What are the risk factors for Obesity Hypoventilation Syndrome (OHS)?

Risk Factors for Obesity Hypoventilation Syndrome

The primary risk factor for obesity hypoventilation syndrome (OHS) is severe obesity, particularly with a body mass index (BMI) ≥30 kg/m², which causes mechanical and physiological changes to the respiratory system that lead to chronic daytime hypercapnia. 1

Major Risk Factors

Obesity-Related Factors

• Body Mass Index (BMI) ≥30 kg/m² - required for diagnosis 1
• Higher BMI correlates with increased risk, with severe obesity being the most significant factor

Sleep-Disordered Breathing

• Obstructive Sleep Apnea (OSA) - present in approximately 90% of OHS patients 1
• Severe OSA (AHI >30 events/hour) - present in about 70% of OHS patients 1
• Sleep-disordered breathing without severe OSA occurs in approximately 30% of cases 1

Respiratory System Impairments

• Restrictive pulmonary function pattern characterized by:
  • Reduced lung volumes, especially forced vital capacity (FVC)
  • Normal or elevated FEV1/FVC ratio (>0.7)
  • Reduced total lung capacity (TLC) 1
• Mechanical limitations due to obesity affecting chest wall compliance 2
• Increased work of breathing and carbon dioxide production 3
• Respiratory muscle dysfunction 3

Ventilatory Control Abnormalities

• Reduced ventilatory response to hypercapnia 3
• Alterations in respiratory drive 1, 2

Clinical Indicators and Associated Factors

Laboratory Findings

• Serum bicarbonate >27 mmol/L - most characteristic laboratory finding, representing a response to chronic acidosis 1
• Daytime hypercapnia (PaCO₂ >45 mmHg) 1
• Daytime hypoxemia of variable degree 1

Comorbidities Associated with Higher Risk

• Cardiovascular conditions:
  • Chronic heart failure
  • Pulmonary hypertension
  • Coronary artery disease 1
• Metabolic disorders 1

Clinical Pitfalls to Avoid

• Do not attribute respiratory difficulties in obese patients solely to physical deconditioning 1
• Do not overlook OHS in patients with respiratory failure - patients hospitalized with respiratory failure and suspected OHS should receive NIV at discharge until completion of outpatient diagnostic workup 1
• Do not miss the diagnosis - OHS is often overlooked and confused with other conditions associated with hypoventilation, particularly COPD 4

Diagnostic Approach

1. Screen high-risk obese patients with serum bicarbonate levels (>27 mmol/L suggests OHS)
2. Confirm with arterial blood gas showing daytime hypercapnia (PaCO₂ >45 mmHg)
3. Perform polysomnography to determine the specific pattern of respiratory disorder during sleep
4. Exclude other causes of hypoventilation (OHS is a diagnosis of exclusion) 1

Early recognition and treatment of OHS is crucial as it is associated with significant morbidity and mortality compared to eucapnic obese patients 5.