Despite using the data from two multiple-night sleep studies, our results show that there are patients who will still cross thresholds even when multiple-night studies are repeated. This was the case either when using the mean, where approximately half of patients changed category; or the maximum ODI, where approximately one third changed category. This further calls into question the use of arbitrary thresholds to decide any aspect of a patient’s management, as is common practice at present .
Even amongst patients selected to have moderate OSA on their first CPAP withdrawal, three patients had a mean ODI < 15/h on their second period of CPAP withdrawal, which is below the threshold required for reimbursed CPAP therapy in some countries . As might be expected, using the maximum ODI from these four nights reduced this to only one patient.
Randomised controlled trials have shown CPAP to be effective in reducing symptoms in patients with OSA categorised as mild on the basis of a single-night sleep study [13, 14]. If OSA severity had been measured on multiple nights rather than on a single night in these trials, then some patients with mild OSA might have been re-categorised as moderate OSA, due to regression to the mean of sleep apnoea severity. Furthermore, randomised controlled trials have shown CPAP to be similarly effective across the spectrum of severity of OSA [13, 14], and the correlation between OSA severity and sleepiness is weak [6, 7, 15]. We therefore think that the severity of symptoms and likely impact on quality of life are much more important than the severity of OSA on sleep studies when making management decisions.
Threshold-based categorisations of OSA have shown at the epidemiological level that only the more severe OSA tends to be associated with cardiovascular events , and therefore might justify continuing this high threshold to guide treatment. However, the SAVE study showed that CPAP does not reduce cardiovascular events ; therefore, it still seems reasonable that symptoms should remain the principal reason to treat OSA.
There are limitations of this study. This was an opportunistic study looking at data from a previously published study and therefore is exploratory. Returning OSA was assessed using overnight pulse oximetry and not polysomnography, which is currently the gold standard technique for diagnosing OSA in some countries. Pulse oximetry was chosen for this study as it is more practical to measure this at home over multiple nights. Without using polysomnography, differences in total sleep time, differing sleep architecture, and differing time in a supine position could all account for differences in the ODI, and are in fact the likely explanation for some of the observed night-to-night variation . However, our use of mean values from multiple-night sleep studies should have reduced any such variability. In many sleep centres that use polygraphy, sleep length, sleep state, and posture issues will be part of the variation they observe anyway. Our study only selected patients with an ODI > 20/h on their first period of CPAP withdrawal; thus the results might have shown regression downwards towards the mean, although the means of the two periods were in fact almost identical. It is possible that patients’ weight changed between the first and second withdrawal period, which might have contributed to changes in OSA severity. However, given that the average time between CPAP withdrawal periods was short, and the means of the two periods were almost identical, we feel this is unlikely to explain the changes in OSA severity.
It has been pointed out by others that overnight pulse oximetry in isolation is not recommended for the diagnosis of OSA ; however, the correlation between ODI and AHI is very high, and our observations are likely to apply to more comprehensive sleep studies. One clear difference between the two periods was that all patients received sham oxygen therapy (air at 5 L/min) during their second CPAP withdrawal, which might have affected sleep. However, if this decreased sleep, we might have expected a different ODI during the second withdrawal period, which was not the case.