Beneath the Bells and Whistles

Anyone who has spent any time as an oxygen provider is no doubt familiar with the concept of the oxygen-conserving device. Since its introduction in 1986, the conserver has gone from being a luxury item to a mainstay of long-term oxygen therapy. With its elevated status, however, has come an influx of new devices, each touting their own feature set and specific benefits. How does one evaluate the effectiveness of a conserving device then? Or, harder still, choose the proper one for a given patient?

Fortunately, beneath all the bells and whistles lay some specific principles that most, if not all, conserving device manufacturers base their designs. By the time you finish reading this article, you can face your equipment sales representative armed with the information you need to ask the proper questions and make an informed purchase decision.

Conserving Device Theory

First, conserving device operation is based on the knowledge that oxygen delivered while the patient is exhaling is wasted, therefore, a third or less of that oxygen is actually making it into the lungs. A further understanding of lung physiology tells us that, of the oxygen delivered during inspiration, only a small portion reaches the alveoli to participate in the gas exchange process. The rest is exhaled.

Working from this knowledge, the first conserving devices delivered high flows of oxygen for a very short time period (the pulse) at the earliest stages of inspiration, thereby eliminating the oxygen that would normally be exhaled. Because the actual volume of oxygen delivered is so much less than continuous flow, the conserver could give an increase in tank duration of three to seven times.

This principle highlights the main factors in both the clinical and conserving efficiency of a device including: pulse volume, sensitivity and response time. While it would seem that pulse volume would be the most important factor in the equation, it is actually the interrelation between sensitivity and response time that most directly affects a device's clinical efficacy and conservation ratio. A large pulse delivered late in the inspiratory cycle will be mostly wasted; a smaller pulse delivered at the initial stage of inspiration can deliver the same amount of oxygen to the alveoli and yield a higher conservation ratio at the same time. Therefore, more sensitive units will usually have smaller pulse volumes and higher conservation ratios. The reverse is true for less sensitive conservers.

Knowing this, you may wonder how this relates to the current conserver options. Each type of device uses the principles described above in different variations, trading off efficiency in one area for better performance in another, whether it is weight, size, conservation ratio or bolus size. There are three main criteria by which to classify a conserver and determine which features are most important to you.

1. Electronic and Pneumatic

This is the best-known classification of oxygen conservers. Put simply, an electronic conserver uses a circuit board and software-controlled valves and sensors to sense a breath and deliver oxygen. A pneumatic device uses mechanical means to accomplish the same task.

Both types have their advantages. The use of electronics allows conservers to respond more quickly and deliver sooner than most pneumatics; as such, they can saturate the patient with smaller pulse volumes and will yield a higher savings ratio. Pneumatic conservers, on the other hand, often sacrifice sensitivity and conservation ratio in favor of lighter weight and battery-free operation. Either type, when properly made, can saturate a patient, though electronic conservers tend to be the more economical choice in the long-term by virtue of superior conservation ability.

2. Pulse, Demand and Hybrid

At the inception of conserving device technology, the division between pulse, demand and hybrid delivery methods was much more clearly defined. The improvement of technology may have blurred the line somewhat, but all conservers still lean towards one characteristic more than others.

A pulse type conserver delivers a clearly definable pulse of oxygen during inspiration. No oxygen is delivered anywhere else in the cycle.

A demand type conserver is similar to a SCUBA valve the patient breathes in and the oxygen turns on; the patient exhales and it turns off. Because of the additional oxygen delivered in the latter part of inhalation, true demand devices have a low-conservation ratio. Few of these types are still made today, and they are not considered top-sellers. It is important to note that many people will refer to any conserving device as a demand device because they activate on demand at patient inspiration.

The hybrid delivery method is most common among pneumatic conservers, particularly ones that utilize dual-lumen cannulas. A small flow of oxygen is always present throughout the cycle to cancel false inspiratory signals and help reduce double pulsing. A pulse of oxygen is then delivered at the earlier part of inspiration, in addition to the continuous flow. This methodology can often be effective, but conservation ratios tend to be lower because of the wasted oxygen during exhalation.

