Thursday, September 23, 2010

6.25 High Altitude, Underwater, High Temp Affect on Heart

Physician's Notebooks 6 See Homepage
25: Heart & Lungs Response to Environment, Update 19 December 2018
The following descending list of headings in order of reading will help locate by search & find  or scrolling down.

tourist travel to altitude over 10,000 feet 
Carrying an oxygen canister if you hike over 1 mile high could be life saving 
Affects of Underwater
Affect of Increased Body Heat on Heart
Pregnancy stresses the maternal heart

IntroductionAt normal temperature, the atmosphere (1 atmosphere unit) weighs down as a 760-mm barometer column of quicksilver mercury (Hg), the standard measure 760 torr at sea level. This weight pressing on a body would suffocate, if not for the muscles of respiration and rigid chest tissue to resist its pressing. In deep-sea diving, every 33 feet in depth adds 1 atmosphere pressure and muscle-breathing motion must overcome it for oxygen-breathing to continue. Thus, oxygen must be delivered at higher than atmosphere pressure to a deep-sea diver.
   The fraction of oxygen gas (FiO2) of inspired air is 0.21 (21%). It can be further increased by increasing the pressure of the inspired air or by increasing the percentage of oxygen in the inspired air above 21%.
   The pressure of oxygen in breathed air determines the pressure of oxygen in arterial blood because the oxygen crosses into the pulmonary arteriole capillaries and then becomes fully oxygenated blood in pulmonary veins and left side of heart and in the aorta. Because of dilution and circulatory factors, a partial pressure of Oxygen in atmosphere in breathed-in air, c. 150 to 160 mm Hg, gives an alveolar PAO2 of c.105mm, and gives a PaO2 of c.95 mm Hg in freshly oxygenated artery blood when we breathe air at sea level.
   One more important measurement is the oxygen saturation of hemoglobin in fresh left-sided artery blood (SaO2 measured by pulse oximeter on finger). Oxygen mixes into blood in two forms. It dissolves physically in fluid blood plasma but only to 0.3%, not enough to satisfy the oxygen needs of tissues. So the body uses hemoglobin in red blood cell to form a loose, reversible chemical bond with oxygen, oxyhemoglobin. When all hemoglobin molecules in a red blood cell (RBC) are converted to oxyhemoglobin the SaO2 is 100%, and as oxygen gets transferred from hemoglobin to tissues during the course of blood circulation the SaO2 falls so that by the time blood reaches the veins its oxyhemoglobin saturation is 75%. (It is “mixed venous oxygen saturation”) The body normally uses 25% of its oxygen in artery blood at rest at sea level. The remainder is available as reserve for exercise or circulatory obstruction or low oxygen in breathed air as when we ascend heights.
   The relationship of the pressure fraction of oxygen that goes into our lungs from the atmosphere (PAlveolarO2), and that of fresh artery oxygen in blood in the aorta (PaO2), and also the oxygen saturation of hemoglobin in fresh artery blood (SaO2) is of importance in considering our response to change in altitude. We will want to know how altitude affects oxygenation of vital organ, especially brain and heart. And also we want to know how breathing pure oxygen at atmosphere or high pressure could improve this oxygenation. Down to PAO2 80 mm Hg, the SaO2 remains relatively unchanged, in comfortable healthy range above 95%, but below 80 mm Hg it starts to drop and below PAO2 60 mm Hg the drop becomes a plunge and may kill. 
High-Altitude: In 2019 you could be forced to breathe air at 35,000 feet (10,668 meters) in your intercontinental jet that springs a cabin leak, or up to 29,029 feet (8848 m.) atop high mountain like Mt. Everest, or at 15,000 feet (4,572 m.) up to Inca ruins in Machu Picchu/Cuzco Peru area, or at 14,000 feet (4267 m.) on Pikes Peak or at 13,000 feet (3962 m.) around Lhasa Tibet or Lake Titicaca, Peru, or at 10,000 feet (3160m) in a Rocky Mountain cabin in Colorado or on a vacation in Mexico City at 7000 feet or at 5280-foot (1609 m.) one-mile high Denver Colorado, or other place at high altitude in this jet-set world.
   It is useful to be aware what happens to blood oxygen number at varying altitude. For PAlveolarO2, there is direct connection between altitude and number so you can figure out yourself the in-between numbers. Also to focus attention on danger, a key PAO2 number is 60 (13,000 feet or c.4,000 m.). Below 60 mm Hg, which gives SaO2 of 90% at rest, the SaO2 drops off the graph and unless breathing pure oxygen a light skin person will turn blue (pigmented skin, blue in lips) and shortly lose consciousness and die. 
 Altitude: Partial Pressure Oxygen & O2 Sat of Hgb Fresh Artery Blood

