Intermittent Flow Devices
Many Variables Exist in Oxygen Delivery Systems
We are all familiar with continuous flow oxygen – you set the dial to a liter flow setting and oxygen isperpetually delivered to the user at that set flow rate. It’s a pretty basic concept, which has helped continuous flow oxygen delivery remain a “gold standard” in oxygen therapy. Intermittent flow oxygen delivery systems were introduced in the 1980s as a way to maximize oxygen availability – by delivering oxygen to the user during inhalation only. Delivered oxygen was no longer wasted during exhalation so storage tanks and por- table devices were able to provide oxygen for a longer period of time before the device or tank needed refilling. Manufacturers were also able to create smaller and lighter products to help the user remain active and ambulatory without needing to haul around large concentrators or storage bottles. However, with this introduction of intermittent flow systems, new variables in the delivery of oxygen were introduced that impacted therapy outcomes. This article will explore and explain some of these variables to help you understand some of the impor- tant characteristics of intermittent flow oxygen delivery.
Intermittent flow oxygen delivery – often referred to as “pulse flow”, “pulse dose”, or some other variation containing the word “pulse”– requires the oxygen system to be able to sense the user’s inhalation, deliver a volume of oxygen within the user’s inhala- tion phase, and turn off oxygen delivery so that oxygen does not get delivered during exhalation. Portable oxygen concentrators (POCs), battery powered conservers and liquid oxygen systems use electronic components to achieve all three outcomes. Other non-electric devices use control valves to meter the oxygen delivered the inhalation.
Sensitivity Trigger to User’s Inhalation
One performance variable introduced by intermittent flow systems was triggering sensitivity, or the ability of the device to sense the user’s inhalation and then respond by turning on the flow of oxygen. The more sensitive a device, the earlier that oxygen can be delivered. If the device is too sensi- tive, the unit may “auto-trigger”, delivering oxygen at an inappropriate time. If a device is not sensitive enough, oxygen may not be delivered at all!
Once the user has started inhaling, an ade- quately sensitive device will be able to trigger oxygen delivery very quickly, ensuring that the pulse of oxygen gets to the user while they are still in the first part of their inhale cycle. Shallow breathing, such as when the user is sleeping, may result in the pulse of oxygen being delivered late in the inhalation phase, or not at all. This is one reason why when considering sleeping with an intermit- tent flow device, it is recommended to have an overnight oximetry study while on the device before purchasing or using the device long term.
Volume of Pulse Effects Oxygen Delivery
Another performance variable introduced by intermittent flow systems were pulse waveform characteristics – or the shape and volume of the pulses. Each intermittent flow system has a unique way of delivering its volume of oxygen. For example, one system set to “4” may deliver a pulse at a very high flow rate, like 12 LPM, for a very short time, like 200 milliseconds (ms). This results in a pulse volume of 40 milliliters (mL). Another system also set to “4” may deliver its pulse volume at 5 LPM for 600 ms, totaling 50 mL of oxygen delivered. So on two devices set to the same numerical setting, we have oxygen pulses delivered at different flow rates, for different lengths, resulting in different pulse volumes delivered.
Pulse volumes directly impact the user’s inspired oxygen (FiO2), and SpO2 (oxygen saturation). Some systems, when set to a spe- cific setting, deliver the same pulse volume regardless of how fast the user is breathing. These are considered fixed-pulse devices. Other systems, when set to a specific setting, reduce the pulse volume delivered as the user breathes faster. These are considered minute-volume devices. There are a small number of products even feature both types of delivery methods!
Considering the number of intermittent flow oxygen systems currently available, and that each one has its own unique pulse flow characteristics, the variability in performance by these systems is huge. It is no wonder some users of pulse systems are able to use one device but not another. Unfortunately this also leads to some people discrediting pulse flow delivery altogether, “Well, I tried Device X and it didn’t work, so I must not be able to use pulse flow.” This isn’t accurate at all, since it may just be that that particular device isn’t able to meet the user’s needs, whereas another system might have the ability to oxygenate that user at all of their activity levels.
The Timing is Important Too
When a pulse volume of oxygen is deliv- ered, the timing of the pulse is important. Delivering the pulse within the first part of inhalation in imperative, as any oxygen delivered in the later half may not reach the user’s lungs. This is why oxygen pulses are often delivered within 200–600 ms, though some devices may have settings with delivery times lasting much longer.
Electronic intermittent flow systems are programmed to stop delivery after a certain amount of time has passed. Most pneumatic devices require the user to exhale against the oxygen flow before turning off flow delivery. In both cases, when flow delivery is stopped is ultimately dependent on the product’s design characteristics. At faster breath rates, or in cases where the pulse is triggered later in inhalation, flow may be stopped well after the user had begun exhaling, meaning that some or all of the delivered oxygen has been wasted.
Learn the Pros and Cons of Systems
There are many pros and some cons to intermittent flow delivery systems. At their best, they allow the user to have a light- weight, long-lasting oxygen system at their side, giving them freedom and mobility that stationary concentrators and large tanks cannot provide. At their worst, the user is unable to stay oxygenated and ends up with an expensive paperweight. Look for another system that could meet your needs. As always education is important, and in the current oxygen environment where common misconceptions still exist among many folks working with oxygen equipment, you must be your own advocate in learning about what systems may be beneficial for you.