To adequately understand OCDs, it is first beneficial to think about continuous flow oxygen (CFO) delivery, which for decades was considered the ‘gold standard’ of LTOT. As CFO is applied via nasal cannula, some of the oxygen delivered during inhalation is mixed with inspired air for a net Fraction of Inspired Oxygen (FIO2) in the lungs. Unfortunately, determining FIO2 isn’t quite as simple as calculating the flow of oxygen and the flow of air. Several factors conspire to complicate the process.
During CFO the flow rate is fixed, so as a patient breathes faster, creating a shorter inhalation time, the amount of oxygen inhaled per breath decreases. As a result, the net FIO2 drops.
This chart highlights the dead space and pooling factors, and indicates what is considered to be “useful” oxygen, during continuous flow therapy.
During the latter portion of inhalation, gas entering the airway never reaches the lungs. Instead, it remains in the air passages that lead to the alveoli, only to be exhaled before they reach these gas exchange units. This includes some of the oxygen volume delivered from CFO therapy, meaning that that oxygen was wasted. The amount of oxygen wasted varies with the patient’s anatomy and breathing pattern.
Oxygen delivered from CFO therapy late in the exhalation phase, when the patient’s expiratory flow rate is relatively low (or during the pause after total exhalation), may not be wasted. The oxygen exiting the small diameter cannula is traveling at a relatively high velocity and some amount of oxygen is ‘pooled’ in and around the nose, nasopharynx and upper airway. This oxygen volume is able to be inspired at the beginning of inhalation. Patient disease, anatomy and breathing pattern, as well as environmental conditions (like wind) can vary this effect.
Early Conserving Concepts
Early conserving pioneers estimated that only the oxygen delivered during inhalation was considered useful. Assuming a typical breath rate of 20 breaths per minute, and an Inspiratory:Expiratory (I:E) ratio of 1:2, that would mean that 1 second delivery was required. If a 2 LPM setting was used, this meant that on each breath, 33 mL of oxygen was “useful”.
Others recognized that, while 1 second of inhalation time was spent breathing in air, the last portion of the delivered oxygen never reached the lungs; it remained in dead space, only to be exhaled. Product developers theorized that it was possible to cut the delivery time roughly in half while keeping the delivery flow rate the same. Since continuous flow oxygen could not be providing “useful” oxygen for the full inhalation time, reducing the delivery time from 1.0 seconds to approximately 0.5 seconds would reduce the delivered oxygen volume from 33 mL to ~16 mL, and, in theory, the patient should still get the same therapeutic benefit of CFO therapy. This theory led to the development of oxygen conserving devices that could claim a much higher savings ratio though they delivered oxygen in a very similar manner to other products.
To further complicate matters, with continuous flow therapy, there is oxygen flowing throughout exhalation also. While most of that is blown out into the atmosphere, the oxygen delivered at the very end of exhalation may pool in and around the nose, and then is inhaled when the patient starts their next breath. The amount of pooled oxygen will vary greatly (patients’ breathing pattern, their anatomy, the oxygen flow rate, the wind, etc., all are factors), so it is very difficult to estimate how much oxygen is required to achieve an equivalent dose amount to a 2 LPM continuous flow rate of oxygen. But it was suspected that the oxygen dose would need to be more than 16 mL to maintain equivalent therapy to CFO delivery. This theory led to the idea of demand/hybrid delivery OCDs delivering a bolus volume early in inhalation to account for the lost volume from the lack of pooled oxygen around the nose at the start on the inspiratory phase.
Gerald Durkan, whose product development work led to the products and patents now marketed by Sunrise Medical, thought that it would be advantageous to deliver the same 33 mL dose resulting from 2 LPM continuous flow oxygen therapy early in inhalation, but at a higher flow rate and for a much shorter duration. With this method of delivery, the delivered pulse volume could easily be changed by varying the duration of the delivery. Most, but not all, pulse devices today operate in this manner, increasing the delivery time with an increase in the setting number.
Initially, OCDs were not well received. Many patients and clinicians experienced limited success with the new devices. Many problems were related to technical issues that occur with any new product on the market. Recent research related to the testing of how OCD products operate provides another possible cause for poor results using different OCDs.
Each manufacturer determines what volume of gas to provide at each setting and often reports that to be equivalent to continuous flow. Testing each unit on the bench found that one device set on 4 was giving 66 ml per breath and another unit set on 4 was giving 34 ml per breath. These types of differences in volume, on the same setting, created confusion and the perception that the units did not work properly.
With some manufacturers promoting savings ratio rather than patient oxygenation, OCDs initially had an acceptance problem. There is a lesson to be learned from this experience. Clinicians need to be informed on the performance capabilities of each piece of equipment that a patient utilizes. The respiratory products industry is growing rapidly. Without direction from knowledgeable clinicians, manufacturers are sometimes left in the dark in regards to how their devices should operate to maximize therapeutic benefits.