How is sleep-related hypoventilation diagnosed?

Diagnosis of Sleep-Related Hypoventilation

Polysomnography with transcutaneous or end-tidal CO2 monitoring is the definitive diagnostic test for sleep-related hypoventilation, and should be used instead of home sleep apnea testing in patients with suspected hypoventilation. 1

Who Should Be Tested

High-Risk Populations Requiring Polysomnography

• Patients with significant cardiorespiratory disease 1
• Those with potential respiratory muscle weakness from neuromuscular conditions 1
• Patients with awake hypoventilation or suspicion of sleep-related hypoventilation 1
• Chronic opioid medication users 1
• Obese patients (particularly BMI >30 kg/m²) with elevated serum bicarbonate levels 1
• Patients with restrictive lung disease or chest wall deformities 1

Screening Tests to Identify At-Risk Patients

Serum bicarbonate ≥27 mmol/L in obese patients with suspected sleep-disordered breathing should prompt arterial blood gas measurement and polysomnography. 1 The kidneys compensate for chronic respiratory acidosis by retaining bicarbonate, making this a useful screening marker 1.

Diagnostic Criteria

Polysomnography Requirements

The study must include 1:

• Electroencephalography, electro-oculography, and electromyography to confirm sleep stages
• Respiratory pattern monitoring (nasal pressure or airflow)
• Oxygen saturation via pulse oximetry
• Transcutaneous PCO2 or end-tidal PCO2 monitoring (essential for hypoventilation diagnosis) 1
• Electrocardiography
• Body position monitoring

Defining Hypoventilation During Sleep

Score hypoventilation if either criterion is met 1:

1. Arterial PaCO2 (or surrogate) >55 mm Hg for ≥10 minutes during sleep 1
2. ≥10 mm Hg increase in PaCO2 (or surrogate) during sleep compared to awake supine value, reaching >50 mm Hg for ≥10 minutes 1

The duration of hypoventilation as a percentage of total sleep time should be reported 1.

CO2 Monitoring Methods

Recommended Surrogates for PaCO2

Transcutaneous PCO2 (PtcCO2) is the preferred surrogate for both diagnostic studies and PAP titration 1. End-tidal PCO2 (PETCO2) is acceptable but has important limitations 1:

• The PaCO2-PETCO2 difference is typically 2-7 mm Hg, higher in patients with lung disease 1
• PETCO2 becomes inaccurate during mouth breathing 1
• Requires continuous suctioning through nasal cannula (side-stream method) 1

Gold Standard Confirmation

Arterial blood gas obtained immediately upon awakening provides definitive evidence of sleep hypoventilation, though this is rarely practical in sleep centers 1. A daytime PaCO2 ≥45 mm Hg confirms awake hypoventilation 1.

Critical Diagnostic Pitfalls

Why Home Sleep Testing Is Inadequate

Home sleep apnea testing cannot diagnose hypoventilation because it lacks CO2 monitoring capability 1. If HSAT is performed and hypoventilation is suspected, polysomnography must follow 1.

Special Considerations in Neuromuscular Disease

In patients with marked respiratory muscle weakness, external sensors may fail to detect respiratory movement during obstructive events, causing true obstructive apneas to be misclassified as "central" 1. Solutions include:

• Recording esophageal pressure during polysomnography 1
• Examining the flow-time curve for inspiratory flattening 1
• Empiric CPAP trial to determine if "central" events respond to positive pressure 1

Interpretation Framework

Distinguishing Hypoventilation from Other Disorders

The key distinction is sustained elevation of CO2 rather than discrete respiratory events 1. Patients may have:

• Pure hypoventilation: Sustained elevated PaCO2 without significant apneas/hypopneas
• Combined disorder: Hypoventilation plus obstructive sleep apnea (common in obesity hypoventilation syndrome, where ~90% have coexisting OSA) 2
• Hypoventilation during specific sleep stages: Often worse during REM sleep when respiratory muscle tone is most reduced 1

Oxygen Desaturation Patterns

Identify the extent of oxygen desaturation decline during different sleep stages, as hypoventilation typically causes progressive desaturation rather than the cyclical pattern seen in obstructive sleep apnea 1.

When Polysomnography Results Are Equivocal

If daytime oxygen saturation is ≥94% and arterial PO2 is ≥68 mm Hg, clinically significant hypoventilation requiring treatment is unlikely 3. Conversely, patients with normal daytime blood gases may still have sleep-related hypoventilation detectable only with overnight monitoring 4.