Effects of Nasal Continuous Positive Airway Pressure Therapy on Respiratory Parameters of Upper Airway Patency in Patients With Obstructive Sleep Apnea Syndrome

Objectives: To assess whether an initial treatment with nasal continuous positive airway pressure (NCPAP) therapy, applied for one night, had any effect on airway patency. Methods: In 18 patients with obstructive sleep apnea syndrome (OSAS), we measured the total resistance of the respiratory system (Rrs) and their relevant lung functions before and after polysomnography, with and without NCPAP therapy. The Rrs was measured at 3 Hz with the forced oscillation technique. The overnight changes in the specific respiratory conductance (SGrs = reciprocal of the Rrs per unit lung volume) was also calculated in the sitting position. Since many reports have suggested that obesity, through fat deposits around the pharynx, can affect the mechanical and neuromuscular properties of the upper airway, we also investigated if the degree of obesity was related to the magnitude of improvement in these parameters. Results: After the first night of NCPAP therapy, the Rrs decreased (sitting: 4.8 ± 0.4 vs 4.3 ± 0.4 cm H2O/L/S, p < 0.05; lying: 6.5 ± 0.4 vs 5.6 ± 0.4 cm H2O/L/s, p < 0.05) and the maximal voluntary ventilation increased in the morning (sitting: 101.6 ± 5.8% vs 106.4 ± 4.5%, p < 0.05; lying: 91.2 ± 5.4% vs 97.9 ± 4.7%, p < 0.05). The overnight difference in the SGrs showed a significant improvement after the initial treatment with NCPAP therapy (p < 0.05). However, the lung volume, flow volume loop, and closing volume in the morning did not change significantly after the therapy. An overnight decrease in the Rrs following NCPAP therapy is significantly correlated with the body mass index (sitting: r = 0.54, p < 0.05; lying: r = 0.61, p < 0.01). Conclusion: The improvements in Rrs without changes in spirometry may reflect improved upper airway patency after NCPAP therapy. The degree of obesity is suggested to be associated with the treatment effect on upper airway in patients with OSAS. (CHEST 1998; 114:691-696) Abbreviations: AHI = apnea hypopnea index; ANOVA = analysis of variance; BMI = body mass index; CV = closing volume; D-Rrs = difference in the Rrs in the morning vs the previous afternoon; D-S Grs = difference in the SGrs in the morning vs the previous afternoon; ESS = Epworth sleepiness scale; FRC = functional residual capacity; MW = maximal voluntary ventilation; NCPAP = nasal continuous positive airway pressure; NS = not significant; OSA = obstructive sleep apnea; OSAS = obstructive sleep apnea syndrome; Rrs = total resistance of the respiratory system;