A PROPOSED 60 SECOND LIMIT FOR BREATH-HOLD DIVING
by CAPT Frank K. Butler, MC, USN
Command Surgeon United States Special Operations Command
MacDill AFB, FL, USA
DR. BUTLER: I need to start with a disclaimer. I am not going to talk about how breath-hold diving is done in the U.S. Navy. Although I hope that the Navy will eventually adopt this approach to breath- hold diving, it is currently just a proposal.
As we talk about freedivers, we sometimes make them out to be a homogenous group. They are emphatically not a homogenous group. The main purpose of spearfishing while freediving is to hunt and gather, but there is also a competitive element. Long breath-hold times are a badge of honor. These divers take pride in being the person who can hold their breath the longest.
Contrasting this are the commercial freedivers, primarily the Korean and the Japanese Amas. These women have been freediving for centuries. This is not a recreational activity for them. It is a way of life, and they have a drastically different approach to it, as we will see.
Next we have military combat swimmers. Back in World War II, when you did not want the allied countries to invade your territory, what you would do is to place lots of obstacles, often large concrete blocks with spikes sticking out from the top, on your beaches. Beach obstacles and landing craft full of soldiers or Marines do not mix, so a way had to be developed to clear the beaches for landings. A whole new discipline was therefore born in the Navy, the frogman. It was their job to go in attach explosives to these obstacles before the amphibious landings so that the landing craft could reach shore safely. It is been a while since we have done any large-scale amphibious landings, but Navy SEALs still train for this mission.
The next group to consider is the elite breath-hold divers. These individuals are the world’s best freedivers, the ones who set records for dive depth and duration.
Finally, we have the rest of us, the non-elite freedivers who are just down there to have fun and to experience the underwater environment in a different way. As we consider the science and the sport of freediving, we need to consider each of these communities somewhat differently.
There are several disadvantages of breath-hold diving. The first is air hunger, something experienced by most breath-hold divers on most dives. Air hunger can detract from the enjoyment of the freediving experience.
Less commonly, but more dangerously, is the risk of hypoxic loss of consciousness on a breath-hold dive. A brief review of the safety of breath-hold diving in recent years will show the magnitude of this problem.
Consider for a moment how the diving medical community approaches safe no-decompression limits for scuba diving. Much research effort goes into defining the safe no-decompression limits for divers and the published limits are quite conservative when compared to the results of research. For example, no-decompression research exposures at 60 fsw [18 m] produced no incidents of decompression sickness in 29 exposures of 66 min bottom time duration. Despite this fact, there are no dive computers that I am aware of that allow a 60 fsw no-decompression dive of 66 min. Generally, diving physicians and scientists have been very conservative about recommendations that they make for decompression, choosing to favor diver safety over maximized bottom time. This philosophy has carried over into the dive instruction industry.
In contrast, what does the diving medical community say about the maximum length of time that you can safely hold your breath on a dive? Not much. Part of the reason for that is the basic premise of breath-hold diving: Either your body or your freediving experience will tell you when you should end your dive and come up and take a breath.
So, how well is this premise working? I am going to present a selection of case reports that come from my experience, from the experiences of diving medicine colleagues, or from published media articles.
An experienced, 25-year-old freediver died in 2003 performing breath-hold diving in Ginnie Springs, FL. An experienced 32-year old freediver died recently while doing breath-hold dives in the Cayman Islands. The 31-year-old son of a champion freediver died practicing breath-holding in chest-deep water in a swimming pool. Drs. Edmonds and Walker reported 12 deaths in breath-hold divers in Australia between 1987 and 1996. Two more died in Queensland in December, 2002 and January, 2003.
A 22-year-old University of Georgia student died in June 2001 performing breath-hold dives, again in Ginnie Springs, FL. A world-class woman freediver died attempting a deep breath-hold dive in the Dominican Republic in October, 2002.
A 19-year old college triathlete and lifeguard died in the swimming pool at the University of North Carolina while breath-hold diving. A member of the Mexican national water polo team died practicing breath-hold diving in a swimming pool. Martin County, FL banned breath-hold diving in public pools because of a death there.
The list continues. The excellent work that Dr. Pollock and his colleagues at DAN have done gathering information on breath-hold dive fatalities has produced at least eight more deaths that can be attributed to breath-hold diving.
These reports are troubling, but maybe the military breath-hold divers are doing better. Or maybe not. The next individual, as described in a 1997 case report was an instructor at the SEAL training command. There is a 60 ft [18 m] training tower there that they use to teach the students free and buoyant ascents. This instructor was an excellent breath-hold diver and it was his practice to freedive down to the bottom of the pool, sit down, fold his arms, and watch the students as they did their free ascents. On the dive in question, the other instructors on the surface noticed that he had been down for a long time and appeared to be slumped over. We do not really know exactly how long he held his breath, but he was in cardiopulmonary arrest when they got him to the surface. CPR was done and he was intubated. He was revived on the scene, but went on to die the next day at the hospital in Coronado, CA.
Next is an individual who was practicing breath-hold diving in preparation for SEAL training. He was in the pool in Camp Pendleton, CA when he died. He was a bodyguard for the US Marine Corps commandant, which means that he was an outstanding Marine – lost to breath-hold diving.
The next case occurred in a SEAL student in August, 2001. He was doing underwater knot tying, in which the students go down and tie knots that they will use in preparing explosive charges to blow up beach obstacles. He gave the instructor an “OK” and started to swim for the surface, but as he came up, he lost consciousness while still in the water. He went on to have an anoxic seizure and develop cardiac arrest at the surface. He was resuscitated but had some residual cognitive deficits that precluded him from remaining in SEAL training.
A Navy Experimental Diving Unit [Panama City, FL] diver, a very experienced diver, drowned while free diving in a Florida spring. A Naval Academy midshipman, practicing breath-hold diving in preparation for SEAL training, died in the pool at the Academy. Another SEAL was treated at the Naval Hospital in Pensacola, FL, having suffered permanent neurological residual from a breath-hold dive incident in a pool. Yet another SEAL lost consciousness during a breath-hold dive and was in the intensive care unit for a week with adult respiratory distress syndrome (ARDS).
So, overall, is this an acceptable safety record?
Well, it is difficult to say. The problem is that we do not have the denominator data. In addition, all of the details of the events are not known. Alcohol may have been a factor in some of the fatality reports. Medical issues could also have caused loss of consciousness as well, although the cases we have discussed here involved mostly young, fit divers.
As a general statement, however, when talking about recreational diving or military training, I think there should be little or no tolerance for breath-hold dive fatalities.
If we were going to try to make breath-hold diving safer, where would we start? Let me start with a question. How long can an individual hold his or her breath before becoming unconscious?
CAPT David Southerland of the Navy Dive School is fond of saying that most questions in medicine can be answered with two words: “It depends.”
That is definitely the case with this question. How long a diver can hold his or her breath underwater depends. Is the diver in air or immersed? Resting or exercising? Breathing air or oxygen before the breath-hold? From a family of elite breath-hold divers? Following a specific training regimen? What training regimen? Are techniques like hyperventilation or lung packing being used?
Breath-hold duration is frequently based on resting, or armchair, performance. Figure 1 depicts the arterial oxygen saturation measured with pulse oximetry (SpO2) as a function of breath-hold time in a single, non-elite, male breath-hold diver (unpublished data). The individual hyperventilated on room air for 30 s prior to breath-hold. The subject remained at rest during one breath-hold, and pedaled a cycle ergometer at a rate to consume approximately 400 kcal·h-1 during the other.
