From the Guidelines
Obesity hypoventilation syndrome (OHS) should be diagnosed in patients with obesity (BMI > 30 kg/m2), sleep-disordered breathing, and awake daytime hypercapnia (awake resting PaCO2 > 45 mm Hg at sea level), after excluding other causes for hypoventilation, as defined by the American Thoracic Society clinical practice guideline 1. The diagnostic process for OHS involves several key steps, including:
- Arterial blood gas analysis to confirm hypercapnia
- Polysomnography or sleep respiratory polygraphy to determine the pattern of sleep-disordered breathing and hypoventilation
- Evaluation for coexistent obstructive sleep apnea (OSA), which is present in approximately 90% of patients with OHS 1
- Additional testing, such as pulmonary function tests, chest imaging, and thyroid function tests, to rule out other respiratory disorders and metabolic causes Key clinical features that suggest OHS include:
- Severe obesity
- Daytime sleepiness
- Morning headaches
- Signs of right heart failure
- Serum bicarbonate levels above 27 mEq/L, which can serve as a screening tool for chronic respiratory acidosis compensation
- Nocturnal oximetry showing significant desaturations, which may raise suspicion but cannot definitively diagnose OHS Early diagnosis of OHS is crucial, as it is associated with increased cardiovascular morbidity, hospitalization rates, and mortality compared to simple obesity or obstructive sleep apnea alone 1.
From the Research
Diagnosis of Obesity Hypoventilation Syndrome
The diagnosis of Obesity Hypoventilation Syndrome (OHS) is based on a combination of clinical criteria, including:
- Obesity (body mass index ≥30 kg/m²) 2, 3, 4, 5
- Daytime hypercapnia (arterial carbon dioxide tension ≥45 mmHg) 2, 3, 4, 5
- Sleep-disordered breathing, such as obstructive sleep apnea (OSA) 2, 3, 4, 5
- Absence of other known causes of hypoventilation 2, 3, 4, 5
Diagnostic Criteria
The diagnostic criteria for OHS may include:
- Arterial blood gas analysis to measure daytime hypercapnia 2, 4
- Sleep study, such as polysomnography, to diagnose sleep-disordered breathing 2, 3
- Measurement of venous bicarbonate levels, which may be useful in screening for OHS 3
- Calculation of HCO3 levels, which may be a sensitive and specific predictor of OHS 4
Predictors of OHS
Several predictors of OHS have been identified, including:
- High body mass index (BMI) 2, 4
- Low forced expiratory volume (FEV1) and forced vital capacity (FVC) 4
- High apnea-hypopnea index (AHI) 4
- Low mean and minimum nocturnal oxygen saturation (SpO2) 4
- High calculated HCO3 levels (> 27 mmol/L) 4
Importance of Diagnosis
The diagnosis of OHS is important because it is associated with high morbidity and mortality if left untreated 2, 3, 5, 6. Early diagnosis and treatment of OHS can improve clinical symptoms, quality of life, and gas exchange, and reduce the risk of adverse postoperative events 2, 5.