Physiologic Evaluation of 4 Weeks of Nocturnal Nasal Positive Pressure Ventilation in Stable Hypercapnic Patients with Chronic Obstructive Pulmonary Disease

Background: The long-term daily use of noninvasive mechanical ventilation (NIMV) to treat chronic respiratory failure in chronic obstructive pulmonary disease (COPD) patients is not widely recommended, partly because of a lack of clear clinical results and partly because the physiological mechanisms by which the daily application of NIMV would be helpful in these patients have not yet been clarified. Objectives: We designed a physiological study in order to assess the effects of supervised long-term NIMV on gas exchange, respiratory muscle function, pulmonary mechanics and to ascertain the possible effect of the treatment in responders and nonresponders. Methods: Fourteen consecutive inpatients with stable hypercapnic COPD (pH = 7.37 ± 0.01; PaCO 2 = 56.73 ± 6.48 mm Hg) underwent 4 weeks of nocturnal NIMV delivered with a bilevel ventilator ‘physiologically’ set to reduce tidal transdiaphragmatic pressure (Pdi) by at least 50% and the amount of dynamic intrinsic positive end-expiratory pressure by 70%. Various measurements were compared with those obtained in a control group of consecutive patients with comparable baseline characteristics who refused NIMV and underwent breathing exercises for the same period of time. Results: By the end of the 4 weeks NIMV had induced a slight but significant (p < 0.01) reduction in resting PaCO 2 (53.78 ± 5.64 mm Hg) associated with a decrease in the pressure time product of the diaphragm per minute (from 172 ± 60 to 136 ± 61 cm H 2 O/l/s; p < 0.05). This latter value was primarily due to a significant shortening of the inspiratory duty cycle, while Pdi and lung mechanics were not modified. Eight of the 13 NIMV-treated patients (1 dropped out for nonmedical reasons) had a clear reduction in PaCO 2 (>3 mm Hg or >5% from enrollment) and were classified as responders. The acute reduction in PaCO 2 during the first trial with NIMV resulted to be a strong index of the final response. The subgroup of responders had a significantly increased maximal Pdi (from 41 ± 19 to 49 ± 23 cm H 2 O, p < 0.05) and an enhanced ability of the ventilatory pump to clear CO 2 (9.7 ± 3.4 vs. 7.2 ± 2.9 cm H 2 O × s/min; p < 0.01). No significant changes were observed in the control group. Conclusions: These results suggest that in a remarkable and identifyable proportion of patients with stable hypercapnic COPD, nocturnal NIMV may decrease resting PaCO 2 , reraising the role of chronically supporting the respiratory pump.