THE 36 MOST COMMON DIVING PHYSIOLOGY MYTHS
by
Dr. Jolie Bookspan
Misinformation can be fun on April Fool's, but not in diving. Here are 36 of the most common divers' myths, mistakes, and mix-ups about gases, heat, cold, immersion, fitness, nutrition, and decompression. How many do you know?
DECOMPRESSION MYTHS
1. HALF-TIMES. Are half-tines just theoretical numbers made up to pretend to explain nitrogen in your body after a dive, or do they really exist? They are real and not just for nitrogen. Many substances such as carbon monoxide, drugs, and alcohol enter and leave your body in real and measurable units of time called half-lives, or half-times. For example, Novocain has a short half-life to work quickly and then stop affecting you quickly. Valium's half-life is longer, about 24 hours, allowing its effects to persist in your body longer. The very long half-life of carbon monoxide makes it difficult to get rid of in your body. Hyperbaric oxygen treatment is used for carbon monoxide poisoning because of its effects is to reduce the half-life. In diving, nitrogen has been experimentally demonstrated to enter and leave your body in half-time fashion.
2. BUBBLES. When bubbles form in your body after a dive, do they form in your tissues and pass into your circulation? Probably not. Bubbles, even though tiny, are too big to physically pass through blood vessel walls. They may dissolve for passage through vessel walls then reform into bubbles, but they are too big to fit through your vessel walls.
3. SLOW TISSUE. Is fat a slow tissue because it is not very vascular? Not at all. Fat is very vascular to be able to furnish your daily energy during rest and exercise. You grow a mile of extra blood vessels for every pound or so of fat you gain.
Fat is a slow tissue due to gas solubility. Because of high gas solubility, fat holds much nitrogen and takes time to uptake and offgas it all. This is a property of fat and is true even for fatty areas with the same degree of blood supply as leaner tissue.
4. TABLES. Here's an oldie, but still common one: The US Navy tables risk of decompression sickness incidence. Used properly, the Navy standard air decompression tables do not have a 5% risk. The incidence is less than 1/10 of 1 %. A very small risk.
IMMERSION MYTHS
5. THE "P PHENOMENON". When you get in the water and feel the urge to 'go,' is that all 'in your head'? Not at all. It occurs from several physiologic mechanisms, and becomes stronger as the water gets colder. It is also not true that if you put a sleeping person's hand in a glass of water they wet themselves. (But it is always worth a try for the sake of science.)
6. DIVING REFLEX. When a child survives after 30 minutes under icy water was the dive reflex responsible? Does the dive reflex extend underwater breath holding time? Does it protect your brain against low oxygen states? No to all three. Although the dive reflex protects marine mammals from hypoxia, it does not reduce the human requirement for oxygen or extend underwater breath holding time. Human survival after very cold water drowning is due to the cold exposure not the dive reflex. In humans, the dive reflex reduces heart rate and reduces blood flow to your arms and legs, primarily as a protection against cold.
7. SHALLOW WATER BLACKOUT. You may have read that "shallow water blackout" means passing out from breath holding, due to decreasing oxygen levels. But the term "shallow water blackout" originally was used in 1944 by Barlow and MacIntosh for something very different - blackout from carbon dioxide retention. During World War II British divers using oxygen rebreathers were passing out without warning. The Royal Navy called it *shallow* water blackout because these rebreathers could only be used in shallow water because of the high oxygen content. Most of the cases weren't deep enough to have been O2 toxicity, which had previously been the prime suspect. The problem subsided after improving the carbon dioxide absorption canisters. Although the term already had an established meaning, it was later applied to unconsciousness from too low oxygen (hypoxia) in breath-hold diving, especially following excessive hyperventilation. The mix-up has perpetuated into common usage.
8. VALSALVA (OR VALSALVA'S) MANEUVER. You were probably taught that the Valsalva maneuver is a technique to equalize, or "pop" your ears. It is described as done by breathing out against a closed mouth with nostrils pinched shut. That forces air from your mouth up through your Eustachian tubes to your middle ear, increasing air pressure on the inside of the eardrum to match increasing water pressure on the outside. But it is probably technically incorrect to call this a Valsalva maneuver.
The Valsalva maneuver is named for Antonio Maria Valsalva (1666-1723), Italian anatomist. The technique originally described by Valsalva was to forcibly exhale against a closed glottis, by closing the vocal cords together, as in a cough. This technique would not equalize the ears. Now, both techniques are commonly called a Valsalva maneuver.
Either technique may increase pressure in the chest cavity, impeding venous return of blood to the heart, and because of that, is often used to study cardiovascular effects of decreased cardiac filling and output. It is possible that English physician Joseph Toynbee (1815-1866) may have developed the maneuver for exhaling against a closed nose and mouth. To make things confusing, Toynbee also developed a different, gentler equalization method that we call the Toynbee, consisting of swallowing with the nose and mouth shut.
