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Questions and Answers on
Physiology and Medical Aspects of Scuba Diving


Lawrence Martin, M.D. Copyright 1997


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Stress, Hyperventilation, and Hypothermia


Stress is a term frequently used by divers to describe uncomfortable or dangerous experiences. The dictionary defines stress as "physical, mental or emotional strain or tension." The diver who says "I was stressed" usually means something like: "I found myself in a difficult situation and was uncomfortable, anxious, fearful." Or, "I was not in full control of the situation, my heart was pounding, and for a while I felt pretty tense." The sudden onset of stress is usually accompanied by specific physiologic changes such as sweating, headache, fast heart beat and over breathing (hyperventilation).


Stress is such an important concept in diving - avoiding it when possible, confronting stress without panic when it does occur - that the subject is an integral part of "Stress and Rescue" courses offered by the scuba training agencies. Recognition and proper handling of stress are keys to safer, more enjoyable diving.

Stressful situations leading to panic probably account for many preventable fatal scuba accidents. In Stress and Rescue courses divers are taught that in any stressful situation you must STOP - BREATHE - THINK - ACT, in order to avoid a panic reaction.




Panic - defined as a chaotic and inappropriate response to a stressful situation - puts the diver's life in jeopardy. Rather than belabor this point I will provide examples of several stressful situations, comparing reactions by the panicked or near-panicked diver and the stress-trained diver. (The diver's thoughts are in single quotation marks.)

1) Howard, diving alone, finds himself snagged in some kelp 45 feet below the surface. As he tries to swim away he is pulled back deeper into the kelp bed.

Panic reaction. 'I'm caught! What is this? I can't get away! Oh my god! I've got to twist around and get out of this stuff!' With each attempt at moving away Howard finds himself more restricted. He begins to over ventilate and run low on air. He's not sure if the kelp is around his body or just his BC. He decides to ditch the BC and make an emergency ascent. He risks drowning.

Stress-trained reaction. STOP. 'I'm caught in some kelp. Can't seem to move. Have to stop and figure this out.' BREATHE. 'Must control my breathing. Take slow, deep breaths while I work on this. I should have at least 1000 psi left.' THINK. 'Since I can't move away, I have two options. Try to cut it with my knife or try to ditch my BC and tank.' ACT. Howard slides his right hand down his leg and grasps his knife, which he always takes with him into the kelp bed. Slowly, carefully, he brings it up to waist level and begins to cut all kelp he can see or feel. Making slight rotational movements he continues to cut a wider and wider area. In a short few minutes he is able to turn completely and cut the last remaining kelp, around his ankle. Then he sheathes the knife and makes a slow ascent to the surface.

2) John, 50 feet down, finds that he has run out of air. His buddy is nowhere to be seen.

Panic reaction. 'Oh my god I'm going to die! How did this happen? I can't breathe! Where the hell's my buddy? He left me!' John sees the surface far away and starts kicking up as fast as possible. Panicked and unthinking, he holds his breath and experiences an air embolism on reaching the surface.

Stress-trained reaction. STOP. 'I'm out of air, for sure. And I can't see my buddy. No time to start looking for him.' BREATHE. 'That's the problem. Can't inhale water.' THINK. 'I must have a good half minute of air in my lungs. I remember the first rule of diving: never hold your breath. OK, I've got to make an emergency ascent. Must be sure to exhale all the way up. Better ditch my weight belt.' ACT. John unbuckles his weight belt and it falls quickly away. He uses the power inflator to inflate his BC a little, finding there is enough air remaining for that purpose. He slowly kicks upward while continuously exhaling, all the while concentrating on the bubbles coming out of his lungs. In only a few seconds he surfaces, and then manually inflates his BC. He has saved himself by coping properly with an extremely stressful situation.

3) Bill, a newly certified diver, signs on for a morning wreck trip with Macho Divers. He is diving alone but is told he can buddy up on the boat. Sure enough, he is teamed with Rambo, who has made "hundreds of dives" in the area. Near the wreck site the dive master informs the group that the first dive will be to 90 feet on the wreck, over twice the depth Bill has ever reached before.

Near-panic. Bill is anxious: 'I can't back out now.' He rationalizes: 'This dive should really be no different from the two I made to 40 feet in my certification course.' He is fearful: 'What if I can't keep up with my buddy? Do I follow him inside the wreck? Or wait for him outside? Will I have enough air to last? Oh damn, what choice do I have? If I say something they'll think I'm chicken. I'll just go down and see what happens. But what if there's a problem?' Doubt-ridden, anxious and hyperventilating, Bill makes the dive. A few minutes later, while hovering over the wreck and trying to keep up with Rambo, Bill is shocked to discover his air gauge reads only 1100 psi. He aborts the dive and ascends quickly, forgetting to make a safety stop.

