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Heart Rate Variability, or simply HRV, is becoming more and more used in different fields, from an instrument able to diagnose cardiovascular anomalies, to a tool instrumental in improving high performance sports training. Many of us who are seriously into physical activities and sports practicing are familiar with or even have used one of the many apps available for watches or smart phones. But do we really know what they are?
Well, HRV is simply the variance in the interval between two heat beats. Or to be more precise, the changes in the interval between successive normal heartbeats. Usually, this is assessed through the timing between QRS complexes, which are main spikes as seen in a continuous electrocardiographic recording (ECG). HRV is the result of the balance between the sympathetic and the parasympathetic branches of the autonomic nervous system (ANS), as well as of other non-neural sources of variation. From a practical standpoint, it is as simple as a collection of time intervals, or for those more familiar with mathematical terms, it is a time series (a series of data points indexed by time).
Typically, these time intervals are not fixed or static but can vary widely moment to moment in response to input from the sympathetic and the parasympathetic branches of the autonomic nervous system, which control the contraction and relaxation of the cardiac muscle. Each system is activated by multiple receptors, responding to arterial pressure (baroreceptors), oxygen, and CO2 levels, as well as endogenous substances, emotions, immunological alterations, among other stimuli.
The parasympathetic branch, which regulates your energy and helps your body recover during rest periods, is mediated mainly by acetylcholine and is responsible for maintaining homeostatic heart frequencies and contractility without exhausting. It is responsible for short-term fluctuations of the heart frequency, and it operates in a frequency between 9 to 25 cycles per minute or 0.15Hz to 0.4Hz. Note that Hertz (Hz) is an international metric of frequency and is defined as one cycle per second (cpm).
Conversely, the sympathetic branch, which regulates your “fight or flight” stress impulses, is mediated mainly by norepinephrine, and its activity is triggered by stress, increasing cardiac energy demand by increasing heart rate . This branch is responsible for longer-term fluctuations of the heart frequency, operating in a lower bandwidth of 0.04Hz to 0.15Hz (or 2 to 9 cycles per minute).
HRV is commonly studied in the time and frequency domains, that is as a function of time and frequency. Different HRV indicators have been associated with sympathetic or parasympathetic activity. Additionally, there is an important association between specific frequencies and the baroreflex function, which provides a rapid negative feedback loop to adjust our heart frequency in order to maintain blood pressure at nearly constant levels.
But wait, what do time and frequency domains mean?
Let us start with the simpler one, time domain. In mathematics, the domain of a function is the set into which all of the input of the function is constrained to fall; therefore, when the term time domain is used, it simply means that we are doing our analysis based on time intervals extracted from the ECG recording. There are many indicators that have been created to study the variance in the intervals but here we will discuss only two of them, SDNN and RMSSD.
SDNN is the standard deviation, a measure of variation, of the time interval between heart beats (the R-R interval), and reflects all the cyclic components responsible for variability. The greater the SDNN, that is the variation between beats, the better. RMSSD is the square root of the mean squared differences of successive R-R intervals, or the variance of the variance of our time series, and is considered to be related to the parasympathetic activity. Both, SDNN and RMSSD are powerful indicators of our cardiovascular health, since the loss of variance between intervals is associated with many cardiovascular and/or inflammatory diseases. As shown below in Figure 3, we want our heart rate frequency to fluctuate a lot over time.
Now that the main time domain indicators have been defined, let us move to the frequency domain. But first, we need to understand one important math concept: any function or time series—think of the graph of a curve—can be re-written as a summation of sine and cosine functions, which are used to model phenomena such as sound and light waves. This idea was first introduced by Jean Baptiste Fourier in 1882 in his work Théorie analytique de la chaleur and later became known as Fourier series.
Figure 1 shows an example, using an arbitrarily chosen function:fx=x3+ x2+3x+5, plotted in the interval ]-π to π [. The resulting data shown as a curve can be reconstructed using a summation of sines and cosine functions through a Fourier series. In this case, twenty sine/cosine functions were used to approximate the curve, while in Figure 2, fifty functions were used.
It is easy to see from the diagrams that, the more functions we use, the greater the precision in the reconstruction of the original data. In both cases, we are using the sum of sine and cosine or “wave” functions to approximate the curve fx=x3+ x2+3x+5 in an ordered way. Each of the component sine and cosine functions correspond to a specific frequency. In this way, a curve can be broken down into its constituent frequencies.