HEAT MYTHS
9. SWEAT GLANDS. Who has more sweat glands, men or women? Neither. It's a popular myth that men have more. But although men often seem to have many more, if you look at cellular anatomy studies you will find women have no fewer than men.
10. SWEATING. If men sweat more than women will they be cooler in the heat? Usually not. Evaporation of sweat is more important to cooling than just sweating. Many men are capable of sweating more than can be evaporated. Not only does women's lesser sweating put them at no greater risk of overheating, the conserved fluids and electrolytes are to their advantage.
11. SUSCEPTIBILITY TO HEAT STRESS. Who is more susceptible to heat stress men or women? Women are not more susceptible than men, whether they sweat less or not. Women have several effective cooling mechanisms beside sweating. Early studies compared out-of-shape women to in-shape men yielding false conclusions about women that perpetuate today. The person usually at risk in the heat is the large, heavy, male. You have probably seen such a person red faced and streaming sweat when others were comfortable.
COLD MYTHS
12. COLD AND DCS. Does cold make you more susceptible to decompression sickness? No. And yes. Cold decreases your body's ability to take up and give off nitrogen. Being uniformly cold throughout your dive may not increase decompression risk. However a diver starting a dive warm could absorb more nitrogen than in a cooler thermal state. If that diver then chills, common toward the end of a dive, eliminating that additional nitrogen gas burden slows, possibly increasing decompression sickness risk.
13. SIGNS OF HYPOTHERMIA. Which of the following mean you have hypothermia? Shivering, cold hands, cold feet, blue lips, weakness, teeth chattering, feeling miserable and cold, dexterity decrements, numb fingers? None of them. A core temperature below 95 degrees F (35 degrees C) determines hypothermia. Studies finding cooler skin in women than men does not mean women are more susceptible to hypothermia. Cool skin helps stop heat loss by lowering the gradient from your skin to the outside. It is one of several advantages women have in the cold.
14. OCCURRENCE OF HYPOTHERMIA. How common is hypothermia in cold water diving? A constant hazard? The cause of most cold water diving accidents? Hypothermia is not at all common in divers. Getting cold is very common. Chilling to the point of endangering your health can occur long before hypothermia.
15. GETTING WARM. If you pour warm water in your suit will that help you stay warm? Yes. The additional heat gained is important for rewarming. You will be warmer than before and will build back a heat reserve, an important part of cold water diving. It is also not the case that you should avoid wearing a coat indoors if you are cold, on the assumption that you will be colder, once outside. The heat you gain is beneficial and gives you that much more heat to lose in the cold. Even if you get warm enough to sweat a bit, you will not lose more heat than you gained. You will still be warmer than you started.
16. FAT AS INSULATION. Does fat help you in the cold? Yes and not just if you are obese. Fat is one of your major protections against cold. Any amount of fat you have under your skin is helpful protection against cold. Thin people usually begin shivering in water one to two degrees warmer than better insulated people.
Body insulation increases directly with the average thickness of the fat layer under the skin and with deep body fat. People with thicker fat layers lose less core heat at rest and during exercise both in cold air and cold water. Thicker people tolerate a lower temperature before shivering, and their core temperature does not drop as fast during swimming in cold water compared to thinner people. Thin people raise their metabolic rate higher than fatter people in a none too successful attempt to keep as warm as the more calorically challenged. There is no question that the advantage is to the young and the globular.
17. SURFACE AREA TO MASS RATIO. How much does the surface area to mass ratio matter in the cold? It is not the deciding factor in human heat loss, as in smaller animals, and especially not in cold water. Whether a woman or man has a larger ratio will not determine their susceptibility to cold. There are too many other variables. The ratio is independent of gender and varies little from one human to the next except for large size differences such as between child and adult.
18. LOSE MOST OF YOUR HEAT FROM YOUR HEAD? This is a popular myth. Head heat loss is not the majority of heat lost. Not even close. The heat you lose from your head is small compared to the rest of your body, and varies with temperature and exercise.
Head heat loss is linear with temperature, meaning the lower the temperature, the higher percentage head heat loss. At 0 degrees Centigrade, up to about 30 to 35% of heat could be lost through your head at rest. When exercising at about a work rate of 50% of aerobic capacity, head heat loss falls to less than half that.
Even though head heat loss is less than one-third to one-fifth of total heat loss, that is a good amount considering that your head is only about 7-9% of your body total surface area. (But, some people have bigger heads than others in proportion to their stature.) To reduce heat loss, wear a hat.
