The Kairos of the upper airway: The critical time of mid-to-late expiration in the treatment of obstructive sleep apnea.

Obstructive sleep apnea is traditionally described as an inspiratory disorder because obstructive events are most commonly visualized in-vivo during inspiration. However, pharyngeal narrowing has also been well-documented during expiration. Thus, an important unresolved question is which phase of the respiratory cycle is most relevant for positive airway pressure (PAP) therapy. Here we propose that the upper airway has a "Kairos" (ancient Greek for "critical time"), a brief interval in late expiration where therapy must be maintained to optimize pharyngeal patency. Our hypothesis is supported by several observations: 1) end-expiration has been associated with the smallest upper airway cross-sectional area throughout the respiratory cycle, and increasing surface area through PAP raises dilating wall force; 2) end-expiration coincides with the lowest end-expiratory lung volume, thus minimizing caudal traction on the upper airway; 3) reductions in expiratory pressure produced by some expiratory pressure relief algorithms have been associated with diminished pharyngeal airway size. Therefore, optimizing positive pressure in late expiration may maximize airway size and stiffness, thus preparing the airway for the subsequent inspiration, when negative intraluminal pressure would otherwise promote collapse. These mechanisms suggest that preventing late expiratory narrowing may be more important for maintaining pharyngeal patency than previously appreciated. Pilot studies examining novel PAP technologies preserving expiratory pressure during the "Kairos" have provided evidence that upper airway stability can be maintained despite substantial reductions in inspiratory pressure. Assuming these concepts are correct, PAP therapies should be calibrated not only to pressure magnitude, but also to the timing of pressure delivery.