Obesity hypoventilation syndrome
Obesity hypoventilation syndrome refers to sleep-related nonobstructive hypoventilation with hypercapnia (partial pressure of carbon dioxide [PaCO2] more than 45 mmHg or serum bicarbonate concentration more than 27 mmol/L) in obese people (body mass index [BMI] more than 30 kg/m2), for which no other cause of hypoventilation can be identified (eg kyphoscoliosis, drugs, neuromuscular disease). If obesity hypoventilation syndrome is suspected, seek expert advice.
Patients with obesity hypoventilation syndrome typically have the following characteristics:
- marked obesity
- cyanosis or plethora
- right heart failure or biventricular heart failure
- excessive daytime sleepiness.
Diagnosis of obesity hypoventilation syndrome requires a sleep study, usually with a transcutaneous carbon dioxide monitor, and often with arterial blood gases taken in the evening and morning. In the absence of transcutaneous or arterial carbon dioxide levels, serum bicarbonate concentration can be used, except in patients with disorders affecting bicarbonate concentration (eg kidney failure).
Many patients exhibit features of both obesity hypoventilation syndrome and obstructive sleep apnoea, with a combination of obstructive apnoeas and progressive hypoxaemia or hypercapnia through the night.
Treatment for obesity hypoventilation syndrome involves weight loss and continuous positive airway pressure (CPAP) under specialist guidance. A study comparing CPAP with bilevel positive airway pressure (BPAP) found that the two modalities had equivalent benefit1. CPAP is usually preferred as initial therapy because BPAP is more expensive and more complex to use. BPAP can be considered in patients who do not respond to CPAP.
Rarely, supplemental oxygen may be needed at low flow rates for patients with persistent hypoxaemia despite CPAP. The oxygen must be titrated to a target range to prevent oxygen-induced hypercapnia.
Although respiratory stimulants have been used in clinical trials to increase the sensitivity of ventilatory chemoreceptors, long-term studies to support this therapy are not available at the time of writing.