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Should Tech Divers Be Thinking More About Equalization—Like Freedivers Do?

Compared to Frenzel equalization used by freedivers, Valsalva equalization can be hard on the ears and eustachian tubes, and could cause problems for tech divers with undetected PFOs. Here scuba instructor turned freediver Charly Stringer argues the case for Frenzel and greater equalization awareness. Community comments included.



By Charly Stringer

Header photo by Derk Remmers

Technical divers plan out their dives extensively before entering the water, they think of every small detail, from gas percentages to time limits. 

Freedivers plan too, they plan how deep they will go in the session and they plan out their long term training goals. But, what freedivers focus on probably the most is equalisation. This is something that scuba divers rarely have to think about, but in freediving, bad equalisation can ruin a dive session and stop you in your tracks when trying to get deeper.

But, should technical divers be taking a leaf out of their bubble-less friends’ books and thinking more about their equalisation? What if I was to tell you that the way you’re equalising during a tech dive could affect your likelihood of getting bent?

Your first response might be: “Don’t talk crazy, what has equalising the ears got to do with decompression illness (DCI)?”

Good question, but hear me out…

Photo by Julian Mühlenhaus.

Equalisation techniques in freediving vs scuba diving

First, let’s explore the differences in equalisation techniques used by freedivers and scuba divers. Some people don’t know that there are different ways to equalise the ears and sinuses, it’s something they just do, without paying much attention to which body parts they are using. But actually, there are two main ways to equalise, and they are very different:

Photo by Markus Dirschl @markus.dirschl


The Valsalva maneuver involves blocking the airways and pushing with the abdominals to create enough pressure to equalise the ears. This technique can be quite forceful and can put strain on the respiratory muscles and the heart.

It is the most common equalisation technique with scuba divers and with beginner freedivers because it is the simplest to perform. For scuba diving, the Valsalva technique is adequate because the diver has time to stop, breathe, and take their time.

For freediving, however, Valsalva is not desirable for equalising the ears. Firstly, it’s not very relaxing because of how strenuous it is. Secondly, pressure increases as the freediver gets deeper, it becomes almost impossible to perform the Valsalva technique because it requires a good amount of air in the lungs to be performed. As the diver gets deeper, the air in the lungs compresses and therefore can’t be used to Valsalva, unlike in scuba where the diver can simply breathe more air to the lungs. 

Valsalva can sometimes be ineffective as it doesn’t activate muscles which open the Eustachian tubes, so it may not work if the tubes are already locked by a pressure differential.


The Frenzel maneuver is a bit more technical. It involves bringing air up into the throat, closing the glottis, and then contracting the throat. This creates pressure in the nasal cavity, which opens the eustachian tubes, and equalises the middle ear pressure. This technique is a little trickier but is much easier on the body, assuming that the diver can keep up with the descent. As all the work is done in the throat area, and it doesn’t require the lungs to be so full, it can be performed a lot deeper underwater. It’s also much easier to stay relaxed this way, making it perfect for freedivers.

Photo by Nacho Palaez Mella : @nachopelaezphotography

Some people will perform the Frenzel technique naturally, however it is less common. A lot of people who have scuba dived for many years, come to freediving with confidence that their equalisation will not be a problem, because they have been Valsalvaing successfully for all that time. As they have had almost unlimited air, and the luxury of taking their time to equalise before moving deeper, Valsalva has been good to them. But when your body has a history of autopilot Valsalva, it’s hard to break that habit when switching to Frenzel for freediving, and they can struggle.

So Frenzel has become the technique of choice for freediving. Many freedivers that want to improve, and get deeper in their diving spend hours practicing equalisation. Those that naturally Valsalva have to really create an awareness to switch over to Frenzel and for some people this comes very quickly, for others it can take time. There are coaches that specialise in teaching equalisation. This is something that scuba divers never really have to practice or focus on.

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Decompression illness, Valsalva, and PFOs

We know that most tech divers are using Valsalva, and we know that Valsalva can put pressure on the heart. But could this be a factor in getting bent? Well, it could if they also have a PFO…

Patent foramen ovale (PFO) is a hole in the heart between the upper right and left chambers. This is something we all have while we’re in the womb, but for most people it closes up after birth.

Studies have shown that divers who suffer from decompression illness (DCI) have a PFO prevalence twice that of the general population, and most of those divers have a PFO that is a centimeter in diameter or larger.

