Nasal CPAP treatment for obstructive sleep apnea: Developing a new perspective on dosing strategies and compliance
Introduction
The obstructive sleep apnea and hypopnea syndrome (referred to as OSA in this paper) is characterized by repetitive episodes of upper airway obstruction that occur during sleep, usually associated with a reduction in blood oxygen saturation and/or an arousal from sleep. This produces a cyclic pattern of breathing and not breathing, sleeping and not sleeping, which can occur hundreds of times each night. The two most commonly recognized symptoms are loud snoring at night and excessive sleepiness during the day. Obese males are most at risk. Depending upon the definition used, OSA is found in approximately 4% of the general middle-aged adult population [1] and up to 24% of the elderly population [2] in the US. The potential consequences can be serious and are broad in scope (see Table 1). There is growing evidence that OSA worsens over time when left untreated [3], [4], [5].
Medical awareness of the disorder has been growing steadily over the last few decades, and the ability to recognize and diagnose OSA has been improving. Clinically and polysomnographically, the focus is on the rate of respiratory events per hour of sleep (also known as the respiratory disturbance index, or RDI). Despite current disagreement on whether this index adequately represents the severity of the disorder [6], [7], reducing RDI to a target of less than five is the current standard. Though some OSA treatments (listed in Table 2) have been successful in reducing RDI compared to pretreatment levels, in many cases, posttreatment RDI remains high enough to classify the patient as “apneic.” When using RDI≥5 events per hour as a yardstick for comparison, nasal continuous positive airway pressure (CPAP) is the most effective treatment for OSA [8], [9], [10].
Despite its efficacy, CPAP compliance is disappointingly low. Compliance is usually defined as the number of hours per night that the CPAP machine is either powered “on” or “on at the prescribed pressure.” Generally, about 70–75% of patients “accept” CPAP, or agree to use CPAP after a short trial period. Of those, about 40–60% will continue to use it 1 year later. If this is so, this means that over 50% of patients started on CPAP may not be using it 1 year later [11]. Of those that are using it at 1 year, few are using it to the extent prescribed. For example, one study showed that only 6% of OSA patients used the machine for at least 7 h on at least 70% of nights [12]. From a behavioral medicine standpoint, the majority of OSA patients “fail” to comply fully with treatment recommendations, and many follow only a partial compliance pattern. This has generally been assumed to reflect a significant health problem, and it is reasonable to assert that CPAP compliance could be improved. However, the question is, by how much should CPAP compliance be improved? What is a clinically meaningful amount of CPAP use?
The current definition of CPAP “dose” is focused on pressure, when in truth, CPAP “dose” is likely to be function of time as well as pressure. However, relationships between CPAP “dose” and treatment outcomes (dose–response relationships) are incompletely understood. For example, we do not now know the therapeutic effect of increasing or decreasing CPAP dose, or even the shape of the dose–response curve. The low threshold for a clinical response is unknown, as is the upper limit above which no further clinical benefit can be gained. The field would benefit greatly from a better understanding of the role played by compliance within the context of dose–response relationships and CPAP treatment.
Section snippets
Nasal CPAP: the treatment of choice
First described in 1981, CPAP effectively keeps the airway open, acting as a “pneumatic splint” [13]. Patients who tolerate CPAP often experience rapid and dramatic upsurges in alertness, mood and quality of life. These improvements are attributed to the restoration of normal sleeping and breathing patterns. As Beninati and Sanders [14] point out, a CPAP prescription consists of several elements: pressure titration study (full night vs. split night), decision regarding endpoints for pressure
Where should we go from here?
As mentioned earlier, despite the medical recommendation for full compliance, the majority of OSA patients follow a partial compliance pattern. How do measures of sleep apnea severity and important symptoms change as a function of varying levels of use? What is the low-end threshold below which no clinical benefit is obtained? What is the upper threshold above which no additional clinical benefit is obtained? For what subgroups of patients do these relationships hold? It may be noteworthy that
Acknowledgements
This work was supported by grants CA23100, HL36005, AG02711, RR00827 and MH18399.
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