Obesity hypoventilation syndrome (OHS) is a condition seen in some obese patients (BMI > 30 kg/m2) with daytime hypercapnia (PaCO2 of > 45 mm Hg) and sleep-disordered breathing. The diagnosis requires that all other causes of hypoventilation be ruled out, such as obstructive or restrictive ventilatory disorders, neurologic, neuromuscular and endocrine etiologies. Although the weight criteria for OHS is a BMI of > 30 kg/m2, most patients have a BMI of > 35 kg/m2 with higher prevalence among patients at a higher BMI. Testing involves daytime wake arterial blood gas analysis and a nocturnal sleep study with continuous CO2 monitoring. Sleep-disordered breathing is typically OSA (90%) with the remainder being sleep-related hypoventilation. Additional testing including imaging, echocardiogram and pulmonary function testing may be obtained to further evaluate the etiology of hypoventilation. A serum bicarbonate level of less than 27 mEq/L has a 97% negative predictive value, and it may help exclude the diagnosis in those with a low suspicion of OHS. However, in those strongly suspected to have OHS, confirmation with arterial blood gases is recommended.
The estimated prevalence of OHS in the United States is about 0.4% of the adult population, but this may be underestimated due to misdiagnosis, failure to screen, or lack of confirmatory testing. With increasing rates of obesity worldwide, OHS prevalence is expected to rise.
Underlying pathophysiologic mechanisms that leads to the hypoventilation are complex and not entirely understood. They include the mechanical load of obesity on the respiratory system, impaired central respiratory drive, sleep-disordered breathing and hormonal mechanisms. Interestingly, the term Pickwickian syndrome made popular by writer Charles Dickens in his book ‘The Posthumous Papers of the Pickwick Club’ is often used synonymously with OHS, however they are not interchangeable for the character (Joe) likely had advanced OHS with signs of right heart failure.
The morbidity and mortality seen in the OHS population makes them substantially sicker than the eucapnic obese patient with or without OSA. If undetected and untreated, OHS patients can develop pulmonary hypertension (World Health Organization Group 3) with subsequent right ventricular failure or cor pulmonale. These patients have high healthcare utilization with recurrent admissions for acute on chronic hypercapnic respiratory failure and heart failure and post acute long-term care needs.
The mainstays of treatment include weight loss and positive airway pressure therapy. The targeted goal for sustained weight loss is 25 to 30% of body weight. In addition to aggressive counseling regarding weight loss, referral for bariatric surgery may be considered. For the OHS patient with significant OSA and milder hypoventilation, continuous positive airway pressure (CPAP) is initial therapy. In those with more hypoventilation and milder (or absent) OSA, non-invasive positive pressure ventilation (NIPPV) is indicated. In patients newly diagnosed in the hospital with OHS after respiratory failure, efforts should be undertaken to discharge the patient with NIPPV, followed by an outpatient sleep study.
OHS can be a difficult disease to recognize and treat. Adherence and acceptance for both positive airway pressure therapy and weight loss strategies may be challenging. However, early recognition and optimal management are paramount to improving outcomes.
Four Facts About Sleep Apnea
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