Starling resistor and stability of ventilation during sleep.

The upper airway can be described as a collapsible segment (the pharynx) in terposed between two rigid bony (the cavum) or cartilaginous (the trachea) segments. Due to this structure, the pharynx behaves as a collapsible tube in which airflow does nor depend on the downstream pressure, bur is limited to a maximum value which depends only on the upstream pressure and on the pressure surrounding the collapsible segment; this behavior, known as a Sta rling resistor can be modeled by the waterfall effect. Thus, the upper airw ays can be in three different conditions: an occluded condition, in which n o flow is possible, a patent condition, in which flow depends on the differ ence between upstream and downstream pressures (according to Poiseuille's l aw), and a situation in which flow is limited The behavior of the upper air way is largely dependent on its anatomic structure. but functional factors play a critical role. Among these sleep state is both a determinant of the collapsibility of the pharynx, and determined by the simulation of upper ai rway mechanoreceptors whose activity depends on the activity of respiratory muscles Thus the interplay of three factors: ventilatory drive, upper airw ay collapsibility, and arousal threshold can predict most of the situations of stable and unstable ventilatory behavior during sleep. The level of the arousal threshold governs the stability of the ventilatory pattern, as it determines whether a combination of flow, respiratory effort, and blood gas es can be maintained or is interrupted by an arousal. (C) 1998 Elsevier, Pa ris.