Ventilatory Weaning

Part of: Mechanical Ventilation

• Everyday someone is on the ventilator, assess for liberation unless patient is on $F_{\text{i}}\mathrm{O}{}_2>45\%$, patient is unstable, or on high amounts of vasopressors:
  • Lifting sedation
  • Weaning/liberation trial

Spontaneous Breathing Trial (SBT)

Readiness Criteria

System	Criteria
Respiratory	$F_{\text{i}}\mathrm{O}{}_2$ < 50% with PEEP ≤ 10 cm (higher PEEPs may be acceptable in obese patients); $P_{\text{a}}\mathrm{CO}{}_2$ or $\text{et}\mathrm{CO}{}_2$ is normal or close to baseline for patients with chronic hypercapnoea
Cardiovascular	No ongoing myocardial ischaemia; HR <140; Not on high-level vasopressors
Neurological	Patient is arousable and ideally following commands
Renal	No uncontrolled acid-base disturbances

SBT Settings

• Pressure support ventilation with 5 cm PS and 5 cm PEEP (PSV 5/5)
• If a patient can tolerate the above settings for 30 minutes, they are likely ready for extubation

Passing Criteria

• Adequate oxygenation: saturating >88% without requiring more than ~50% $F_{\text{i}}\mathrm{O}{}_2$
• Adequate ventilation: no ↓ in minute ventilation, no ↓ in tidal volume, $\text{et}\mathrm{CO}{}_2$ does not increase by >10 mmHg
• No signs of severe fatigue:
  • Agitation, diaphoresis, use of accessory muscles
  • RSBI ($=\text{RR}/V_{\text{T}}$) <105¹
• No obvious complications (arrhythmia, hypotension, severe hypertension)
• Complete an ABG:
  • pH > 7.35 and $P_{\text{a}}\mathrm{CO}{}_2<45$ and $P\mathrm{O}{}_2>60$ → likely to succeed extubation

If Apnoea Develops During SBT

Causes:

• Patient was hyperventilated prior to the trial → place back on standard ventilator mode, decrease backup rate to stimulate spontaneous breaths, then repeat SBT
• Cheyne-Stokes breathing pattern
• Over-sedated

On Failing a SBT

If a patient fails a SBT

• Place the patient back on full ventilator support immediately
• Repeat SBT later in the day only if something easily manipulable can be corrected (e.g. sedation), otherwise repeat the following morning

Causes of failing a SBT:

Category	Causes
Pulmonary	Volume overload/pulmonary oedema, bronchospasm, pleural effusion, VAP, atelectasis/mucous plugging, small ETT, occult ETT occlusion
Cardiovascular	Angina, pulmonary embolism
Neurological/Psychiatric	Anxiety, chronic tachypnoea
Metabolic	Metabolic acidosis, elimination of chronic compensatory metabolic alkalosis (e.g. COPD), electrolyte abnormalities (especially hypophosphataemia)
Ventilator	Dyssynchrony or inadequate ventilator support

Investigations to consider:

• Electrolytes including CMP
• Review of fluid balance and examination for volume overload
• Chest imaging (CXR, POCUS)
• Review acid-base status and compare to baseline bicarbonate
• CT angiography if considering PE

On Passing a SBT

Passing means the patient is strong enough to sustain the work of breathing, but also consider:

1. Risk of post-extubation laryngeal oedema
2. Will the patient be able to maintain their airway

Figure 19. Assessment of risk of post-extubation laryngeal oedema

Assessment of ability to maintain airway is based on four factors (subjective):

1. Patient’s mental status
2. Is the patient producing plenty of secretions (e.g. requiring suctioning < q2hrly)?
3. Does the patient have a history of hypercapnoea?
4. Does the patient have a strong cough (assessed while suctioning)?

Extubation

Checklist for Extubation

1. Optimise sedation
   • Ideal target: following commands, mildly distressed by ETT when sedation held
   • Consider cross-tapering from propofol onto dexmedetomidine
2. Optimise volume status
   • Extubation increases preload and blood pressure
   • Examine fluid charts for fluid overload; consider diuresis before extubation if required
3. Optimise acid-base status
   • Treat metabolic acidosis prior to extubation (patient compensates with respiratory alkalosis)
   • Patients with chronic hypercapnoea and chronic compensatory respiratory alkalosis should ideally be restored to their baseline bicarbonate level prior to extubation
4. Review chest X-ray
   • Observe for any treatable disease process (e.g. pleural effusion)
5. Review insulin regimen and glycaemic control
   • Extubation often involves discontinuation of enteral nutrition
6. Other considerations
   • Suction stomach
   • Check for cuff leak if indicated; air leak on deflation of ETT cuff suggests absence of tracheal swelling

Post-Extubation Support

• Most patients can go from extubation to high flow nasal prongs (HFNP); however BiPAP can be considered in patients with heart failure or COPD
• For HFNP to be effective:
  • HFNP needs to be continued for a substantial amount of time (24-48 hours) unless the patient is already on night-time BiPAP
  • The flow rate should be increased as high as can be tolerated by the patient (ideally 50-60 L/min)

Unplanned Extubation

• Accidental extubation (e.g. while turning or transporting patient) → generally requires re-intubation
• Self-extubation (patient intentionally removes their own ETT):
  1. Stop all sedative infusions
  2. Place the patient on BiPAP
  3. Observe
  4. Re-intubate if clinically indicated

Footnotes

1. RSBI has a specificity of 44% predicting extubation failure. Should be considered a red flag, yet some patients can still be extubated despite a high RSBI (e.g. in patients with interstitial lung disease who have chronic tachypnoea) ↩