Figure 3 shows the intervals between heart beats i.e., the R-R intervals extracted from a ECG recording and plotted against the time domain, which shows the variation of the heart rate frequency over time.
The next step is to approximate the waveform or curve formed by the heart beat data displayed in Figure 3 as a summation of functions through the Fourier series. This enables us to determine all the frequencies that are acting in our system. The frequency domain indicators are thought of as the “power” associated with each frequency—think of it as the relative contribution of that frequency in the re-construction of the curve formed by the data and calculated through a Fourier transform. These are usually plotted in a spectrogram, a graph where the x axis corresponds to the frequencies and the y axis to each frequency’s power or contribution, like the one displayed in Figure 4.
Now we are ready to define each one of the frequency domain indicators, categorized according to its associated frequency. They are:
- Ultra-low and Very-low Frequencies: 0.01 to 0.04 Hz, not relevant in most cases, due to the relatively short ECG recording usually used.
- Low Frequencies (LF): 0.04 to 0.15 Hz
- High Frequencies (HF): 0.15 to 0.4 Hz
In the beginning of this article, we noted that the branches of the autonomic nervous system (ANS), that is the sympathetic and the parasympathetic branches operate in different frequencies. Based on that, we can see that different HRV indicators in the frequency domain might be associated with sympathetic or parasympathetic activity.
High frequencies are highly impacted by the respiratory pattern, while low frequencies are affected by both the sympathetic and the parasympathetic branches of the ANS. Therefore, by analyzing the power or contribution associated with different frequencies, we can make inferences about the activity of each of the branches of the ANS and their interaction with other systems.
So, why does that matter?
Going back to the beginning of this article, most of the applications, i.e., “apps” used by smart phones and watches to track HRV perform their analysis are based on SDNN, which is a simple-to-calculate and powerful HRV indicator. It’s use is based on the idea that after a workout, the activity of the sympathetic branch temporarily prevails, reducing the overall variability and hence the SDNN.
In this sense, the tool can be used to avoid overtraining, adjusting the intensity of the training session according to the monitored HRV, until the measured SDNN returns to its pre-training values. The same concept can be used to measure stress levels. In theory, all other factors being the same, the more stressed we are, the less variability, and therefore a lower SDNN can be expected. In both cases, the exercise and the stress will likely induce a temporary preponderance of the sympathetic branch.
Now that we know how to analyze it and we understand that different systems are likely to be associated with different frequencies, we can understand why historically HRV was seen as a good measure of imbalances in the ANS. Probably the best way to describe HRV would be as a surrogate measure of the complex interaction between the brain and the cardiovascular system.
So how does that relate to diving?
It has been demonstrated by many studies that a reduced HRV is related to decreased life expectancy. A reduction in HRV has been reported in several cardiological and non-cardiological diseases, ranging from diabetes to renal failure, to mention a few [2,3,4]. A reduction in HRV, when analyzed in the frequency domain has also been associated with inflammatory processes [4, 5].
It is interesting to note, however, that due to huge inter-individual variance, it is difficult to establish expected HRV parameters for a population, and although some interesting studies have been published over the years, there is no consensus on standard values for each one of the HRV parameters. On the other hand, intrasubject analysis, that is the variation of HRV for the same individual over time, can offer very important insights.
Scuba diving is known to trigger oxidative and inflammatory processes, causing a variety of alterations in our physiology, ranging from loss of endothelial function , that is the capacity of the vascular endothelium to respond to vasodilator stimulus, to the activation of the innate immune system and production of microparticles  i.e., particles shed by different cells, which carry nuclear components of their originating cells, like RNA and DNA, and are involved in cell signaling and communication.
As one could imagine, scuba diving is also related to alterations in HRV  and by studying the pattern of these alterations we could infer how our bodies are responding to a dive and, in particular, to the decompression. Our recent study demonstrated that HRV is negatively associated with the production of microparticles and that, using a model built with machine learning, it was possible to predict the pre to post dive variation of the HRV, based on the variation of specific inflammatory markers, linking inflammation and oxidative stress to HRV in scuba diving.
