Sleep Apnea - Respiratory Care Pathophysiology (Video Notes)

Sleep Apnea Pathophysiology - Comprehensive Study Notes

Sleep-Related Breathing Disorders (Overview)

Sleep-related breathing disorders are characterized by abnormal breathing patterns during sleep.
Includes four main categories:
- Obstructive sleep apnea syndrome (OSA)
- Central sleep apnea syndrome (CSA)
- Mixed sleep apnea
- Sleep related hypoventilation/hypoxemia syndromes (SRHHS)
Important to recognize patterns of airflow, respiratory effort, and oxygenation during sleep for diagnosis and management.

Obstructive Sleep Apnea (OSA)

Definition
- OSA is a blockage of ventilation due to upper airway obstruction.
- There is no airflow despite chest and abdomen movement.
Pathophysiology
- When the genioglossus muscle fails to oppose the collapse tendency of the airway during inspiration, the tongue moves into the oropharyngeal area and obstructs the airway.
Risk factors
- Excessive weight
- Neck size (>17 inches)
- Anatomic narrowing of the upper airway:
- Excessive pharyngeal tissue
- Enlarged tonsils or adenoids
- Deviated nasal septum
- Laryngeal stenosis
- Vocal cord dysfunction
- Enlarged tongue (macroglossia)
- Recessed jaw (micrognathia)
- Age > 65 years
- Family history
- Smoking
- Alcohol and sedatives
Signs and symptoms
- Loud snoring
- Observed breathing pauses during sleep
- Abrupt awakenings with shortness of breath
- Insomnia, moodiness, irritability
- Concentration problems, memory impairment
- Morning headaches, dry mouth or sore throat
- Nausea, excessive daytime sleepiness (hypersomnia)
- Intellectual and personality changes; depression
- Nocturnal enuresis; sexual impotence; night sweats
Diagnosis: Apnea–hypopnea index (AHI) and related metrics
- AHI is the number of apneas and hypopneas per hour of sleep; also reported are:
- Respiratory disturbance index (RDI)
- Arousal index (arousals per hour)
- Percentage of each sleep stage
- Frequency of SaO2, mean SaO2, and nadir SaO2
- AASM severity thresholds:
- Normal: AHI < 5
- Mild: 5 \,\le\, AHI < 15
- Moderate: 15 \,\le\, AHI < 30
- Severe: AHI \,>\, 30
Diagnosis tools
- History of snoring, sleep disturbance, daytime sleepiness
- Examination of upper airway
- Pulmonary function tests (PFTs) to evaluate airway obstruction
- Blood tests for polycythemia, thyroid function, bicarbonate retention
- Arterial blood gas (ABG) to assess resting oxygenation and acid-base status
- Polysomnography (sleep study)
- In-home portable monitoring
Sleep study and monitoring
- Polysomnography monitors sleep stages and physiological parameters
- Involves:
- EEG, EOG, EMG (polysomnography)
- Respiratory effort (chest/abdomen)
- Oral and nasal airflow
- Snoring
- Pulse oximetry (SpO2)
- ECG
- Body position
- Limb movements
Epworth Sleepiness Scale (for OSA risk assessment)
- Scoring approach:
- 0 = would never doze
- 1 = slight chance of dozing
- 2 = moderate chance of dozing
- 3 = high chance of dozing
- Situations include: sitting and reading, watching TV, sitting inactive in public, passenger in car, lying down to rest, sitting and talking to someone, sitting quietly after lunch, in a car stopped in traffic
- Total score interpretation:
- 0–10: Normal
- 10–12: Borderline
- 12–24: Abnormal
Mallampati classification (airway assessment prior to anesthesia, relevant for airway management and OSA risk)
- Class 1: Soft palate, fauces, uvula, and pillars are easily seen.
- Class 2: Soft palate, fauces, and portion of uvula seen.
- Class 3: Soft palate and base of uvula seen.
- Class 4: Only hard palate seen.

Central Sleep Apnea (CSA)

Definition
- CSA is a disorder characterized by repetitive stopping or reduction of both airflow and ventilatory effort during sleep.
Types
- Primary CSA: Idiopathic or unknown cause
- Secondary CSA: Associated with other conditions
- Cheyne–Stokes breathing (CHF)
- Encephalitis
- Brain stem neoplasm or infarction
- Spinal surgery
- Hypothyroidism
- Drug or substance abuse
- High altitude periodic breathing
CSA subcategories
- Hyperventilation-related CSA (most common)
- Hypoventilation-related CSA
Hyperventilation-related CSA
- Includes primary CSA and Cheyne–Stokes breathing, and high-altitude periodic breathing
- Patients develop alternating cycles of apnea or hypopneas with hyperpnea
Hypoventilation-related CSA
- Usually secondary to underlying conditions such as:
- Central nervous system (CNS) disease
- CNS-suppressing drugs
- Neuromuscular diseases
- Severe disorders of pulmonary mechanics (e.g., COPD, pulmonary fibrosis)

Mixed Sleep Apnea

Definition
- Mixed sleep apnea is a combination of obstructive and central sleep apnea.
Typical pattern
- Usually begins as CSA, followed by the onset of OSA

Sleep-Related Hypoventilation/Hypoxemia Syndromes (SRHHS)

Broad range of sleep disorders within this category
Examples/typical conditions:
- Obesity hypoventilation syndrome (OHS; Pickwickian syndrome)
- COPD overlap: overlap between COPD and sleep apnea

