What are the characteristics of Pickwickian syndrome (obesity hypoventilation syndrome)?

Characteristics of Pickwickian Syndrome (Obesity Hypoventilation Syndrome)

Pickwickian syndrome, or obesity hypoventilation syndrome (OHS), is defined by the triad of obesity (BMI > 30 kg/m²), sleep-disordered breathing, and awake daytime hypercapnia (PaCO₂ > 45 mm Hg at sea level) after excluding other causes for hypoventilation. 1

Core Diagnostic Criteria

• Obesity: BMI > 30 kg/m² (with severe obesity being a major risk factor)
• Hypercapnia: Awake resting PaCO₂ > 45 mm Hg at sea level
• Sleep-disordered breathing: Requires polysomnography or sleep respiratory polygraphy for confirmation
• Exclusion: Other causes of hypoventilation must be ruled out

Clinical Presentation

• Respiratory symptoms:

  • Daytime hypercapnia
  • Progressive dyspnea
  • Severe hypoxemia (both during wakefulness and sleep)
  • Sleep-disordered breathing

• Sleep patterns:

  • Approximately 90% of patients have coexistent obstructive sleep apnea (OSA)
  • Nearly 70% have severe OSA (AHI > 30 events/h) 1
  • High apnea-hypopnea index (AHI) - often >30 events/hour 2

• Laboratory findings:

  • Elevated serum bicarbonate (>27 mmol/L) - useful screening tool 1, 3
  • Arterial blood gas showing hypoxemia with hypercapnia 2

Complications and Sequelae

OHS is the most severe form of obesity-induced respiratory compromise and leads to serious sequelae, including:

• Increased mortality rates
• Chronic heart failure
• Pulmonary hypertension
• Hospitalization due to acute-on-chronic hypercapnic respiratory failure 1
• Higher healthcare expenses compared to eucapnic obese patients 4
• Early cardiovascular mortality 4

Pathophysiological Mechanisms

OHS arises from a complex interaction of multiple factors:

• Sleep-disordered breathing (primarily OSA)
• Diminished respiratory drive
• Obesity-related respiratory impairment (chest wall restriction)
• Ventilation-perfusion mismatch
• Increased work of breathing due to excess weight on the chest and abdomen 5

Screening and Diagnosis

• For high pretest probability patients: Measure PaCO₂ directly with arterial blood gases 1
• For low to moderate probability patients (<20%):
  • Use serum bicarbonate as initial screening
  • If bicarbonate <27 mmol/L, OHS is unlikely
  • If bicarbonate >27 mmol/L, proceed to arterial blood gas measurement 1, 3
• Sleep study: Polysomnography is required to determine the pattern of sleep-disordered breathing and establish optimal treatment settings 1

Treatment Approach

1. Positive airway pressure therapy:

   • CPAP is first-line for patients with OHS and coexistent severe OSA 1
   • Noninvasive ventilation (NIV) for patients with OHS without severe OSA or when CPAP is inadequate 3

2. Weight loss interventions:

   • Target sustained weight loss of 25-30% of body weight
   • Bariatric surgery is most effective for achieving resolution of OHS 1, 3
   • Lifestyle modifications alone rarely achieve sufficient weight loss 3

3. For hospitalized patients with respiratory failure:

   • Discharge with NIV until outpatient diagnostic procedures and PAP titration can be performed (ideally within 2-3 months) 1

Clinical Pitfalls to Avoid

• Assuming shortness of breath in obesity is due to deconditioning alone 3
• Delaying diagnosis until late in the disease course 4, 6
• Using CPAP in patients with OHS without severe OSA (may be inadequate) 3
• Inadequate follow-up after interventions 3
• Failing to screen for OHS in high-risk obese patients 6

Early recognition and treatment of OHS are crucial as effective management can significantly improve outcomes and reduce the high burden of morbidity and mortality associated with untreated disease 4, 6.