FRCA Notes

Obstructive Sleep Apnoea

OSA is a key anaesthetic topic and a growing issue (no pun intended).

The curriculum asks us to describe 'the causes, pathophysiology and management of obstructive sleep apnoea and the surgical procedures used to treat it', as well as the principles of sleep studies.

It featured as an SAQ in 2017, where there were marks on offer for knowledge of STOP-BANG scoring, the cardiovascular consequences of OSA and minimising perioperative risks in OSA.

Resources

  • OSA is defined by repeated episodes of complete or partial airway obstruction leading to apnoea/hypopnoea and oxygen desaturation
  • The prevalence is ∽10% in the adult population
    • Approximately double this in the surgical population
    • The prevalence increases to 30-50% in the bariatric surgical population

Non-modifiable Modifiable Paediatric-specific
Age 40-70yrs BMI >35kg/m2 Adeno-tonsillar hypertrophy
Male gender Neck circumference >40cm Craniofacial deformities
e.g. Down's syndrome, Pierre-Robin sequence
Surgical patient Alcohol Neuromuscular disease
Pregnancy Smoking
Menopause Low physical activity
Family history Unemployment


In health

  • The oropharynx is a muscular tube which depends on muscle tone for patency, particularly:
    • The hyoid muscles (geniohyoid, sternohyoid)
    • The muscles of the tongue i.e. genioglossus
    • The muscles of the palate (tensor palatini, levator palatini)
  • During inspiration, negative pressure is generated by the action of the diaphragm/intercostal muscles
  • This would promote oropharyngeal collapse
  • However, time-coordinated contraction of oropharyngeal dilator and abductor muscles prevents collapse

In OSA

  • During (predominantly REM) sleep there is loss of background muscle tone
  • Loss of balance between collapsing and dilating forces arises due to:
    • Narrower airway e.g. from increased pharyngeal adipose tissue, oedema, inflammatory process e.g. tonsillitis
    • Increased insiratory pressures e.g. due to obesity, narrower airway
    • Anatomical defects e.g. acromegaly, macroglossia, retrognathia
    • Decreased tone of oropharyngeal muscles
    • Neuro-depressants e.g. drugs, alcohol, myopathy, bulbar stroke
  • There is impaired neuromuscular coordination and the normal time-coordinated increase in airway tone which occurs in inspiration does not (fully) occur

  • The combination of narrow airway, reduced muscle tone and poor inspiratory coordination lead to either or both:
    • Partial collapse leading to snoring and hypopnea
    • Complete collapse leading to apnoea (≥10s without airflow)
  • There is consequent hypercapnoea and hypoxia

  • Breathing resumes when there is arousal from sleep due to:
    • Central chemoreceptor detection of rising CO2 levels
    • Peripheral chemoreceptor detection of falling O2 levels
    • Increased oropharyngeal muscle tone due to increased inspiratory effort

  • The temporary arousal during sleep causes increased sympathomimetic activity, leading to:
    • Tachycardia
    • Increased arterial blood pressure (often higher than day-time blood pressure)
    • A period of hyperventilation
    • Increased adrenocortical tone

  • The cycle then repeats as sleep deepens again

Clinical assessment

  • Presence of predisposing factors (see above)
  • A constellation of symptoms arising from disturbed sleep, as well as hyoxia and hypercapnoea during sleep:
At night During the day
Witnessed apnoea Unrefreshing sleep
Snoring Tiredness/fatigue
Insomnia Daytime somnolence
Nocturia Morning headaches
Restless sleep Mood/personality change
Behavioural issues (paediatrics)

Screening tools

  • Of these, STOP-BANG is the most suitable diagnostic screening tool:

    • Category Threshold
    Snoring Loud snoring
    Tiredness Daytime tiredness
    Observed Witnessed apnoea
    Pressure On treatment for hypertension
    BMI >35kg/m2
    Age >50yrs
    Neck circumference >40cm
    Gender Male

  • A total score of 0-2 is considered low risk for OSA
  • A total score of 3-4 is considered moderate risk for OSA
  • A total score of ≥5 predicts severe OSA
  • Overall, a score >3 has an 84% sensitivity of predicting OSA

