Nasal CPAP treatment for obstructive sleep apnea

Abstract

Nasal continuous positive airway pressure (CPAP) is presently considered as the “treatment of choice” for obstructive sleep apnea (OSA). Though some OSA patients adhere to treatment recommendations and ultimately respond quite well to CPAP therapy, there is a substantial subgroup for which compliance is a particularly difficult issue. Despite receiving recommendations to the contrary and for reasons that are incompletely understood, the majority of OSA patients settle for a partial compliance pattern. Whether a partial compliance schedule is beneficial or harmful is virtually unexamined: Unlike other medical treatments, there are few data concerning the “dose–response relationship” of CPAP to its desired effects. We argue that CPAP “dose” is a function not only of CPAP pressure but of time-on-CPAP as well. Critical questions that remain unanswered are what “dose” of CPAP is needed to effect an appropriate treatment outcome and which treatment outcomes should form the basis of our recommendations. Recent placebo-controlled studies comparing CPAP to suboptimal CPAP pressures may be informative in this regard. Directions for future research are suggested.

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.