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:

Dr. 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.