Obstructive Sleep Apnoea

Sleep disorder defined as cessation of airflow for more than 10 seconds:

Epidemiology and Risk Factors

OSA is:

  • Highly prevalent in Australian population
    • 24% of men
    • 9% of women
  • Responsible for significant societal costs
  • Underdiagnosed and undertreated

Risk factors for OSA include:

  • Severe obesity
    Major risk factor.
  • Male
  • Middle-aged
  • Alcohol use
  • Sedative use

Non-obese individuals with sleep apnoea usually have another structural problem, such as:

  • Craniofacial abnormality
    • Micrognathia
    • Pierre Robin sequence
    • Down syndrome
    • Marfan syndrome
    • Acromegaly
  • Tonsillar hypertrophy
  • Chronic nasal obstruction

STOP-BANG Score

STOP-BANG assesses the probability of a having OSA, and may be used perioperatively to predict baseline risk of respiratory events:

  • Each factor gets 1 point
  • Snoring
    Snoring loudly overnight.
  • Tired
    Daytime somnolence.
  • Observed
    To stop breathing/choking during sleep.
  • Pressure
    Presence of hypertension.
  • BMI
    ⩾35kg/m2.
  • Age
    ⩾50.
  • Neck size
  • ⩾43cm (male), 41cm (female).
  • Gender
    Male.

Interpretation:

  • Low risk
    0-2 points; very high negative predictive value.
  • Moderate risk
    3-4 points.
  • High risk
    Any of:
    • 5-8 points
      More likely than not to have severe OSA.
    • 2 points + male
    • 2 points + BMI
    • 2 points + neck circumference

Pathophysiology

Four main pathophysiological causes:

  • Anatomical compromise
    Narrow or crowded upper airway, from:
    • Craniofacial morphology
    • Hyoid position
    • Airway surface tension
    • Tongue scalloping
  • Non-anatomical
    • Low arousal threshold
    • Ineffective pharyngeal dilator
      Pharyngeal closing pressure:
      • Evaluates the CPAP required to maintain airway patency
      • CPAP is gradually ↓ during stable sleep, until airflow limitation occurs
      • Patients with OSA commonly have a pharyngeal closing pressure close to atmospheric pressure
      • OSA is rare in patients with Pcrit lower than -5cmH2O
    • Unstable control of breathing
      Loop gain:
      • General term describing behaviour of a feedback-control system
      • With respect to OSA, describes the change in ventilatory response due to ventilatory disturbance
        Consists of:
        • CO2 gain in tissues
        • Circulatory time
          Time taken for change in CO2 to mix with blood and reach chemoreceptors
        • Chemoreceptor sensitivity
      • High loop gain leads to exaggerated ventilatory responses

Clinical Manifestations

Patients (or partners) may complain of:

  • Snoring
  • Daytime somnolence
  • Neurocognitive impairment

Examination:

  • Weight and BMI
  • ENT exam
    • Septal deviations
    • Turbinate hypertrophy
    • Oral cavity
      • Mallampati score
      • Macroglossia
      • Enlarged uvula
      • Tonsillar hypertrophy

Risk Stratification by Symptoms

Note grade drops by 1 if compliant with CPAP, i.e. moderate OSA by AHI becomes mild if CPAP compliant

OSA is graded by AHI, but in absence of a sleep study symptoms can provide a guide:

  • Low risk
    • Snores
      • ⩽3 nights per week
      • Tolerated by partner
    • BMI ⩽30
    • Absence of CVS disease
    • Absence of daytime somnolence
  • Moderate risk
    • Snores
      • Regularly
      • Partner may sleep separately
    • BMI 30-40
    • Witnessed apnoeas
  • High risk
    • Snores
      In all positions.
    • BMI >40
    • Frequent apnoeas
    • Coexistent:
      • CVS disease
      • OHS
      • Polycythaemia
    • Daytime somnolence
    • Expected intubation difficulty

Investigations

Polysomogram is:

  • Diagnostic
  • Used to grade severity
    Given as the apnoea-hypopnoea index (AHI), the total number of apnoeas or hypopnoeas per hour of sleep.
    • Mild: 5-15
    • Moderate: 16-30
    • Severe: ⩾31
  • Other useful values
    • Minimum SpO2
    • T90
      Time with SpO2 <90%.
  • A highly involved study
    • Waiting lists may be extensive which makes it often impractical
  • An evaluation of multiple parameters:
    • Infrared camera
      Monitors body position.
    • Airway microphone
      Monitors airflow.
    • SpO2
    • Chest girth measurement
      Evaluates respiratory effort.
    • EEG
      For monitoring of sleep cycles.
    • Electrooculogram
      Monitors eye movement to separate REM from non-REM sleep.
    • ECG
      Monitors cardiac activity.
    • Chin EMG
      Upper airway muscle activity.
    • Leg EMG
      Monitors leg movements.