19. ALWAYS COLDER WITH EXERCISE IN COLD WATER? It is popular for divers to say that exercise always makes you colder in cold water. Not so. In general, it is easier to chill than overheat in the water. However, exercise in cold water can generate enough heat to match or surpass the heat you lose. It is also possible to overheat, as swimmers doing laps in warm pools and divers sweating into their masks can tell you. One Navy study looked at overinsulated divers swimming in cold water and found they needed a bit of heat extraction to prevent overheating. Desert Storm divers in the Persian Gulf tried wearing ice vests for heat extraction. It is true that exercise makes you lose more heat than if you were not exercising, but it is very important to remember that losing heat does not mean that you are chilling. Exercise also generates heat. It all depends whether you lose more, or gain more. Either can happen, depending on many variables.
20. COLDER BREATHING HELIUM? There is much discussion whether you get colder breathing helium than breathing air. Helium has greater thermal conductivity than air. Undeniably, you lose more heat when surrounded by helium than by air, because heat conductance is the major factor in skin heat loss. Therefore helium is not used in dry suits. However, respiratory heat loss depends on heat capacity, and not at all on conductance. The thermal capacity of helium per gram is higher than that of air. However, there are fewer grams of helium for the same volume breathed because it is far less dense, making thermal capacity less compared to the same volume of air. Less heat would be lost breathing helium, so it should not chill you to breathe, as commonly thought. In a helmet or full face mask, your face may feel cool, making it hard to separate out the lesser loss through breathing.
Depth affects gas density, and so, heat loss through the breathing medium, and to be more confusing, you also need to account for interactions of respiratory heat loss through convection and evaporation. With helium you may also be more aware of the cold that is so common in diving, than when dulled by narcosis while breathing non-helium mixes. Remember too, it is generally not feasible to breathe air at depths where helium is used, so hard to compare in actual use. The short answer seems to be that breathing mixtures of helium at depths encountered by technical divers does not seem to result in greater cooling than breathing air. Helium feels colder to your skin than air, but it carries away less heat when you breathe it. So there.
DIVING FITNESS MYTHS
21. UNDERWATER HEART RATE. It's true that your heart rate is lower in the water than while standing in air. Your heart rate is also lower while swimming than doing comparable exercise on land. Are you getting less of a workout? Luckily, heart rate is not the only indicator of work intensity. During immersion the amount of blood returning to your heart increases for several reasons. The increased blood volume produces a reflex drop in heart rate. However the total blood volume output from your heart along with several other measures of work intensity can equal that of comparable land based exercise. Your water exercise can be as much a workout as on land but without the impact. It all depends how hard you work.
22. BONES. Is the reduced weight bearing and impact of water workouts better for your bones? In certain cases of orthopedic illness or injury, often probably yes. But remember that on land, the longitudinal weight of your body is part of the mechanical loading your bones need to keep and build density. Muscles pulling against bone during weight-bearing resistive exercise is crucial to keep your bones dense and strong. Exercise os one of the most important components of an osteoporosis prevention program.
23. STRENGTH. Who has greater arm strength? A man with larger biceps muscles or a women with smaller biceps? Believe it or not, there's not enough information from that description to tell. Strength is not determined just by cross sectional area of a muscle. Women and older men increase strength more through neural adaptations than size increments. The biceps of a woman or older man may be larger or smaller than that of a comparably strong young male.
24. FAT. Who has more body fat on average, men or women? A 120 pound woman with 20% fat carries 24 pounds of fat. A 180 pound man with only 15% fat tops that with 27 pounds of fat. A 190 pound man would have 28 1/2 pounds of fat. It's not yet known whether percentage fat or absolute fat amount is more problematic to decompression issues - if either are important - another area that is still unknown, but prone to myths.
25. FLEXIBILITY. Muscle bound? Exercise and weight lifting do not make your muscles stiff and inflexible. Inactivity is the culprit. It is also a myth that swimming, in itself, stretches you. To stretch you must do range of motion work - in other words, do stretches.
26. SWIMMING WITH WRIST WEIGHTS. Are hand and ankle weights the way to get more out of your swim? No, they are ineffective and potentially troublesome. Like shadow boxing with hand weights, the resistance of the weights is in the wrong direction. Boxers and swimmers are better off with resistance devices that oppose forward motion, not weigh your arms down so that you build patterns that lift upward. Ankle weights for kicking have similar problems. Hand and wrist weights in a pool can throw off form, and contribute to shoulder injury. Tying weights on is also not the safest thing to do in the water without an air supply. To increase swimming fitness, increase your resistance against forward movement by wearing drag suits, webbed gloves, tethers, dive gear, or commercial or home-made resistance devices.
DIVING NUTRITION MYTHS
27. SPORT DRINKS AND DEHYDRATION. A common rumor in diving is that sport drinks like Gatorade will dehydrate you, and are therefore bad for divers. Sport drinks will not dehydrate you. They are specifically formulated to rehydrate you and do a good job of that. If you like them, go ahead and drink. In hot weather don't forget to drink extra water also.