Stress-trained reaction. STOP. 'First dive to 90 feet? This is not a safe situation at my level of training.' BREATHE. 'No need to panic. Glad I'm not in the water. My breathing is even; no one around me even senses my concern. THINK. 'I've never been that deep and now is not the time to go, especially with this Rambo buddy. No chance he's going to stay close by me. I'm getting goose bumps just thinking about all the bad things that can happen down there.' ACT. Bill tells the dive master he's a new diver and isn't comfortable with this first dive to 90 feet; he would rather sit it out and make the second dive to 50 feet, with the whole group. "No problem" says the dive master. Rambo is teamed up with someone else. Bill has a safe and successful shallow dive later that morning.

4) Mary loses sight of her buddy during a drift dive. She becomes disoriented and decides to surface. Back on the surface, she sees that the dive boat is far away. There is about a two-foot chop on the surface.

Panic. Mary starts screaming: "Help! Help!" She flails her arms wildly but no one sees her. Her regulator falls behind the tank and she cannot grasp it. She forgets to use her snorkel. A wavelet rushes over her and she swallows a mouthful of water. She begins choking and coughing, and feels certain she is going to die. When finally spotted by the boat captain she is water logged and choking. Mary is picked up and rushed to shore for medical examination.

Stress-trained reaction. STOP. 'Well, I'm far from the boat. Not a good situation.' BREATHE. 'Must take slow, deep breaths. Don't want to hyperventilate. Could get dizzy that way.' THINK. 'They won't hear me if I yell, and I don't have any signaling device. Surely they'll find me soon. I just have to be patient. As long as I have air in my tank there's no danger of drowning. Let's see...um, that's good, 1500 psi. OK, I'll be fine. Just be patient.' ACT. Mary inflates her BC and tucks the regulator mouthpiece into her vest so she can get it without fumbling. She begins breathing through her snorkel, and lays back to relax and conserve energy. Periodically, she waves her arms back and forth, the universal diver's signal for "I need help," in case someone from the boat looks her way. She spends a relaxing 10 minutes in the water until picked up by the boat.

There are many potentially stressful situations that can occur when diving. Every month the pages of Skin Diver and other scuba magazines report how experienced and novice divers handle them, sometimes well, sometimes not so well. The key steps to handling any stressful situation: first, recognize when it occurs; second, STOP - BREATHE - THINK - ACT.


A stressful situation can develop within seconds, or build up over minutes to hours. True emergencies, such as an entrapped or injured diver, or an out-of-air situation, are obvious stressful situations. However, any time there is an element of fear, tension or uncertainty there is a stressful situation which, if not handled properly, can lead to a bad outcome.

If you experience these feelings you are in, or are approaching, a stressful situation. This is not necessarily bad. Stressful situations are part and parcel of diving. The important thing is to recognize stress so it can be contained. For example, the mere occurrence of any of the following questions in the mind of the diver signals a potentially stressful situation. Remember to STOP - BREATHE - THINK - ACT.

before a dive

  • Should I make that third dive this afternoon?
  • Should I rent this equipment even if it has no octopus?
  • Should I dive with this cold/sinus infection?
  • The water looks rough from shore. Should I do this boat dive today?
  • Should I do this night dive? They all say it's going to be great, but I'm really tired.

during a dive

  • Should I take a chance and enter that cave/wreck?
  • Should I turn around with only 1000 psi in my tank, or go further?
  • My computer just died. Should I abort this dive or just stay with my buddy until he's ready to surface?
  • Should I try to pet that moray eel?
  • Should I try to pick up that spiny urchin hiding under the rock?

Handling stress does not mean taking no risk, or always sitting out a dive when a doubt arises, or insisting only on perfect conditions. After all, diving by definition is riskier than not diving. Handling stress means doing what is best for you in any difficult or tense situation, consistent with your level of training; above all, it means never, never panicking.

What constitutes stress, and a stressful situation, is largely diver-dependent. An open water student may experience stress on her very first pool dive. Someone who has only dove in Caribbean waters may feel stress in a cold water lake. A highly experienced diving instructor may feel stress while diving in a cave. The point is, every scuba diver is at risk for encountering stressful situations. Every diver needs to learn to handle stress. STOP - BREATHE - THINK - ACT.


