Elevated pulse flow resistance in awake obese subjects with obstructive sleep apnea

We measured resistance to air flow using the pulse technique in 5 normal subjects and in 12 male patients with obstructive sleep apnea while awake. At the end of an expiration, a constant flow pulse of 0.3 L/s of air was blown into a subject's mouth, while transpulmonary pressure and total transrespiratory pressure were recorded. Resistance was calculated by dividing the initial pressure step by the pulse flow rate. Nocturnal sleep studies were performed in patients with an ear oximeter, esophageal catheter, and face mask pneumotachometer; in the normal subjects, the studies were performed with an oximeter. A desaturation index was calculated by multiplying the average maximal percent desaturation during each apneic episode by the average number of desaturation episodes per hour of sleep. Transpulmonary and total transrespiratory system pulse flow resistance (RI and Rtot) were highly correlated with desaturation index (r = 0.94, p less than 0.001 and r = 0.88, p less than 0.001, respectively). The elevated pulse flow airway resistance in our patients may be caused by upper airway narrowing because of compression by fat, by lax tone in pharyngeal muscles, or by both. Measurements of RI and Rtot may be useful to detect and follow subjects with obstructive sleep apnea.