Figure 1: Arterial oxygen saturation and breath-hold duration during resting and exercising breath-hold (unpublished data)
Oxygen saturation remains quite stable the baseline value of 98-100% for the first 1:30 min:s in both cases. It begins to drop after that much more sharply for the exercising case. The breath-hold duration is cut in half with exercise, from four minutes to two minutes. Increasing the oxygen consumption, not surprisingly, decreases breath-hold time and results in more rapid oxygen desaturation. The SpO2 at breath-hold breakpoint was 80% during rest and 85% with exercise.
A more dramatic example is seen in the information provided on Tanya Streeter’s website. In August, 2002, on her 525 ft (160 m) No Limits record dive, during which she rode a weighted sled down and a buoyant bag up, her breath-hold time was 3:26 min:s. In July, 2003, on her Constant Weight Without Fins record dive, which entails more exercise, the dive depth was shallower (115 ft [35 m]) and the breath-hold time was shorter by about half (1:44 min:s). Here again, exercise cut breath-hold time approximately in half.
With this understanding, let us go back and restate our question more precisely. How long can an exercising, immersed diver hold his or her breath without losing consciousness?
The published literature provides insights on this question. Many studies have been done on the Ama breath-hold divers. Stanek et al. (1) studied four AMA divers, and found that in 92 routine dives lasting from 0:15 to 0:44 min:s, arterial saturation did not drop from pre-dive values of 98%. It is interesting that these Ama divers do not routinely attempt heroic breath-hold times. This is their living. They are going to be breath-hold diving their entire lives, and want to do it safely.
When Stanek et al. (1) asked the Ama divers to hold their breath for as long as possible, they found that in 15 dives, the average breath-hold time was only 1:09 min:s, but the mean arterial saturation had dropped to 73%. Remember that once the oxygen saturation begins to falls, it does so rapidly. The authors concluded that “…it seemed that at least 60 s of breath-hold preceded the onset of desaturation of arterial blood in these divers.”
A quote from Drs. Lundgren and Ferrigno (2) in 1999: “The professional breath-hold divers of Japan and Korea have an excellent safety record as they limit their dives to not much longer than one minute to avoid hypoxia.”
Dr. Wong (3) published a study of pearl divers in the Tuamato Archipelago, near Tahiti in 1999. It is the practice of these divers to hyperventilate for three to 10 min before beginning their breath-hold dives. They also typically make a weighted decent, which reduces the amount of exertion needed for descent. They averaged 1:30 min:s on their freedives in this series, with an impressive maximum time of 2:35 min:s. Less impressive is their safety record, with five episodes of hypoxic loss of consciousness in one six-hour workday among the 235 working divers. One of these episodes resulted in a fatality.
Drs. Lanphier and Rahn (4) from Buffalo published a breath-hold study in 1963. They looked at seven exposures of 60 s with immersed and working divers and had no loss of consciousness. They then did six exposures at 80 s. There were again no episodes of loss of consciousness on these dives, but two of the six divers had symptoms of hypoxia at the end of the dive. So, going out to 80 s on an immersed exercising dive, at least in this study, gave us a 33% incidence of symptomatic hypoxia, with a lowest PaO2 of 24 mm Hg and a lowest arterial saturation of 58%.
Drs. Pollock and Vann (5) from Duke reported a study in 2000 that was funded by the U.S. Special Operations Command. Seven divers performed immersed, exercising breath-hold dives after hyperventilation and had a mean breath-hold time of 1:26 min:s. There were no episodes of hypoxic loss of consciousness in these seven dives. When discussing the topic of safe limits for breath-hold dives, they proposed a two minute limit at rest and a one minute limit for exercising divers.
Dr. Qvist et al. (6) in 1993 studied arterial blood gases during immersed, exercising dives performed by five Korean Ama divers. Once again, these divers did not seek to achieve maximal breath-hold times, with their average dive being approximately 0:30 min:s. The investigators then requested that the divers hold their breath as long as possible for a series of 37 dives. The mean breath-hold time on these dives was found to be 1:02 min:s with a maximum time of 1:24 min:s. The PaO2 after this longest dive was 33 mm Hg and the arterial oxygen saturation was 59%. The authors stated in conclusion: “In the current study, dives that lasted longer than 55 s were associated with large and potentially dangerous decreases of arterial oxygen pressure and content.”