28. DILUTION AND QUANTITY. Must you dilute sport drinks? Should you only drink a small amount from a small cup? No to both. Although there is nothing wrong with diluting commercial sport drinks to suit your taste, they are specifically formulated to be helpful straight from the bottle. For rehydration in the heat the more you drink, the better.
29. PROTEIN. Do divers need lots of protein? Not unless they want to increase their risk of dehydration, fat gain, and osteoporosis. Most people in Western culture--many vegetarians included--eat double to triple the protein they need every day. Extra protein does not convert into muscle any more than the protein that helps manufacture skin pigment would turn you brown. Muscle needs very little protein for growth. Excess protein, especially animal protein, leaches calcium from your bones. Then it converts into fat for storage. It can't turn back into protein later. The breakdown products are later excreted in a process that is tough on your kidneys and takes large amounts of water with it. For good nutrition, moderate your protein intake, and emphasize vegetable sources.
GAS MYTHS
30. CARBON DIOXIDE. Does high pressure carbon dioxide smell like acid? No. It is when carbon dioxide mixes with water, in or out of your nose, that it produces carbonic acid: CO2 + H2O = H2CO3. Carbonic acid smells, but carbon dioxide by itself has no odor regardless of pressure.
31. OXYGEN. Does breathing 100% oxygen make you happy? Help football players? Make you feel good after nitrox dives? Help libido? No on all four. Oxygen has no euphoriant properties and no ability to boost athletic performance before or after short intense efforts like football. It does nothing at all and in studies, the subjects could not tell if they were breathing oxygen or room air.
32. NITROGEN. Is nitrogen an inert gas? Nope. If nitrogen were really inert you wouldn't have protein or explosives. Nitrogen is a very stable molecule. It usually forms compounds only at high temperatures or pressures. Because of relative lack of reactivity nitrogen gas behaves as an inert atmosphere. However, unlike helium, neon, and argon, nitrogen is not truly inert.
BODY MYTHS
33. BLOOD. Is your blood the same concentration as sea water? No, not even fish have the same blood composition as sea water. Your blood is much less concentrated, has minerals and other important particles in different relative amounts than sea water, and of course, has many things in it that sea water does not have. Your blood is also not similar in concentration to the hypothesized amounts in primordial oceans.
GENDER MYTHS
34. SUSCEPTIBILITY TO HYPOTHERMIA. Who is more susceptible to hypothermia men or women? In many studies, it's men. Both men and women protect their core temperature in different but effective ways. There is evidence that women are better protected than men particularly in the water. The largest commercial fleet of divers in the world are the women free divers of the Orient. Distance swimming in cold water has a long history of records set by women from the English Channel to the Bering Strait.
35. CURVES. Divers sometimes ask if women's curves present a greater surface area for heat loss rendering a disadvantage for women in the cold. The answer is no. The nature and mass of the tissue greatly limit heat loss. Furthermore, men's bodies are also full of curves, from muscles to other normal structures. Men's genitals don't fare well in the cold for similar reasons as fingers and ears. "Frostbite Shorts." under various names, is a documented medical malady.
36. GENDER AND BUOYANCY. Although some divers ask if women's breasts affect their horizontal 'trim' underwater, it is not the case. Scientific techniques that analyze center of gravity and buoyancy reveal that men seem to have a greater predisposition to streamline or 'trim' problems. Men usually carry their fat on their upper bodies compared to women with fat distributed on both upper and lower body. Men's longer, leaner legs are more likely to sink, causing increased drag in the water. An interesting study by Pendergast on competition swimmers found male swimmers to have a poorer comparative power output than female swimmers due to their buoyancy distribution. Their typically less horizontal position increased drag.
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How many myths did you know? To find out more about these, and many more issues in underwater physiology, read the book "Diving Physiology In Plain English" by Dr. Jolie Bookspan, formerly research physiologist for the US Navy.
"Diving Physiology In Plain English" is available from:
Undersea and Hyperbaric Medical Society (UHMS)
10531 Metropolitan Avenue
Kensington, MD 20895-2627 USA
(301) 942-2980
fax (301) 942-7804
UHMS@radix.org or uhms@uhms.org
$30 plus $5 shipping.
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About The Author
Dr. Jolie Bookspan earned Master's and Doctoral degrees in exercise physiology and underwater physiology, a fellowship in cold immersion, and post docs in saturation decompression and altitude. Five years of her work involved unraveling the results of extension of oxygen tolerance research in humans. Far from the ivory tower, her father taught her to dive in the Hudson River in the late 1960's. After serving in the Army she went on to become research scientist for the Navy, demonstrating that even skilled scientists can make the same mistake twice. She taught anatomy at a college in México in the mountains where the entrance exam was getting up there without a nosebleed, and has conducted intensive work in cold chambers and other scientific thing-a-ma-bobs which funding organizations have gone out of their way to meticulously ignore.
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