In the past decade, many studies have demonstrated that the presence of inflammatory processes are linked to lowering HRV (either in the time or frequency domains). In our study we demonstrated the inflammatory and oxidative process related to diving are also related to changes in HRV and, interestingly enough, to a preponderance of the sympathetic branch in cases where the volunteers presented more intense responses to the decompression. This fact is also probably linked to the loss of endothelial function, long observed to happen after diving, although the mechanisms are, at this point, not completely clear.
There is still a lot to be understood about the relationship of HRV alterations and diving. The hyperoxia, i.e., exposure to pressures of oxygen higher than 0.5 ATA associated with diving, has its own effects on HRV, making interpretation of HRV variations in diving even more complex. The long-term goal of our research is to better understand individual responses to decompression. We believe HRV variations can be a powerful tool to achieve this objective.
Our team has been working in cooperation with DAN Europe, which has a huge database of diving profiles and outcomes, and some interesting models are being created to model the oxidative and inflammatory processes, but there is still a long way to go before these models can be used in any practical application. However, it is a promising field, and its comprehension will surely help in the full understanding of decompression physiology, making this subject certainly something interesting for the diving community. We could even dream about being able to adjust our dive profiles based on individual responses, right? Watch this space.
- Ernst G. Heart-Rate Variability—More than Heart Beats? Front Public Heal. 2017;5(September):1-12. doi:10.3389/fpubh.2017.00240
- Malliani A, Pagani M, Lombardi F, Cerutti S. Cardiovascular neural regulation explored in the frequency domain. Circulation. 1991;84(2):482-492. doi:10.1161/01.CIR.84.2.482
- Appel ML, Berger RD, Saul JP, Smith JM, Cohen RJ. Beat to beat variability in cardiovascular variables: Noise or music? J Am Coll Cardiol. 1989;14(5):1139-1148. doi:10.1016/0735-1097(89)90408-7
- Sloan RP. Heart Rate Variability Predicts Levels of Inflammatory Markers: Evidence for the Vagal Anti-Inflammatory Pathway. 2015;(Bernik 2002):94-100. doi:10.1016/j.bbi.2014.12.017.Heart
- Adam Moser, Kevin Range and DMY. Relationship between Heart Rate Variability, Interleukin-6, and Soluble Tissue Factor in Healthy Subjects. Bone. 2008;23(1):1-7. doi:10.1038/jid.2014.371
- Brubakk AO, Duplancic D, Valic Z, et al. A single air dive reduces arterial endothelial function in man. J Physiol. 2005;566(3):901-906. doi:10.1113/jphysiol.2005.089862
- Thom SR, Bennett M, Banham ND, et al. Association of microparticles and neutrophil activation with decompression sickness. J Appl Physiol. 2015;119(5):427-434. doi:10.1152/japplphysiol.00380.2015
- Schirato SR, El-dash I, El-dash V, Natali JE, Starzynski PN, Chaui-berlinck JG. Heart rate variability changes as an indicator of decompression-related physiological stress. Undersea Hyperb Med. 2018;Mar-Apr 20:173-182.
- Schirato et al. Association between Heart Rate Variability and decompression-induced physiological stress. Front. Physiol. Front. Physiol., 03 July 2020
- Schirato’s Talk at the 2019 GUE Conference: Heart Rate Variability: What it is and Why it Matters
- From GUE’s membership magazine QUEST: Decompression: Revisiting Old Assumptions by S.Rhein Schirato
Brazilian scientist, Sergio Rhein Schirato, is a hedge fund manager and a researcher at the Laboratory of Energetics and Theoretical Physiology of the Biosciences Institute of the University of Sao Paulo (USP). He holds a PhD in Sciences, a Masters in Finance jointly granted by New York University and London School of Economics and post graduation in applied Math. His current research includes the application of neural networks in decompression modeling and heart rate variability. Additionally, he is a GUE Fundamentals and Rec 1, 2 and 3 instructor, as well as GUE Rebreather certified diver.
Fact or Fiction? Part 2: Interview with World Record Holder Ahmed Gabr
by Michael Menduno
The day before this issue of InDepth was to go LIVE with Gabr’s world record story—I had reached out to him to comment but he declined—I received an email from Gabr. He apologized for any previous confusion and told me that he had checked my credentials and legitimacy and was now willing to grant me an interview. An hour later, we were speaking on FaceTime. Gabr was animated and very talkative. He would frequently go off on tangents and I had to keep bringing him back to my questions. At first, he seemed a bit nervous, but he started to relax more as the conversation went on.