You might be surprised to learn that over a quarter of people have a PFO, though in some cases these are small and not psychologically relvant. Most of which will never have symptoms, and it will never cause them any health issues. However, for scuba divers, the risk of getting DCI increases five to 13-fold (some researchers place this risk at closer to 3-5 times) by having a PFO because of an increased risk of inert gas shunting (bubbles moving from the right heart chamber to the left). 

Photo by Derk Remmers.

For recreational scuba divers that stick to their no decompression limits, the risk of DCI is higher with a PFO, but it’s still very low. However, technical divers have a higher risk of DCI because of the depths they are diving to and the amount of time in which they are staying at those depths i.e. greater gas loading. There is less room for error in tech; every detail needs to be planned out and stuck to… make a mistake and you’re likely to get into trouble.

So where does Valsalva come into play?

A study funded by DAN Europe on PFOs and decompression sickness in sport divers found that divers who suffered cerebral DCS (in which a large number of grade 2 PFOs were found) frequently induced sustained and severely strained Valsalva maneuvers to equalise their ears whilst diving. Of course, since Valsalva is the most commonly used equalizing technique for scuba, there are likely many divers are using it without incident.

Diagram of  PFO closure procedure.

A PFO can be determined by using something called a “bubble study”. During this study the physician will inject the patient’s vein with air and get the patient to perform the Valsalva maneuver. This raises the pressures in the right side of the heart and will show bubbles in the left atrium if the patient has a PFO. 

So the fact that they are using the Valsalva technique to prove that a patient has a PFO by deliberately pushing bubbles through it, is a possible indication that we shouldn’t be using it at depth; because we know that inert gas being shunted through the PFO causes a higher risk of DCI when diving. [Ed.note: However, bubbles more likely to form on ascent not at depth]

So, it seems that the way tech divers are equalising could actually be a risk factor to consider when it comes to preventing DCI.

Preventative Measures

Based on the knowledge that over a quarter of people have a PFO and that most don’t know that they have one, we can assume that over a quarter of tech divers have a PFO, and may not find out until they potentially suffer from DCI in cases where they are psychologically relevant. Few divers will likely consult their doctor to check to see if they might have a PFO in the absence of symptoms. 

So, what could tech divers do to reduce the risk of getting DCI, in addition to planning their dives extensively and diving conservatively? They could switch their equalisation technique from Valsalva to Frenzel as a preventative measure. There are really no negative points to switching over: Frenzelling is easy when you get the hang of it, it could potentially lower your risk of DCI, and has other benefits too, such as being gentler on the ears, conserving more energy, and requiring less air to perform.

Photo by Julian Mühlenhaus, GUE Creative Trip 2021.

To be clear, I am in no way suggesting that switching to Frenzel is circumvent to surgical procedures if you do have a PFO. If you know that you do have one, you should follow your doctor’s advice when it comes to diving.

As mentioned earlier, there are freediving coaches that specialise in teaching people how to Frenzel, as well as Youtube videos and articles. With some practice, Frenzel can become the new autopilot equalisation when diving, causing a lot less strain on the heart and therefore, reducing the risk of getting bent.

Comments from The Field:

We asked a few people we thought were knowledgeable to respond to Charly’s piece. Here is what they had to say.

Photo by Derk Remmers.

Eric Albinsson

Instructor Development Programs Specialist, PADI Americas 

“The author’s rationale for avoiding Valsalva as a means to lessen the risk of DCO in cases where the diver has a PFO seems to make sense given how forceful the procedure actually is (can be) relative to other techniques.  Even DAN states that with Valsalva there’s risk of other injuries occurring—See Beat the Squeeze: Equalize Like a Pro

 In my view, everyone can benefit from transitioning from Valsalva to Frenzel, or similar technique, simply because not only is it generally easier to do regardless of head orientation, but it’s generally viewed as gentler on the ears and as divers, whether tech, rec, freediver or airplane flyers, we should be as gentle as possible on our ears if we want to keep diving as much as possible.  I also found this WebMD page that alleges the gentler aspect of Frenzel. May your ears, mask and visibility always be clear.”

