From the Guidelines
Obesity Hypoventilation Syndrome (OHS) is diagnosed through a combination of clinical evaluation and specific tests, requiring three key elements: obesity (BMI ≥30 kg/m²), daytime hypercapnia (arterial PCO₂ >45 mmHg), and sleep-disordered breathing, with exclusion of other causes of hypoventilation. The diagnostic criteria for OHS, as outlined in the American Thoracic Society clinical practice guideline 1, involve a combination of clinical evaluation and specific tests.
Key Diagnostic Elements
- Obesity, defined as a body mass index (BMI) greater than 30 kg/m²
- Daytime hypercapnia, defined as an arterial carbon dioxide level (PaCO₂) greater than 45 mmHg at sea level
- Sleep-disordered breathing, which can be assessed through polysomnography or sleep respiratory polygraphy
Diagnostic Approach
Initial evaluation should include:
- Arterial blood gas analysis to confirm hypercapnia
- Pulmonary function tests to rule out other pulmonary disorders
- Overnight polysomnography to assess for obstructive sleep apnea, which coexists in 90% of cases Additional testing may include:
- Chest imaging
- Thyroid function tests
- Serum bicarbonate levels, which are typically elevated due to renal compensation for chronic respiratory acidosis
Clinical Features
Clinical features that should raise suspicion for OHS include:
- Excessive daytime sleepiness
- Morning headaches
- Signs of right heart failure
Importance of Early Diagnosis
Early diagnosis is crucial as OHS is associated with significant morbidity and mortality if left untreated, as noted in the guideline 1. The pathophysiology involves a complex interaction between obesity-related mechanical limitations to breathing, leptin resistance affecting respiratory drive, and sleep-disordered breathing leading to chronic hypercapnia and hypoxemia.
Screening for OHS
For obese patients with sleep-disordered breathing and a high pretest probability of having OHS, measuring PaCO₂ rather than serum bicarbonate or SpO₂ is suggested to diagnose OHS, as recommended in the guideline 1. However, for patients with low to moderate probability of having OHS, using a serum bicarbonate level of less than 27 mmol/L to exclude the diagnosis of OHS is suggested, as outlined in the guideline 1.
Treatment
Treatment for OHS typically involves positive airway pressure (PAP) therapy, with continuous positive airway pressure (CPAP) being the first-line treatment for stable ambulatory patients with OHS and concomitant severe obstructive sleep apnea, as recommended in the guideline 1. Weight-loss interventions that produce sustained weight loss of 25-30% of actual body weight are also suggested to achieve resolution of hypoventilation, as noted in the guideline 1.
From the Research
Diagnostic Criteria for Obesity Hypoventilation Syndrome (OHS)
The diagnostic criteria for OHS include:
- Obesity, defined as a body mass index (BMI) of 30 kg/m² or higher 2, 3
- Daytime hypercapnia, defined as an arterial carbon dioxide tension (PaCO2) of 45 mmHg or higher 2, 3
- Sleep-disordered breathing, in the absence of other known causes of hypercapnia 2, 4, 5
- The absence of significant lung or respiratory muscle disease 5
Key Characteristics of OHS
Key characteristics of OHS include:
- Daytime hypercapnia and sleep-disordered breathing 2, 4, 5
- Obesity-related changes in the respiratory system, alterations in respiratory drive, and breathing abnormalities during sleep 2
- High prevalence of metabolic and cardiovascular comorbidities, including heart failure, coronary disease, and pulmonary hypertension 2
Diagnosis of OHS
The diagnosis of OHS is typically established after: