Introduction Obesity Hypoventilation Syndrome (OHS) is defined as sleep disordered breathing, obesity, and daytime hypercapnia, without another cause of ventilatory impairment.1 Recent studies have shown that a raised base excess (≥2) or raised venous bicarbonate without daytime hypercapnia, represents a subgroup with OHS without overt respiratory failure.2 A readily available sleep study parameter indicating the presence of OHS rather than requiring biochemistry would be ideal. We assessed the use of time spent with oxygen saturations ≤90% from standard sleep study data and its relationship with a biochemical diagnosis of OHS.
Methods We prospectively collected data on sleep clinic patients referred for assessment of possible obstructive sleep apnoea. Patients underwent sleep studies as per standard practice, and the%time spent with saturations <90% was noted (more or less than 30% of the night). Venous bicarbonate or arterial blood gas was checked. Those with evidence of OHS on blood testing had assessment to exclude co-existent respiratory disease.
Results Data was collected from 190 patients, 71% male, average age 31 (10.8, range 25–75) and mean BMI 39 kg/m2 (8.7, 25–76). There was biochemical evidence of OHS in 54 patients (22%) (Venous bicarbonate >27, BE≥2, pCO2 ≥6kPa). Four patients were excluded: COPD (2), Myasthenia gravis (1) and thoracic scoliosis (1).
Table 1 shows the results. Saturations of ≤90% for ≥30% of night had a sensitivity for diagnosing OHS of 59%, specificity 47%. The positive predictive value was 31% and negative predictive value was 74%.
Conclusions The parameter of “time spent with saturations below 90%” on sleep study is not particularly sensitive or specific for identifying patients with OHS in isolation. We cannot find other literature which has assessed this variable. It does not seem that it can replace blood biochemical measurement in the diagnosis of OHS. This condition still has many unanswered questions remaining including best method of diagnosis and management.
References 1 Mokhlesi B, Tulaimat A. Recent advances in obesity hypoventilation syndrome. Chest 2007;132(4):1322–36
2 Manuel A, Hart N, Stradling J. Is a raised bicarbonate, without hypercapnia, part of the physiological spectrum of obesity-related hypoventilation? Thorax 2014;69(Suppl 2): A29