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 x 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.