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Guide to Prescribing Home Oxygen
by Thomas L. Petty, M.D.
Contents
The Key to Prescibing Home Oxygen
Conserving Device Technology
Patient Considerations in Selecting Equipment
Conserving Device Technology
Conserving Devices; Demand vs. Pulse
Oxygen conserving devices can substantially increase the range of a liquid portable or small high- pressure gas system. Some devices are mechanical/pneumatic and others are battery powered. The initiation of inspiration causes a pulse flow of oxygen to be delivered. Not all conserving devices deliver the same amount of oxygen. Checking for adequacy of oxygenation by the use of pulse oximetry, particularly during exercise, should be done when conserving devices are used. The classification of conservers is often misleading. Some conservers are referred to as demand devices because they are triggered by patient demand even though they only deliver a fixed pulse. In reality, there are two general classifications of oxygen conservers. Those that deliver a fixed volume per breath are classified as pulsing devices. Those that deliver a variable volume (commensurate with the length of inspiration) are demand devices. Other demand devices deliver for a fixed time that is approximately the length of inhalation.
Pulse Devices
Some pulse devices deliver every breath, others on alternate breaths. The alternate breath devices deliver a fixed volume (36 ml) 3 out of 4 breaths at a setting of 3, 2 out of 4 at a setting of 2 and 1 out of 4 at a setting of 1 and activate with each breath on a setting of 4. Other devices in this category deliver a fixed volume of up to 16 ml times the setting number (1 through 4) each time the conserver is triggered.
All pulse type devices share some common characteristics:
- They deliver fixed volumes for each flow setting each time a pulse is triggered.
- They do not deliver any more or less volume as the length of the patient's inspiration time varies.
- They tend to deliver less than prescribed of oxygen per minute volume at low breath rates but can deliver more at high breath rates, e.g., during exercise.
- They tend to require more patient attention to conserver function to permit manually switching from conserver mode to continuous flow in case of conserver malfunction.
The settings used on the flow control may or may not provide the same delivered minute volume during inspiration as that provided by continuous flow. This is particularly true of pulse type devices.
Demand
Devices
By comparison, demand devices will vary the amount of oxygen delivered on each and every breath consistent with the length of time the patient inhales. Demand type devices such as the Puritan-Bennett C550 and CR50 deliver a 12ml bolus followed by continuous flow throughout the remainder of inspiration. No oxygen is delivered during exhalation.
Demand devices also exhibit other shared characteristics:
- Following the initial bolus they deliver at the equivalent flow rate of continuous flow for the remainder of the inspiration.
- They deliver a variable volume at each flow setting depending on the length of inspiration.
- They have lower levels of savings at low breath rates but at a given setting can save as much as or more than pulse devices at high breath rates and high I.E. ratios.
- They tend to deliver volumes equal to or greater than those received with continuous flow therapy for most settings.
- They generally revert automatically to continuous flow without patient interaction in the event of conserver malfunction.