Clinician Talk

Oxygen Conserving in 2011

Almost 30 years old and still sometimes misunderstood.

Oxygen conserving technologies intended for use in the management of patients receiving long term oxygen therapy (LTOT) were first introduced commercially in the early 1980s. Reservoir nasal cannula systems were introduced in 1983 and mechanical oxygen conserving devices (OCD) followed in 1984. Despite the relatively early introduction, the use of OCDs was rather limited prior to 1998, the first year following the passing of the Balanced Budget Act of 1997, which included a 30 percent cut to homeoxygen payments.

According to industry data, in 1994 there were only 18,000 OCDs in use and in 1998 approximately 80,000 new OCDs were placed into service. Today, nearly every modern portable oxygen device incorporates an oxygen conserving technology. OCDs are routinely used with ambulatory LTOT patients to a level that is arguably the standard of care in the United States. Regardless of the method or model of low flow oxygen delivery system (continuous flow and/or OCD), the real goal of supplemental oxygen is to increase the FiO2, which in turn is intended to raise the patient’s blood oxygen level (i.e., SpO2) to a physiologic target determined by the physician. In prior studies and frequently in current clinical practice, target blood oxygen levelsare generally a PaO2 > 60 mmHg or a SpO2 >90 percent.

Lingering Confusion

After nearly 30 years of use and hundreds of thousands of applications, there still remains some confusion about the role and clinical effectiveness of oxygen conserving technologies. This confusion persists despite an abundance of favorable peer reviewed science, along with recognition and acceptance in both expert consensus and clinical practice guidelines.

The technical differences and specifications based on the different mathematical assumptions, technical designs and conserving methods employed by the different manufacturers are often cited as primary causes of the confusion, which is a valid argument. Much of this may be rooted in the lack of standardized terminology and definitions associated with oxygen conserving technologies. Manufacturer brand, model or feature specific terms and descriptions are often mixed in with more common terms that explain device functions and specifications. Marketing often gets confused with science and the result can leave the clinician unsure of what feature is clinically relevant and what is simply marketing. This lack of standardized terminology and definitions is not uncommon.

In their landmark paper on ventilators, Chatburn and Primiano point out similar issues existing in mechanical ventilation, where myriad new ventilation modes and terms are introduced to promote a specific brand or model of ventilator. They argue that many of these modes and terms generally fall into a small number of already established modes and classifications. The net result in ventilation, like in oxygen conserving, is confusion among many clinicians.

Recent estimates suggest there are approximately 35 different commercially available oxygen conserving technologies. This volume of devices does add a level of complexity; however the delivery of low fl ow oxygen is really not very complicated. As previously noted, the goal of oxygen therapy is to raise the FiO2 to a level that produces the intended SpO2. In low fl ow O2 delivery, this is a relatively simple process; deliver enough O2 per breath to increase the FiO2. If one steps back from the devices and especially the marketing claims (which include devices promoted to but don’t respond to saturation changes and others misrepresenting pulse dosing settings for continuous fl ow) there are three key elements to determining FiO2 in low flow o2 delivery: (1) source gas, (2) ambient inspired gas, and (3) deadspace gas. The third of these, deadspace gas, is not typically considered part of the FiO2 formula but is ever present and therefore, for the purpose of this discussion is included to help demonstrate the dilution of the gases as they enter the airway.

Source gas is derived from the O2 delivery device, which can be flow in liters/min or a pulse-dose volume of gas. Ambient gas is the room air being inspired with each breath (tidal volume) and the deadspace gas the mixed gas remaining in the upper airways at the end of exhalation. The mixing of these three gases produces the FiO2 (see Figure 1, below).

Figure 1
Source Gas Tidal Volume Dead Space FiO2
0.99 0.21 0.15

In this discussion of FiO2, deadspace is constant, tidal volume is highly variable, and source gas is variable (when adjusted). If the O2 setting remains constant and tidal volume increases, the FiO2 decreases. If tidal volume decreases, the FiO2 increases. The changes are simply the result of air dilution, which is true regardless if the O2 source is continuous flow or pulse-dose. Changes in tidal volume, respiratory rate and I:E ratio all can impact the FiO2 in low flow devices and therefore insuring the O2 source is set appropriately to the patients activity has long been a recommendation in the clinical research and the expert consensus reports.

As clinicians supporting LTOT patients, we can’t control their respiratory rates, activities, etc. and therefore must exercise control over the technology. This starts by having a detailed knowledge of the performance specifications and characteristics of the oxygen technologies dispensed to the patient. Knowing how the particular device operates, delivers oxygen and responds to the patient’s respiratory pattern are essentials in selecting and matching the oxygen system to fit the patient’s clinical and lifestyle needs.

Separating the science from the marketing is critical and the first step in better understanding and working with new and changing oxygen technologies. As one of my Navy drill instructors reminded us in boot camp, “you have to be smarter than the equipment you are working with.”

This article originally appeared in the May 2011 issue of HME Business.

About the Author

Joseph Lewarski is vice president and general manager of the Respiratory Group of medical equipment manufacturer Invacare Corp.

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