An article from Alert Diver in 2005 (7) noted that: “We really do not have a definitive study on the issue of a safe breath-hold limit for exercising free drivers but the number best supported by the data at this time is 60 s.”
A recent chapter on military diving (8) stated: “The practice of immersed, exercising divers holding their breath in excess of 60 s should be re-evaluated.” In the interest of full disclosure, I authored the Alert Diver article and co-authored the Bove chapter.
Let us finish up with four proposals for your consideration.
Proposal One is that the diving medical community establish 60 s as a maximum recommended breath-hold time for non-competitive breath- hold diving
I know that two groups are not going to be big fans of this idea: breath-hold spearfishers and elite, competitive breath-hold divers. I certainly do not have the expertise to presume to make any recommendations for a group of world-class athletes like these two communities, but let us acknowledge that these divers are just that. They are on the cutting edge of their sport, attempting heroic breath-hold times, and are responsible for establishing their own limits and their own safety techniques. They also are willing to accept a greater level of risk than we would for recreational diving or military operations.
I compare them to elite mountaineers who attempt the summit of Everest. Historically, the average mortality on summit attempts has been around 10%. There were just over 100 climbers in base camp at Everest in 1989. Eleven of them were killed. So these are people who do extraordinary things, and accept extraordinary risks to do them.
Non-elite divers do not need to take similar risks just to go out and look at the reef creatures.
Proposal Two, assuming that we are going to terminate our breath-hold dives at 60 s, is to allow 30 s of hyperventilation. There is no evidence to suggest that you increase your risk of hypoxic loss of consciousness by hyperventilation – as long as you end your dive at 60 s. This also makes the dive much more comfortable. It is a totally different experience to go down and stay for 60 s without air hunger. Additionally, it allows you to have better task focus, whether that task is a military one or personal one.
Proposal Three is to better define the maximal safe breath-hold time for immersed, exercising divers in the laboratory setting. A study at Buffalo or Duke could standardize the oxygen consumption in a group of divers to the appropriate level for underwater swimming, monitor oxygen saturation during the exposure, and determine if the divers can safely go for longer durations. One of the issues that we would have to come to grips with is: “How safe is safe enough?” If you perform 50 exposures breath- holding to 90 s and have two losses of consciousness, is that safe enough? That question would have to be answered prior to doing the study.
Proposal Four is that we do a better job of collecting data from breath-hold dive events, competitions, and accidents and analyze this data appropriately. The researchers at DAN have made a very strong start on this. More data on this topic from non-controlled breath-hold diving events and accidents would allow us to get a more precise risk analysis and a better definition of a safe breath-hold limit for immersed, exercising divers.
- Stanek KS, Guyton GP, Hurford WE, Park YS, Ahn DW, Qvist J, Falke KJ, Hong SK, Kobayashi K, Kobayashi H, et al. Continuous pulse oximetry in the breath-hold diving women of Korea and Japan. Undersea Hyperb Med 1993; 20(4): 297-307.
Ferrigno M, Lundgren C. Human breath-hold diving. In: Lundgren CEG, Miller JN, eds. The Lung at Depth. New York, Marcel Dekker, 1999.
Wong RM. Taravana revisited: decompression sickness after breath-hold diving. SPUMS 1999; 29(3): 126-131.
Lanphier Eh, Rahn H. Alveolar gas exchange during breath-hold diving. J Appl Physiol 1963; 18(3): 471-477.
Vann RD, Pollock NW, Natoli MJ, Corkey WB: Oxygen-enhanced breath-hold diving. Center for Hyperbaric Meddicine and Environmental Physiology Report. Durham; Duke University Medical Center; 24 March 2000.
