The clinical presentation of patients with obesity-hypoventilation syndrome (OHS) is heterogeneous in terms of severity, ranging from those with few symptoms referred for suspected sleep apnea–hypopnea syndrome (SAHS), to those diagnosed after admission to an intensive care unit for hypercapnic encephalopathy. Published clinical series indicate that OHS is associated with major morbidity and mortality, with respiratory or cardiovascular failure causing fatal outcome. It is likely that failure to suspect the respiratory disorder together with cardiovascular comorbidity at the time of diagnosis contribute to an unfavorable prognosis in specific patients.1–4

Several observational series estimating mortality in this group and identifying associated risk factors have been published.1–4 Ojeda Castillejo et al. recently published an interesting prospective study in Archivos de Bronconeumología on the evolution of patients with OHS, and we would like to comment on several aspects of this series.5 This study has probably the longest follow-up time (mean >7 years) of those hitherto published; patients were closely monitored, and seemingly appropriate ventilation criteria were established. As regards results, the authors found that the OHS group without SAHS had higher mortality, and that, in contrast to earlier studies, persistence of reduced forced vital capacity (FVC) had prognostic value. Moreover, and contrary to what might be expected,2–4 they found that baseline paO2 had no prognostic value. Although the results of studies such as this are relevant and necessary to understand the natural history of patients with OHS, the absence of some data limits interpretation of the results:

• 1.

  Patients were recruited either after they had been stabilized following hospital admission for respiratory acidosis, or during a visit to the clinic. Hospitalized patients may have more comorbidities than those recruited in the clinic.3,4 However, the authors do not provide data on comorbidities.

• 2.

  In the mortality analysis, the authors do not indicate if they included patients who did not comply with non-invasive mechanical ventilation (NIV) or continued positive airway pressure (CPAP) therapy–a total of 9 subjects. This is another relevant detail, as lack of compliance has been associated with higher mortality.1,3 The distribution of non-compliers by patient group is also unknown.

• 3.

  The many causes of hypoxemia under NIV include central hypoventilation and ventilation/perfusion disorders. In this respect, there are clinical practice guidelines that recommend a support pressure setting of at least 10mmHg (inspiratory positive airway pressure [IPAP]–expiratory positive airway pressure [EPAP]) before assessing whether oxygen should be added. In the paper by Ojeda Castillejo et al., it is not clear if this consideration has been taken into account.

• 4.

  ANOVA or the Student's t-test with Bonferroni correction is more appropriate for a comparative analysis of FVC values over time.

To summarize, to estimate prognostic factors of mortality in patients with OHS, NIV or CPAP settings must be correct, and a detailed study of potentially relevant risk factors should be made. Additionally, and while awaiting the results of ongoing studies, it will be interesting to see if nocturnal monitoring of NIV in poor responders (by analyzing the ventilator software with or without simultaneous polygraphy) will have a positive effect on their quality of life and prognosis for survival.