Kirk Krack

Performance Freediving Academy CEO/Founder, Performance Freediving International President/Founder

“I agree with what Charly wrote. Frenzel is superior to Valsalva and has many benefits as described including the advantage regarding PFO’s. The main advantage is it provides superior pressure that’s more immediate without straining the chest wall and thereby compressing the heart as it is all conducted at the throat. Essential you’re compressing a volume of air the size of your thumb vs almost two gallons which helps create higher pressure. Think of a thee stage air compressor. The first stage is the low pressure (0-1500psi) and the piston is quite large and slow whereas the third stage (2500-3000) is quite large and fast. 

Additionally, it’s somewhat simple to learn and I’d say most professional scuba divers naturally move to this type of equalization because the body will tend to find efficiency. For those that don’t use Frenzel it can be a short and simple online course.”

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Steve Lewis

Training Director, RAID Inc.

“Bold statements but no citations except for the related DAN study on PFO, which doesn’t make the case. By all means use the Frenzel technique (I do at times) but not to circumvent dealing surgically with a hole in your heart.”

Ted Harty

Principal & Founder Immersion Freediving 

“Frenzel is a superior equalizing method in every way. The only downside is that for the people that don’t naturally do it, it’s tricky to learn. If the tech diver or scuba diver has ZERO issues equalizing than I would just keep doing what they are doing, but I know lots of scuba divers have equalization issues, that would be fixed with switching to Frenzel but so few people in the scuba industry really understand the differences between the two. I honestly look at this article as yet even another reason to use Frenzel over Valsalva!” 

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Dive Deeper


DAN Europe: EqualEasy – Equalisation Awareness

DAN US Smart Guide to Ear Equalization: Beat the Squeeze: Equalize Like a Pro (Downloadable)

Instagram Live: EqualizationsTechniques from Ted Harty

YouTube: How to Frenzel Equalize: an equalisation tutorial from a Professional Freediver by Adam Stern

Go Freediving: The Definitive Guide to the Frenzel Technique – With Emma Farrell

Ted Harty’s Immersion Freediving OnLine Course: Make your equalizing problems a thing of the past.

FreeDive Passion (Dahab): Personal Coaching


Journal of Applied Physiology: Patent foramen ovale and decompression sickness in sports divers (DAN Europe)

InDepth: Everything You Wanted To Know About PFOs and Decompression Illness Patent Foramen Ovale (PFO) – Divers Alert Network (Discusses PFOs and Valsalva): The “Bubble Study” for Patent Foramen Ovale 

Charly is a writer who’s originally from the UK but has been based in Dahab, Egypt for the past four years. She taught scuba diving for three years in Cyprus, Thailand, and Egypt before discovering her love of freediving. She still scuba dives for fun but these days she’s more focused on her freediving training. When she’s not in the water, diving, she’s on her laptop, writing about diving.


Personal Coaching from a Professional Freediver | Dahab | Freedive Passion


Hyperbaric Chambers Are Turning Away Divers. Will There Be One Nearby When You Need It?

Unfortunately, it’s hard to make a business case for treating divers versus wound and burn care victims. As a result, many hyperbaric chambers no longer treat divers, leaving fewer facilities available for divers in need and increasing their post-dive time to treatment. InDEPTH editor Ashley Stewart reports on this growing crisis in the US and what can be done!




By Ashley Stewart

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Steven Wells was diving on the scuttled wreck of the USS Oriskany off the coast of Florida in 2016 when a problem with his buoyancy compensator caused a rapid ascent to the surface.

Wells’ dive buddies followed the emergency action plan for the Oriskany listed on the Florida Fish and Wildlife Conservation Commission’s website at the time and brought Wells straight to Naval Air Station Pensacola, the nearest facility with a hyperbaric chamber. The facility turned him away because there was no one there to run it.

Wells was taken 30 minutes away to Baptist Hospital, which also has a chamber capable of treating his injuries, but the hospital had years earlier decided only to use it for wound care. Doctors there decided Wells would be taken by ambulance more than an hour away to Mobile, Alabama, the nearest facility that accepts divers.

By the time Wells arrived at the only chamber that would help him, it was too late.

Steven Wells

“I got a call from the hospital saying, ‘Your husband is on life support. You need to get here now,’” Rachel Wells said of her late-husband of more than 23 years. 

Julio Garcia — the program director of Springhill Medical Center’s wound care and hyperbaric facility where Steven Wells was to be treated — told InDEPTH that while no one can be certain how sooner treatment would have affected the outcome of Wells’ case, it would have given him the best chance for a full recovery.