  • Other screening tools exist, namely:

Formal diagnosis

  • Polysomnography is considered the gold standard, but is labour-intensive, expensive and has a long waiting list
  • It measures:
    • ECG | EEG | EMG
    • Eye movements
    • Pulse oximetry
    • Snoring volume
    • Oro-nasal airflow

  • Therefore overnight oximetry ('sleep studies') are often used instead, as they are more accessible and cheaper, measuring either:
    1. Pulse oximetry alone → oxygen desaturation index
    2. Pulse oximetry + airflow + abdominal effort → oxygen desaturation index and apnoea-hypopnoea index

  • Both tests can provide data on the frequency and severity of:
    • Apnoea, which is cessation of airflow >90% of the baseline for >2 breaths, or for >10s
    • Hypopnoea, which is a flow reduction by ≥30%, or >2 breaths or >10s with either a ≥3% oxygen desaturation or an arousal on EEG
    • Desaturations of >4%, or to <90%, per hour of sleep
  • These give rise to:

  • The apnoea-hypopnoea index (AHI)
  • Characterised as:
    • Mild: AHI ≥5
    • Moderate: AHI ≥15
    • Severe: AHI ≥30

  • The oxygen desaturation index (ODI)
  • Characterised as:
    • Mild: ≥5
    • Moderate: ≥15
    • Severe: ≥30

  • A diagnosis of OSA is made with:
    1. An AHI or ODI >5 + associated symptoms
    2. An AHI or ODI >15 irrespective of symptoms

Respiratory

  • Chronic hypoxia
  • Chronic hypercapnoea
  • Reset respiratory drive centre

Cardiovascular

Neuro-cognitive

  • Increased risk cerebrovascular disease due to polycythaemia and hyperviscosity
  • Reduced seizure threshold
  • Chronic REM sleep deprivation associated with:
    • Cognitive dysfunction
    • Reduced QoL
    • Mood disturbance inc. increased risk of depression

Endocrine

  • Development of IGT and dyslipidaemia
  • Impaired HPA axis functioning causes increased ACTH and cortisol levels
  • Gonadal dysfunction (both genders)
  • PCOS and hypothyroidism (females)

Haematological

  • Activation of endothelial inflammatory response
    • Increased platelet aggregation
    • Increased pro-inflammatory cytokine release

  • Polycythaemia (due to chronic hypoxia)
  • Increased risk of VTE (up to 10x)

Endocrine


Conservative

  • Education
  • Treat modifiable risk factors inc. smoking and alcohol cessation
  • Weight loss and exercise
  • Avoid sedative drugs

Non-surgical

  • Mandibular advancement devices

  • CPAP 5-20cmH2O is the gold standard treatment
    • Should be used at least 6hrs/night
    • Patients on CPAP should have had their treatment efficacy checked within the last year
    • It is associated with:
      • Reduced AHI
      • Improved QoL
      • Reversal of associated cardiovascular and cerebrovascular conditions

Surgical

  • Tonsillectomy e.g. in children
  • Uvuloplasty or uvulopalatoplasty using LASER (little evidence of benefit)
  • Orthognathic surgery to correct craniofacial abnormalities
  • Bariatric surgery

Perioperative management of the patient with obstructive sleep apnoea


  • One should risk stratify patients, with various ways of doing so, in order to inform perioperative journey
  • This will inform aspects of care such as degree of pre-operative intervention, specialty referral, or need for enhanced care beds post-operatively

History and examination

  • Full history and examination
  • Ascertain presence of other comorbidities:
    • Cardiovascular e.g. HTN, IHD, CCF
    • Metabolic e.g. diabetes, metabolic syndrome
  • Establish functional status

Investigations

  • Sleep studies ± blood gas ± CPAP

  • Bloods
    • FBC - polycythaemia
    • Blood gas - raised venous bicarbonate an indicator of chronic hypercarbia
    • U&E - renal impairment from HTN
    • LFT - hepatic congestion or NASH
    • Blood glucose ± HbA1c - concurrent diabetes

  • ECG e.g. AF, RVH, RAD, RV strain
  • TTE to assess RV function and for the presence of pulmonary HTN
  • Measures of functional capacity e.g. CPET

