Commentary on oxygen delivery during hyperbaric medicine

It is important not to become fixated on some absolute need, or ability, to achieve a 100% oxygen concentration following compression of the monoplace hyperbaric chamber. Some have gone to great lengths to record and describe how painstakingly long it has taken the chamber to eventually reach 100% upon arrival at prescribed treatment pressure, as if it had some bearing on therapeutic effectiveness. Such preoccupation is more a failing academic exercise than any measure of dosing relevance. While seemingly counterintuitive, 100% oxygen concentration is not achievable. If an oxygen analyzer is reading 100% it is out of calibration.

Depending upon a given clinical facility’s source and manner of storage, its oxygen purity will vary within an approximate range of 95-99.5% and defined as Medical Grade. It may be somewhat lower but never higher. Certain esoteric research and analytic settings require a value higher than 99.5%. This value is termed Research Grade and reaches 99.999%, also referred to as “five nines’” purity. It facilitates chemical consistency rather than medical hygiene and avoids ruining sensitive analytical results, chemical reactions and physical measurements. It does, however, require a significant degree of additional and relatively expensive processing beyond that incorporated in the production of Medical Grade oxygen.

Cryogenic bulk liquid oxygen (LOX) systems commonly used to support hospital needs have a minimum permissible 99.0% oxygen purity and required range of 99.0-99.5% to comply with United States Pharmacopeia’s Medical Grade standard. This same standard is in effect for high pressure cylinder stored oxygen. Some hospitals rely on a high-volume low-pressure (HVLP) oxygen production, particularly where access to LOX is difficult or impossibly expensive to acquire. This system forces air through a molecular sieve (such as zeolite) to extract oxygen and achieve 90-95% purity. The last HVLP system this writer observed was during a visit to King Edward VII hospital in Bermuda. Here, the successful practice of multiplace chamber hyperbaric medicine is undertaken using identical chamber pressures to those with access to higher oxygen purity.

The balance of the gas to 100% in each of the above settings is primarily argon gas, with trace amounts of nitrogen. So, patients are breathing oxygen, just not 100% oxygen. Now some comments regarding oxygen concentrations with respect to effective therapeutic dosing.

The U.S. Navy introduced minimal recompression-oxygen breathing in 1965 to improve decompression illness clinical outcomes. These treatment tables were adopted by other navies and civilian entities and have since evolved as widely accepted standard of care. Injured divers breathe oxygen in air-filled chambers through an oral nasal aviator-type mask. This system has long been associated with inhaled oxygen levels falling well short of supplied oxygen purity. Stephenson, et al. (1996) studied mask delivery levels using a precision/paramagnetic measuring device in injured divers being treated in accordance with the above protocols. Values were found to range between 80-88%. All divers fully recovered. Sheffield, et al. (1977) had previously compared inhaled oxygen delivery involving two oral nasal mask types, with levels ranging from 84-96%.

Throughout the intervening decades there has been nothing to suggest that values less than 99.5% oxygen have adversely influenced outcomes in decompression-injured divers. The same can be said for clinically referred patients using identical methods of oxygen delivery.

In recent years, a hood has become an increasingly preferred method of oxygen breathing. It enhances delivery by fully isolating a patient’s head and neck, thereby overcoming mask fit/comfort/compliance issues. That said, Stephenson’s study also included analysis of this treatment approach, noting a range of 95-99% purity, so indeed more effective but not completely so. Another advantage of the hood is that it allows chamber pressure to be reduced from the common 2.34/2.4 ATA to 2.0 ATA, as the ‘mismatch’ between chamber pressure and oxygen pressure is largely overcome; 2.0 ATA oxygen being efficacious for deficient wound healing and other clinical indications. Importantly, a multiplace chamber treatment pressure of 2.0 ATA all but eliminates the risk of decompression sickness for those who are employed as inside attendants.With the above narrative as background, we return to the monoplace chamber. The compression gas (oxygen) flow rate (not to be confused with chamber compression rate) should be high to accelerate air (21% oxygen) displacement. Rates of 300-400 lpm are recommended. Upon arrival at treatment pressure the oxygen level at the patient’s head (oxygen’s entry point) will already be above 90% and continue to increase. We deduced this during our clinical trials incorporating the Sechrist chamber Models 2500 and 3200, where a chamber exhaust line monitoring point was established. We were primarily interested in confirming that enrolled subjects were correctly randomized to either sham (air) or experimental (oxygen) groups. Oxygen levels in the experimental group at the exhaust line (patient’s foot end) uniformly reached and often exceeded 95% within 10 minutes of arrival at the 2.0 ATA study pressure. Head levels would be correspondingly greater, of course. Under these conditions, late radiation soft tissue injury healing rates were higher in the experimental/oxygen group, and to a statistically significant degree.

Being transfixed on that 100% number is, therefore, misguided from an oxygen purity requirement and therapeutic relevance. This is one example of the fact that optimal dosing of hyperbaric oxygen is incompletely researched. We are a far cry from determining a precise dose of oxygen for a given indication, in terms of hyperbaric oxygen pressure, exposure duration, daily frequency and treatment course.

There is a more important aspect to all of this. Chamber relative humidity (RH defined as the mass of water vapor specific to the volume of gas, regardless of temperature) holds the key to static electricity accumulation and its discharge potential. The lower the chamber flow rate the higher the RH; a function of the patient’s insensible moisture loss. A high RH, 60% and above, is a good thing. It acts as a conductor, permitting any accumulating static to readily and harmlessly dissipate into the surrounding atmosphere’s higher moisture content. Higher flow rates have the opposite effect. They result in lower RH levels as patient-generated moisture is more readily eliminated, thereby creating an environment suitable for static accumulation and ever-present risk of sudden discharge.

It is therefore recommended that the monoplace chamber be compressed with high flow oxygen, and at a compression rate patients comfortably tolerate. Upon arrival at the prescribed treatment pressure, oxygen flow should be immediately reduced to a level consistent with patient comfort, or an oxygen conservation mode if available. Patients may be breathing slightly less than 99.0% oxygen for several minutes, but what they are breathing is therapeutic. And they are doing so in a safer surrounding environment.

One patient management goal should be to operate the chamber through the treatment period at the lowest tolerable oxygen flow rate. If a patient complains of being too warm at these lower rates, rather than increase flow for subsequent treatments, switch out that thick blanket for a thin sheet. If this proves insufficient, then titrate increasing flow rates to patient comfort. Don’t simply adjust upwards to what might be an unnecessarily high rate.