For the purposes of this discussion HBOT is “Hyperbaric Oxygen Therapy”, and mHBOT is “Mild Hyperbaric Oxygen Therapy”. It does not mean “Medium Hyperbaric Oxygen Therapy”, this is a play on the meaning of the label and how it relates directly to dosage. The differentiation between these two applications comes down to dosage. Full dose, and mild dose.
Dosage is affected by two main factors. Pressure and duration. The pressure being the treatments depth or pressure protocol, and the duration the length of time exposed to the treatment protocols pressure.
Different conditions respond differently to different dosage. Some require higher pressure and some require lower pressure.
Not intended to pre conclude, but below is a release from one of the UHMS affiliate branches SAUHMA, (South African Undersea and Hyperbaric Medicine Association), regarding the use of soft shell fabric hyperbaric chambers marketed for use in mild hyperbaric oxygen therapy, (mHBOT), or in one or two cases referred to as “medium” hyperbaric oxygen therapy.
Its rather a damning position and raises a rather contentious issue within the greater hyperbaric industry. There is no consensus on this particular subject. Some are all for it, and others are vehemently against it as you can tell from the release found here. The conflicting part of this is, both sides have a point.
To understand dosage and what the difference in applied pressure or pressure capability makes, reproduced below is a modified excerpt from the overview explaining what dosage is and why low pressure chambers (1,4 ata or less) are limited in their scope of application.
Dose brings us to J.S. Haldane, the father of decompression theory. In his work on decompression theory, Haldane proposed what are called tissue compartments. These are essentially theoretical “tissues” or more accurately described, “compartments”, represented by a mathematical algorithm. In his compartment algorithm he includes mathematically represented theoretical tissue groups based on gas absorption rates according to Henry’s law previously discussed. He represents the slowest possible rate and the fastest possible rate, and then in between rates, thus theoretically covering and accounting for any possible tissue likely to exist in mammalian biology. When we talk about fast or slow we mean how fast or slow they saturate with a given gas when the pressure is increased.
It isn’t an absolute requirement to understand the intricacies of saturation and decompression theory though. This information is included to make readers of aware of the existence of the theory rather than and in depth explanation. Knowing what it is, is enough for this discussion.
Any diving manual will be quite clear in their affirmation that these theoretical tissues or compartments and are not actual tissues. Over the years, since his original 5 compartment model, with tissue half times of 5, 10, 20, 40, and 75 minutes (half time to saturation), dive computers and dive tables have begun representing more “tissue” half times, such as found in the Buhlmann ZHL-16 algorithm (16 compartments), (Uwatec Aladin computers used this in the 1990’s), with some research models now representing as many as 256 compartments.
Divers, and students of diving, increasingly tend to casually assign specific tissues to the various compartments although not strictly academic. Slower tissues generally refer to tissues such as bone which is slow to absorb gas, adipose tissue which is fairly slow but faster than bone, and at the fast end of the spectrum, blood, which will saturate faster than bone. Brain tissue is also a fast tissue. Blood flow to specific tissues is the determining factor when considering their half times or time until they become halfway saturated.
A good explanation of tissue halftimes can be read on the NHS Scottish Diving Medicine website here:
While decompression theory is primarily involved with inert gas (nitrogen), oxygen saturation into plasma and tissue will occur based on the same underlying principal of gas diffusion in tissue/liquid under pressure, (Henry’s Law). More on this is covered in our previous publication FLYING AND DIVING – A SOJOURN INTO PHYSICS AND PHYSIOLOGY
This is how dose is considered
For treatments involving slow tissue such as bones, in the case of fractures, a higher-pressure gradient is needed to saturate that tissue to an effective extent. Higher pressure shortens the saturation half-life with a steeper inward gradient facilitating shorter treatment times. Steeper inward gradients are needed because blood flow to these tissues are limited. The blood they do recieve needs to carry more oxygen per unit volume. Henry’s Law again.
