Key Facts

Obesity hypoventilation syndrome (OHS) is defined by the triad of obesity (BMI ≥30 kg/m²), chronic daytime hypercapnia (PaCO₂ >6.0 kPa), and sleep-disordered breathing, in the absence of other identifiable causes of hypoventilation.

Epidemiology

• Prevalence: ~0.15-0.4% of general population; ~10-20% of obese patients with OSA
• Affects ~30% of patients with severe obesity (BMI >40)
• Increasing prevalence in parallel with obesity epidemic
• Often undiagnosed: many cases only recognised during acute hospital admission
• Present in ~8-20% of patients referred for sleep studies

Aetiology

• Severe obesity → mechanical respiratory impairment
• Coexistent OSA in ~90% (OHS-OSA phenotype)
• ~10% have sleep hypoventilation without significant OSA (OHS-non-OSA phenotype)
• Central chemoreceptor insensitivity to CO₂ (acquired, possibly leptin-mediated)

Pathophysiology

• Mechanical: excess adipose tissue reduces chest wall compliance and restricts diaphragmatic excursion → reduced tidal volume, increased work of breathing → alveolar hypoventilation
• Central: acquired blunting of hypercapnic ventilatory response (possibly due to leptin resistance)
• Metabolic: chronic CO₂ retention → renal bicarbonate retention (compensatory metabolic alkalosis) → further blunting of central drive
• OSA component: repetitive nocturnal apnoeas worsen overnight CO₂ retention
• Consequences: pulmonary hypertension, cor pulmonale, polycythaemia, hypoxaemia

Typical Presentation

• Morbid obesity (BMI often >40 kg/m²)
• Excessive daytime somnolence
• Morning headaches (hypercapnia)
• Dyspnoea on exertion
• Loud snoring, witnessed apnoeas (if OSA component)
• Poor concentration, cognitive impairment

Clinical Signs

• Morbid obesity, large neck circumference
• Central cyanosis
• Plethora (polycythaemia)
• Raised JVP, peripheral oedema, hepatomegaly (cor pulmonale)
• Reduced chest wall expansion
• Somnolent/drowsy patient

Acute Presentation (Acute-on-Chronic Respiratory Failure)

• Often presents as acute hypercapnic respiratory failure during intercurrent illness
• Confusion, drowsiness, flapping tremor (CO₂ narcosis)
• May require emergency NIV
• High hospital mortality (~18% in-hospital)

Red Flags

• Acute hypercapnic respiratory failure (pH <7.35, PaCO₂ >6 kPa)
• Signs of cor pulmonale (raised JVP, oedema, hepatomegaly)
• Polycythaemia on FBC (Hb >160 g/L)
• Serum bicarbonate >27 mmol/L in obese patient
• Overnight SpO₂ <80% for prolonged periods

Bedside

• ABG: daytime awake PaCO₂ >6.0 kPa confirms chronic hypercapnia; raised HCO₃⁻ (>27 mmol/L) suggests chronicity
• Pulse oximetry: daytime and overnight
• BMI: ≥30 (usually ≥35-40)
• Epworth Sleepiness Scale: typically elevated

Bloods

• FBC: polycythaemia (chronic hypoxia)
• U&Es: serum bicarbonate >27 mmol/L
• TFTs: exclude hypothyroidism
• HbA1c: diabetes screening
• BNP/NT-proBNP: if heart failure/pulmonary hypertension suspected

Sleep Study

• Overnight oximetry: sustained desaturation ± sawtooth pattern (OSA)
• Polysomnography with CO₂ monitoring: gold standard; quantify AHI, confirm nocturnal hypoventilation (rise in PtcCO₂ >1.3 kPa during sleep)

Imaging/Special Tests

• CXR: cardiomegaly, elevated hemidiaphragms
• Echocardiography: pulmonary hypertension, RV dysfunction, LV dysfunction
• Spirometry: restrictive pattern (reduced FVC, normal FEV₁/FVC ratio); exclude obstructive disease
• Respiratory muscle assessments: MIP, SNIP (exclude neuromuscular weakness)

Non-pharmacological

• Weight loss: most important long-term intervention; 10% loss can significantly improve ventilation
• Dietary counselling: high-protein, calorie-restricted diet
• Exercise: supervised programme as tolerated

Pharmacological

• Limited pharmacological options
• Acetazolamide 250mg BD: respiratory stimulant (carbonic anhydrase inhibitor); may reduce bicarbonate and stimulate ventilation; limited evidence
• Treat comorbidities: diabetes (metformin, GLP-1 agonists which also promote weight loss), hypertension, heart failure
• Avoid sedatives, opioids (worsen hypoventilation)

Ventilatory Support

CPAP:

• First-line if predominantly OSA phenotype (AHI >30) without severe resting hypercapnia
• Resolves hypercapnia in ~50% of OHS patients with severe OSA

NIV (bilevel positive airway pressure — BiPAP):

• For persistent hypercapnia despite CPAP, or OHS without significant OSA
• Typical settings: IPAP 18-25 cmH₂O, EPAP 6-10 cmH₂O, back-up rate 14-16
• Nocturnal use (minimum 4-6 hours/night)
• Improves daytime PaCO₂, symptoms, and survival

LTOT:

• If persistent hypoxaemia (PaO₂ ≤7.3 kPa on stable ABGs) despite NIV
• Use ≥15 hours/day

Surgical/Interventional

• Bariatric surgery: potentially curative; significant improvement or resolution of OHS in >60-80%
  • Indicated for BMI ≥40 or ≥35 with comorbidities
  • Roux-en-Y gastric bypass or sleeve gastrectomy
  • Peri-operative risks: must optimise NIV pre-operatively

Referral Criteria

• Respiratory/sleep specialist for all suspected OHS
• Bariatric surgery assessment if BMI ≥40 (or ≥35 with comorbidities)
• Dietitian and weight management service
• ICU if acute hypercapnic respiratory failure

• Untreated OHS has significantly higher mortality than simple obesity or OSA alone
• 18-month mortality untreated: ~23% (vs ~9% with treatment)
• In-hospital mortality for acute presentation: ~18%
• NIV/CPAP improves survival, daytime gas exchange, and quality of life
• Bariatric surgery: can normalise ABG and cure OHS in 60-80%
• Weight loss of 25-30% typically resolves hypercapnia
• Pulmonary hypertension and cor pulmonale are partially reversible with NIV and weight loss
• Long-term prognosis depends on adherence to NIV and weight management

OHS Diagnostic Criteria

OHS Phenotypes

Distinguishing OHS from OSA