Each year in the US, there are about 400 serious cases of decompression illness (DCI) — a category including both arterial gas embolism and decompression sickness — in divers, according to one 2020 paper. The Divers Alert Network (DAN) hotline dealt with 587 cases annually over the past five years.

The availability of hyperbaric chambers to treat decompression illness is something many divers take for granted. We try to avoid dive-related injuries through training, but expect treatment to be available when we need it. 

The reality — as Steven and Rachel Wells tragically learned — is that only a minority of divers are close to care for diving-related injuries, according to medical professionals in the field. The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

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The estimates vary, but it’s generally believed there are about 1,500 hyperbaric medicine facilities in the US and only 67 are currently treating diving accidents, according to DAN.

“The problem is only getting worse, not better,” Garcia, the Springhill Medical Center program director, said. Garcia has been sounding the alarm about this problem for more than a decade. His hospital takes patients from as far away as Florida cave country and treated 20 DCI cases in 2022. Those patients had an average transportation time of 11.5 hours, according to an InDEPTH analysis of Garcia’s records.

Florida stands out because it’s a popular diving destination, DAN Research Director Frauke Tillmans said, but the situation is not much better across the US. Many of the 1,500 hyperbaric medicine facilities, like Pensacola’s Baptist Hospital, have transitioned to treating wound care only for economic reasons. Emergency hyperbaric services are expensive to train and staff, and come with increased liability.

Patient briefing before treatment at the Environmental Medicine and Physiology Unit at Simon Fraser University. 

Time to treatment can be important in DCI cases

Time is of the essence when treating DCI. Divers Alert Network Director of Medical Services Camilo Saraiva told InDEPTH time to treatment is a “pivotal determinant” when it comes to outcomes for DCI patients. “Swift intervention significantly influences the effectiveness of therapeutic recompression,” Saraiva said.

Decompression sickness, for example, results from rapid changes in pressure and can form gas bubbles in body tissues. Initiating recompression therapy minimizes bubble size and number, Saraiva said, enhancing their elimination and reducing the risk of further vascular obstruction and tissue damage.

“The timely provision of hyperbaric oxygen therapy not only aids in bubble resolution but also mitigates the potential for neurological deficits and other severe complications, highlighting the critical role of early treatment in optimizing outcomes for DCI patients,” Saraiva said.

The 2018 paper “In water-recompression” stated delays to recompression in military and experimental diving are typically less than two hours and more than 90% of cases are completely resolved during the first treatment.

Frank K. Butler and Richard E. Moon, hyperbaric medicine experts, wrote in a 2020 letter to the Undersea and Hyperbaric Medicine journal editors suggesting a minority of patients who need life-saving hyperbaric oxygen treatment (HBO2) are close to a major hospital with a 24-hour emergency hyperbaric facility.

Julio Garcia’s log on patient time to treatment at Springhill Medical Center. Click to enlarge

“Despite the urgent need for treatment, most hyperbaric chambers will decline to accept emergent patients at present,” Butler and Moon wrote. “Patients may eventually receive HBO2 but after a significant delay and a transfer of several hundred miles. Many never receive indicated HBO2, often resulting in poorer patient outcomes.”

Patients who are delayed treatment, they wrote, face the possibility in some cases of “death, permanent neurological damage, permanent loss of vision, or loss of an extremity, most of which would have been readily preventable had emergent HBO2 been administered.”

Why fewer chambers treat dive injuries

As recently as two decades ago, according to Butler and Moon, the majority of hyperbaric treatment facilities were available 24/7 to treat emergency patients. The percentage of those facilities now treating emergency patients is unclear, but it’s universally agreed the number has fallen significantly.

The reasons for the loss of emergency HBO2 facilities, Butler and Moon suggest, include “a better economic return when chambers focus on wound care patients as opposed to emergencies; the greater legal liability involved with treating high-acuity emergency patients; and the increased training and staffing requirements that would be required to manage critically ill patients — especially diving injuries and iatrogenic gas embolism patients.”

A letter from an administrator at Baptist Hospital — which sent Steve Wells to Springhill Medical Center — viewed by InDEPTH shows the hospital discontinued hyperbaric emergency services in December 2010, citing lack of staffing for specialty trained hyperbaric physicians who can provide 24-hour patient care. Baptist has yet to respond to InDEPTH’s request for comment.

