😴 Sleep‑Related Breathing Disorders OSA · CSA · OHS · Diagnostic Testing & Management for Medical Students

1. Overview & Epidemiology

• Sleep‑disordered breathing (SDB) affects ~1 billion people worldwide; Obstructive Sleep Apnea (OSA) is most common.
• Underdiagnosed: ~80‑90% of moderate‑severe OSA remains undiagnosed.
• Consequences: Hypertension, atrial fibrillation, heart failure, stroke, metabolic syndrome, daytime sleepiness, motor vehicle accidents.

2. Obstructive Sleep Apnea (OSA)

Pathophysiology

• Recurrent collapse of the pharyngeal airway during sleep due to ↓ pharyngeal dilator muscle tone + negative intraluminal pressure.
• Apnea → hypoxemia, hypercapnia → sympathetic surge, arousal → sleep fragmentation.
• Risk factors: Obesity (strongest), male sex, age >50, retrognathia/micrognathia, large neck circumference (>17" male, >16" female), tonsillar hypertrophy, family history.

Clinical Presentation

Screening: STOP‑BANG Questionnaire

Letter	Question	High‑Risk Answer
S	Do you Snore loudly?	Yes
T	Do you feel Tired, fatigued, or sleepy during the day?	Yes
O	Has anyone Observed you stop breathing during sleep?	Yes
P	Do you have high blood Pressure or on treatment?	Yes
B	BMI >35 kg/m²?	Yes
A	Age >50 years?	Yes
N	Neck circumference >40 cm (male) or >36 cm (female)?	Yes
G	Gender male?	Yes

• Score ≥3: High risk of OSA → diagnostic testing.
• Score 5‑8: High probability of moderate‑severe OSA.

Diagnosis: Polysomnography (PSG) vs. Home Sleep Apnea Test (HSAT)

Parameter	In‑Lab PSG (Gold Standard)	HSAT (Level III)
Channels monitored	EEG, EOG, EMG, ECG, airflow, respiratory effort, SpO₂, snoring, leg movement, body position	Airflow, respiratory effort, SpO₂, heart rate (no EEG)
Advantages	Comprehensive; can diagnose other sleep disorders (PLMD, narcolepsy); sleep staging	Convenient, lower cost, done at home
Limitations	Cost, limited access, "first‑night effect"	Underestimates AHI (cannot detect EEG arousals); not suitable for central apnea, severe comorbidities
Indications	Suspected central sleep apnea, comorbid cardiopulmonary disease, neuromuscular disease, negative/inconclusive HSAT	High pretest probability of uncomplicated moderate‑severe OSA

Key PSG Definitions

• Apnea: Cessation of airflow ≥10 seconds.
• Hypopnea: ≥30% reduction in airflow for ≥10 sec with ≥3% O₂ desaturation or arousal.
• Apnea‑Hypopnea Index (AHI): Number of apneas + hypopneas per hour of sleep.
  • Mild OSA: AHI 5‑14
  • Moderate OSA: AHI 15‑29
  • Severe OSA: AHI ≥30
• Respiratory Disturbance Index (RDI): AHI + Respiratory Effort‑Related Arousals (RERAs). Used when hypopnea definition requires desaturation.
• Oxygen Desaturation Index (ODI): Number of ≥3‑4% desaturations per hour.

Management of OSA

First‑Line: Positive Airway Pressure (PAP) Therapy

• CPAP (Continuous Positive Airway Pressure): Fixed pressure splints airway open. Gold standard for OSA.
• APAP (Auto‑titrating PAP): Adjusts pressure automatically based on detected events; useful for variable pressure needs.
• BiPAP (Bilevel PAP): Higher IPAP for inspiration, lower EPAP for expiration. Indicated for pressure intolerance, hypoventilation (OHS), or coexisting central apnea.
• Adherence: Goal ≥4 hours/night on ≥70% of nights. Mask fit and humidification are key.

Alternative & Adjunctive Therapies

Therapy	Indication	Notes
Oral Appliance (Mandibular Advancement Device)	Mild‑moderate OSA, CPAP intolerant	Advances mandible → ↑ upper airway size. Custom‑fitted by dentist.
Weight Loss	All overweight/obese OSA patients	10% weight loss → ~26% ↓ AHI. Bariatric surgery may be curative.
Positional Therapy	Positional OSA (supine AHI ≥2× non‑supine)	Devices that prevent supine sleep (e.g., NightShift, tennis ball shirt).
Hypoglossal Nerve Stimulation (Inspire)	Moderate‑severe OSA, CPAP failure, BMI <32‑35, no complete concentric collapse on DISE	Implanted device stimulates hypoglossal nerve → tongue protrusion during inspiration.
Upper Airway Surgery (UPPP, MMA)	Anatomic obstruction (tonsillar hypertrophy, retrognathia), CPAP failure	UPPP = uvulopalatopharyngoplasty; MMA = maxillomandibular advancement (most effective surgical option).

