Knowing when a patient can breathe alone
Patients on a mechanical ventilator are taken off it as soon as safely possible, because every extra day raises the risk of pneumonia, muscle weakness and other complications. Removing the breathing tube too early is also risky: about 10 to 20% of patients need to be reintubated. Intensive care teams therefore test whether a patient is ready with a spontaneous breathing trial, in which the ventilator gives little or no support.
The index
In 1991, Karl Yang and Martin Tobin noticed that patients who fail a breathing trial tend to breathe fast and shallow: the tired respiratory muscles cannot take deep breaths, so the brain compensates with more of them. They summarized this pattern in one number:
RSBI = respiratory rate ÷ tidal volume (in liters)
A value below 105 predicted successful weaning in their study.
Worked example
During the first minute of a breathing trial, a patient breathes 25 times a minute with a tidal volume of 400 mL:
- RSBI = 25 ÷ 0.4 = 62.5, shown as 63 breaths/min/L.
- Minute ventilation = 25 × 0.4 = 10.0 L/min.
The index is well below 105, which supports continuing the trial. A patient breathing 35 times a minute with 250 mL breaths would have an RSBI of 140, a warning sign of respiratory muscle fatigue.
How it is used today
- The index is measured early in the trial, usually during the first one or two minutes, with minimal pressure support or a T-piece.
- Later studies found it less accurate than in the original report, especially when measured with higher pressure support, in older patients or after long ventilation.
- Most teams now use it as one item in a broader assessment: the cause of respiratory failure has improved, oxygen levels are adequate, the patient is awake enough to protect their airway, and they can cough out secretions.
- The decision to remove the breathing tube is based on how the patient tolerates a 30 to 120 minute trial, not on the index alone.
Measuring it correctly
Read the respiratory rate and the exhaled tidal volume from the ventilator, averaged over a minute, not from a single breath. Convert milliliters to liters before dividing. Agitation, pain, fever or a full stomach can all raise the respiratory rate temporarily.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.