EFFECTS OF NONINVASIVE VENTILATION ON PULMONARY GAS-EXCHANGE AND HEMODYNAMICS DURING ACUTE HYPERCAPNIC EXACERBATIONS OF CHRONIC OBSTRUCTIVEPULMONARY-DISEASE

Noninvasive positive pressure ventilation (NIPPV) can replace tracheal intubation in acute exacerbations of chronic obstructive pulmonary di sease (COPD) with severe hypercapnic respiratory failure. However, the underlying mechanisms by which NIPPV improves pulmonary gas exchange are not known. We studied 10 male COPD patients (68 +/- 8 [SD] yr) wit h acute severe hypercapnic respiratory failure within 36 h after hospi tal admission. Measurements of pulmonary gas exchange, hemodynamics, a nd respiratory mechanics were done: (1) breathing spontaneously (basel ine); (2) after 15 and 30 min of NIPPV with pressure support (inspirat ory pressure = 12 +/- 2 cm H2O, PEEP = 3 +/- 2 cm H2O); and (3) 15 min after NIPPV withdrawal. Patients were ventilated using a full face ma sk, keeping Fl(O2) constant (0.23 +/- 0.02) in all conditions. Compare d with baseline, during NIPPV (15 min) we observed a moderate increase in Pa-O2 (from 50 +/- 6 to 57 +/- 9 mm Hg; p < 0.05), and a fall in P a-CO2 (from 66 +/- 10 to 59 +/- 10 mm Hg; p < 0.0001), but AaPO(2) inc reased (from 39 +/- 13 to 48 +/- 13 mm Hg; p < 0.001). Breathing frequ ency decreased (from 26 +/- 5 to 19 +/- 3 breaths/min; p < 0.0001), ti dal volume increased (from 311 +/- 42 to 520 +/- 133 mi; p < 0.0001), and minute ventilation increased (from 8.0 to 1.7 to 9.6 +/- 2.0 L/min ; p < 0.05). Cardiac output fell during NIPPV in all patients (from 6. 7 +/- 1.6 to 5.8 +/- 1.3 L/min; p < 0.0025) with no impact on mixed ve nous PO2. NO substantial changes in (V) over dot A/(Q) over dot mismat ching (multiple inert gas elimination technique) were observed. While oxygen uptake showed a trend to decrease, the respiratory exchange rat io (R) increased (from 0.78 +/- 0.17 to 0.90 +/- 0.22; p < 0.001). The effects of NIPPV were unchanged at 30 min compared with 15 min and we re reversed after 15 min of NIPPV withdrawal. We conclude that improve ment in respiratory blood gases during NIPPV is essentially due to hig her alveolar ventilation (p < 0.001) and not to improvement in (V) ove r dot A/(Q) over dot relationships. The increase in AsPO2 was explaine d by the rise in R due to an increased clearance of body stores of CO2 during NIPPV. Our results indicate that attainment of an efficient br eathing pattern rather than high inspiratory pressures should be the p rimary goal to improve arterial blood gases during NIPPV in this type of patient.