Guide — Mechanical Ventilation
Weaning & Spontaneous Breathing Trials
Liberation from the ventilator is a daily question, not a single event. This guide builds the sequence — screen for readiness, run a spontaneous breathing trial, and assess the airway separately — and shows why a passed trial is still not an extubation order.
10 min read · Mechanical Ventilation
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Getting a patient off the ventilator sooner reduces complications, but pulling the tube too early and reintubating is its own harm. The discipline that balances those two risks is an ordered one: screen for readiness every day, run a spontaneous breathing trial when the patient passes, and — separately — decide whether the airway itself is ready to come out.
Keep the three steps distinct. The screen finds candidates, the trial tests the lungs, and the airway assessment protects the extubation. Collapsing them is where patients get hurt.
Key Concepts — The Readiness Screen
Before anyone spends effort on a trial, the patient has to pass a daily readiness screen. The classic criteria:
| Domain | Readiness Criterion |
|---|---|
| Cause of failure | The reason for intubation has resolved or is clearly improving. |
| Oxygenation | Adequate on modest support: FiO₂ ≤ 0.4–0.5, PEEP ≤ 5–8 cmH₂O, P/F > 150–200 (per protocol). |
| Hemodynamics | Stable with no or low-dose vasopressors. |
| Respiratory drive | The patient can initiate breaths. |
| Acid-base | pH > 7.25. |
Pair the screen with a spontaneous awakening trial — interrupting sedation and testing breathing together, the “wake up and breathe” approach, shortens ventilator days and improves outcomes.
Assessment & Findings — Running the SBT
A patient who passes the screen earns a spontaneous breathing trial — a real test of breathing on minimal support. Put them on pressure support 5–8 cmH₂O (or a T-piece, per protocol) for 30–120 minutes, and watch the patient, not just the clock.
The rapid shallow breathing index (RSBI) — respiratory rate divided by tidal volume in liters, measured during unsupported breathing — helps predict the outcome. An RSBI < 105 predicts weaning success (Yang-Tobin). It is one screen among many, not a verdict on its own.
| Sign | Failure Threshold |
|---|---|
| Respiratory rate | > 35 /min |
| Oxygen saturation | SpO₂ < 90% |
| Heart rate | > 140 /min or a sustained change > 20% |
| Blood pressure | SBP > 180 or < 90 mmHg |
| Distress | Anxiety, diaphoresis, or paradoxical (abdominal) breathing |
Any of these means stop the trial and return the patient to comfortable settings — the trial has given you its answer.
RT Priorities & Interventions
Passing the SBT clears the lungs, not the airway. Extubation is a separate decision with its own checklist.
- Assess the airway before pulling the tube. Mental status (can the patient protect the airway and follow commands?), cough strength, secretion burden (how much and how often you suction), and a cuff-leak test when upper-airway edema is a concern.
- Extubate to the right support. In selected high-risk patients (hypercapnia, heart failure, multiple comorbidities), applying NIV prophylactically right after extubation prevents respiratory failure — prevention, not rescue.
- Monitor closely afterward. Stridor, rising work of breathing, and accumulating secretions in the first hours are the warning signs; have a reintubation plan ready.
- When the SBT fails, regroup. Return to comfortable supportive settings, hunt the reversible cause — fluid overload, bronchospasm, delirium, weakness, infection — and retry in about 24 hours. Failure is information, not defeat.
Common Pitfalls
- Treating a passed SBT as automatic extubation. The lungs can breathe and the airway can still fail — a weak cough, copious secretions, or airway edema send a “passing” patient back to the tube.
- Trusting RSBI as a yes-or-no answer. An RSBI < 105 predicts success but is one data point; a borderline number does not overrule a comfortable patient protecting the airway, or vice versa.
- Skipping the sedation interruption. Running an SBT through deep sedation underestimates the patient; pair the spontaneous awakening trial with the breathing trial.
- Reading a failed trial as defeat. A failed SBT points to a reversible cause — fluid overload, bronchospasm, delirium, weakness, infection. Fix it and retry.
- Not hunting the cause after a failure. The reason is usually findable; volume status, delirium, and weakness top the list.
Board Exam Pearls
- RSBI = respiratory rate ÷ tidal volume (in liters), measured unsupported. < 105 predicts weaning success (Yang-Tobin). Memorize the number and the formula.
- Pair the SAT and SBT — spontaneous awakening (sedation off) plus spontaneous breathing trial. The “wake up and breathe” protocol shortens ventilator days.
- SBT = 30–120 minutes on low pressure support (5–8 cmH₂O) or T-piece. Failure: RR > 35, SpO₂ < 90%, HR > 140 or > 20% change, SBP > 180 or < 90, distress.
- Passing the SBT is not the same as extubation — assess cough, secretions, mental status, and cuff leak separately.
- Prophylactic NIV after extubation prevents failure in high-risk patients; it is not a rescue for a patient already failing after extubation.
FAQ
What has to be true before I attempt an SBT?
The patient should pass a daily readiness screen: the cause of respiratory failure has resolved or is improving, oxygenation is adequate on modest support (FiO₂ ≤ 0.4–0.5, PEEP ≤ 5–8, P/F > 150–200 per protocol), hemodynamics are stable on little or no vasopressor, the patient can initiate breaths, and the pH is > 7.25. Pair the screen with a sedation interruption so the trial tests the real patient.
How is an SBT actually run?
Put the patient on minimal support — pressure support 5–8 cmH₂O or a T-piece, per your protocol — for 30 to 120 minutes, and watch closely. Failure signs include a respiratory rate over 35, SpO₂ under 90%, heart rate over 140 or a sustained change beyond 20%, systolic pressure over 180 or under 90, and anxiety, diaphoresis, or paradoxical breathing. Any of these ends the trial.
What is the RSBI and how do I use it?
The rapid shallow breathing index is the respiratory rate divided by the tidal volume in liters, measured during unsupported breathing. An RSBI below 105 predicts a successful wean (the Yang-Tobin index). Treat it as one screening number among several — it supports the decision, it does not make it alone.
My patient passed the SBT — can I extubate?
Not automatically. Passing the trial shows the lungs can do the work; extubation also requires an airway that can protect itself. Check mental status, cough strength, secretion burden, and a cuff leak when airway edema is a concern. In high-risk patients, plan for prophylactic NIV right after extubation to prevent failure.
Put it to work
RSBI is the one weaning number you will calculate at the bedside again and again. Run a respiratory rate and tidal volume through it and see where the 105 threshold falls.
Open the RSBI Calculator →Related Resources
Sources
- Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991;324(21):1445-1450.
- Ouellette DR, Patel S, Girard TD, et al. Liberation from mechanical ventilation in critically ill adults: an official ATS/ACCP clinical practice guideline. Chest. 2017;151(1):166-180.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Discontinuing ventilatory support chapter.