Note: The following column was published in the April 2006 Respiratory supplement “Inspiration!”
Oxygen Conserving Devices (OCD) are one of the most misunderstood products used in Long Term Oxygen Therapy (LTOT), yet are the standard of care for portable oxygen systems used in the home. LTOT devices have entered the market with little or no clinical studies that determine how each device operates, and there is little information on the appropriate application of the devices to ensure patient oxygenation. Most home care clinicians view an OCD as a commodity item that is selected for features or price. Most physicians do not know how the devices operate and believe they are similar to continuous flow oxygen therapy. All OCDs operate differently compared to continuous flow and to other OCDs and have specific capabilities and limitations dependent on the product. In the absence of significant clinical research on each OCD, the clinician must become informed on the capabilities, applications and limitations of the OCD they choose to use with their patients.
The first intermittent flow oxygen conserving device was introduced in 1984 with the goal of making a portable oxygen system lighter and last longer. The concept is self-evident since a breathing cycle includes inhalation and exhalation. To provide oxygen continuously is inherently wasteful. Limitations of technology required that the introduction of more sophisticated oxygen delivery systems wait until engineers developed a method of sensing inhalation and controlling the delivery of oxygen to the patient.
Understanding the capabilities of a specific OCD with the knowledge of the respiratory rate and tidal volume will improve the potential of oxygenating a patient at varying activity levels that change volume and rate.
Introducing a new medical product requires a submission to the FDA for approval. If the product is novel and has no predicate device already approved, a comprehensive investigation is required to prove safety and efficacy. If the new product is similar to another product that has FDA approval, the submission process is simpler — the product needs onlyto show significant equivalency to the existing device. The first intermittent flow oxygen conserving device used continuous flow oxygen delivery as the predicate device. Using a standard breathing pattern, the amount of oxygen delivered to a patient using continuous flow can be calculated. A patient with a 500 milliliter tidal volume, a 20 breath per minute rate and an inspiratory to expiratory ratio (I:E) of 1:2 will have a one second inspiratory time. Two liters per minute (LPM) flow provides 33 ml per second. If the patient has a one second inspiratory time, and a 2 LPM flow is giving 33 milliliter per second, a conserving device that gives 33 milliliter in one second is equivalent to continuous flow. Right? This logic was used for the first intermittent flow device approval and it was accepted.
Yeah but … most of the new OCDs do not give 33 milliliter on the 2 LPM setting. The settings on the OCD dose selector dial do not correlate to continuous flow or other OCDs. The selection number is to be used as a reference point for setting an appropriate dose. This has been a hard concept for the market to accept and the “two is not two” is making its rounds to home care clinicians. The physicians, on the other hand, still write prescriptions for a number based on old habits. If they write to titrate to a target saturation, home care clinicians would be able to adjust the number on the dial for patient needs.
First experiences for some clinicians may not have been positive. Since OCDs were promoted to be equivalent to continuous flow, if patients did not maintain saturation on the continuous flow number set on a conserving device, clinicians felt the device did not work. At medical conventions it was common to hear a clinician say, “I tried a conserving device and it didn’t work.” The failure typically was not the conserving device; it was the understanding of how the conserving device operated. If the two settings on the conserving device are not giving the same volume of gas as the continuous flow device, simply increasing the setting on the OCD will increase the volume and probably result in equivalent to continuous flow. A concern was that if the number on the conserving device was different than continuous flow or the prescription, the home care provider was changing the prescription. The prescription was changed when the OCD was used. OCDs are volume delivery, not flow delivery, and there needs to be an understanding of these differences for effective use of oxygen conserving devices.
Through research we have learned that OCDs are not just basic oxygen therapy. OCDs provide a dose of oxygen, which is considered a drug, and have the potential to deliver this drug more effectively. Early studies focused on the equivalency concept. Small studies were conducted with the goal of showing that a conserving device set on the same number as a continuous flow device would provide equivalent oxygen saturations. Some of these studies showed equivalency. Most of the studies did not focus on patient activity (the main use of OCDs in home care) and if they did, the results varied. Recent studies have looked at performance of devices tested on a mechanical lung. By controlling lung variables, it was possible to determine performance differences and capabilities of each conserving device. Knowing how a device operates gives the clinician the capability to determine which device to use and what to expect from product performance. If a clinician knows that a conserving device cannot provide the volume of gas a patient might need with exercise he or she can prevent a perceived failure of the device.
OCDs have the potential of being very sophisticated drug delivery systems. These OCDs are similar to small ventilators since they deliver a volume of gas at a specific location in the respiratory cycle. The better we understand the mechanics of breathing and the multiple variables that influence gas exchange, the better our capability to oxygenate the patient at most activity levels. The two major variables that will affect oxygenation with a conserving device are respiratory rate and tidal volume. These are two simple parameters that respiratory therapists deal with in practice, yet are rarely measured in home care for oxygen patients. Understanding the capabilities of a specific OCD with the knowledge of the respiratory rate and tidal volume will improve the potential of oxygenating a patient at varying activity levels that change volume and rate. There are numerous other variables that will impact the performance of oxygen conserving devices, and it is impractical to expect an OCD to be sophisticated enough to respond to all patient physiology — yet we have moved past the steel cylinders and brass regulators of the past and are moving to the next generation of oxygen therapy.
There is scientific evidence for oxygen conservation and a quick review of the literature will provide some information on previous research. This research is limited and typically not specific to products that are used in home care today. The home care clinician needs to use all resources available to determine the capabilities, applications and limitations of conserving devices to provide the best LTOT. These clinicians should encourage manufacturers to provide independent research on key issues related to their products’ performance and related to patient oxygenation at activity levels. A conserving device is not about how much it saves, it is a tool used to improve the LTOT patient’s ability to live a healthier and more normal life with their oxygen.