[Respiratory muscle force and resistance in patients with SAHS. The effect of using nighttime CPAP]

During nighttime episodes of obstructive apnea in patients with sleep apnea-hypopnea syndrome (SAHS), repeated and progressive inspiratory efforts are made. Such intense nighttime activity can have a deleterious effect on daytime function of respiratory muscles.

The objective of this study was to evaluate daytime respiratory muscle function in a group of SAHS patients before and after two months of treatment with nighttime continuous positive airway pressure (CPAP).

We enrolled 12 patients with SAHS and 10 normal subjects (control group). To evaluate respiratory muscle strength we measured maximum esophageal pressure (Pesmax), transdiaphragmatic pressure (Pdimax) and inspiratory pressure in the mouth (PM). Respiratory muscle resistance was assessed using peak pressure in the mouth (PMPeak), time of tolerance (Tlim) and maximum inspiratory pressure-time index (PTimax). We also analyzed the nighttime function of respiratory muscles during apneic episodes in 10 of the 12 SAHS patients. We propose and define an index of nighttime respiratory muscle activity (RMian) as the product of the tension-time index for the diaphragm observed at the end of nighttime apneic episodes (TTdiapnea) and the apnea-hypopnea index (AHI).

Respiratory muscle strength was similar in the two groups and no changes were observed in SAHS patients after treatment with nighttime CPAP. However, tolerance was lower in SAHS patients (PMpeak--30%, Tlim--31% and PTimax--49%). Two months of nighttime CPAP normalized all three variables in these patients. MRian was related to percent improvement in PMpeak after treatment with nighttime CPAP in SAHS patients (r = 0.66, p < 0.04).

SAHS has an adverse effect on the daytime endurance of respiratory muscles that is proportional to the increase of nighttime mechanical muscle activity. The application of nighttime CPAP is restorative, probably because it allows respiratory muscles to rest.