A Special Guest Article By Neal W. Pollock, Ph.D.
The risk of fatal loss of consciousness in fit and frequently highly competent swimmers was well described by Albert Craig in 1961.1,2 Blackout in swimming pools is not a new problem, but it is one that requires eternal vigilance. More importantly, terminology has recently become confusing and misleading. We can clear up some of the confusion; the need for vigilance will remain.
Metabolic gases (oxygen [O2] and carbon dioxide [CO2]) are fundamental components of our physiological processes. We consume O2 and produce CO2. While CO2 is commonly thought of as a waste product, it is critical in maintaining the acid-base balance in our tissues. For this reason, we maintain CO2 in the body at concentrations 140-160 times greater than the concentration in air.
The respiratory cycle regulates the levels of O2 and CO2 in our bodies by focusing on CO2. Expiration (breathing out) eliminates CO2 and inspiration (breathing in) restores O2. It is the rise of CO2 in the blood that stimulates breathing. When a breath-hold swimmer takes in a full breath of air and begins voluntary breath-hold, the urge to break the breath-hold is almost exclusively driven by rising CO2 levels. A normal healthy individual can hold his or her breath as long as possible with no significant risk. The point at which the urge to breathe is absolutely undeniable is reached far before the O2 level in the blood falls low enough to threaten consciousness. This is the exquisite nature of respiratory control.
What many swimmers who become interested in breath-hold quickly realize is that hyperventilation (ventilation of the lungs in excess of metabolic need) can dramatically increase breath-hold time. The effect works whether the hyperventilation is by faster or deeper breathing than that being demanded by normal body signals. Here, though, lies the start of the confusion. Contrary to the belief of some, hyperventilation is only trivially increasing to O2 stores in the body. What it does do is to dramatically reduce the CO2 content that is normally so much higher than in air. The result of lowering the body CO2 levels at the start of breath-hold is that it takes longer to reach the levels required to drive the urge to breathe. Excessive hyperventilation can delay the urge to breathe so long that the person can lose consciousness due to low O2 levels (hypoxia) with absolutely no warning. Stated another way, hyperventilation eliminates the safety buffer between the normal CO2-driven urge to breathe and minimum safe O2 levels. Excessive hyperventilation is effectively removing one of our most important protective mechanisms, a critical one when in water since the medium is unforgiving for an unconscious person.
The lack of postmortem physical evidence makes it difficult to confirm the use or magnitude of hyperventilation. While reasonable confidence can be established through witness reports or known patterns of victim practice, excessive hyperventilation must often remain only as a suspicion, even when blackout appears to have occurred in apparently healthy individuals with no other obvious insults or injuries. Still, even with conservative analyses, blackout is consistently one of the most common disabling agents found in fatal events captured in the Divers Alert Network breath-hold incident database.3,4
Terminology is another point of confusion. The term “shallow water blackout” has been picked up by the aquatics community to talk about almost any case of unexplained loss of consciousness. This is problematic because the term has already been used to describe two other conditions. The first was for problems observed in British closed-circuit oxygen rebreather divers. Device failures resulted in a buildup of CO2 that led to intoxication and loss of consciousness. More recently, for a problem experienced by breath-hold divers traveling vertically through a substantial depth range. In essence, descending through the water column compresses the gas in the lungs, driving more gas into the blood. Most importantly, this increases the amount of O2 available to be consumed. Problematically, though, as the breath-hold diver ascends through the water column the blood O2 level falls much faster than it would without the vertical excursion. Since the relative pressure change is greatest in the shallowest water, it is normal for blackout to occur in the final stage or just after surfacing, hence the adoption of the term. The blackouts occurring in most swimming pool cases, particularly those in shallow pools, are almost undoubtedly driven by excessive hyperventilation alone, with no meaningful contribution of pressure change to the event.
Turning to considerations for safe practice, it is important to note that, effectively, every respiratory cycle includes a brief period of functional breath-hold. Trying to ban breath-hold as some have proposed is a fairly irrational response to the problem. As discussed earlier, a normal person cannot voluntarily breath-hold long enough to lose consciousness without hyperventilation. The focus of safety programs should be to educate swimmers about the risk so they appreciate the importance of conservative practice. Not only is the understanding more likely to keep them safer than a ban that is impossible to enforce, it may help them explain the hazards to others who discover the effect of hyperventilation on their own.