(Height in Feet) (PAlvO2 in mm Hg) (SaO2 after rapid ascent)
30 to 35,000              c.30                   <40% (collapse)
15,000                   c.50                    88% (short of breath)
13,000                   c.60     90% (increase depth of breaths)
10,000                   c.80         93% (normally not noticed)
 5,000                   c.90               95% (essentially normal) 
Intercontinental commercial jets normally cruise at 35,000 feet and the cabins are sealed and pressurized so that inside is equivalent 8,000 feet altitude, which is normally not noticed. Present regulation is for crew to receive oxygen by mask if cabin depressurizes to equivalent 10,000 feet and passenger to receive the oxygen if it depressurizes to 15,000. These would mostly be sudden changes. If the aircraft suddenly lost pressure, you would have up to 1 minute to breathe before passing out. It emphasizes not to waste time, initially helping other persons. First, put on your own mask and breathe.
   Concerning tourist travel to altitude over 10,000 feet (usual for Peru Andes, some part of Rocky Mountains, and Nepal), the key word is gradual acclimatization. Anyone who plans tourism to such area should get heart and lung checkup. And no sudden aircraft ascent (to Pikes Peak, Machu Picchu, Lake Titicaca, Lhasa)! At least 2 hours is the safe time interval between sea-level and up to 10,000 feet for a normal person.
   Continuous mask oxygen is not normally needed at 3000 meters (c.10,000 feet) but pure oxygen should be available because anyone who overworks or a person in less than best health may get sudden shortness of breath that verges into pulmonary edema and deadly mountain sickness. Carrying an oxygen canister if you hike over 1 mile high could be life saving for a hiker who got a sudden attack of breathlessness that is preliminary to high altitude pulmonary edema.
   The key to working successfully at 3000 meters (c.10,000 feet) is: slow down the labor rate; acclimatize which means do not start laborers to work the full schedule immediately on arriving at 3000 m; be sure no laborer has low red blood cell level, chronic lung disease, or heart disease. No tobacco smoker can function as laborer at 3000 m. 