After we briefly talked about his military background—note that he made the dive while still a colonel in the Egyptian army—I began by asking Gabr why he thought these allegations were surfacing now, six years after he conducted his world record dive. He said he believed it was because a documentary of his dive had been completed, and his detractors wanted to get back at him. “Their agenda is to discredit me and the movie,” he said. I asked him if he knew who Scuba Sam was, and he told me that he was a former disgruntled member of his team that had fled Egypt under legal suspicion and wanted to get back at him.
Gabr verified that the head cam video included in Scuba Sam’s email was accurate. I asked him why he hadn’t carried a camera and filmed the dive to begin with. He explained at the time he had an 8-year old child and a pregnant wife, and he said it would have been a huge stressor for him to know that they might see the video if he had died. For that reason, he hadn’t videoed his training dives either.
What About The Depth Timer?
I asked Gabr about Scuba Sam’s first allegation, whether his Scubapro 330 m Depth Timer with blacked out corners—that only showed his Maximum Operating Depth—was metric or imperial. He answered by first saying that computers were not required as part of the validation and launched into a detailed explanation of Guinness’s measurement methodology.
Guinness’s procedure for validating the dive was twofold. First, Gabr had to retrieve one of the unique, signed, and wrapped depth tags from the certified descent line. More on procedure surrounding the tags later. In addition, Gabr’s deep support diver, who met him at 90 m/295 ft, instead of 110 m/361 ft, as planned, was supposed to video him giving a special hand sign indication “3-5-0,” which was requested by the Guinness adjudicator shortly before the dive. In addition, he was to display the retrieved tag, which was the 335 (335 m/1094 ft) tag as shown in the video. He said that only the adjudicator, deep support diver, and Gabr knew the sign. Computers were not required. However, Gabr told me that he was so excited that he held out his Depth Timer (DT) which showed a max depth of 330. The question is whether it was feet or meters.
I asked him why he had blackened out the three corners of the Depth Timer so that only the max depth was showing. Note that only the last digit of the current depth reading, and part of the preceding digit, in the upper left-hand corner is showing. He explained that he did that before his last training dive because he was worried that breathing helium beyond 300 meters would give him hallucinations, so he wanted to keep his brain focused by having a single number. Is max depth the best number? His account contradicts the explanation that two of his team members independently gave me. They reported that Gabr told them it was to insure the integrity of the device under pressure as per the manufacturer.
Gabr explained that he made the dive with four measuring devices: two computers, the Depth Timer, and a Swiss military watch which was rated to extreme depth. He told me that he clipped one of the computers on the line at 90 m/295 ft on his way to depth, as he was worried that it would fail and he wanted it to keep track of his overall dive time compared to his runtime table. He later retrieved the computer which is shown in his hand with double enders in the video. Gabr then said that the main computer died (went blank) at 290m/796 ft during his descent.
OMG! How did he monitor his depth with no working computer and a depth timer only showing MOD? Gabr explained that he used the rope as his “depth tool,” and his watch to keep the time?!? Wow. Can you imagine? What if you got blown off the line? The rope was NOT marked in 3 m/10 ft increments. Rather, according to the engineering report, it was marked in 5-meter increments from 320-350m, and 50-meter increments from 300-150 m, then 10m increments from 120-100 m, and the 3m increments from 96-90 m, where he met his first safety diver. In other words, it would be very difficult to follow as a depth guide, particularly if he were suffering confusion. Gabr reported to me that he was mentally confused and feeling drained at depth.
Note also that Gabr’s first three planned 1-min deep stops (186 m, 171 m, 150 m) were not marked on the rope, nor his stops at 140, 130, 123 (see the partial runtime deco table shown in Part 1). On the video, Gabr gave his first safety diver the OK upon meeting him at 90 m and later reported that the dive went according to plan. No mention of computer problems or the lack of a depth-keeping device at depth.
Again, I asked him about the Depth Timer, whether it was the imperial (feet) version as alleged, or the metric version. Specifically, if it were the metric unit, why wasn’t the decimal showing before the last digit of current depth? [See the analysis in Part 1] Note that the Scubapro metric unit shows the current depth shallower than 100 m in three digits XX.X, to the tenth of a meter. Gabr said that the device “wasn’t in feet.”
