This section outlines the various ways oxygen delivery can be conserved during the application of oxygen therapy.
Titrate Correct Flow for Patients on Continuous Flow Oxygen
In the past, patients were simply prescribed 2 LPM continuous flow for oxygen delivery. This “rubber stamp” was a default setting to start patients on oxygen therapy. If the patient’s oxygen levels were appropriate, rarely did the clinician test to see if a lower setting would have accomplished the same objective. Doing so would have conserved some oxygen for patients who were able to stay oxygenated at lower flow rate settings. With the advent of conserving devices, CFO therapy is used much less than in the past, but these same issues still exist today.
Reservoir cannulas, often referred to as moustache or pendant cannulas depending on the design and use characteristics, have approximately 20 mL of reservoir space that stores oxygen during exhalation. On inhalation, the patient receives that stored oxygen, adding a bolus volume to the ongoing continuous flow delivery. These devices are simple, effective, and, when used properly, can allow a patient to receive the same therapy at a lower CFO flow rate, thus conserving oxygen. Numerous studies have proven their effectiveness, yet the appearance of the devices has been a limiting factor for most patients.
Intermittent Flow Devices
Intermittent flow devices operate by turning oxygen delivery “on” during some portion of inhalation and “off” for the balance of the breathing cycle. In this way, oxygen that would otherwise be wasted as the patient exhales is conserved. This method often allows a given supply of oxygen to last 2-4 times as long as it would if it were delivered continuously. One benefit of the use of intermittent flow conserving devices is that a smaller oxygen supply may be carried, “lightening the load” for the patient.
With intermittent flow devices, the way in which oxygen is delivered to the patient differs greatly from one device to another, and NO device delivers oxygen in the same way as continuous flow (let it be said that this is not necessarily a bad thing).
Intermittent flow devices can be separated into two broad categories, pulse and demand, and within these categories there are many variants.
Pulse Delivery Devices
Pulse delivery devices deliver oxygen in the form of a relatively high flow rate bolus beginning early in inhalation. For example, the original CRYO/2 device, when set at the “2” setting, delivered oxygen at a rate of 10 LPM for only 0.2 seconds. Some pulse delivery devices vary the dose of oxygen by changing the duration of the bolus. Others increase the peak flow rate at which the dose is delivered as the user increases the setting number. Some devices do a combination of both.
Certain pulse delivery devices, in addition to allowing for oxygen conservation, may have an added benefit over continuous flow therapy. On a continuous flow system the flow rate stays constant, and as a patient’s respiratory rate increases, the inspiratory time decreases, so the volume of oxygen inhaled per breath actually decreases. However, on most pulse delivery devices, the delivered volume at a given setting stays the same regardless of the patient’s breathing rate, so the oxygen dose that is delivered stays the same, regardless if the patient is breathing at 15BPM or 30BPM. As a patient moves from rest to activity and their breath rate increases, a pulse device operating in this manner may maintain oxygenation better than continuous flow or a demand device. This, however, has not been proven clinically. Therefore, it is impossible to say that pulse type delivery is equivalent or otherwise to continuous flow delivery across a wide variety of breathing patterns. Device manufacturers however, label their products with the same setting numbers used for continuous flow (“1, 2, 3…”), and so there is often confusion about why a conserving device set at “2” is not oxygenating a patient like continuous flow at 2 LPM does.
Also important to note here is that a few pulse delivery OCDs are manufactured such that they decrease the delivered dose per breath as the patient’s breath rate increases. These devices are sometimes known as minute-volume delivery devices, as they generally deliver the same total oxygen volume over one minute regardless of the patient’s breath rate.
Pulse vs Demand/Hybrid Devices at 20 and 30 BPM
Demand Delivery Devices
Demand delivery devices have evolved from the initial, intuitive idea of creating an oxygen conserving device that simply used the patient’s breathing signals to turn the device’s oxygen delivery on during inhalation and off during exhalation. Because of the lack of pooling, however, this has not proved to be an adequate approach. For this reason, most current demand conservers deliver some amount of bolus volume at the onset of delivery, to make up for lost pooled oxygen. These demand devices are sometimes called hybrids, because they act like both a pulse and demand device, delivering a fixed pulse volume at the onset of inhalation and then continuing to deliver oxygen at a “tail” flow rate until the device senses the beginning of exhalation.
This is a surgical procedure that bypasses the upper airway’s dead space by inserting a catheter through a small hole made in the front of the neck and into the trachea. Oxygen conservation is achieved as the patient is usually able to be given the equivalent of CFO therapy with nasal breathing at a lower continuous flow setting. Studies have also shown that a single lumen intermittent flow conserving device will work with transtracheal delivery, however the oxygen savings is about the same as using nasal breathing with that same intermittent flow device. Because patients on oxygen often can be self-conscious about how they appear when using their oxygen equipment, a notable benefit of transtracheal oxygen therapy is an increase in patient compliance to therapy due to the lack of cannula interface with the patient’s cheeks and ears and the fact that the public usually is not able to see the catheter.