- Qvist J, Hurford WE, Park YS, Radermacher P, Falke KJ, Ahn DW, Guyton GP, Stanek KS, Hong SK, Weber RF, et al. Arterial blood gas tensions during breath-hold diving in the Korean Ama. J Appl Physiol 1993; 75(1): 285-293.
Butler FK: Breath-hold diving: a proposed 60-second rule. Alert Diver 2004; September/October: 35-39.
Butler FK, Smith DJ. United States Navy Diving Techniques and Equipment. In: Bove AA, ed. Diving Medicine, fourth ed. W.B. Saunders: Philadelphia, 2004: 547-574.
DR. FOTHERGILL: You did not mention one population, and that was the U.S. Navy diving population. How do you speculate if you put in the proposal of the 60-second breath-hold limit would impact extended training procedures in dive school and SEAL training?
DR. BUTLER: This proposal has actually been made, but not implemented to date. I would like to say that we have not lost the battle. We have just not won it yet.
DR. SCHAGATAY: I wonder why focus on time, because for one thing we know that all these various individuals are untrained. I think that the recommendation of time would focus on the wrong thing because what you have to learn to focus on are your body signals. To have a recommendation that everyone surfaces at the first involuntary contraction would be much more useful system for most divers, for the ones with the short breath-holding ability during exercise and for the ones with a good natural ability. So I do not see what purpose it would be to have a recommendation of 30 s or 60 s or three minutes at all actually.
DR. BUTLER: If we established a limit based on the first involuntary contraction, what time would that give us?
DR. SCHAGATAY: Does not matter, because the interesting thing is to limit the risk.
DR. BUTLER: I do not know. That could be unnecessarily restrictive, but it is an interesting counterproposal.
DR. SCHAGATAY: It would be an average of maybe 60 s anyway, but I think in itself the recommendation should focus on the individual ability to make it a good system.
DR. BUTLER: Right. Well, I do not necessarily disagree with that – it is an interesting physiologic alarm clock. However, I do not think it is the practice of any breath-hold dive communities that I know of to do that. The idea is typically to hold your breath through that symptom.
DR. SCHAGATAY: I am training children with free diving, and we have also in the very young kids we can see some kids that can breath-hold for 15 s and some who can easily do three minutes. I also measured the saturation in some of them just to see what they are doing, and they have a very different situation. I mean, they – if they learn how to recognize the signals, they will surface in time.
DR. BUTLER: It would be interesting to compare the two and see what they each give us for limits. I just have no feel for exactly what sort of limit that would give us and how safe it would be.
DR. LUNDGREN: If I could make a very brief remark from here. We heard of some, and I certainly have no feeling for how many it would involve, who really do not feel anything.
DR. BUTLER: Yes. Certainly there are a few of those out there. If I do not hyperventilate, I start to get that urge to breathe fairly early on. We would have to define whether or not we allowed the divers to hyperventilate before the dive.
DR. SCHAGATAY: It only works when you do not have them hyperventilate first.
DR. BUTLER: Well, if you did not hyperventilate, my guess is that you would get a safer but a more restrictive limit. But, again, I have not seen data to describe the time to onset of that particular symptom in a study.
DR. YOUNGBLOOD: Thanks, first of all, for an excellent and provocative presentation. I would like to point out one thing, unlikely though it might be, in the support for limited hyperventilation. That is it gives you a cushion if you should be so unlikely as to get entangled in a fishing line or have a boat come roaring up overhead and need some extra time, just as long as you do not use it up for things that you should not.
DR. BUTLER. Thank you for that point.
DR. SMITH: It would seem to me that we could certainly do more preventive and diagnostic work on people that are interested in doing this, and maybe that is something DAN is already doing. If not, they could look into it. People would pay to have it done. I mean, basically, everything from a stress test to heart rate variability testing.
