Snoring during NREM sleep: respiratory timing, esophageal pressure and EEG arousal

Eight men who were regular heavy snorers were monitored while awake and during nocturnal sleep. All subjects were known to be free of lung disease, obstructive sleep apnea syndrome, and oxygen drops during sleep. For each subject, two snoring periods of 3-31 min with a mean of 12.7 min were randomly selected for comparison with periods of normal, non-snoring NREM sleep breathing. A mean of 150 respiratory cycles per period were analyzed. For each respiratory cycle, respiratory inductive plethysmography and measurements of peak flow, laryngeal sounds, and esophageal pressure (Pes) were used to calculate breathing frequency (bf), inspiratory time (Ti), expiratory time (Te), total respiratory cycle length (Ttot), and Pes at its nadir. During NREM sleep silent breathing, the Ti/Te ratio was analogous to that already measured in normal subjects. With the onset of snoring, an immediate increase in Ti, Te, and Pes nadir were noted. Mean peak Pes nadir increased 120 +/- 37%, and mean Ttot increased by 18%. Through the duration of the snoring period, a further increase in Ti (mean = 10.4 +/- 3.4%) and a decrease in Te were noted, with a mean change in Ti/Ttot of 12 +/- 3.1%. The shape of the esophageal pressure wave during expiration shifted from its normal dynamics. The percentage of Te decreased by a mean of -9.8 +/- 2.3% (P less than 0.0001), and the rise time in Pes increased a mean of 37%. When Pes nadir was the most negative, a mean peak flow decrease of 43 +/- 13.6% from baseline was observed. Tidal volume had decreased by a mean of 22% and minute ventilation by a mean of 21% at the end of the snoring period. Separate investigation of each subject indicates that the evolutions of Ti, Te and Pes during snoring were not the same for all subjects. At least two different groups of snorers exist; these groups may be differentiated by the evolution of Pes over time during snoring. Modifications in the 'braking' role of inspiratory muscles during expiration may explain the changes in the Pes wave dynamics snoring which lead to repetitive EEG arousals, termination of snoring periods, and some sleep fragmentation.