Spontaneous Breathing Trial


 Chris Nickson

Spontaneous breathing trials (SBT) are used to identify patients who are likely to fail liberation from mechanical ventilation


  • SBT is “the defacto litmus test for determining readiness to breathe without a ventilator”
  • Ideally, during an SBT we want to observe the patient under  conditions of respiratory load that would simulate those following extubation


IDENTIFICATION OF PATIENTS SUITABLE FOR SBT


Patients that pass the following daily ‘wean screen’ should undergo SBT:


  • lung disease is stable/ resolving
  • low FiO2 (< 0.5) and PEEP (< 5-8cmH2O) requirement
  • haemodynamic stability (little to low inopressors)
  • able to initiate spontaneous breaths (good neuromuscular function)
  • This indicates patients suitable for a spontaneous breathing trial, those who pass also to be assessed for extubation.


METHOD


SBT involves the following steps:


  • It be conducted while the patient is still connected to the ventilator circuit, or the patient can be removed from the circuit to an independent source of oxygen (T-piece)
  • When using the ventilator a PS of 5 – 7 cmH2O and 1-5 cmH20 PEEP (so called ‘minimal ventilator settings’) will overcome increased work of breathing through the circuit (i.e. ETT)
  • If still on the ventilator the patient should have ‘minimal ventilator settings”
  • Initial trial should last 30 – 120 minutes
  • If it is not clear that the patient has passed at 120 minutes the SBT should be considered a failure
  • In general, the shorter the intubation time the shorter the SBT required
  • 80% of patients who tolerate this time can be permanently removed from the ventilator


CRITERIA TO STOP SBT


No single parameter should be used to judge SBT success or failure, but a combination of the following are often used:


  • Respiratory rate RR >38 bpm for 5 minutes or <6bpm
  • SpO2 < 92%
  • Tidal volume (TV) < 325 mL
  • Heart rate: HR > 140 OR 25% above baseline OR HR<60
  • Blood pressure: SBP 40 mm Hg above baseline
  • Worsening agitation, anxiety or discomfort despite reassurance
  • Rapid shallow breathing index (RSBI) = RR/ TV
  • Most consistent and powerful predictor
  • RSBI > 105 min/L predicted failure well, but if used rigidly may slow the weaning process

READ ALSO: Assessing patient readiness for extubation

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