What about living in such area for more than a few days? This question should occur even at 1 mile high in Denver, where millions live, apparently well. What long term risk?
   First consider the red blood cell and its hemoglobin concentration. The advice above is for normal Hgb of 12 to 17 gm%. If you start off anemic, do not ascend to any height above 8,000 feet until you improve your red blood cell count. Normally persons who travel from sea level to above 10,000 feet and stay there increase the Hemoglobin concentration in the blood within 1 week by losing some fluid of the blood and then increase it more by producing more red blood cells after several weeks. Healthy testing in Cuzco Peru is average Hematocrit 50%, an increase of 10%. This is part of acclimatization to high altitude.
   The stress of breathing at such high altitude will be increased because the total atmospheric pressure to push air into mouth and nostrils is low (nearly halved at Pikes Peak) so you have to make stronger voluntary breathing muscular movement to produce a balancing negative chest cavity pressure to draw air into lungs from mouth and nostrils. So a person living at >10,000 feet develops muscular “barrel” chest (especially if born at high altitude). He also develops high pressure in pulmonary artery circulation, which at sea level might be harmful.
   What about the Heart during life at high altitude? Since transport of oxygen by the circulation depends not only on PAlvO2 but also on quantity of freshly oxygenated artery blood pumped by the left heart per minute (ie, Cardiac Output), we might expect an equivalent increase in Cardiac Output as a response to ascending to high altitude. But what actually happens to Cardiac Output, and its components, Stroke Volume (SV) and Heart Rate (HR), measured in healthy volunteers who ascend to Pikes Peak?
   By 1 hour after ascent, the exercising Cardiac Output rises from 12 to 15 L/min (+25%) and the increase is due to an increase in the exercising heart rate from max 120 to max 150 bpm and there is no change in the Stroke Volume (amount of blood the heart ejects in 1 squeeze). This shows there is no change in heart muscle efficiency (ie, Ejection Fraction); the increase is purely due to a reflex sympathetic nervous system response that ups the heart rate. Its implication is that a healthy, well-exercised person should have no trouble with quick ascent to 14,000 feet except increased HR and moderate shortness of breath with light exertion that at sea level does not cause it. But anyone who cannot increase his heart rate to maximum with exercise (older person with coronary artery narrowing, someone taking beta blocker or calcium channel blocker or digitalis medication, or with cardiac arrhythmia like atrial fibrillation or certain types of pacemaker) could experience sudden heart failure or infarction after quick ascent. But let us continue the experiment that gave the surprising long term effect.
   In the volunteers, after the initial rise in Cardiac Output due to the max Heart Rate, which peaks at 1 hour after quick ascent, the Cardiac Output drops progressively to reach a low, both at rest and during exercise; after 10 days it is 20% below the corresponding value at sea level!  Heart rate at rest returns to normal and is not the cause of this low Cardiac Output; instead, the Stroke Volume declines an equivalent 20%. This was further investigated and it was found that there is no change in the heart’s Ejection Fraction but that the dropped SV is due to decreased blood plasma fluid volume with less filling of Heart.
   What these experiments tell us is that, in a person with normal heart function, the cardiovascular system acclimatizes on ascent to heights of 14,000 feet after 10-day period by improving its red blood cell oxygen carrying capacity and oxygen extraction ability and it does so at no increase in heart work by decreasing the fluid load on the heart by progressive healthy adaptive dehydration. What it says is that if you avoid the stress of rapid ascent and take several hours to go by car, and avoid over-exercise, and if you are not anemic or do not have marked Coronary Artery Disease, your body will adapt to the altitude with little cost to coronary artery delivery demand to heart muscle. But let us go more into that based on more experimental data
   The following experiment tested the limits of heart muscle at extreme altitude with acclimatization. Eight healthy volunteers made a simulated ascent to the peak of Mt Everest, 29,000 feet in airtight chamber with controlled oxygen content and pressure. They started the ascent under atmospheric air and pressure (oxygen 21%, atmosphere at 760 torr). Over 40 days the air pressure of their ascent in the chamber was reduced progressively to 240 mm Hg, equivalent to an altitude higher than 29,000 feet. At rest and during exercise while at top of their ascent, their averaged PaO2 was 30 and 27 mm Hg respectively corresponding to SaO2 58 and 49%. Despite this extreme ascent their tests of heart muscle function were within normal limits as compared to sea level, though as previously shown their hearts worked well at 20% less expense of oxygen due to adaptive changes in blood plasma volume and oxygen extraction and utilization. This experiment is good news for old age mountain climbers. But keep in mind the key here is acclimatization and normal heart and normal hemoglobin to start with. Had this ascent simulated a rapid helicopter ascent the result might have been catastrophe.
   What about stress on coronary artery circulation during and after high ascent? This should be a question of interest to a person with Coronary Artery Disease who contemplates high altitude or mountain climb. An interesting experiment tested if low arterial blood oxygen stimulates coronary artery flow: 19 young, healthy men had coronary artery blood flow monitored and it was compared when they breathed 21% oxygen air and then when they breathed 10% oxygen air at sea level. (10% oxygen air at sea level is equivalent in artery oxygenation to sudden exposure to open-air breathing at altitude of 18,000 feet) Coronary blood flow doubled its rate of delivery to muscle.
   In interpreting this experiment you need to know that oxygenation of heart muscle requires a balance between demand and supply. Demand is produced by the increasing heart rate and strength of heart contraction (SV) and the increasing blood pressure. Supply is satisfied by increased flow of blood in coronary arteries based on their ability to dilate to meet demand.
   The message of this experiment is that if you do not have bad coronary arteries (normal maximal exercise EKG stress test) and are not anemic, you should be able to do an ascent to 18,000 feet. However, this is mainly for person who has to ascend to that altitude because of work (eg, soldier in the Himalayan war between India and China in 1970s). For tourist, it is wisest to take time for acclimatization. But it was an acute experiment of an immediate effect of ascent to the heights. Of more lasting interest is the longer affect on coronary artery circulation due to life at high altitude.
   In an experiment with volunteers after 1 week living at >10,000 feet, the initial increase in coronary blood flow they showed on acute ascent was found to decrease to the extent that after 1 week the coronary flow was 20% below what it had been at sea level! This remarkable finding is actually not surprising when we recall the decrease in Cardiac Output in similar circumstance due to increased efficiency of the heart’s adaptation to a low oxygen environment. In the case of the coronary circulation, after 1 week at very high altitude, its reduction in flow rate by 20% is due to increased efficiency of heart muscle extracting oxygen from coronary artery blood plus lower demand due to lower cardiac output. The increased O2 extraction has been proven by measuring the arterio-venous oxygen differences across the coronary artery circulation in the volunteers. This is even better news for older persons and those with coronary disease because it implies that if you allow time for acclimatization and do not initially over-exercise during first few days at high altitude, your coronary arteries will not be stressed, and in fact will not need to carry as much blood per unit time as at sea level because of increased efficiency of oxygen extraction from blood. Once again a warning! This assumes one has normal red blood cell and hemoglobin concentration (ie, is not anemic).
   But let’s go further. So far we have given experiment involving young, healthy volunteer under ideally controlled altitude ascent. What about with coronary artery disease in real world of tourism? How would I advise a 59-year-old man who plans skiing vacation in Colorado? Discussions with physicians practicing at Aspen (7900 feet), Vail (8200 feet), and Breckenridge (9300 feet) give impression of no increase of acute myocardial infarction in transient skier or other tourist. Holter monitor EKG test by telemetry on 149 men skiing at Vail Colorado between 10,000 and 11,200 feet high (92 over age 40, 19 in their 50's and 5 in 60's) showed heart rate exceeding 150 per minute (>80% of the age adjusted predicted max rate) in over half the men but not one suffered an Acute Myocardial Infarction and only one (with history of AMI 3 months earlier) showed temporary EKG changes of acute coronary heart syndrome.
Going to patients who recently suffered Acute Myocardial Infarct, a study of 9 recent AMI victims compared EKG treadmill exercise test at their normal residence in Denver (5300 feet) followed by serial repeat studies during 5 days at 10,200 feet. There was decreased heart function at the higher altitude but all the men were able to complete the test and none developed angina pectoris. (Note that all 9 men normally lived 1 mile high, which conditioned them to withstand low atmospheric oxygen levels of high altitude)
   This brings us to evidence that life at high altitude promotes Healthy Longevity. The age-adjusted death rate in New Mexico declined progressively from low to high altitude. (The higher up, the longer lived) Comparing the mortality above 7200 feet, it was 72% of the mortality rate below 3700 feet. The explanation based on autopsy result is that a lifetime at high altitude stimulates growth of coronary artery branches, and may have an anti-atherosclerotic effect, which protects person at higher altitudes.

