Fig. 1.
Schematic diagram of the experimental protocols (top traces) and polysomnographic responses during non-rapid eye movement (NREM) sleep in an apneic male subject. 1) Baseline (far left): nasal pressure (PN) was adjusted to effective continuous positive airway pressure (CPAP) and held for at least 3 min (holding pressure) to establish a stable nonflow-limited breathing pattern (see flow trace bottom far left). 2) Passive upper airway obstruction (UAO; middle left): a series of brief (5 breaths) drops in PN from holding pressure were performed without concomitant activation of EMG. During these drops, UAO ensued, as indicated by inspiratory flow limitation as in the flow trace below. 3) Active stable UAO (middle right) without arousal: PN was reduced stepwise by 1–2 cmH2O for at least 10 min to a level that produced a stable flow-limited breathing pattern with maximal concomitant cyclic inspiratory activation of EMG, but no intermittent hypoxia. This pressure is referred to as the cycling threshold, because any further reduction in PN was associated with an unstable breathing pattern, as characterized by the presence of either hypopneas, apneas, intermittent hypoxia, or arousal (see panel 4 far right: active unstable UAO). Our analytic approach is also shown. The difference in peak inspiratory airflow (V̇imax) between cycling threshold and passive drop at same pressure level was defined as upper airway ventilatory response (ΔV̇imax). Comparisons in timing parameters [respiratory rate (f), inspiratory duty cycle (Ti/Ttot)] between holding pressure and cycling threshold were taken as respiratory timing responses to UAO. EMGAN, electromyogram of alae nasi; SpO2, oxyhemoglobin saturation from pulse oximetry.