Impaired detection of added inspiratory resistance in patients with obstructive sleep apnea.

The ability to detect added inspiratory flow-resistive loads during wakefulness was examined in 5 men with laboratory-documented obstructive sleep apnea. The patients were eucapnic, and standard measurements of lung volumes and flow rates were within the normal range. The patients breathed through a circuit, to which each of 5 flow resistances of 0.12 to 5.5 cm H2O/L/s was added at 20- to 30-s intervals for 1 inspiration, following a cue. Each resistance was presented 10 times in random sequence, and the percentage of trials in which each load was detected by the patient was calculated. The threshold level of load detection, defined as the change in resistance that could be detected 50% of the time, was higher in the patients (2.37 +/- 0.41 cmH2O/L/s, mean +/- SE) than in 9 healthy subjects (0.65 +/- 0.08; p less than 0.001), and this difference persisted when the threshold load was corrected for background resistance (1.05 +/- 0.11 versus 0.45 +/- 0.07; p less than 0.001). In addition, the ventilatory response to hypercapnia was less in the patients (1.82 +/- 0.22 L/min/mm Hg) than in the normal subjects (3.87 +/- 0.40; p less than 0.005), and there was a significant negative correlation (r = 0.97; p less than 0.001) between the load detection threshold and the ventilatory response to CO2. The results add to the developing concept that subtle defects in respiratory control can be demonstrated even during wakefulness in eucapnic patients with obstructive sleep apnea.