We had a University of Florida freshman football player who is the star of the team fall over dead in practice. We read about it everyday in the paper. Obviously, some of the injuries could be related to that as a function of low oxygen saturation, a little more acidosis in the heart triggering arrhythmia. So if we really knew beforehand – I mean, I can see the future being, it would be just like pilots. A pilot they do it because they have got somebody else usually in the plane with them. But for the divers to have an annual, just look at your blood work, blood pressure, and general indices indicating what condition you are in. And one of the neatest things out there now is heart rate variability as a diagnostic indicator of whether you are really at your best on a given day or not. I could see a time when people would actually determine that, and if your baseline is at a certain level on a given day, you are 20 or 30% less, it is a real wakeup call.
I agree with Dr. Schagatay, it is so individual. To expect somebody like some of the people sitting back here to go down for one minute is not at all realistic. And yet there are a lot of people, and I see them all the time, free diving in spearfishing are really asking for trouble at two and a half and three minutes and what not.
DR. BUTLER: To reiterate, there is no presumption of making any recommendations for the elite, world class breath-hold diver. This is for a recreational freediver or an individual who shows up at SEAL training with little or no experience in breath-hold diving.
In my research, I really did not find anybody who became hypoxic in 60 s, but it all depends on your oxygen consumption. If you have an individual who is really flailing around on the bottom, I am not one hundred percent sure that you could not have a hypoxic loss of consciousness in less than 60 s. I have just never heard or read of it happening.
DR. SMITH: A huge factor in the spearfishing community is even take a one or two-knot current in the water when you are used to doing a 40-50 ft [12-15 m] dive looking for grouper when it is at slack tide, just having to swim upstream and then having to go down and you put that extra effort, that is a real problem. I can see one thing in the future would be very interesting. I would think we could create a waterproof oxygen saturation monitor that you could put over the carotid artery that could feed back the vibratory signal that as the vibration became stronger you would realize that my sat has gone from 98 to 88% and now it is down to 80% and I better be getting to the top real soon.
DR. BUTLER: I think that is a very interesting proposal. Other people have proposed variations of that. If the Navy or the military community acknowledged that there was a problem in this area, it would be not technologically hard to do.
DR. VANN: What you are asking for is essentially population statistics. And that is the kind of information you need before you get into this – not scientific information. You need that before you get into the more political questions about what is safety. I think what you propose is not really inconsistent at all with what Dr. Schagatay was saying because you are going to get that same information on the first contraction from your study. So you are going to get them both. And you can play either way.
Remember, there is a difference between doing the science and then making safe decisions. That should be done in totally different context. And mixing them up is just asking for trouble.
DR. BUTLER: I agree with that premise, but if you look back at the research that is been done on breath-hold diving, most of it has not been focused on the issue of how long a diver can hold his or her breath before they become unconscious. Most of the studies on breath-hold duration focus on the effect of various factors such as training and water temperature on breath-hold break time. As you train, you increase your breath-hold duration, but there is no evidence that I can find that says that (in the absence of perhaps some specific things like lung packing) training increases the time required for the diver to suffer a hypoxic loss of consciousness.
So – another way of looking at training is that as you get to be a better and better breath-hold diver, you are getting closer and closer to the point where the lights go out. I think that that is something that has to be included in the discussion as well.
If you were going to make a judgment evaluation of how safe is safe enough, that is a topic that I am sure that we could talk about for a long time. A single episode of hypoxic loss of consciousness could represent a fatality in an unmonitored situation.
DR. LINDHOLM: I would like to make a few comments, partly in support of Dr. Butler’s idea about the time limit. One, the first is, that you cannot use contractions the same, because if you are diving at depth, if you get contractions and you start to surface, it is a big difference if you are at the surface or if you are at 30 m (100 ft), or if you are at 100 m (300 ft). So using the urge to breathe as a signal is probably not a good idea.
