Weaning, SBT and extubation are not the same thing
"Weaning begins the moment ventilation begins. The commonest error in the ICU is not weaning too fast — it is leaving the tube in too long. Every day of unnecessary ventilation adds pneumonia, delirium and weakness."
Summarised from Marino's The ICU Book; with the Washington Manual of Critical Care and Irwin & Rippe's Intensive Care Medicine.- Weaning — the process of reducing ventilator support as the patient resumes the work of breathing.
- Spontaneous breathing trial (SBT) — a test of the patient breathing with minimal/no support to see if they are ready.
- Extubation — removing the tube. A patient can pass an SBT (ready to breathe) yet fail extubation (airway problem).
Screen daily before any trial
| Domain | Criterion |
|---|---|
| Cause | The reason for intubation is improving or resolved |
| Oxygenation | PaO₂/FiO₂ > 150–200; FiO₂ ≤ 0.4–0.5; PEEP ≤ 5–8 cmH₂O |
| Ventilation | pH acceptable; not severely hypercapnic; adequate cough |
| Haemodynamics | Stable; no/low vasopressors; no active ischaemia |
| Neurological | Arousable, able to protect airway (sedation minimised) |
The daily readiness screen is deliberately generous — it identifies who deserves an SBT. The SBT itself is the real test.
The decisive test of readiness
Method
Either a T-piece (no support) or low pressure-support (PSV ~5–8 cmH₂O ± low PEEP) for 30 minutes to 2 hours.
Pass
Comfortable, stable gas exchange and haemodynamics, RR < 35, SpO₂ ≥ 90%, no distress → proceed to extubation assessment.
Fail
Tachypnoea (RR > 35), desaturation, tachycardia/hypertension, agitation, diaphoresis, accessory muscle use → return to full rest, find the cause, retry next day.
Do not grind a failing patient — put them back on comfortable support for ≥24 h, correct the limiting factor, then re-trial. Repeated failed trials exhaust the patient.
The rapid shallow breathing index
| Predictor | Threshold favouring success |
|---|---|
| RSBI = RR ÷ Vₜ(L) | < 105 (rapid shallow breathing = likely failure) |
| Tidal volume | > 5 mL/kg |
| Respiratory rate | < 35/min |
| Negative inspiratory force | More negative than −20 to −30 cmH₂O |
| Minute ventilation | < 10 L/min |
No predictor replaces the SBT. RSBI is a useful adjunct, not a gate — a well-conducted spontaneous breathing trial is the best test of readiness.
Coordinate sedation interruption with the breathing trial
Spontaneous Awakening Trial (SAT)
Daily interruption of sedation — if the patient tolerates it (calm, no distress), proceed to the SBT.
Spontaneous Breathing Trial (SBT)
Performed on the awakened patient. The paired "wake-and-breathe" strategy (ABC trial) shortens ventilation and ICU stay and improves survival.
SAT + SBT are the "A/B" of ICU liberation, alongside choice of light sedation, delirium monitoring, early mobility and family engagement.
Passing the SBT is necessary but not sufficient
Before pulling the tube, confirm the airway will be safe once the tube is gone:
- Conscious level & airway protection — follows commands, adequate gag/cough.
- Cough strength & secretions — a weak cough or copious secretions predicts failure.
- Cuff-leak test — a small/absent leak with the cuff deflated suggests laryngeal oedema and risk of post-extubation stridor.
If the cuff-leak is low in a high-risk patient (traumatic/prolonged intubation, reintubation, airway oedema), give systemic corticosteroids ~4–12 h before extubation and have reintubation equipment ready.
Preventing reintubation in the high-risk patient
| Tool | Role |
|---|---|
| High-flow nasal cannula | Comfort, mild support; reduces reintubation in some low/moderate-risk groups |
| Prophylactic NIV | In high-risk patients (hypercapnia, COPD, heart failure) immediately after extubation — reduces reintubation |
| Rescue NIV for established failure | Generally not advised — may delay necessary reintubation and worsen outcome |
Timing matters: NIV works best used prophylactically in the right patient, not as a last-ditch rescue once respiratory failure is established.
Why patients fail — think in systems
| Category | Examples |
|---|---|
| Respiratory load | High resistance/secretions, dynamic hyperinflation, low compliance, residual lung disease |
| Cardiac | Weaning-induced pulmonary oedema — the negative intrathoracic pressure of spontaneous breathing raises LV afterload/preload → failure |
| Neuromuscular | ICU-acquired weakness, critical illness polyneuromyopathy, diaphragm dysfunction, over-sedation |
| Metabolic/other | Electrolytes (PO₄, K, Mg), anaemia, hypothyroidism, sepsis, overfeeding (↑ CO₂) |
| Psychological | Anxiety, delirium, sleep deprivation |
The WIND classification groups patients by weaning difficulty: simple (first attempt), difficult (up to 3 SBTs / 7 days), and prolonged (>3 SBTs or >7 days) — prolonged weaning needs a systematic, tracheostomy-inclusive plan.
Where it goes wrong
- Leaving the tube in because "the numbers aren't perfect" — over-ventilation causes real harm.
- Not pairing an SAT with the SBT (sedation masks readiness).
- Confusing passing an SBT with being extubatable — the airway (cuff leak, cough, secretions) is separate.
- Using NIV as a rescue after established post-extubation failure → delayed reintubation.
- Missing weaning-induced cardiac pulmonary oedema as the reason for repeated SBT failure.
High-yield one-liners
Q: RSBI formula & threshold?
Respiratory rate ÷ tidal volume (L); < 105 predicts weaning success.
Q: What is the SBT and its duration?
T-piece or low PSV for 30 min–2 h to test readiness.
Q: Purpose of the cuff-leak test?
Detect laryngeal oedema / risk of post-extubation stridor; steroids if low in high-risk patients.
Q: Best use of NIV around extubation?
Prophylactic in high-risk patients — not as rescue for established failure.
Q: Cardiac cause of weaning failure?
Weaning-induced pulmonary oedema (raised LV afterload from negative intrathoracic pressure).
Side-by-side comparisons
The distinctions examiners probe: the two trials, and the two ways a liberation attempt can fail.
SAT vs SBT
| Feature | SAT (awakening) | SBT (breathing) |
|---|---|---|
| Tests | Readiness to be off sedation | Readiness to breathe unsupported |
| Intervention | Interrupt sedation | T-piece / low PSV |
| Sequence | First | After a passed SAT |
Weaning failure vs Extubation failure
| Feature | Weaning failure | Extubation failure |
|---|---|---|
| Problem | Cannot sustain spontaneous breathing (SBT fails) | Passed SBT but fails after tube removed |
| Typical cause | Respiratory load, cardiac, neuromuscular | Airway (stridor, secretions, weak cough) |
| Timing | During the trial | Within 48–72 h of extubation → reintubation |
References
- Marino PL. Marino's The ICU Book. 5th ed. Wolters Kluwer; 2025.
- Kollef MH, Isakow W, Burks AC, Despotovic VN (eds). The Washington Manual of Critical Care. 4th ed. Wolters Kluwer; 2024.
- Irwin RS, Lilly CM, Mayo PH, Rippe JM (eds). Irwin & Rippe's Intensive Care Medicine. 9th ed. Wolters Kluwer; 2023.
- Boles JM, Bion J, Connors A, et al. Weaning from mechanical ventilation (Task Force). Eur Respir J. 2007;29:1033–1056.
- Girard TD, Kress JP, Fuchs BD, et al. (ABC trial). Awakening and Breathing Controlled trial. Lancet. 2008;371:126–134.