Regarding safe limits, the available research suggests that hyperventilation restricted to no more than two or three full ventilatory exchanges (maximum breath in and out) is unlikely to reduce CO2 levels enough to produce a significant risk of loss of consciousness. Regarding terminology, “hyperventilation-induced blackout” is most appropriate if pre-breath-hold hyperventilation is known or suspected. Simply “blackout” or “hypoxic blackout” are also better alternatives to ‘shallow water blackout’ unless a significant vertical migration (perhaps greater than 15 feet in depth) was involved in the event and blackout occurred during the final stage of ascent.
Ultimately, the keys to breath-hold safety are proper education, thoughtful practice, and awareness of events. These keys hold for swimmers, instructors, lifeguards, and other leaders.
References
1. Craig AB Jr. Causes of loss of consciousness during underwater swimming. J Appl Physiol. 1961; 16(4): 583-6.
2. Craig AB Jr. Underwater swimming and loss of consciousness. JAMA. 1961; 176(4): 255-8.
3. Pollock NW, Dunford RG, Denoble PJ, Caruso JL. DAN Annual Diving Report – 2009 Edition. Durham, NC: Divers Alert Network, 2013; 153 pp.
4. Pollock NW, Dunford RG, Denoble PJ, Dovenbarger JA, Caruso JL. Annual Diving Report – 2008 Edition. Durham, NC: Divers Alert Network, 2008; 139 pp.
Dr. Pollock is a research physiologist at Duke University and Research Director at Divers Alert Network (DAN), in Durham, NC.
UPDATE: Dr. Pollock answers questions regarding this article: HERE
It should also be noted that high intense swimming could also lower the levels of oxygen and if done before breathold it could reduce the magin. Reduced body temperature will also increase or digesting of food will increase oxygen consumption and also lead to smaller margins. These reduced margins could lead to black out one day on a distance what you made without problem earlier.
Breath holding should be conducted with a safety diver educated in rescue with BTT at armlengths distance. Talk to your closest freediving club to learn more.
I am very concerned about the message the NDPA and Neal Pollock, PhD. are sending to swimmers: If you do not intentionally hyperventilate prior to a breath-hold, you will be safe from blacking-out.
–What about the competitive swimmer who ignores the urge to breathe?
–What about those who stimulate genetic triggers with breath-holding such as long Q-T interval leading to cardiac arrhythmia, and cardiac arrest?
–What about those who practice multiple, repeated breath-holding especially dynamic apnea depleting their O2 levels so they blackout before they reach their CO2 trigger point to breathe?
–What about those who unintentionally hyperventilate?
These people may end up dead thinking they were safe because they did not actively hyperventilate. Hyperventilation is one contributing cause of blackouts in shallow water from breath-holding. I urge caution until you have studied multiple survivors of Shallow Water Blackout (SWB). In your cases you are assuming hyperventilation, but the dead cannot be interviewed. I have cases of children diving up and down for toys on the bottom of the pool over and over with no voluntary hyperventilation, both blacked out and one succumbed.
Another issue with allowing prolonged breath-holding without hyperventilation is the fact that if blackout occurs chances of survival are low. It is hard to detect by an observer because of water ripples on the surface, and with the hypoxic state along with warm pool water (promoting brain metabolism), there are only a few minutes for a successful rescue.
Please do not add to the deaths from underwater breath-holding by falsely reassuring swimmers they are safe. SWB does not equate with HIH. I would not want even a single death on my conscience when your name is National Drowning Prevention Alliance.
Sincerely,
Rhonda Milner, M.D.
Founder and Chairman of Shallow Water Blackout Prevention shallowwaterblackoutprevention.org
Since I have been studying the Shallow Water Blackout phenomenon, I have seen several opinions about the risks of underwater swimming. Some say, underwater swimming is very important in training and not dangerous. These people may not know about SWB. Others say that to ban hyperventilation is enough to prevent the fatalities. But, the majority of those who understand SWB, believe that the only way to prevent these senseless accidents and deaths are to ban all hypoxic underwater activities.
Voluntary hyperventilation is just one of the risks that can cause fainting underwater.
For example, excessive breath holding underwater can stimulate the vagus nerve (without prior hyperventilation) and can cause syncope/fainting (without decreased CO2 levels), cardiac arrhythmia, arrest, and sudden cardiac death. Strenuous physical activity before breath-holding can cause a rapid lowering of the oxygen level pre-breath-holding which will hasten fainting. The higher water temperature and the physical activity during the underwater breath-holding (dynamic apnea) also cause a rapid decrease in the oxygen level increasing the risk of blackout even without hyperventilation. A competitive swimmer’s (who often performs prolonged underwater swimming) brain receptors may change and decrease the answer for hypoxia and hypercarbia. We don’t know all the physiologic changes caused by underwater long breath-holding.