For faster tissues such as brain tissue, a lower inward gradient is needed to achieve a similar saturation level, owing to their naturally shorter half times because of greater blood supply. There is no need to push the half time with a higher pressure. In fact neurological conditions respond better to lower pressure. Just how low is the key question though. Some believe that fabric low pressure chambers are just too low with the recommended treatment pressure for neurological conditions at 1,5 ata to 1,75 ata. A considerably higher pressure than low pressure fabric chambers are capable of achieving. This is in contrast to their steel hulled cousins which can be pressurised from zero to as much as 6 ata. A far wider range of treatment pressures and by default, a wider range of conditions which can be treated more effectively.
In some cases a slightly longer duration at lower pressure will achieve a relatively deep saturation level in faster tissues owing to faster half times. High gradient dosage such as found in a 3ATA treatment protocol, will of course be shorter treatment times than a lower dose of say, 1.75ATA when oxygen tolerance is considered.
In the case of sports injury’s, it is normally slower tissues that are injured such as tendons and ligaments. These usually require shorter duration and higher-pressure treatments as opposed to something like brain injury, which can benefit from as low as 1,5ata to 2ata for longer periods, with chronic ongoing neuro-degenerative conditions benefiting from ongoing multiple low pressure, longer duration treatments.
White brain matter differs also from grey matter, experiencing lower circulation and drainage. Accordingly, a moderately higher pressure may be required than that for grey matter conditions, to facilitate a shorter tissue half time. Between 1.75ata and 2ata would suffice. Again outside the range of a soft chamber.
This raises the question of just how effective is a 1,3ata treatment in a low pressure chamber for something like wound healing or damaged bones and ligaments? In contrast to some claims, it cannot possibly be as effective as a full pressure and full dose system, yet practitioners often charge the same price as their bigger cousins. While a steel hulled full capability chamber can offer low pressure treatments, not the same can be said the other way around. Low pressure fabric chambers are limited in this regard. the simple truth is they are not the same despite them being promoted as such.
Practitioners who claim they are, are deliberately misleading the public. If in this position, try a simple test. Ask them to explain why it’s called mild HBOT. If they say its the same – walk away. It simply isn’t. The soft chamber cannot reach the pressures required for optimal effectiveness.
Soft Shell Chamber
While Decompression sickness and decompression theory may seem like another topic the crossover should be clear. Henry’s Law is the underlying scientific principal at play. Whether dealing with inert or metabolically active gas, the saturation into tissue is governed by the same principal. Decompression theory also deals with the coming out of solution of inert gas, which we don’t have a need to observe in the case of pure oxygen breathing.
Notwithstanding the above comments, mild hyperbaric oxygen therapy (mHBOT) does have it’s place. Various studies employing randomised and placebo controlled groups have been dismissed because they made use of low pressure air at 1,3ata. What this means is that even those who claim 1,3ata is not hyperbaric, it is indeed hyperbaric. If not then why would valuable results have been dismissed? They were dismissed because it was found that even air at 1,3ata was found to have had some effect on the body.
Hyperbaric merely means higher than atmospheric pressure. Soft shell chambers do indeed offer a higher than atmospheric pressure. And according to the physics, oxygen levels will rise accordingly, and it is beneficial. It’s beneficial to a lesser degree though and practitioners need to be forthcoming about this fact. To tout something as the same as its big cousin in misleading.
Furthermore, most soft chamber models aren’t designed with pure oxygen in mind and are hence not rated for use with pure oxygen. Use of pure oxygen has inherent fire risk and only devices designed and certified for use with pure oxygen should be used with pure oxygen. It’s called oxygen service and most of the soft chambers around are simply not oxygen service. It is unsafe to use pure oxygen in such chambers not rated for its use.
They are further limited to 1,3ata and hence cannot reach a significant pressure for most conditions and treatments with the exception of altitude sickness. Despite that, we support their use in general well-being therapy as well as neuro degenerative disorders as maintenance treatments following a course of full dose therapy. They can be used in the home and will cause no harm and there is no harm in trying something that may bring relief to some. The point of contention being what they are promoted and advertised as. They have their place.
Something else to note is that there is no current evidence they are any good, but there is also no long term statistical evidence yet of an increasing number of patients who do benefit. While dosage is considered in pressure and treatment duration terms, we have no data regarding the effect of years of repetitive use. Optimism is prudent. Since there is an established physiological effect, it is reasonable to remain hopeful that through years of use it may lead to a positive cumulative effect.
Hayden Dunstan
cumbria-hyperbaric.org 
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