Background: Our objective was to study the long-term effects of non-invasive positive pressure ventilation (NPPV) on lung function and gas exchange in patients with the obesity hypoventilation syndrome (OHS).
Design: Prospective observational study in OHS patients performing NPPV over a period of 24 months.
Results: We studied 35 clinically stable OHS patients with a mean body mass index (BMI) 45.9+/-8.8 kg/m(2) and daytime PaCO(2) at room air of 6.92+/-0.48 kPa at baseline. Nocturnal NPPV was initiated with pressure-cycled devices on IPAP 24+/-3 cm H(2)O, EPAP 6+/-2 cm H(2)O and respiratory frequency of 18.8+/-3.7/min. After 12 and 24 months of NPPV hypercapnia was persistently normalized and hypoxemia was markedly improved while no changes in calculated alveolar-arterial oxygen difference occurred. Augmented ventilation was followed by a significant reduction in hemoglobin and hematocrit (P<0.001 each). Daily duration of ventilator use significantly correlated with the decrease in PaCO(2) after 12 months (r = 0.37; P<0.05) and 24 months (r = 0.47; P<0.05). Vital capacity (VC) and expiratory reserve volume (ERV) significantly increased after 12 and 24 months NPPV compared to the baseline values, though BMI was only slightly reduced. The 2-year survival rate was 91% with three patients (9%) discontinuing NPPV during the study period.
Conclusion: Long-term domiciliary NPPV normalizes hypercapnia and markedly improves hypoxemia as well as polycythemia in OHS patients. In addition, NPPV leads to a significant reduction in restrictive ventilatory disturbance, predominantly by increasing ERV. Application of high inspiratory pressures and good adherence to therapy are presumed to be the basis for the beneficial effects of NPPV in OHS.