Undoubtedly, with our present knowledge, we believe, that hyperventilation prior to breath-holding could be a main risk factor for loss of consciousness, but not the only one.
To prohibit only the voluntary hyperventilation is not enough. It is very important to educate athletes not to hyperventilate, but it would be a mistake to focus only on hyperventilation.
There are many articles published on the issue of hypoxic training, and there is no clear evidence that the prolonged underwater breath-holding would be any advantage. This practice has been used for decades all over the world, but it doesn’t mean this practice is right. The only safe training to increase lung capacity is aerobic training, not anaerobic. The consequences of underwater hypoxic training can lead to known serious consequences including death.
My son survived a SWB. He did a breath-exercise (slow and deep inhalations and exhalations before every underwater lap) and fainted after the 5th lap. He did not perform the classic hyperventilation (fast, superficial breathing). The pool’s water was higher than usual; there was not enough time for O2 recovery between laps; he is a well-trained swimmer, who can hold his breath for a long time; he was practicing dynamic apnea; the underwater laps were repeated within a short time, and there was no close and continuous supervision although this was during his water polo practice. The (involuntary) hyperventilation was only one of the risk factors that caused his accident. The risk factors in this case: repetition, deceased recovery time, warm water, competition, hyperventilation, well-training, physical activity, lack of awareness, and rescue time. He survived, but almost did not. Now, every time when he goes to the pool, I tell him: Do not hyperventilate AND never swim underwater.
I strongly believe, banning all hypoxic underwater activity (regardless of hyperventilation) can prevent deaths and accidents caused by Shallow Water Blackout.
Szilvia Vasas MD, pulmonologist
Dr. Pollock answers questions posed by Dr. Milner and Tom Griffiths, Ed.D.
http://ndpa.org/responses-to-questionsstatements-regarding-blackout-events/
I have read through all your responses to Milner & Griffiths and feel these responses and initial article are very biased to the Free Diving world.
What about the common household practises? After losing my nephew last year and asking questions to everyday kids and families like, “have you ever held your breath in the water” or “do you do laps underwater & how far do you go, how long?” most people and kids responded, “Sure thing!, its a NORMAL practice isn’t it?” Some responses were, “I even hold my breath in the bathtub!” With so many people unaware of risks and dangers, we need to have plain and simplified awareness strategies implemented. I am not a Dr, but am working beside many in regards to informative information, facts and terminologies, and am finding that the recent details you have presented are very biased toward Free Divers, and professional athletes. The general population and kids that this has happened to, don’t understand the meaning of hyperventilation, voluntary or involuntary. They don’t understand how kids have blacked out/drowned, when they could swim! For these impressionable youngsters, banning the long breath hold practice, is the safest way to avoid risk. We are promoting the safest way we know for the time being is a rule of thumb – One Breath Hold – One Lap (short) – One Time Only! then to take a good break to recover.
In public pools, banning the practice & training of breath holding of any kind in ‘trading hours’ is also the only real way to stop impressionable copycats. This type of training should only be done when the everyday public are not in the area.
I am open to discussions on this subject, I understand we are all aiming for the same result of, preventing further senseless drowning victims from long breath holding. But understand, if the breath hold didn’t occur in the water in the 1st place, then the dead would still be here!
