What are the diagnostic criteria for Obese Hypoventilation Syndrome (OHS)?

From the Guidelines

Obese hypoventilation syndrome (OHS) is diagnosed when a patient with obesity (BMI ≥30 kg/m²) has daytime hypercapnia (PaCO₂ >45 mmHg) and sleep-disordered breathing, after excluding other causes of hypoventilation. The diagnostic process begins with arterial blood gas analysis to confirm hypercapnia, which is essential for diagnosis, as stated in the American Thoracic Society clinical practice guideline 1. Polysomnography should be performed to identify sleep-disordered breathing, particularly obstructive sleep apnea, which coexists in 90% of OHS patients 1. Additional testing includes pulmonary function tests to rule out other pulmonary disorders, chest imaging to exclude thoracic restrictive disorders, and thyroid function tests to eliminate hypothyroidism as a cause.

Key Diagnostic Features

• Serum bicarbonate levels are often elevated (>27 mEq/L) due to renal compensation for chronic respiratory acidosis, making this a useful screening tool in obese patients with suspected hypoventilation 1.
• Clinical features supporting the diagnosis include excessive daytime sleepiness, morning headaches, fatigue, and signs of right heart failure in advanced cases.
• Early diagnosis is critical as OHS is associated with increased morbidity and mortality compared to simple obesity or obstructive sleep apnea alone, with complications including pulmonary hypertension, cor pulmonale, and increased risk of hospitalization and death 1.

Diagnostic Approach

• The two tests required to diagnose OHS are a sleep study (polysomnography or respiratory polygraphy) to establish the presence of SDB and a measurement of arterial blood gases during wakefulness to establish the presence of hypercapnia 1.
• Simple tests to screen for OHS, such as serum bicarbonate levels and pulse oximetry, can be used to identify patients at risk, but arterial blood gas analysis and polysomnography are necessary for definitive diagnosis 1.

From the Research

Diagnosis of Obese Hypoventilation Syndrome

The diagnosis of obese hypoventilation syndrome (OHS) is based on the presence of obesity, daytime hypercapnia, and sleep-disordered breathing in the absence of other causes of hypoventilation 2, 3, 4, 5, 6.

• The diagnostic criteria for OHS include a body mass index (BMI) of 30 kg/m² or higher, arterial carbon dioxide tension of 45 mmHg or higher, and sleep-disordered breathing 2, 3, 4.
• The diagnosis is typically established after arterial blood gases and a sleep study 2.
• Other causes of hypoventilation, such as chronic obstructive pulmonary disease (COPD), must be ruled out before making a diagnosis of OHS 3, 4.

Diagnostic Tests

The following diagnostic tests are used to diagnose OHS:

• Arterial blood gases to measure carbon dioxide and oxygen levels 2, 4, 6
• Sleep study to assess sleep-disordered breathing 2, 3, 4
• Venous bicarbonate measurement to screen for OHS 3

Clinical Characteristics

Patients with OHS often have the following clinical characteristics:

• Obesity (BMI ≥ 30 kg/m²) 2, 3, 4, 5, 6
• Daytime hypercapnia (arterial carbon dioxide tension ≥ 45 mmHg) 2, 3, 4, 5, 6
• Sleep-disordered breathing 2, 3, 4, 5, 6
• Comorbidities such as metabolic and cardiovascular diseases, including heart failure, coronary disease, and pulmonary hypertension 2, 4, 5, 6