Weaning is not a vibe. It is a screen, a trial, and a separate decision about the airway. Most weaning arguments at the bedside happen because somebody collapsed those three steps into one.
Here is the sequence, in the order it actually runs.
RSBI: One Number, Narrow Job
The rapid shallow breathing index is respiratory rate divided by tidal volume in liters. Count the frequency and measure the spontaneous tidal volume, then divide.
A patient breathing 24 times a minute with a 400 mL tidal volume: 24 ÷ 0.4 = 60.
The classic threshold from Yang and Tobin is 105. Under 105 predicts weaning success. Over 105 predicts failure. That number came from measurements taken during one minute of unsupported spontaneous breathing, which matters more than most people remember.
Measure it on minimal or no support. Pressure support inflates tidal volume, which shrinks the index and hands you a falsely reassuring number. An RSBI of 70 on 12 of pressure support is not an RSBI of 70.
Known caveats: small endotracheal tubes raise resistance and drive the number up, and smaller patients breathing smaller tidal volumes run higher indices without being sicker. RSBI is a screening tool with a mediocre positive predictive value. It is one data point among several, not a verdict.
The Readiness Screen
Before anybody talks about a trial, the basics have to be true:
- The reason for intubation is resolved or clearly improving
- Adequate oxygenation on modest settings, generally FiO2 at or below 0.4–0.5 with PEEP at or below 5–8 cmH2O
- Hemodynamically stable, off vasopressors or on low and stable doses
- Intact respiratory drive, triggering their own breaths
- Acid-base and electrolytes not actively working against them
Exact cutoffs vary by unit and by protocol. Some services screen with a PaO2/FiO2 ratio, others with an SpO2 target. Use yours.
Pair the spontaneous awakening trial with the spontaneous breathing trial where protocol allows. Sedation that is still on board is a weaning failure waiting to be misattributed to the lungs.
Running the SBT
Spontaneous breathing trials run 30 to 120 minutes on low pressure support (commonly 5–8 cmH2O), CPAP around 5, or a T-piece. Longer is not better. Most failures announce themselves in the first several minutes.
Stay in the room for the start. The first two minutes tell you most of what the next two hours will.
Call the trial and return to full support for sustained tachypnea, desaturation, a significant rise or fall in heart rate or blood pressure, agitation, diaphoresis, accessory muscle recruitment, or paradoxical abdominal motion. Thresholds differ between protocols, but the pattern does not: rising work, falling reserve.
Classic bedside parameters still have a place alongside RSBI. Negative inspiratory force more negative than roughly -20 to -30 cmH2O and a vital capacity above about 10–15 mL/kg suggest reserve. Both are effort dependent, which means a sedated or uncooperative patient can produce a number that says nothing about their lungs.
Passing the Trial Is Not the Same as Getting the Tube Out
Weaning asks whether the lungs and the pump can carry the load. Extubation asks whether the patient can own their own airway. Those are different questions with different failure modes.
Before the tube comes out, look at mental status, cough strength, and secretion burden. A patient who passes a beautiful SBT and needs suctioning every fifteen minutes is not an extubation, they are a reintubation with a delay built in.
Consider a cuff leak test for patients at risk of laryngeal edema: prolonged intubation, traumatic intubation, large tube, known airway swelling. Absent leak does not mandate keeping the tube, but it changes what you have ready at the bedside.
Have the plan for failure before you pull. Who is reintubating, what equipment is staged, whether noninvasive support is going on immediately after.
Field Card
- RSBI = f ÷ Vt in liters, measured on minimal support
- Under 105 favors success, over 105 favors failure
- Screen first: cause resolved, oxygenation modest, stable, awake, driving
- SBT: 30–120 minutes, low PS, CPAP, or T-piece
- Stop for rising rate, falling sats, hemodynamic swings, distress
- Extubation is a separate call: airway, cough, secretions, mental status
Every ICU has its own weaning protocol and its own cutoffs. Yours wins over ours. Confirm thresholds and stopping rules against your unit policy before you act on anything here.
Study reference, not medical advice. More reference cards live in the FIELD NOTES archive.
For the people who liberate patients for a living: the WEAN LIFE tee is in supply.