Overnight oximetry:

  • Easily accessible
  • Good correlation
  • Grades oxygen desaturation index (ODI), counting an event as a fall in SpO2 by (usually) ⩾4%
    • ⩽5/hr: Normal
    • 5-15: Mild
    • 16-30: Consider treatment

    • 30: Recommend CPAP

ABG:

  • Respiratory acidosis/Metabolic alkalosis
    Evaluating for presence of obesity hypoventilation syndrome.

FNE:

  • Allows assessment of oral and nasal cavity

Management

Largely focuses on correcting anatomical causes:

  • CPAP
    ↑ pharyngeal pressure above closing pressure, maintaining airway patency.
    • Indicated in moderate to severe disease
    • Improves sleep quality and quality of life
    • Reduces pulmonary complications

Surgery:

  • Indicated in patients with severe OSA unable or unwilling to use CPAP
  • Options depend on location and degree of obstruction, but include:
    • Septoplasty/turbinate reduction
      May improve nasal breathing and CPAP compliance.
    • UPPP/UPF
      ~40% reduction in AHI.
    • Tongue base reduction
      ~54% reduction in AHI. Usually combined with another procedure.

Anaesthetic Considerations

  • Respiratory and cardiac risk is elevated perioperatively
    Risk is substantially elevated with unrecognised severe OSA; up to 14× risk of cardiac death, 7× risk of CCF, 3× risk of pneumonia, 2× risk of sepsis, etc.
  • Treating patients with OSA in the perioperative period appears effective in ↓ their perioperative risk
  • A
    • CPAP available
      Ensure CPAP or BiPAP device that the patient uses is brought to hospital and used perioperatively.
    • Difficult airway
      Intubation difficult in 10-25%.
  • B
    • Significantly ↑ risk of perioperative pulmonary and cardiovascular complications including:
      • Aspiration pneumonia
      • ARDS
      • PE
      • Reintubation
        Double risk.
    • Treatment prior to surgery may ↓ perioperative risk
    • Perioperative respiratory monitoring is reasonable
      Lacks strong evidence base.
  • C
    • ↑ CVS complications
      1.5-2× rate of cardiac complications.
  • D
    • No clear evidence for general anaesthesia or regional anaesthesia (or vice versa)
    • Sensitive to CNS depressants: may precipitate airway collapse
      • Anaesthetic agents
      • Benzodiazepines
      • Opioids

Risk Stratification

Low Risk OSA
Moderate Risk OSA
High Risk OSA

Complications

Hypoxaemia and hypercarbia in severe OSA has several cardiovascular sequelae:

  • Arrhythmia
    Due to changed autonomic tone.
    • Bradycardia
    • Second-degree heart block
    • Ventricular ectopy
  • Pulmonary hypertension
    • Directly
      Due to hypoxic pulmonary vasoconstriction.
    • Indirectly
      • ↑ sympathetic tone
        Due to hypoxia.
      • ↑ transmural pressure from more neagative intrathoracic pressure due to inspiratory effort against upper airway obstruction

Obesity Hypoventilation Syndrome:

  • Present in 15% Risk ↑ BMI, >50% with BMI >50.
    • Requires HDU monitoring post-operatively
    • CPAP pre-operatively for stabilisation

Multiple associations with:

  • Hypertension
  • CVA
  • MVA

Prognosis

References

  1. De Hert, S., Staender, S., Fritsch, G., Hinkelbein, J., Afshari, A., Bettelli, G., Wappler, F. (2018). Pre-operative evaluation of adults undergoing elective noncardiac surgery: Updated guideline from the European Society of Anaesthesiology. European Journal of Anaesthesiology (Vol. 35).
  2. Osman AM, Carter SG, Carberry JC, Eckert DJ. Obstructive sleep apnea: current perspectives. Nature and Science of Sleep. doi:10.2147/NSS.S124657
  3. Jun JC, Chopra S, Schwartz AR. Sleep apnoea. European Respiratory Review. 2016;25(139):12-18. doi:10.1183/16000617.0077-2015