3. Fixed Pulse vs. Minute Volume

Until recently, all conserving devices fell into the category of fixed-pulse; that is, the amount of oxygen per pulse remains the same regardless of breath rate. As breath rate increases, so does the amount of oxygen delivered per minute. Minute volume conservers, on the other hand, make a certain amount of oxygen available per minute, regardless of the breath rate. As the breath rate increases, the size of the pulse decreases.

Minute volume conservers have a distinct advantage in the conservation category some pneumatics that use this technology can give savings ratios of up to six to 1 on certain liter flow settings. Also, because the amount of oxygen that is delivered is static, the tank duration is always predictable.

At lower breath rates and settings, minute volume conservers can saturate patients, as well as their fixed-volume counterparts. At higher breath rates, however, the decreasing size of each pulse can be problematic for patients with greater oxygen needs. Plus, the variability of the pulse size makes titrating a patient on the device problematic.

Choosing the Best Device for Your Patient

With these factors in mind, what is the best oxygen-conserving device? Like most consumer products, "best" is largely subjective and is based on a matter of lifestyle and individual preference. Not all people would want to drive the same car, whether because of size, ease of operation, or functionality; therefore, a single device is not likely to satisfy every patient.

A user who is active and needs maximum tank duration might want the superior conservation of an electronic, pulse conserver, as might a less active patient who wants a lighter weight cylinder without sacrificing too much duration. A user who has trouble remembering to change batteries and does not care as much about duration could benefit from a pneumatic conserver.

The important point to remember is that patients should be titrated on any device before they are sent out into the world. You might also wish to ask follow-up questions in the first few months: Do they feel out of breath? Do they turn up the settings while active? Do they find it easy to use? While it is impossible to stock every conserver to satisfy every need, a brief look at your patient base might show you the similarities among them and help you narrow the possibilities down to two or three models. This will allow you to satisfy 99 percent of your new referrals, and you can deal with the 1% on a case-by-case basis.

Glossary

Sensitivity: The amount of negative pressure or vacuum (usually measured in centimeters of water) a conserver requires to respond to patient inspiration.

Response Time: The amount of time from the initial part of inspiration a conserver takes to respond to patient inspiration.

Pulse Volume (Bolus): The size of the oxygen pulse (usually measured in cc or ml.)

Conservation (Savings) Ratio: The increase in time a conserver yields over continuous flow on the same setting; usually written "x:y" (3:1 means a cylinder will last three times longer with the conserver than it would on continuous flow.)

Electronic: A conserver that runs off battery power and uses electronics to sense inspiration and deliver oxygen.

Pneumatic: A conserver that uses mechanical means (normally air pressure) to sense inspiration and deliver oxygen.

Pulse: A conserver that delivers a definable pulse of oxygen at inspiration and nowhere else in the cycle.

Demand: A conserver that turns on during inspiration (negative pressure) and turns off during exhalation (positive pressure) much like a SCUBA valve.

Hybrid: A conserver that delivers a minimal continuous flow of oxygen throughout, as well as a larger pulse during inspiration.

Fixed Pulse: A conserver that delivers a predetermined, fixed amount of oxygen on every breath, regardless of breath rate.

Minute Volume: A conserver that delivers a fixed amount of oxygen per minute; pulse sizes will vary depending on breath rate.

Seven Questions to Ask Before Buying

By compiling the answers to the Questions 1-5, you will be able to accurately compare the operation of each device based on hard data, not sales pitches. Questions 6-7 are important to ensure both the clinical efficacy and the safety of the device.

1. What is the sensitivity of the conserver?

2. What is the pulse volume at each setting?

3. What is the maximum breath per minute rate the conserver can deliver?

4. Is this fixed pulse or minute volume operation?

5. What is the conservation ratio at each setting?

6. Are there clinical studies available that demonstrate the efficacy of the device?

7. Does this device have FDA approval?

This article originally appeared in the November 2004 issue of HME Business.

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