What happened, he explained, was that the depth timer began to flood on ascent, and as a result, the decimal point indicating that it was a metric unit and the temp reading had disappeared by the time he reached 90 m/295 ft. He said that eventually the face of the depth timer went blank. Again, Gabr pressed the point, “Why do I need to fake something that is not even required [for the record]?”
One source told me that Gabr had been wearing his metric depth timer on his previous training dives with a standard strap mounting. However, the depth timer in the video has bungee cords. Was the metric device swapped out for an imperial version for this dive?
What About the Gas?
I asked Gabr about arriving at his 90 m/295 ft stop, where he met his deep safety diver, roughly eight-minutes (21 meters shallower) earlier than the plan. He explained that it wasn’t an issue and was within his calculated safety limit. He said that he had reached the 335 m/1099 ft depth tag between 12-14 minutes, and in the absence of a working depth monitor, he ended up making his first stop from depth at 150 m/492 ft using the rope markings. The plan called for slowing his ascent rates as he ascended through various depth zones.
I next asked him about the video which shows him breathing his back gas (trimix 4/85) as he is met by his safety diver at approximately 90 m: “Why were you breathing your bottom mix?” Gabr answered immediately, “I ran out of deco gas.” He then launched into an explanation about Isobaric Counter Diffusion (ICD), and why it wasn’t a big concern for him to switch back to his back gas because the differences in nitrogen were minor. But what about the oxygen?!?
Note also, that none of the tanks had SPGs. I asked him about this. “It’s a weak point for failure,” he explained. However team members told me that all cylinders were triple checked, analyzed, and marked for proper contents prior to the dive.
In the video, the safety diver descends to Gabr, asks in sign, Are you Ok? Gabr, who appears very relaxed, responds with an OK sign. No mention of an out-of-gas issue. Gabr then displays his retrieved 335 tag, shows his Depth Timer reading 330, and makes the sign requested by the Guinness adjudicator 3-5-0. “The first thing I wanted to do was secure the record, and do the whole video process,” Gabr explained. He next secured the tag to his support diver to prepare it for its trip topside. “My nightmare was that my deep support would lose the tag so I zip-tied it to his D-ring.”
After the Guinness protocol was accomplished, the safety diver switched out Gabr’s empty stages and passed him his stage bottle with trimix 12/75 deco gas, that was supposed to be breathed from 120 m up to 90 m. Gabr then went back on his deco gas. The support diver also sent up a status slate to the surface indicating Gabr was OK. The 90 m safety diver met them soon after, and facilitated Gabr’s gas switch to trimix 16/69.
There were several things that were a bit inexplicable. First, if you had just completed a 330+ meter diver, lost a computer, and then ran out of deco gas prior to your 90 m stop while you were facing another 14-hours of decompression, would you give your safety diver, who was carrying back-up deco gas the OK sign? Again, according to Guinness and other news reports, Gabr said that the dive went as planned. Second, were Gabr’s priorities. Wouldn’t you switch to your deco gas and then do the record protocol?
Finally, most troubling, Gabr had four 12L cylinders of 12/75: three staged on the line and one in a carried stage bottle. According to the plan, Gabr was to breathe 12/75 from 120 m to 90 m, for a period of 15 min at an average depth of 110 m or 12 atm. You do the math. For example, with a high 20L surface consumption rate for example, he should have burned through 20L/min x 12 ATM x 15 min=3600 liters. That’s roughly 1.5 12L cylinders, not four. Was he in that depth range longer? Gabr acknowledged that he had burned through lots of gas between those depths and was crazily coughing and had some chest pain. He thinks he had mild pulmonary edema.
Lines, Tags and Fitness
The fourth allegation concerns the angle of the descent line. The video, 50-minutes into the dive, shows the line at what is estimated to be 25º angle, which performing a trigonometric analysis, would mean that the 335m tag would be at approximately 303 m/989 ft short of the record. Gabr explained to me that the Guinness adjudicator and the calibration engineer who was on board measured the angle of the descent line at 15 minutes into the dive. Accordingly they subtracted 2.65 meters to arrive at the record of 332.35 m/1094 ft and 4.5 inches.