Another study on hyperventilation was done by P.G. Landsberg about 30 years ago, and his final recommendation was to have the spearfishermen or sports divers of South Africa to use a watch with an alarm clock.
[Landsberg PG. Hyperventilation: an unpredictable danger to the sports diver. In: Lundgren CEG, Ferrigno M, eds. Physiology of Breath-hold Diving. Undersea Hyperbaric Medical Society: Bethesda, MD, 1987: 256-267.]
I would also agree with Dr. Vann that the problem is that we do not have any population data. There has yet been no study where you look at exercise, different work loads, and you correlate with lung volumes and look at bigger material. There has been a few studies with up to maybe 20 subjects, but there are no real good population studies. So what duration that would be appropriate (60 s?, 75 s?), that is another concern.
And finally, I would also like to say that if you hyperventilate, you increase the oxygen stores, as was mentioned here, so you have a bigger margin, but you probably also reduce your oxygen consumption, because you are very much more relaxed. In fact, those contractions can, at least for me, they cause a little effort. It is painful. If you completely relax, you reduce your oxygen consumption. So you probably, if you hyperventilate, you actually push the duration forward even more. Not to say that hyperventilation is safe, but it is a good point to have that you might need to reevaluate how we perform the safe training dives.
DR. BUTLER: Thank you.
MR. KRACK: I certainly appreciate the presentation, where you are coming from with it. I guess I worry about things that are perceived as rules that would make things to be safe or the perception of safe when if, for example, even in hyperventilating for 30 s it is possible to experience a blackout. When we put in rules that would say 60 s, one minute breath holds. But that mitigates the idea of really the overall problem that I would say most of those facilities being without lack of, and I will always go back to this, direct supervision, buddy contact. If I do not dive with a buddy but I maintain my dives within the one minute rule, I am safe. I think we always have to come back to the idea that buddy contact and direct supervision is the key to it.
A one minute breath hold could be a relaxed 30 m [98 ft] down and 30 m back up in a typical dive profile. Persons trying to better their depth are going to increase their speed, which would work against them at that point. So I can understand the pros to the ideas, but there would certainly be a lot of cons to it that need to be addressed.
DR. BUTLER: I appreciate your point. That is why I took some pains to exclude the elite breath-hold divers from the proposals.
MR. KRACK: I would not say that 100 ft [30 m] indicates an extreme breath-hold diver. That is becoming a very common recreational freedive nowadays.
DR. BUTLER: I am not trying to include a depth consideration in this discussion. It really is more about time and oxygen consumption. Some of the techniques used by elite breath-hold divers, such as riding a sled down and a balloon up, reduce oxygen consumption significantly.
What the studies that I presented had in common was that they looked at exercising, immersed divers. They were engaged in an activity that produced an oxygen consumption that we could then extrapolate to other immersed, exercising divers, however roughly
One point about the freediving spearfishers, many of whom are also elite breath-hold divers, is that when they are stalking, that may also have the effect of minimizing oxygen consumption as well. They drop down to a reef and wait for the fish to come to them. Believe me, the fish can swim faster than you can, so that approach makes a lot of sense. So when you see elite spearfishers with two and a half minute breath-hold times, I think it is a mistake for the average novice freediver to extrapolate that to his situation if he is swimming continuously underwater.
It is all about oxygen consumption and there are lots of factors that can change that, such as cold water versus warm water. There was a study in Buffalo that looked at the increase in oxygen consumption just from being immersed in cold water and it went up by a factor of 250%. How warm the water is where you are diving can make a huge difference in your oxygen consumption and how long you can hold your breath safely.
NOTE: To access the entire proceedings of the UHMS DAN 2006 Breath-hold Proceedings, visit Divers Alert Network.
In: Lindholm P, Pollock NW, Lundgren CEG, eds. Breath-hold diving. Proceedings of the Undersea and Hyperbaric Medical Society/Divers Alert Network 2006 June 20-21 Workshop. Durham, NC: Divers Alert Network; 2006.