Sharon
Shallow Water Blackout Australia
I have to say, I have agonised all weekend of how to comment on this article, which I feel is misleading and I totally agree with Dr Rhonda Milner, Szilvia Vasas MD & Tom Griffiths from the Facebook comments also. My 12 year old nephew blacked out in his backyard pool and quickly drowned last year whilst he was being supervised! Jack did not intentionally hyperventilate, he was doing laps underwater and continued to do laps, WITHOUT having a good break between each breath hold, this is known as the repetitive & continuous side to blacking out, rather than intentional hyperventilating with deep fast breaths before hand. He then challenged himself to sit on the bottom of the pool for another breath hold, immediately after doing what we thought were safe underwater laps and unfortunately fainted and quickly drowned, as his trigger to breathe set off “after” he blacked out and Jack took in a large volume of water to the lungs whilst unconscious and did not display any warnings to alarm his mother. His mother, my heartbroken sister, who was supervising him did not realise that he had fainted/blacked out, she thought he was breath holding as he had only just gone back under. Being a 12 year boy, fit and a strong swimmer, who did not take a fall, or have any medical issues that were present at the time, was examined and the result of death, was due to the breath holding and advised “Shallow Water Blackout”, the 1st we had ever known of such a thing. He drowned in less than 1 meter of water in his back yard pool! How does this even happen to someone who can swim, someone who didn’t injure himself, someone being watched??!!…Well we have learnt a lot about SWB in the past 16 months, and I can personally say, that HIH/HIB does not solely fit your description, and you may very well be misleading a lot of people. In Australia, our National Royal Life Saving Team and Doctors, did lengthy investigations and have since agreed that Hypoxic Blackout be renamed to Shallow Water Blackout – SWB. We managed to have Life Saving Manuals updated to include SWB, along with Fact Sheets or how to Prevent SWB etc. SWB does not just occur to Elite Professional Swimmers, Divers, Polo/Hockey Players etc, it CAN and HAS happened to the everyday family members, enjoying the water. I have met a number of families that are living with the same grief after an SWB tragedy and have to say, Rhonda Milner, your response above has definitely nailed every point I wanted to raise! Australia will soon start seeing the same No Long Breath Holding signs in public pools, and swim centres are already personally banning the long breath holding practices and any training that is done, is and should be done after hours where there can be no copy cat minors, also with strict safety precautions in place. Shallow Water Blackout does NOT just occur from voluntary hyperventilation and breath holding. Believe me, OUR FAMILY FOUND OUT THE MEANING AND HOW, A VERY HARD WAY! For more information, you can also view our facebook page: “Shallow Water Blackout SWB” or our website http://www.shallowwaterblackout.org. We are supported by our international partners in the US (Shallow Water Blackout Prevention) and Australia’s Royal Life Saving Society.
I hope to see you acknowledge that HIH/HIB should not replace a history of SWB and the recognition SWB is finally vastly gaining around the world. HIH is very misleading in the way that is biased to only recognise hyperventilation as a sole cause to black out. This is incorrect and personally offensive to our journey.
Sharon Washbourne – SWB Australia
Shallow Water Blackout Australia
shallowwaterblackout.org
A site dedicated to educating Australians about drowning risks from ‘Shallow Water Blackout’ due to breath holding
Several comments with respect to the discussion. As I understand it, Dr. Pollock indicates that a person can not resist the urge to breath without hyperventilation, but what he fails to address is what happens when the person finally succumbs to this unavoidable urge to breath while they are submerged underwater. I believe this is would best be answered as they would begin experiencing respiratory impairment from submersion/immersion in liquid or in short they would begin the drowning process. A second factor that is equally important when reviewing drowning incidents, the lifeguards often indicate that they saw the victim but thought that they were “holding their breath”. This often leads to an excessive delay in responding to a drowning incident.
I have read Pollock’s article and other articles in the past and I’ve also read the responses to this posted article as well as Pollock’s rebuttal. I am a widow of a SWB victim free diver, and now give short 1/2 talks about SWB to Junior Guards and High School students in the San Clemente and Dana Point, CA area. After a few years now of talks, there has been much discussion by parents and the kids themselves in our community. Unfortunately, many people do not know that CO2 is the trigger for that urge to breath. I think most people, even “experienced” and skilled people, do not know this. At a past high school I was teaching at, even the Anatomy/Physiology teacher was unaware that CO2 was the trigger. Using the term SWB is important for discussion purposes. As people become more educated about the basics of respiration and physiology, they will learn to differentiate a breath-hold under pressure, such as in a free-dive, or a breath-hold while doing under water laps, and therefore, begin to use more specific terminology. I have to tell you that our community, and thanks to the Junior Guard groups, state, county and city, as well as Fred Sweggles, a local reporter, the dialogue and discussions continue to occur. Articles and dialogue like the ones that arise from these articles are amazing. Everyone here, Rhonda Milner, Tom Griffith, and Neal Pollock your points are all valid and I appreciate your time and energy that is given to define this often unknown phenomena. Thank you. I hope to see more media, such as CNN and 60 Minutes, cover this “little known cause” of death. As Tom Griffith has pointed out to me, media has a strong influence on our country (and other countries). Instead of showing only the final results of World Record breath holds, how about an interview about the safety and training behind it? This would help immensely with those backyard pool drownings of kids who are trying for the World Record. Thank you again!!
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Even professional swimmers fail to hold their breathe is this type of swimming. Laws of swimming is a must to follow.
[…] a refresher, when the victim’s medical history is irrelevant, blackouts occur due to prior hyperventilation, followed immediately by rapid, repetitive breath-holding. Hyperventilation can be voluntary, […]
[…] Loss of Consciousness in Breath-Holding Swimmers […]
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