Affects of Underwater: I only want to give useful point about holding breath if you are forced by accident under water. If you have time to take deep breath (Your commercial jet is about to crash at sea, the ferry is about to sink in accident and you can’t swim), do deep inspiration and expiration and then breathe-in to your limit with open mouth and include air that will make your cheeks puff out when you close your mouth just before going under water. The inclusion of the extra air to puff cheeks, in my experiment gives 1 minute more of no-breathing time compared to breathing without puffing cheeks if you are using sea-level atmosphere oxygen. If you have access to pure oxygen (as you may in crashing commercial jet), you will even get more breath-holding time. Of course, if you go underwater the only thing to do is get to surface ASAP. Here, holding onto flotation device is best; but do not engage in strenuous physical activity because you will use up your oxygen faster.
   A magician held his breath for the world record 17 minutes and 4 seconds: Before his attempt, he was allowed to inhale pure oxygen for up to 30 minutes. The existing breath-holding record by just preliminary several minutes deep breathing atmospheric air is 8 minutes, 58 seconds.
   What the above tells us is that if we have access to pure oxygen a 20- to 30-min- breathing of the 100% oxygen may give at least 15 minutes breath-holding time, and by just deep breathing room air, as I suggest above, a well trained person may get up to 8 minutes breath-holding time. This assumes minimal physical activity and optimal physical condition and absence of anemia or heart & lungs weakness.
Affect of Increased Body Heat on Heart:  With the first degree increase of temperature between 37 and 38 degrees Celsius (98.6 - 100.4 F), Cardiac Output more than doubles, the stroke volume and heart rate increase, and the worst aspect of it is that oxygen and nutrient-giving artery blood is diverted from vital organ such as kidney, liver, and ultimately from heart and brain to skin in order to lower the fever. Thus, fever can tip a person with borderline heart function into heart failure, can damage kidney and liver, and can make an old person with poor brain circulation delirious. And if you combine fever or hot environment with erect position and heavy exercise, you have catastrophe. Instead of wasting time explaining why, here are the orders. 1) Treat fever of inflammation quickly with acetaminophen. And be prepared to use cool towels for person whose heart may not tolerate fever; 2) When someone is ill with fever they should be in bed; 3) Never exercise for health or pleasure in hot environment, especially dry environment or if feverish.

Pregnancy stresses the maternal heart because especially as the fetus nears birth the pregnant woman’s heart must support fetal blood supply. Therefore a pregnant woman’s Cardiac Output increases markedly as a normal adaptation. For most women, pregnancy is no cardiovascular problem. In rare case where weak heart intersects with pregnancy the best management, which I have seen work is for the pregnant woman to get off her feet and rest in chair or bed (ideally head of bed slightly elevated and legs bent below knees) as soon as she learns she is pregnant, and stay there until baby is delivered. (Of course within the limits of your living situation) If all is going well, avoid CT, other X-rays and unknown risk to fetus of high magnetic field MRI. The EKG and Echocardiogram are harmless and useful if indicated by history or symptom of heart problem.
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