Sleep Apnea Diagnosis (General Diagnostic Approach)

History
- Snoring, sleep disturbance, daytime sleepiness
Examination
- Upper airway assessment
Objective testing
- Polysomnography (overnight sleep study)
- In-home portable monitoring as an alternative in selected cases
Laboratory tests
- Blood tests for polycythemia, thyroid function, bicarbonate retention
- ABG to assess resting, wakeful oxygenation and acid-base status

Sleep Study: Setup and Measured Variables

Polysomnography measures multiple streams of data over time
Monitored variables include:
- Sleep stages via EEG, EOG, and EMG
- Respiratory efforts (movement of chest and abdomen)
- Airflow through nose and mouth
- Snoring
- Pulse oximetry (SpO2)
- Electrocardiogram (ECG)
- Body position
- Limb movements
Example visualization elements (as per study diagrams):
- Sleep stages: wake, REM, non-REM (N1–N3)
- REM periods with characteristic EEG/EOG/EMG patterns
- Snore and respiratory events aligned with changes in SpO2
- Typical 30-second epochs used for scoring

Pulmonary Function Tests and Flow–Volume Loops in OSA

PFTs
- In sleep, restrictive patterns can be difficult to study during actual sleep
Flow–Volume Loop in OSA
- Sawtooth pattern on the inspiratory/expiratory flow loop can be observed in some sleep-related breathing disorders
- Example interpretation:
- Flow (ml/s) vs Volume (ml) plots showing inspiratory/expiratory phases with sawtooth-like irregularities

Arterial Blood Gases (ABG) and Imaging

ABGs
- Mild to moderate sleep-disordered breathing: typically near-normal ABG values
- Severe disease: may show chronic ventilatory failure with hypoxemia; acute on chronic changes possible
Chest X-ray (CXR)
- Often normal in early disease
- May show enlarged heart due to pulmonary hypertension and polycythemia; can reflect right- or left-sided heart failure with prolonged apnea

Treatments for Obstructive Sleep Apnea (OSA)

Continuous Positive Airway Pressure (CPAP)
- Most common treatment
- Delivers a constant positive airway pressure throughout the ventilatory cycle to prevent airway collapse
Other treatment approaches
- Weight reduction
- Sleep posture optimization (elevating head/avoid supine position)
- Oxygen therapy (supplemental oxygen as needed)
- Surgery to correct anatomical defects:
- Mandibular advancement surgery
- Nasal surgery
- Tracheostomy (in select severe cases)
- Uvulopalatopharyngoplasty (UPPP): removal of soft palate tissue including the uvula
CPAP setup and mechanism (illustrative explanation)
- Provides continuous positive airway pressure to keep upper airway open during sleep

Central Sleep Apnea: Treatment Considerations

For hyperventilation-related CSA, CPAP is considered the first-line therapy
For hypoventilation-related CSA, noninvasive ventilatory support is preferred:
- BPAP (Bilevel Positive Airway Pressure) or VPAP (Variable Positive Airway Pressure) with a backup respiratory rate
- Backup rate helps prevent abrupt shifts in PaO2, PaCO2, and pH

BPAP / VPAP (Bilevel/Variable Positive Airway Pressure)

Functionality
- Delivers two different pressures: IPAP during inhalation and EPAP during exhalation
Typical pressure relationship
- IPAP > EPAP; commonly the difference (pressure support) is ≥ 4\;\text{cmH2O}
Example provided
- IPAP: +12 cmH2O
- EPAP: +5 cmH2O
- Pressure support (PS): PS = IPAP - EPAP = 12 - 5 = 7\;\text{cmH2O}
Nomenclature
- IPAP: inspiratory positive airway pressure
- EPAP: expiratory positive airway pressure

Key Formulas and Numerical References

Apnea–Hypopnea Index (AHI)
- AHI = \frac{N{apneas} + N{hypopneas}}{T_{sleep}}\,.
Severity thresholds (AASM definitions)
- Normal: AHI < 5
- Mild: 5 \le AHI < 15
- Moderate: 15 \le AHI < 30
- Severe: AHI \ge 30
Pressure Support in BPAP/VPAP
- PS = IPAP - EPAP
- Example: IPAP = 12\;\text{cmH2O},\; EPAP = 5\;\text{cmH2O} \Rightarrow PS = 7\;\text{cmH2O}
Epworth Sleepiness Scale interpretation
- Total score interpretations as described above (0–10 normal, 10–12 borderline, 12–24 abnormal)

Connections to Foundational Principles and Real-World Relevance

OSA pathophysiology links upper airway anatomy, neuromuscular control, and sleep physiology; obesity and craniofacial anatomy are major contributors.
CSA emphasizes ventilatory control instability, CNS conditions, and cardiovascular coupling (e.g., Cheyne–Stokes in heart failure).
Diagnostic tools (polysomnography, ABG, PFTs, CXR) integrate physiology with clinical presentation to guide treatment.
Treatment choices reflect balancing airway mechanics (airway pressure therapies) with underlying etiologies (neuro-muscular vs. central control problems).
Practical implications:
- CPAP improves nocturnal oxygenation, reduces daytime sleepiness, and lowers cardiovascular risk in many patients with OSA.
- BPAP/VPAP used when hypoventilation or central control issues predominate, with backup rates to stabilize gas exchange.
Ethical and practical considerations include access to sleep studies, adherence to CPAP therapy, and addressing comorbid conditions (e.g., obesity, COPD, heart failure).