Julio R. Garcia at Springhill Medical Center Hyperbaric Center

There’s also the issue of pay. Garcia, the Springhill program director, said the current rate of pay for doctors who administer hyperbaric treatments regardless of length is around $150. A typical hyperbaric treatment for other conditions is about two hours. Diving treatments are usually six or seven, he said. “What doctor wants to get paid $150 to be up all night for seven hours, at that point making less than the technician?” Garcia said. “The fix is that healthcare payers need to pay more for the supervision of the treatment for diving injuries. Make it something that’s worth a doctor’s time besides the goodness of their hearts.”

Silence from lawmakers

Medical and diving organizations in 2020 sent a letter to the House and Senate, federal government agencies, governors of Florida and California, and the American Hospital Association expressing concerns about the lack of availability of chambers to treat diving injuries.

“There are approximately three million recreational scuba divers in the US,” the letter stated. “In the unlikely event that they suffer a diving-related injury, they trust that the US medical system will provide state-of-the-art care for their injuries, but the steadily- decreasing number of hyperbaric treatment facilities in the US willing to treat them emergently for decompression sickness or arterial gas embolism often places them at much greater risk than they realize.”

Garcia has on his own contacted lawmakers, reporters, medical systems — even private space companies like SpaceX because his facility is also the only one nearby treating altitude decompression sickness from space and air travel.

Little has changed, Garcia said.

Garcia showed InDEPTH a 2014 letter from a Defense Health Agency director who said, while there are three Undersea and Hyperbaric Medicine Society-accredited clinic hyperbaric medicine facilities and two additional facilities that can treat civilian emergencies, they are not staffed 24/7 and not designed for patients with other medical illnesses. Garcia at the time requested the creation of a federal grant to support the expansion of 24/7 hyperbaric services, but the director said that was outside of the agencies’ purview. 

The hyperbaric chamber at the University of California San Diego. Photo courtesy of Sherri Ferguson

Two years after this exchange, Steven Wells was taken to and turned away from one of these facilities — the NAS Pensacola, listed on the Florida Fish and Wildlife Conservation Commission’s (FWC) emergency action plan at the time. 

The Florida Fish and Wildlife Conservation Commission website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia.

The FWC website now shows a map of the nearly 4,000 artificial reefs across Florida’s 1,350 miles of coastline. Two chambers, one in Mobile, Alabama, and one is Orlando, cover 500 of those miles densely packed with dive locations, according to Garcia. A report from the University of West Florida estimated the sinking of the Oriskany, scuttled in 2006, generated nearly $4 million for Pensacola and Escambia County in the next year alone.

So many reefs, so few chambers! FWC map screenshot

An FWC spokesperson said the agency provides diver safety reminders and recommended actions on its website “as a courtesy” and is not intended for emergency response. FWC and Visit Florida did not respond to inquiries about how much Florida’s government spends on advertising the artificial reefs and other diving activities, or whether any effort to expand the availability of hyperbaric facilities to treat the divers who show up as a result.

“My question is what is my husband’s life worth compared to your chambers,” Rachel Wells, Steven Wells’ widow said. “Why did he have to die?”


DIVER: A Crisis in Emergency Chamber Availability by Dan Orr (April 2022)

Divenewswire: A Crisis Lurking Below the Surface Emergency Hyperbaric Treatment Availability by Dan Orr (August 2021)

Undersea and Hyperbaric Medicine (2020): Emergency hyperbaric oxygen therapy: A service in need of resuscitation – an open letter by Frank K. Butler, MD, and Richard E. Moon, MD

White paper: Access to emergent hyperbaric oxygen (HBO2) therapy: an urgent problem in health care delivery in the United States (2020)

InDEPTH: A New Look at In-Water Recompression (IWR) (2019) by Reilly Fogarty

Diving and Hyperbaric medicine (2018): In-water Recompression, Doolette DJ and Mitchell SJ 

aquaCORPS (1993): In-Water recompression As An Emergency Field Treatment for Decompression Illness by Richard L. Pyle and David A. Youngblood

InDepth Managing Editor Ashley Stewart is a Seattle-based journalist and tech diver. Ashley started diving with Global Underwater Explorers and writing for InDepth in 2021. She is a GUE Tech 2 and CCR1 diver and on her way to becoming an instructor. In her day job, Ashley is an investigative journalist reporting on technology companies. She can be reached at:

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