Hyperventilation is one of the commonest physiologic responses to stress and anxiety. Hyperventilation simply means breathing more than is necessary for the amount of carbon dioxide your body produces. "Breathing more" means breathing faster and/or deeper than necessary to keep the blood carbon dioxide at an optimum level. To a large extent the BREATHE step in handling stress is a reminder to take slow, deep breaths and not hyper-ventilate.

Hyperventilation lowers the partial pressure of CO2 in the blood (PaCO2) and makes the blood more alkaline than normal. Normal blood PaCO2 ranges between 36 and 44 mm Hg; hyperventilation brings it to less than 36 mm Hg. The lower the PaCO2, the worse the symptoms.

Hyperventilation can occur very quickly, within seconds. You can hyperventilate right now. First, make sure you are sitting down. Take several deep, rapid breaths. Your CO2 level will fall and you will probably begin to feel tingling in your fingers and a little lightheaded (if you keep hyperventilating you will definitely feel lightheaded).

The next few paragraphs describe the consequences of hyperventilation in general, and then the consequences peculiar to diving.

General consequences. So-called involuntary hyperventilation is a common phenomenon. We see it in kids and grownups, in friends, relatives, patients. The person becomes anxious and starts to over breathe. This leads to a low blood CO2 level. If the problem occurs quickly the body doesn't have time to adjust, and several symptoms can manifest: palpitations (feeling your heart beat); numbness and tingling in the fingers and around the mouth; headache and dizziness. In the most severe cases there can be muscle spasms of the arms, low blood pressure and fainting.

The symptoms of acute hyperventilation are often "cured" by having the hyper ventilator breathe into a paper bag held over the mouth and nose; this leads to re-breathing exhaled carbon dioxide, which raises the blood PaCO2 toward normal. The bag should be removed as soon as the symptoms abate and the patient can be reassured that he is OK. At that point the problem causing the stress or anxiety can be investigated. Because the hyperventilator on land has an unlimited air supply, hyperventilation alone is never fatal.

When diving. All the general consequences of hyperventilation apply under water, and more. Under water, of course, the supply of air is very limited. The hyperventilating diver uses air much faster than normal, so the dive is cut short. Instead of a planned bottom time of 30 minutes on a dive to 60 feet, for example, the hyperventilating diver may have only five minutes because of a rapidly diminishing air supply. On one memorable dive to 50 feet an overweighted, hyperventilating diver thought her tank was leaking because 10 minutes into the dive she was down to only 1200 psi! Back on the boat she asked the dive master to check her equipment. Her equipment was fine.

The numbness and tingling from hyperventilation can mimic the bends and cause a stressful situation. If the diver doesn't recognize the problem, anxiety and further hyperventilation can ensue. In the most extreme cases the diver can become lightheaded and lose control of the situation. The regulator could fall out of her mouth, she could experience muscle spasms, end up in a panic state and risk drowning. All from simply over breathing.

Probably most people hyperventilate to some extent when first learning to dive. By appreciating the problem it can be avoided. Be conscious of your breathing. When not stressed, slow, moderate-sized breaths are preferred. Try to avoid deep, sighing breaths, as well as short, shallow breaths.

1. Someone who has symptoms from "hyperventilating" will always have:(choose one answer only)
a. a fast breathing rate
b. deeper breathing then normal
c. numbness or tingling in the fingers
d. an arterial PCO2 lower than 36 mm Hg
e. a history of anxiety
2. An open water scuba student goes to 30 fsw with his instructor. After about five minutes the student finds it difficult to inhale through his regulator and starts to breath fast. The student gives an out-of-air sign and the instructor signals him to surface. On the surface the student appears anxious and is noted to be breathing fast. The instructor helps him get back into the dive boat. The student complains of numbness and tingling in his hands and around the mouth. What is the most likely cause of his problem?
3. What is the principle difference between hyperventilating on land and while scuba diving?



Hypothermia is a lower than normal body temperature. Normal body temperature is about 37C (98.6F). Water temperature less than this will drain heat away from the body and, over time, cause hypothermia. The most susceptible areas for heat loss are the head, underarms and groin. This is why a hood is so important in cold water diving.

Water conducts heat about 25 times faster than air. Because of rapid heat conduction by water, and lack of much natural insulation, the human body cannot defend against low water temperatures as well as against low air temperatures. That is why 75F in the air feels quite comfortable with light clothing, but the same water temperature requires a full wetsuit for comfort.

