The focus of this study was the ability of three contemporary POC devices to detect inhalation and deliver a corresponding pulse, a fundamental objective of pulsed oxygen delivery.
Figure
6 shows the varying ability of these POC devices to synchronize with a shallow
nasal inspiratory volume of 182 mL (breath rate 17.6/min), 53% of the total breath volume. Two of the three devices did not achieve full synchrony at all settings.
This test offers far greater trigger challenge than typical for POC bench evaluations, so discussion is warranted. First, the scenario depicted is that of sleep, where the efficacy and appropriateness of pulsed oxygen devices has been questioned [
2,
19,
24,
25]. Yet the efficacy of the ‘reference’ nocturnal oxygen therapy,
continuous flow oxygen via nasal cannula, also shows high variability during sleep [
26]. Sleep introduces issues with potential to influence
any oxygen therapy delivered via nasal cannula. In normal subjects, sleep is associated with reduced ventilation than when awake, at a similar or slightly increased breath rate, hence the breaths are 6–25% shallower depending on sleep stage [
27]. Similar behaviour is observed in COPD and other nocturnal desaturators, but sometimes with
profound reduction in tidal volume in REM sleep [
15,
28]. Other identified sleep issues include: a worsening of gas exchange ability; ‘mouth breathing’; a displaced cannula; and other sleep breathing disorders (snoring, obstructive apnoea, periodic breathing) [
24,
26,
29,
30]. For such issues sensitive triggering may promote delivery of the pulse within the alveolar duration, or may dictate whether inhalation is detected at all. Continuous flow oxygen
may be less vulnerable to sleep issues, given its delivery is unaffected by breathing behaviours and it offers the (situational) possibility of oxygen pooling. But as noted by Chatburn et al. [
24], a key consideration in achieving efficacious therapy of any oxygen therapy “is not
whether a person desaturates at night, but
why they desaturate”. And despite the controversy, the ambition for the POC category is evolving towards a single-device for home and ambulation, as experience grows with nocturnal pulsed oxygen delivery and in response to user preference [
24,
31,
32].
Second, the test scenario depicts substantial oronasal breathing, where the POC does not ‘see’ the full inhaled volume. Clearly 100% mouth breathing for sustained periods will confound any style of nasal cannula oxygen therapy, hence ‘mouth breathing’ is a commonly cited concern for nasal oxygen therapy during sleep. But from the limited research data available,
exclusive mouth breathing during sleep is infrequent: in one early study in healthy sleeping subjects, 100% mouth breathing was not seen at all [
17], while other sources suggest this may occur in less than 5–10% of normal subjects [
33,
34]. But ventilation shared between nose and mouth during sleep – the scenario represented in Fig.
6 – is frequently seen, particularly in men and increasingly with age [
35]. Our scenario depicted a nasal fraction of 53%, aligned with values seen in older subjects within a study [
17] on mouth breathing in a sleeping normal cohort.
Third, our test case investigating oronasal breathing may be instructive for
daytime situations where oronasal breathing may diminish efficacy of nasal oxygen. In awake healthy subjects, dominant mouth breathing at rest and during exercise is quite rare (5% of subjects), with little apparent increase with age [
36]. But in a population with respiratory compromise, Chadha et al. [
37] found a nasal ventilation fraction at rest (awake) of around 56% compared to 86% for healthy subjects. Leiberman et al. [
38] found that during exertion the nasal fraction decreased in all subjects, but more so in those with respiratory compromise (nasal fraction reduced to 25%). Based on these limited data, it seems oronasal breathing may be common, but it is unusual for the nasal fraction to drop to zero for sustained periods. So it may be that if a POC’s trigger were sufficiently sensitive, it may remain efficacious across the majority of oronasal or ‘mouth breathing’ instances.
Fourth, the interface between cannula and nose possesses ‘geometric’ factors that may affect the capability of the POC to detect inhalation. Consider the nature of the POC trigger: inspiratory flow induces a reduction in pressure at the cannula tip; this pressure is sensed and if below a threshold value, a trigger is asserted. The change in pressure induced at the cannula tip depends not only on the magnitude of nasal flow, but also on various geometric factors, including:
a.Nare internal geometry, itself a function of individual differences, age, race.
b.Nasal valve geometry (depth, area, shape).
d.Cannula insertion depth into the nare, and position relative to nasal valve.
The net effect of these listed factors may result in wide variability of trigger performance between individuals, despite a similar ventilation pattern. This has been evaluated on the bench using replica adult airways [
8,
39]. Across the different replicas, researchers found more than 3-fold variation in the amount of pressure developed for a given nasal flow. A commercial POC included in their investigation proved
unable to trigger on three of the 15 replicas when used with their sleep breathing pattern (520 mL tidal volume) at setting 2 [
8].
Finally, almost all children receiving long term oxygen therapy also require ambulatory oxygen therapy [
40]. Pulsed oxygen delivery is generally considered inappropriate for babies & very small children, but for larger children sensitive triggering may determine whether a child can enjoy the ambulatory benefits of pulsed oxygen delivery.
Overall, a more sensitive trigger may translate to greater likelihood of success across high inter-subject variability in oronasal breathing and in anatomic variation, and across diverse behaviours within a patient. But high sensitivity must not come at the expense of spurious triggering, which can dramatically impair pulsed oxygen efficacy. In this assessment, the most sensitive of the devices tested did not display inadvertent triggering across any of the simulated behaviours.
There are limitations to the bench research presented here. The scope was limited only to the POC’s ability to detect inspiration and trigger a pulse, with no consideration of other pulse parameters such as the pulse’s amplitude, pulse volume, or how much of that volume was successfully delivered within the ‘alveolar’ duration. The tests were conducted in a controlled static laboratory environment free of drafts and ambient vibration. It employed a single bench ‘nose’ with stable cannula positioning. These simplifications allowed us to focus on repeatable and accurate comparison of device triggering, but lack the complexities of real patient breathing and ambient effects, and the results may not relate directly to efficacy of oxygenation.