Is obesity hypoventilation syndrome (OHS) the same as Pickwickian syndrome, and what are the diagnostic criteria and first‑line treatment for an adult with severe obesity (BMI ≥ 30 kg/m²) presenting with chronic daytime hypercapnia?

Obesity Hypoventilation Syndrome and Pickwickian Syndrome Are the Same Condition

Obesity hypoventilation syndrome (OHS) and Pickwickian syndrome are identical terms for the same clinical entity—the historical name "Pickwickian syndrome" has been replaced by the modern medical term "obesity hypoventilation syndrome." 1, 2

Diagnostic Criteria

OHS requires all three of the following components to be present simultaneously: 3, 4

• BMI > 30 kg/m² (obesity)
• Awake daytime arterial PaCO₂ > 45 mm Hg at sea level (chronic hypercapnia)
• Sleep-disordered breathing documented on polysomnography
• Exclusion of other causes of hypoventilation (neuromuscular disease, severe COPD, chest wall disorders, metabolic abnormalities) 3, 5

Risk-Stratified Diagnostic Approach

High Pre-test Probability Patients

For patients with BMI > 40 kg/m² or those with severe obesity plus typical symptoms (daytime somnolence, witnessed apneas, morning headaches): 3

• Proceed directly to arterial blood gas measurement—skip screening tests 3
• Measure arterial PaCO₂ to confirm hypercapnia > 45 mm Hg 3, 4
• Order polysomnography or sleep respiratory polygraphy to characterize sleep-disordered breathing pattern 3

Low-to-Moderate Pre-test Probability Patients

For obese patients with less obvious clinical presentation (estimated prevalence ≤20%): 3

• Screen with serum bicarbonate first 3, 6
  • If bicarbonate < 27 mmol/L: OHS is very unlikely—stop workup 3, 6
  • If bicarbonate ≥ 27 mmol/L: proceed to arterial blood gas measurement 3, 6
• Do not use SpO₂ alone to decide when to measure PaCO₂—insufficient evidence supports this approach 3, 6

Essential Confirmatory Testing

• Polysomnography is mandatory to determine whether obstructive sleep apnea (OSA) coexists and its severity 3, 4
• Approximately 90% of OHS patients have coexistent OSA (AHI > 5 events/h), and 70% have severe OSA (AHI > 30 events/h) 3, 4

First-Line Treatment Algorithm

Treatment selection depends on OSA severity documented on polysomnography: 3, 4

Severe OSA Present (AHI > 30 events/h)

• CPAP is first-line therapy 3, 4
• Perform CPAP titration during polysomnography to determine optimal pressure settings 3
• CPAP alone is often sufficient when severe OSA drives the hypoventilation 4

No Severe OSA (AHI < 30 events/h or no OSA)

• Non-invasive ventilation (BiPAP) is required 3, 4
• Perform BiPAP titration study to establish appropriate inspiratory and expiratory pressure settings 3
• BiPAP provides both pressure support and PEEP, addressing hypoventilation more effectively than CPAP in this subgroup 4

Acute-on-Chronic Hypercapnic Respiratory Failure

For hospitalized patients presenting with acute decompensation: 3

• Initiate NIV immediately—do not delay 3
• Discharge on empiric NIV settings because 3-month mortality is high without therapy 3
• Arrange follow-up sleep study and PAP titration within 3 months of discharge 3
• Never discharge without PAP therapy—this is associated with significantly increased short-term mortality 3

Clinical Significance and Prognosis

OHS carries substantially higher mortality than eucapnic obesity or OSA alone: 3, 4

• Pulmonary hypertension develops in 30-88% of patients 3, 4
• Chronic heart failure and cor pulmonale result from chronic hypoxemia and hypercapnia 3, 4
• Increased hospitalization rates for acute-on-chronic respiratory failure 3
• The condition often remains undiagnosed until late in disease course, making early recognition critical 1

Definitive Treatment Consideration

Sustained weight loss of 25-30% of body weight can lead to resolution of OHS: 3, 6

• Bariatric surgery is the most effective method to achieve this degree of weight loss 3, 6
• Weight loss should be pursued after stabilization with PAP therapy 3
• This represents the only potentially curative intervention for OHS 3

Critical Pitfalls to Avoid

• Do not rely on SpO₂ alone for screening—current evidence does not support using oxygen saturation to trigger PaCO₂ measurement 3, 6
• Do not assume COPD is the sole cause of hypercapnia in obese patients—always consider OHS when BMI > 30 kg/m² 6
• Do not discharge acute presentations without PAP therapy—this significantly increases mortality risk 3
• Do not use CPAP for all OHS patients—those without severe OSA require BiPAP for adequate ventilatory support 3, 4