3. Central Sleep Apnea (CSA)

Pathophysiology & Types

• Cessation of airflow without respiratory effort (no chest/abdominal movement). Due to transient loss of respiratory drive.
• Cheyne‑Stokes Respiration (CSR): Crescendo‑decrescendo pattern of hyperventilation followed by central apnea. Common in heart failure (HFrEF), stroke.
• Treatment‑Emergent Central Sleep Apnea (TECSA): Emerges after initiating PAP therapy for OSA (~5‑15% of patients). Often resolves with continued PAP use.
• Idiopathic CSA / Narcotic‑Induced CSA: Chronic opioid use → respiratory depression.
• High‑Altitude Periodic Breathing: Hypoxia‑induced hyperventilation → hypocapnia below apnea threshold.

Diagnosis

• In‑lab PSG required (HSAT cannot distinguish central vs. obstructive).
• ≥50% of events are central; central AHI ≥5.

Management of CSA

• Treat Underlying Cause: Optimize heart failure (GDMT), reduce/stop opioids, acetazolamide for high‑altitude CSA.
• Positive Airway Pressure: CPAP first‑line for CSA with heart failure (improves cardiac function).
• Adaptive Servo‑Ventilation (ASV): Variable pressure support that stabilizes breathing pattern. Contraindicated in HFrEF with LVEF ≤45% and predominant CSA (SERVE‑HF trial showed ↑ mortality).
• Supplemental Oxygen: Reduces CSA in heart failure and high‑altitude periodic breathing.
• Pharmacologic: Acetazolamide (metabolic acidosis stimulates ventilation), Theophylline (rarely used).
• Phrenic Nerve Stimulation (Remedē): Implantable device for moderate‑severe CSA; stimulates diaphragm during sleep.

4. Obesity Hypoventilation Syndrome (OHS)

Definition & Pathophysiology

• BMI ≥30 kg/m² + daytime hypercapnia (PaCO₂ ≥45 mmHg) + sleep‑disordered breathing (usually OSA) after excluding other causes of hypoventilation (lung disease, neuromuscular, chest wall).
• Mechanisms: Obesity → restrictive chest wall mechanics, leptin resistance → blunted respiratory drive, OSA → sleep fragmentation.
• 90% of OHS patients have concurrent OSA; 10% have pure sleep hypoventilation (no apneas).

Clinical Features & Diagnosis

• Severe obesity, daytime sleepiness, dyspnea, signs of right heart failure (cor pulmonale).
• Often present with acute‑on‑chronic hypercapnic respiratory failure.
• Diagnosis: Daytime ABG showing PaCO₂ ≥45 mmHg + elevated serum bicarbonate (≥27 mEq/L suggests chronic compensation).

Management

• Positive Airway Pressure: First‑line is BiPAP (or Auto‑BiPAP) — provides pressure support to augment ventilation. CPAP alone may be insufficient for hypoventilation.
• Weight Loss: Essential; bariatric surgery often curative.
• Acute Decompensation: May require NIV (BiPAP) or invasive mechanical ventilation.
• Tracheostomy: Reserved for severe, refractory cases.

5. Other Sleep‑Related Breathing Disorders

6. Cardiovascular Consequences of Untreated OSA

Condition	Mechanism	Evidence
Hypertension	Sympathetic surges, endothelial dysfunction	OSA is a secondary cause of HTN; CPAP reduces BP ~2‑3 mmHg
Atrial Fibrillation	Atrial remodeling, autonomic dysregulation	OSA doubles risk of AF; untreated OSA ↑ AF recurrence after cardioversion/ablation
Heart Failure	↑ Afterload, hypoxia, sympathetic activation	CPAP improves LVEF in OSA + HF
Stroke / TIA	Hypercoagulability, atherosclerosis, AF	OSA independent risk factor; post‑stroke OSA common (60‑70%)
Pulmonary Hypertension	Hypoxic vasoconstriction, left heart disease	Usually mild (mean PAP 25‑35 mmHg); improves with CPAP

7. Perioperative Considerations in OSA

• Increased risk: Difficult intubation, postoperative respiratory depression, hypoxemia, arrhythmias.
• Preoperative screening: STOP‑BANG. Known or suspected OSA → consider preoperative PAP initiation.
• Intraoperative: Minimize opioids and sedatives; use regional anesthesia when possible.
• Postoperative: Continuous SpO₂ monitoring; use PAP device in recovery; avoid supine position.
• Ambulatory surgery: OSA alone is not a contraindication to outpatient surgery if well‑controlled.

8. Quick Reference: AHI Severity & PAP Modes

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