TABLE 4

Staging of hypoventilation in obesity

0	At risk	BMI >30 kg·m⁻²	OSA	No hypercapnia
I	Obesity-associated sleep hypoventilation	BMI >30 kg·m⁻²	OSA/hypoventilation during sleep	Intermittent hypercapnia during sleep, full recovery during sleep (P_aCO₂ or P_tcCO₂ morning∼evening) Serum bicarbonate <27 mmol·L⁻¹ during wake
II	Obesity-associated sleep hypoventilation	BMI >30 kg·m⁻²	OSA/hypoventilation during sleep	Intermittent hypercapnia during sleep (P_aCO₂ or P_tcCO₂ morning>evening) Serum bicarbonate ≥27 mmol·L⁻¹ during wake Bicarbonate increased during day
III	Obesity hypoventilation	BMI >30 kg·m⁻²	OSA/hypoventilation during sleep	Sustained hypercapnia (P_CO₂ >45 mmHg) while awake
IV	Obesity hypoventilation syndrome	BMI >30 kg·m⁻²	OSA/hypoventilation during sleep	Sustained hypercapnia while awake, cardiometabolic comorbidities

BMI: body mass index; OSA: obstructive sleep apnoea; P_aCO₂: arterial carbon dioxide tension; P_tcCO₂: transcutaneous carbon dioxide tension; P_CO₂: carbon dioxide tension.