How to Rule Out Obesity Hypoventilation Syndrome
To rule out OHS in an obese patient with sleep-disordered breathing, use serum bicarbonate <27 mmol/L as your screening threshold—if below this level, OHS is very unlikely and you can forego arterial blood gas testing in patients with low to moderate clinical suspicion. 1
Diagnostic Criteria for OHS
OHS requires all three of the following components to be present 1:
- Obesity: BMI >30 kg/m² 1
- Daytime hypercapnia: Awake resting PaCO₂ >45 mm Hg at sea level 1
- Sleep-disordered breathing: Documented on polysomnography 1
- Exclusion: Other causes of hypoventilation must be ruled out 1
Algorithmic Approach to Screening
Step 1: Assess Clinical Probability
High pretest probability patients (typically severely obese with classic symptoms) 1, 2:
- Severe obesity (often BMI >40 kg/m²) 2
- Dyspnea and excessive daytime sleepiness 2
- Loud disruptive snoring with witnessed apneas 2
- Mild hypoxemia during wakefulness and/or significant hypoxemia during sleep 2
- Lower extremity edema 2
- Fatigue and nocturia 2
For these high-risk patients: Proceed directly to arterial blood gas measurement rather than using screening tests. 1
Step 2: Use Serum Bicarbonate for Low-to-Moderate Probability Cases
For patients with <20% probability of OHS 1:
- If serum bicarbonate <27 mmol/L: OHS is very unlikely—you can safely rule out OHS without measuring arterial blood gases 1
- If serum bicarbonate ≥27 mmol/L: Proceed to arterial blood gas measurement to confirm or exclude OHS 1
This bicarbonate threshold of 27 mmol/L allows you to avoid arterial blood gas testing in 64-74% of obese patients with OSA who have low clinical suspicion 1. Research supports this cutoff with 76.6% sensitivity and 74.6% specificity, with a negative predictive value of 88.9% 3.
Step 3: Avoid Using Awake SpO₂ Alone
Do not rely on awake SpO₂ measurements to decide when to measure PaCO₂—insufficient evidence exists for this approach. 1 While nocturnal oxygen desaturation (nadir SpO₂ <80%) has predictive value, awake SpO₂ lacks validated thresholds for screening 1, 3.
Essential Diagnostic Testing
Arterial Blood Gas Analysis
When indicated based on the algorithm above, arterial blood gas is the definitive test to confirm or exclude daytime hypercapnia (PaCO₂ >45 mm Hg) 1. This must be measured during wakefulness at rest 1.
Polysomnography
Even though OHS is defined by daytime hypercapnia, polysomnography is required to 1:
- Determine the pattern of sleep-disordered breathing (obstructive vs. nonobstructive) 1
- Tailor treatment appropriately 1
- Establish optimal positive airway pressure settings 1
Approximately 90% of OHS patients have coexistent obstructive sleep apnea (AHI >5 events/h), with nearly 70% having severe OSA (AHI >30 events/h) 1, 2.
Common Pitfalls to Avoid
Pitfall #1: Assuming normal awake oxygen saturation rules out OHS—patients may have only mild hypoxemia while awake but significant hypoxemia during sleep 2.
Pitfall #2: Using serum bicarbonate alone in high-risk patients—proceed directly to arterial blood gas in severely obese patients with classic symptoms 1.
Pitfall #3: Failing to exclude other causes of hypoventilation such as significant lung disease, neuromuscular disorders, or chest wall abnormalities before diagnosing OHS 1.
Pitfall #4: Missing the diagnosis entirely—OHS prevalence ranges from 8-20% in obese patients referred to sleep centers, yet it remains largely underdiagnosed 1, 4.
Clinical Context
The prevalence of OHS is approximately 0.4% in the general adult population but increases dramatically to 8-20% among obese patients with sleep-disordered breathing 1, 5. Given the obesity epidemic, maintaining high clinical suspicion is essential, as undiagnosed OHS carries increased mortality, chronic heart failure, pulmonary hypertension, and risk of acute-on-chronic hypercapnic respiratory failure 1.