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Loss of Consciousness in Breath-Holding Swimmers by Dr. Pollock, repost for posterity

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Jun 13, 2020

Loss of Consciousness in Breath-Holding Swimmers

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.

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.
The responses:

Last week, the NDPA posted an article by Dr. Neal Pollock entitled “Loss of Consciousness in Breath-Holding Swimmers”. The article received comments on the site and on the NDPA Facebook Page from respected members of the aquatics safety community and Dr. Pollock wanted to respond stating, “I think that it is beneficial for this kind of discussion to be in the open.”

Below are the questions or comments made and Dr. Pollock’s response to them:

Questions from Dr. Rhonda Milner:​

I appreciate Dr. Milner’s comments regarding my article on loss of consciousness in breath-holding swimmers. They provide a great opportunity to continue the discussion on this clearly important topic. I will start with a point-by-point response:

–What about the competitive swimmer who ignores the urge to breathe?
Answer: Without hyperventilation, a normal person cannot voluntarily breath-hold long enough to lose consciousness. The urge to breathe often comes in waves. If the first can be ignored, breath-hold time can be substantially increased. I have talked about using distraction such as attempting to swallow until wave passes in a published review of breath-hold safety (Pollock, 2007). While ignoring the first wave of urge to breathe is feasible, it becomes progressively more difficult. Most important, a normal person cannot ignore the urges long enough to lose consciousness under typical conditions. This is an easily testable response pattern.

–What about those who stimulate genetic triggers with breath-holding such as long Q-T interval leading to cardiac arrhythmia, and cardiac arrest?
Answer: While it is possible for long Q-T to be a factor in fatal events, this is probably a far more rare issue – the zebra – than excessive hyperventilation. One of our human drives is the need to explain things, wherever possible finding something other than ourselves to blame. Identifying a possible organic culprit can often be more attractive that accepting that the simple act of voluntary excessive hyperventilation is responsible. It becomes problematic, though, when too much attention goes to the exotic possibilities. It distracts from attending to the far more likely issue faced by all involved. Unfortunately, the difficulty in conducting appropriate studies will continue to make it difficult to resolve just how frequently (or infrequently) the exotic explanations are likely to be relevant.

–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?
Answer: The situation as described is probably only feasible if substantial (definitely excessive) hyperventilation is conducted to make the repetitive cycles high risk. As discussed in the original article, hyperventilation dramatically drives down CO2 levels without appreciably increasing O2 levels. An important clarification here, though, is that O2 levels will not be appreciably increased above normal levels. Hyperventilation will help to restore normal oxygen levels while rapidly driving down the CO2 levels. So pre-breath-hold hypoxia is not the prime problem. It is still the reduced CO2 levels that delay the urge to breathe past the point at which hypoxia again becomes an issue. Despite the previous clarification, I agree that rapid repetitive breath-holds represents a much higher level of hazard and should be discouraged. In the freediving community, the rule of thumb is that a post-breath-hold recovery period should be no less than the duration of the previous breath-hold. The recovery time should also progressively increase as dive depth and/or fatigue increases. Following these rules and limiting hyperventilation will remove the majority of the hazard. Those who want to be even more conservative should eliminate hyperventilation completely. Most critical in any case is that we understand the true impact of our choices and behavior so we can control risk. We will never eliminate risk from life, but with appropriate knowledge we can control a substantial amount of it.

–What about those who unintentionally hyperventilate?
Answer: Respiratory control is regulated in a breath-by-breath manner, modulated by a host of factors in real-time. Transient hyperventilation is compensated for by a delay in the subsequent drive to ventilate (again, due to the relatively depressed CO2). Modest unintentional hyperventilation is very likely to remain in the zone that erodes but does not eliminate the safety buffer between the CO2-driven urge to breathe and the O2 levels required to maintain consciousness. For a more extreme example such as the hyperventilation induced by immersion in cold water, that could certainly erode much more of the safety buffer. However, the extreme discomfort of cold water is also incompatible with long breath-hold times, probably eliminating serious risk.

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.
Answer: The greatest challenge in considering the risks associated with breath-hold is separating the elements of emotion from levels of evidence. Much of the historical wisdom regarding breath-hold safety was based on an incomplete understanding of both the hazards and of human capability. The modern freediving community has provided tremendous insights into both and helped by promoting guidelines that favor safety without unnecessarily restricting practice. It may sound very scary to those not comfortable with freediving, but these individuals have given us a unprecedented wealth of opportunity to study “survivors” of blackout. I have both discussed the events with many such individuals (and have observed more than a few events). Hyperventilation, whether recognized as such or sometimes hidden behind a euphemistic term like “workup breaths” is a factor so common as to almost be obligatory. The disqualification rules of competition are rigorous enough that there are relatively few blackouts that occur during these events. Blackouts are common during training activity, however. Serious injury is avoided because of the standards of close supervision and incident management readiness are maintained. There is a strong drive in the community to optimize performance by performing close to but on the safe side of the limits. The operational practice within the responsible freediving community (the vast majority of the community) is such that breath-hold divers can discover and expand their capabilities while preserving their safety. It is not a perfect system and some serious accidents and even fatalities do occur, but at a low rate.

The biggest concern is not the breath-hold divers who learn and practice as part of an organized and responsible freediving program. It is the diver who operates with no oversight and may not fully appreciate the risks. Open discussion is necessary to make sure that a clear understanding of hazards and safe practices is immediately accessible. “Just say no” is a good sound bite, but does not prepare those living in our world of self-discovery and peer pressure.

All aquatic activity includes some risk. Proper education is critical to help to manage it. For breath-hold, adequate support and thoughtful practice are critical. More conservative practice should be followed in the absence of direct, close supervision. The well-trained swimmer or breath-hold diver will learn to assess real-time risk and respond appropriately. No one wants to see injury or fatality arising from something that should be a safe activity. We must also remember, though, that there are a lot of benefits of participating in different activities. Safety will always remain an act in balance with the other needs in life. My priority is to make sure that individuals have sufficient and correct information to make decisions in their best interest.


Pollock NW. Breath-hold diving: performance and safety. Diving Hyperb Med. 2008; 38(2): 79-86.
Comment made on Facebook by Tom Griffiths, Ed.D.

The recent article about the loss of consciousness while breath-holding in the water was not only misleading, it may be counterproductive. When the author suggests that the aquatic community describes most unwitnessed cases of unconsciousness as Shallow Water Blackout, this is both unfair and untrue. Through the efforts of the American Red Cross, the YMCAs, The National Swimming Pool Foundation, StarGuard Aquatics, The Redwoods Group, and many, many others, the aquatics community is now beginning to appreciate the dangers of competitive, repetitive, and prolonged breath-holding. While we do know that hyperventilation has precipitated many of these tragic deaths, we also know that many of these highly conditioned athletes and soldiers did not hyperventilate. As we continue to analyze these breath-holding deaths closely, we find that many occur after extremely intense and exhaustive exercise without traditional hyperventilation. The term Shallow Water Blackout has been used for more than 50 years and now that the aquatics industry has embraced it, it’s no time to lose momentum by changing names and qualifying its conditions. While I agree the term is not perfect, it specifically references “shallow water” and “blackout” where most of these accidents occur, particularly in comparison to the open-water. Just about everyone can understand what the term Shallow Water Blackout means. Terms like HIH and HIB are just too generic and academic for the masses to identify with. Decades ago we changed the name of Life Jackets to Personal Floatation Devices (PFD) with very poor results. Today most people still do not know what PFDs are so we are back to calling them what they were named more than a century ago, Life Jackets. We do know that breath-holding is the primary cause of unconsciousness to swimmers and hyperventilation may increase the likelihood of unconsciousness. However, other physical conditions have been present during breath-holding deaths without hyperventilation when swimmers have lost their lives. In addition, as presented at a recent NDPA conference in Pittsburgh, medical researchers told the attendees that simple voluntary breath-holding in the water can precipitate Genetic “Drowning” Triggers like long Q-T Syndrome which lead to sudden death. Indeed, breath-holding deaths in our swimming pools are a complicated matter. Spotlighting Hyperventilation only, saying that it must be present to cause unconsciousness, may lead to an increase in Shallow Water Blackout events.

Dr. Tom Griffiths
President and Founder
Aquatic Safety Research Group, LLC

Dr. Pollock’s response:

r. Griffiths raises three points that require response. First, the lack of physical evidence post-event has driven investigators to look for problems that could contribute to blackout. Long Q-T is one of the exotic ones. Saying that something is possible is very different from saying that it is probable. It is important to not lose sight of the vastly more common problem by exaggerating the focus on the attractive but far less likely one.

Dr. Griffiths also described blackout problems during activities “without traditional hyperventilation.” It does not matter if the hyperventilation (ventilation in excess of metabolic need) is traditional or non-traditional. A swimmer deeply overbreathing to aid his or her recovery after a sprint is hyperventilating. The physiological response is the same. Near-normal levels of oxygen are restored and carbon dioxide levels are dramatically depressed. It is not uncommon to have breath-hold divers swear that they do not hyperventilate only to see them do so. The fact that they call it “work up breathing” or “cleansing breaths” or some other euphemistic term does not change the physiological impact.

Regarding the term shallow water blackout, the current blanket application by the aquatics community is a recent development, and one that is confusing since it is clear at odds with the traditional use. I suggested “blackout” as a reasonable term if the use of hyperventilation could not be established. This is a reasonable and less-pejorative term. “Shallow water blackout” described a very specific set of conditions – loss of consciousness when shallow water was reached while ascending from depth. Without the ascent, the water depth, shallow or deep, is irrelevant. Holding on to a term that is clearly confusing and confounded by other (earlier) usage is not likely to improve clarity of thought or understanding.

Teasing out all of the factors that can generate risk is challenging. Improving understanding of the problem, reducing excessive hyperventilation, and promoting close supervision of breath-hold activities will go a long way to reduce the risk.
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