Normal body temperature is 37C/98.6F. For medical purposes, hypothermia is defined as a central body temperature equal to or less than35C (95F). Hypothermia can be divided into three stages based on the body temperature: mild, moderate and severe (Table 1).

Mild: 34 - 36.5 C (93 - 95 F)

Blood pressure usually normal but may be high or low; victim may have trouble speaking, walking, remembering; shivering often present when temperatures reaches 95 F.

Moderate: 28 - 33.5 C (82.4 - 92 F)

Victim begins t lose consciousness; heart rate slowed; victim may have heart rhythm disturbance with low blood pressure .

Severe: Below 28 C (82.4 F)

Victim in shock; no response to pain; very slow heart rate; victim at high risk for sudden death from heart rhythm disturbance

Up to a point we can maintain normal central body temperature when exposed to cooler temperatures, even though we may feel cold. Feeling cold or chilly is not the same thing as hypothermia, but may be a precursor to true hypothermia. True hypothermia is a medical emergency that, uncorrected, can be fatal. However, water temperature has a variable effect on a given individual; its physiologic effects depend not just on individual tolerance, which varies from person to person, but also on the length of the exposure and the type of protection worn.

For most divers, water temperatures above 90F feel hot, and temperatures between 84F and 89F feel quite comfortable, at least initially. We often see divers enter water in the mid 80s without a wetsuit, or with only a non-insulating protective "skin" worn to avoid abrasions. Even so, heat loss is occurring at these "warm" temperatures, because they are still lower than normal body temperature. For each degree of water temperature lower than 84F, an increasing percentage of people feel the need for thermal protection with a wet suit.

There are data regarding how long an unprotected person can stay immersed in water at various temperatures before hypothermia occurs; obviously the colder the water, the faster body temperature will fall and the more likely the victim will succumb. According to the National Oceanic and Atmospheric Association (NOAA), a body temperature of 91.4 F (33C) is lethal for about 50 percent of all people who reach this level from water exposure.

Recreational divers in warm waters (>75F) will likely not develop hypothermia during one dive. They may, however, feel chilled and uncomfortable, lose some body heat, and enter into a stressful situation. It is not a pleasant way to dive. The major warning sign of impending severe hypothermia is shivering, which usually occurs when body temperature reaches 95F. The shivering diver should exit the water.

It is always better to have a little too much protection than too little. Protection for the diver means some type of worn garment covering most of the skin surface, either a wet or a dry suit. One can always open a wet suit zipper if the water feels too warm. Once chilled, however, there is little one can do except get out of the water.

A wet suit is a form-fitting suit, usually made of closed-cell neoprene, which means it contains air cells that are an inherent part of the fabric. The wet suit allows water to enter between the suit and the diver's skin. This layer of water is then warmed by the diver's skin and serves to retard heat loss during the dive. Wet suits, depending on their thickness and use of hood and gloves, can be protective down to the 50F to 55F range. Note that the closed air cells of neoprene are compressed with depth, so the wet suit loses much of its insulating ability the deeper one goes.

Below 50F, a dry suit is usually necessary for proper thermal protection. (Some divers feel any water temperature less than 60F mandates a dry suit.) No water can enter a dry suit, so the diver stays dry throughout the dive. A dry suit is typically composed of 1/4 inch closed-cell neoprene with nylon backing. Instead of water, a layer of air (delivered from the tank) is kept in place between the inside of the dry suit and the undergarments worn by the diver (thermal underwear is recommended). Boots are also an integral part of the dry suit. The amount of air inside a dry suit is controlled by the diver; since it can markedly affect buoyancy throughout the dive, special training is necessary for proper use.

The range of wet suit thickness' and temperature effectiveness are shown in the following box, compared with the dry suit. Note that the protection afforded by both types of suits will be enhanced if the diver also wears a hood and gloves. The neoprene wet suit sizes are based on the English system of measuring thickness. In most countries wet suits are marketed by metric thickness, e.g., 1, 3, 5 mm, so the corresponding thickness in inches will vary accordingly.

wet suit thickness: water temperature
inches mm range*
1/16 1.6 75-85 F / 24-30.5 C
1/8 3.2 70-85 F / 21-30.5 C
3/16 4.8 65-75 F / 18-24 C
1/4 6.4 50-65 F / 10-18 C
3/8 9.5 45-60 F / 7-15.5 C
Dry Suit < 60 F / 15.5 C
* Tolerance to cold will vary according to the diver's body makeup, plus use of hood and gloves. There are no hard rules about what wet suit to wear at what water temperature ; It is largely a matter of preferences and diver tolerance. The temperatures shown are approximate ranges for comfort for the specified protective gear.


