another increase in minute ventilation occurs as hypoxaemia becomes a stimulus to breather again.
Due to the reduced level of oxygen, the blood vessels in the lungs undergo hypoxic pulmonary vasoconstriction (a mechanism originally evolved to minimize the effects of shunt). Unfortunately, since the whole lung is hypoxic, the whole lung vasoconstricts. This is a Bad Thing ™ because you now have to pump your entire cardiac output through vasoconstricted vessels, resulting in pulmonary hypertension, and some of the vessels develop leaks as a result of increased pressure. This is thought to be one of the main contributors to HAPE (High Altitude Pulmonary Edema (US English spelling)). Another contributor appears to be mechanical stretch damage due to the hyperventilation, which causes exposure of the basement membrane, and release of inflammatory mediators, which is, you guessed it, another Bad Thing™. Overall there is an improvement in the V/Q ratio, and in the DLO2 as the blood flow increases and ventilation increases.
From a cardiovascular point of view, you increase your cardiac output to meet the continuing oxygen demands of the body’s tissues. Your heart rate and stroke volume increase. Visiting Lhasa if you have angina could be considered a serious health risk. (Interestingly, the former Holiday Inn in Lhasa has piped oxygen into the rooms…) You stimulate production of red cells and haemoglobin, which also increases your oxygen carrying capacity. If you produce too much of it, however, and fail to keep up with your fluid hydration, then the polycythaemia can result in sluggish blood flow (particularly in the cold) and additional cardiac work. This process takes days to have effect.
As you ascend, the oxygen haemoglobin (O2-Hb) dissociation curve initially shifts RIGHT at moderate altitude, because of 2,3-DPG, before shifting off to the left as your CO2 decreases, and if you are in extreme cold environments, this will also cause the O2-Hb curve to shift left.
Back to the reduction in CO2 mentioned above. As you become more alkalotic, your kidneys compensate by excreting HCO3, which is a slow pressure. Eventually the new steady CSF pH state is reached as described above. In doing this you excrete more urine. Your urine and sodium loss is also increased as a result of the cold and the hypoxia (renal vasodilatation takes place). Eventually as FiO2 drops further, however, sodium retention and an antidiuretic response occur. Maximal diuresis occurs at FiO2 between 0.12-0.14, right at around 3482m, in fact…
In the muscle, several things happen. Firstly, muscle blood flow increases, partly as a side effect of increased cardiac output, but also due to locally mediated vasodilation (there appears to be some debate about whether there is a rise in the capillaries in muscle, whether this is due to a decrease in muscle fibre size or increased capillary numbers?!….) Muscular myoglobin begins to increase. More mitochondria appear.
An interesting phenomenon also occurs. Periodic breathing is often seen. It usually occurs during sleep and accounts for a great deal of the disturbed sleep seen at altitude. Very similar to Cheyne-Stokes, there are periods of hyperventilation followed by apnoeic episodes, lasting a few seconds. Apparently it happens to everyone once they get above a certain altitude (dependent on the individual person).
That about covers the most important points.
Random aside: at lower latitudes (near the equator), atmospheric pressure calculated using Standard Atmosphere calculations are wrong, and in Chile at the top of some of the mountains, the atmospheric pressure is the equivalent of actually being 300m LOWER than predicted, which obviously affects physiology….. (Oh. Dear. - Ed)
Dalton’s Law: the sum of the partial pressures of all of the gases in a mixture will be equal to the total pressure of that mixture (Ptot = P1 + P2 + P3+….. + Pn) OR any gas in a mixture will exert the same pressure as if it alone occupied the same space as the mixture.
Bibliography
Curtis R. Outdoor Action Guide to High Altitude: Acclimatization and Illnesses [Online] 7/7/1999 [accessed 21/7/2008]. http://www.princeton.edu/~oa/safety/altitude.html
Baillie JK, Simpson A, Thompson R, Bates M, Partridge R. http://www.altitude.org/ 2008? [accessed 21/7/2008]
Hollis S. The Ultimate Travel Company High Altitude Notes [Online] Oct 2006 [accessed 21/7/2008] http://www.scope.org.uk/adventures/docs/altitudenotes.pdf
http://www.biomed.cas.cz/physiolres/pdf/56/56_779.pdf accessed on 21/7/2008
West JB, Readhead A. WORKING AT HIGH ALTITUDE: MEDICAL PROBLEMS, MISCONCEPTIONS, AND SOLUTIONS. [Online] 2005? [accessed 21/7/2008] http://www.ismmed.org/WestReadheadPreprint.pdf accessed on 21/7/2008
Thomas F. Hornbein, Robert B. Schoene, High altitude: an exploration of human adaptation; Marcel Dekker Ltd; 2001
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