Sleep-Related Breathing Disorders
What you must know
Obstructive sleep apnea (OSA) is diagnosed by AHI or RDI thresholds: mild 5-15, moderate 15-30, severe ≥30 events/hour. The AHI counts apneas plus hypopneas per hour of sleep; hypopneas require a ≥30% flow reduction with ≥3% desaturation or an arousal (AASM recommended rule). Central sleep apnea (CSA) includes Cheyne-Stokes respiration (heart failure), treatment-emergent CSA, and primary CSA. Obesity hypoventilation syndrome (OHS) = BMI ≥30 plus awake PaCO2 ≥45 mmHg without other cause.
Know consequences: hypertension, atrial fibrillation, stroke, insulin resistance, and motor vehicle crashes. Treatment: PAP is first-line for moderate-severe OSA; alternatives include oral appliances, positional therapy, weight loss, and hypoglossal nerve stimulation in selected patients.
Traps
- Confusing AHI vs RDI (RDI includes respiratory effort-related arousals).
- Missing OHS when only apneas are considered; OHS requires awake hypercapnia.
- Assuming all central apneas need PAP; Cheyne-Stokes may respond to adaptive servo-ventilation, but ASV is contraindicated in symptomatic heart failure with LVEF ≤45%.
- Forgetting that treatment-emergent central apnea can resolve with continued PAP.
Rule of thumb
"5-15-30" for AHI severity, and "BMI ≥30 + awake PaCO2 ≥45 = OHS."
Worked example
A patient with BMI 38, AHI 22, and awake PaCO2 48 mmHg has both moderate OSA and OHS. PAP must be titrated to normalize ventilation, not just apneas; consider bilevel PAP if PAP alone is inadequate.