The recently described clinical syndrome of obstructive sleep apnea (OSAS), possibly affecting 2% to 10% of the adult population, is characterized by episodic sleep-induced changes in upper airway patency. These produce respiratory compromise ranging from partial to complete occlusion. Because obstruction represents a major challenge to homeostatic respiratory control mechanisms and because it can be completely reversed with a simple nasal mask/airway pressure device known as a nasal CPAP, OSAS provides an extraordinary opportunity to study control of breathing in humans.
Upper airway obstruction in OSAS results from interaction between subtle anatomic airway narrowing, increased wall collapsibility due sleep-induced loss of baseline muscle tone, and insufficient inspiratory phasic dilator muscle contraction to oppose the negative intraluminal pressure resulting from diaphragmatic contraction. This collapsible behavior can be modelled with a Starling resistor; exploring the properties of this model should provide insights into pathophysiology and treatment. One key observation is that a sinusoidally varying driving force (inspiratory diaphragmatic drive) acting through such a collapsible tube results in an inspiratory waveform which "flow limits." Although defined by pressure/flow relationships, flow limitation can be recognized by a flat contour of the inspiratory waveform in the flow/time display.
Appreciation of the information content of this easily obtained and noninvasive measurement has provided three research directions in our laboratory: 1) defining the role of the upper airway in sleep-related changes in respiratory control in normal subjects and patients with cardiorespiratory failure resulting in apnea and chronic hypercapnia; 2) developing diagnostic techniques to screen patients for the occurrence of sleep-related respiratory abnormalities; and 3) developing a technique for closed-loop computer-controlled regulation of the treatment for OSAS, e.g., a variation on nasal CPAP which automatically seeks the optimal pressure on a continuous basis.
Adjunct Professor, Department of Medicine
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