Risk assessment

  • Risk prediction can be facilitated with the Obesity Surgery Mortality Risk Score (OS-MRS)
    • It is only validated in bariatric surgery but may be used in non-bariatric surgery
  • The score can be used to help plan the need for post-operative care

    • Risk factor Score
    BMI >50kg/m2 1
    Male 1
    Hypertension 1
    Age >45yrs 1
    Any risk factor for PE:
    Previous VTE
    Right heart failure
    Pulmonary HTN
    IVC filter    		 Max. 1

  • A score of 4-5 indicates a high-risk patient and should prompt consideration of post-operative admission to critical care

Optimisation

  • Patients who have OSA diagnosed pre-operatively should be prescribed CPAP
    • Ideally ≥3 months' CPAP prior to elective surgery
    • At the least 6 weeks' CPAP prior to surgery

  • Book post-operative bed according to risk stratification
  • Ensure home CPAP brought in if known diagnosis of OSA
  • Ensure senior anaesthetist providing anaesthesia

Choice of technique

  • Regional and local anaesthetic techniques are the gold standard, as patients are at greater risk of complications from GA and its accoutrements such as opioid analgesia
  • Caution with interscalene blocks however, as phrenic nerve paralysis may be troublesome

General equipment

  • As patients tend to be obese, one should have:
    • Suitable gowns and theatre-wear of appropriate size
    • Appropriate transfer equipment including adequate staff numbers to help do so
    • Oxford HELP pillow or ramping device
    • Large IPC's or TEDS
    • Extra-wide extensions or arm gutters for the operating table
    • Gel pads to protect pressure points

Anaesthetic equipment

  • Predict difficult intubation therefore VL and associated difficult airway kit
  • Long spinal/epidural and regional needles
  • Ultrasound machine in case of difficult vascular access
  • Ventilator capable of delivering suitable driving pressure and PEEP

Monitoring and access

  • AAGBI, although need large NIBP cuff which one may have to place on the forearm or calf
  • A-line may be necessary to:
    • Monitor invasive BP in a population at higher risk of cardiovascular disease
    • Measure BP if non-invasive methods are unreliable or impossible
    • Check gas exchange in patients with chronic lung disease
  • Neuromuscular monitoring should be used as higher potential for incomplete reversal of NMBA

Airway

  • OSA is associated with an up to 8x increased incidence of difficult airway management
  • In a similar vein, obesity is associated with a 2x increased incidence of adverse airway events and a higher rate of failure of rescue techniques

  • Assessment for difficult intubation shoud take place, with particular reference to:
    • Predictors of difficult facemask ventilation: BMI >50 kg/m2, Mallampati III or presence of a beard
    • Predictors of difficult intubation: neck circumference >42cm, Mallampati III

Induction

  • There is a higher risk of hypercapnoea and hypoxia, as increased body tissue mass and work of breathing leads to greater oxygen consumption and CO2 production
  • FRC is reduced to (near-)closing capacity which can cause atelectasis and hypoxia, due to:
    • Excess adipose tissue reduces chest wall complication
    • Lying supine
    • GA
    • Pneumoperitoneum

  • Induction should take place in theatre to mitigate unnecessary transfer risk and allow greater space and access to assistance
  • Should induce ramped e.g. Oxford HELP pillow or sitting up, as this maintains FRC, reduces dyspnoea and facilitates BVM/laryngoscopy

  • Likely to require intubation and therefore need appropriate equipment to facilitate this, including video laryngoscopy and adjuncts
    • A SAD may be appropriate; a 2nd generation device should be used and should definitely be used with caution if BMI >40kg/m2
    • An airway plan should be vocalised and DAS guidelines followed

  • FONA may be more difficult with a higher risk of complications
  • It may be appropriate in high risk cases to identify the depth of the cricothyroid membrane, vascular tissue, and mark relevant landmarks to improve the chance of success

Maintenance

  • NAP5 revealed an increased incidence of awareness in obese patients shortly after induction of anaesthesia, attributable to the rapid redistribution of IV agents