You may be called on to assist or aid a hypothermic or chilled diver. There are formal hypothermia protocols and whenever possible victims should be handled by a knowledgeable professional. Issues such as CPR or hypoxia may also need management in any hypothermic diving injury.

Treatment of hypothermia can be divided into passive rewarming, active surface rewarming, and active core rewarming. In the field, away from a medical facility, you will probably only be able to accomplish the first two.

PASSIVE REWARMING - remove all wet clothing; use dry, unheated blankets, dry clothes, and a warm room environment.

ACTIVE SURFACE REWARMING - use heated blankets or hot water bottles applied to skin. If the victim is fully alert, give warm liquids; this will help correct any associated dehydration. Alcohol and caffeine beverages such as coffee and tea should be avoided. Immersion techniques, such as placing the victim in a tub of hot water, are not generally recommended.

ACTIVE CORE REWARMING (by medical personnel only) - instillation of warm fluids into the veins; placement of warm fluids through a tube into the victim's stomach; inhalation of warm respiratory gases; dialysis (removal of blood for warming outside body, then returning the blood to the victim).

4. Listed below are four "mechanisms." Choose the two mechanisms by which wet suits protect against hypothermia.
a. insulation (prevention of radiation heat loss)
b. thermal equilibrium between water under the wet suit and the diver's skin temperature
c. continuous circulation of water under the wetsuit and the diver's skin
d. compression of neoprene air cells with depth, which keeps heat from escaping
5. What is the recommended procedure for a diver who feels chilled and begins to shiver 10 minutes into a planned 40 minute/ 50fsw dive? The diver is wearing a wet suit appropriate for the water temperature.
a. Stay as deep as possible to increase wet suit compression and thus thermal insulation.
b. Exercise by using arms in a swimming motion; plan for a somewhat shorter dive since this technique will increase air consumption
c. Tighten the BC to prevent flow of water between it an the wetsuit.
d. Ignore the sensation until your teeth chatter and you can no longer comfortably hold the regulator in your mouth.
e. Make a sign to your dive buddy that you are chilled and then exit the water. Expect your buddy to follow you up unless there is someone else he/she can buddy up with.



Hyperthermia, or over heating, is much less common than hypothermia. The main reason is that open water (oceans, lakes, rivers) is naturally cooler than body temperature. However, while wearing a thick wet suit on hot days, any diver can become overheated and develop true hyperthermia out of the water. The body temperature rises, which is the same as developing a fever. The threshold body temperature that puts the diver at risk is about 102.2F (39C). Signs of hyperthermia include rapid breathing, feeling warm or hot, mental confusion, fatigue, muscle cramps, nausea, and exhaustion.

The higher the temperature above 102F the worse the problem. Hyperthermia is considered life threatening whenever body temperature reaches 105-106F; such a fever places the diver at risk of cardiovascular collapse, shock and death. Treatment requires that any wet suit be removed as quickly as possible. If recently in the water the diver will be wet, but the skin will be warm and pulse rapid. (If hyperthermia occurs before entering the water the victim will appear hot and dry, with flushed skin and a rapid pulse.)

The major complications from hyperthermia are dehydration and shock. Emergency first aid requires cooling the body temperature by any means available: wrapping in a wet blanket, immersion in water, spraying with cool water. Generally the body temperature should not fall below 101F, as too rapid cooling can precipitate hypothermia. Since a thermometer may not be available in the field, it is best to cool the person slowly rather than quickly.


1. d. Of the answers provided, only a low PaCO2 is invariably present, since it is part of the definition of hyperventilation. All of the other symptoms are often found, but are not part of the definition (they could be due to other causes, including DCS.

2. Hyperventilation. Although hyperventilation and the bends may be difficult to distinguish after a dive, in this situation the shallow depth and short length of time virtually rule out the bends.

3. On land you cannot run out of air so hyperventilation, per se, is not life threatening. Under water a hyperventilating diver can run out of air, panic, or have some other catastrophe, and thereby risk drowning.

4. a. and b. Note that answer d, compression of neoprene air cells at depth, actually diminishes the wet suit's thermal protection.

5. e. Exit the water. Exercise is likely to worsen the hypothermia since it brings cold blood from the extremities to the central part of the body.

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