The fifth allegation is that there was a broken chain of custody with the tags, meaning that they could have been altered or replaced prior to the dive. Gabr disputed the claims. Though he wasn’t present for all of it, he said that the tags were under the control of the adjudicator the whole time, who even had his own locks to secure the tagged line the night before the dive. That is in Guinness’ bailiwick to determine. Gabr said that the adjudicator watched him dress and searched him prior to the dive. Eye witnesses reported that Gabr and his lead support diver were not searched thoroughly for tags before entering the water, and that the diver who escorted Gabr to depth was not searched.
Finally, I asked Gabr about his reported high energy level following the dive, which contrasted sharply with Gomes’ experience who had to be helped to walk (See Part 1). Gabr, who is reportedly very fit, explained that, unlike Gomes, he removed his quads at 21 m/70 ft and so only wore them for 4.5 hours. Gabr said that he also hydrated and ate every hour during his shallower deco stops, which he asserted Gomes’ did not. He also pointed out that at 41-years old at the time of the dive, he was 12-years younger than Gomes when he set his record. Ouch! I later checked with Gomes’ who said that he also hydrated and ate. As discussed in Part 1, Gomes insists the quads were not an issue for him. We ended the post-dive discussion with Gabr pointing out that its difficult to compare one person to another.
I asked Gabr what he intended to do. To my surprise he launched into a story about a sustainability project he was involved in with the first Egyptian to climb Mt. Everest. He then told me about a shipwreck exploration project in the Mediterranean in late September that he planned to do a pilot film about. I finally interrupted him. “But what about the allegations?”
“I have no idea,” he said. “What do you think?” “It’s up to Guinness,” I offered. “No, Guinness is done with it. It’s over,” he said. I asked Gabr if he had been in touch with or heard from Guinness or his project leader. He said he hadn’t, adding that, “They have a bullet-proof system.” He went on. “I am confident in what I did. But it bothers me that my reputation has been targeted. Why do I have to justify myself? I am confident, but I’m being hurt,” he said.
He went on. “Why are they doing this now, after six years,” he asked rhetorically. “Usually people try to discredit a record at the time it is made. But why six years later?”
“Do you think it’s because of the documentary film?” I asked. “I’m sure about that,” Gabr replied. “It’s the same person who was with the team. He was trying to sell helium illegally in my name and got me into trouble. He wants to discredit my whole record and the movie,” he said.
I asked if Netflix was going to release the documentary, which several people had told me about. Gabr said, no, that the documentary, which had investors, had not been sold yet. “Netflix didn’t buy it?” I asked. “No,” he said. I asked him several times if he had a financial stake in the film but did not get a clear answer, though he acknowledged he was under contract.
Gabr then said about his anonymous accusers. “They are not searching for the truth. They only mention half the truth. There was no peer review from professionals. My record would be discredited if I had relied on one of the computers. For sure. But that was not the measurement methodology.”
I asked Gabr if he would be willing to have a peer group review all the information and possibly clear him. Gabr said that he was open to anything. He said that he has been using the same video in his talks as presented in the emails and is always willing to sit and discuss things at every dive show he attends. “I don’t mind talking about my dive,” he said.
There is obviously a lot of information to unpack, and numerous additional details, as well as information protected by confidentiality that I have omitted here. It’s clear to me that I am not in a position to render a judgement, nor is it my job. My job has been to ask questions, lay out what appear to be the facts as best and fairly as I can and point out inconsistencies as I see them.
As I said before in Part 1, it’s up to Guinness to decide if they want to investigate the allegations or not, and on that basis to determine if Gabr completed a record dive or not. However, given the widespread distribution of allegations about the record, and what seem like surprising discrepancies, it’s likely that his record will remain under a shadow until the full truth is known, whether by Guinness or perhaps, as Gabr suggests, through a peer review by professionals.
Michael Menduno is InDepth’s editor-in-chief and an award-winning reporter and technologist who has written about diving and diving technology for 30 years. He coined the term “technical diving.” His magazine aquaCORPS: The Journal for Technical Diving (1990-1996), helped usher tech diving into mainstream sports diving. He also produced the first Tek, EUROTek, and ASIATek conferences, and organized Rebreather Forums 1.0 and 2.0. Michael received the OZTEKMedia Excellence Award in 2011, the EUROTek Lifetime Achievement Award in 2012, and the TEKDive USA Media Award in 2018. In addition to his responsibilities at InDepth, Menduno is a contributing editor for DAN Europe’s Alert Diver magazine and X-Ray Magazine, a staff writer for DeeperBlue.com, and is on the board of the Historical Diving Society (USA).
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