  • In order to reduce awareness, one should ensure:
    • Adequate IV dosing of induction agent e.g. as per SOBA
    • Prompt delivery of maintenance anaesthetic agent
    • Further bolus(es) of anaesthetic agent before airway manipulation or protracted airway manoeuvres
    • Using processed EEG-based depth of anaesthesia monitoring to reduce risk of awareness

  • TIVA with propofol offers a number of potential advantages over volatile anaesthesia for the obese patient
    • Rapid offset of action with ‘clear-headed’ emergence
    • Reduced incidence of laryngospasm
    • Reliable clearance of hypnotic agents
    • Maintained anaesthesia during protracted airway manipulation
    • Reduced PONV
  • Historical pharmacokinetic models for propofol are not validated in obese patients who were excluded from initial development, although the newer Eleveld model may perform better

  • If using volatile agents, use those with a rapid offset of action (low blood:gas partition coefficient) e.g. desflurane or sevoflurane

Ventilation

  • Obesity is an independent risk factor for developing postoperative pulmonary complications
  • Judicious use of recruitment manoeuvres where there is suspicious of atelectasis/collapse
  • Minimising the effect of positioning on ventilation is important, e.g. Trendelenburg position in lower abdominal surgery with laparoscopy
  • Ventilatory strategy may include:
    • Lung protective volumes (6-8ml/kg)
    • PEEP titrated to respiratory and cardiovascular state, typically 8 - 10cmH2O
    • Plateau pressure <22cmH2O
    • Driving pressure <18cmH2O

Extubation

  • The hazards of airway/respiratory problems during emergency are greater in the obese population
  • Patients should be fully reversed, with a TOFr >0.9, and exubated sat uproght and fully awake

Recovery

  • Recover in a head-up/upright position
  • Recover in area where CPAP can be applied if needed
  • Continuous monitoring esp. SpO2

Analgesia

  • Multimodal analgesic strategy is paramount due to increased sensitivity to opioids
  • Opioids, even at low doses (i.e. <10mg morphine equivalents) and regardless of route of administration are associated with significant complications
  • Therefore aim to avoid opioids where possible to limit respiratory depression
  • If required:
    • Use short-acting opioids e.g. fentanyl
    • Consider admitting to an area where continuous SpO2 monitoring can be emplyed

Other post-operative care

  • Robust venous thromboprophylaxis
    • Early mobilisation
    • TEDS/IPCs
    • Chemical prophylaxis based on total body weight dosing

  • Multimodal PONV

  • Early initiation of CPAP may help reduce complications

  • HDU care where possible, especially for those risk-stratified as being at higher risk

  • The risk of any postoperative complication nearly doubles in patients with OSA (OR 1.9)
  • The risk increases with higher ODI:
    • 5-15; 13.8%
    • >15; 17.5%
  • The overall rate of post-operative complication in patients with OSA may be as high as 40%
  • Unsurprisingly, the risk is greater in those with untreated OSA than those who have received treatment i.e. CPAP
Predisposing risk factors for post-operative complications
Morbid obesity
Male gender
Undiagnosed OSA
Untreated or only partially treated OSA
Use of opioid analgesia
Lack of appropriate post-operative monitoring

Respiratory

  • There is an overall 2x increase in the risk of post-operative respiratory complications
  • There are higher rates of:
    • Hypoxia
    • Respiratory failure
    • Unplanned re-intubation
    • Any cardiopulmonary complication

Cardiovascular

  • There is an overall 1.5x increase in the risk of post-operative cardiovascular complications
  • There are higher rates of:
    • Post-operative MI
    • Arrhythmia
    • Cardiac arrest

  • The risk of cardiovascular complications increases with OSA severity:
    • Mild: 19%
    • Moderate: 22%
    • Severe: 30%

Insitutional

  • Higher rate of ICU admission (24 vs 9%)
  • Higher rate of unplanned ICU admission
  • Increased length of ICU stay

  • Increase length of hospital stay, which is reduced by 1 day by using CPAP
  • Higher rate of hospital or ICU re-admission (OR 2.25)

  • Of note, does not seem to increase the rate of unplanned admission following day surgery

  • OSA isn't robustly associated with an ↑ mortality across all patient groups as some studies demonstrated lower mortality, presumed to be on account of greater monitoring