Positive Airway Pressure Treatment for Obstructive Sleep Apnea: Mechanisms of Action and Determinants of Pressure

March 28, 2016 Category: Apnea

continuous positive airway pressureSince the original description of continuous positive airway pressure (CPAP) treatment by Sullivan and coworkers in 1983, positive airway pressure (PAP) remains the mainstay of treatment for mod-erate-to-severe obstructive sleep apnea (OSA) in adults. Despite many advances in technology, the major challenge facing clinicians is improving adherence to PAP treatment. A short review cannot provide in-depth coverage of the large amount of literature that has been published regarding the efficacy and delivery of positive pressure treatment. The PAP treatment of children, central sleep apnea, or restrictive lung diseases will not be discussed. Our goal is to highlight some important concepts and recent developments that may be relevant to the practicing clinician.

PAP provides a “pneumatic splint” by delivering an intraluminal pressure that is positive with reference to the atmospheric pressure. Upper-airway muscle tone either remains the same or decreases with the application of CPAP. Imaging studies- demonstrate that PAP increases upper-airway crosssectional area and volume in awake normal subjects and OSA patients with the largest change in the lateral dimensions. A second mechanism by which PAP may affect upper airway size is by increasing lung volume. The increased lung volume provides a downward traction on the trachea (tracheal tug). This action is believed to stretch upper-airway structures and increase upper-airway size. The importance of this second mechanism for the effect of CPAP on the upper airway remains controversial. However, one study found that increases in lung volume induced by applying negative extrathoracic pressure did reduce the amount of CPAP required to prevent airflow limitation in OSA patients during non-rapid eye movement (NREM) sleep.

A number of factors can affect the level of PAP required to keep the upper airway open during sleep. Higher levels of PAP are necessary in the supine (compared to nonsupine) position and during rapid eye movement (REM) compared to NREM sleep. In general, supine REM sleep is the situation requiring the highest pressure. Both elevation of the head of the bed and weight loss have been shown to decrease the required level of pressure to keep the upper airway open. Weight gain may necessitate an increase in a formerly adequate treatment PAP. Of interest, neither moderate alcohol ingestion” nor the benzodiazepine receptor agonist zolpidem impair the efficacy of an otherwise effective pressure level.