We do this when the patient is stable on FiO2 =40% and PEEP =5, and is no longer being actively resuscitated.
There are several possible ways to do the trial - we commonly put the patient on "Tube compensation" mode, but the patient could be placed on CPAP=5 or simply a trach collar/T-piece of 40% FiO2. It is important that the patient is comfortable during the trial (ie. not agitated or in pain) - it is not necessary to stop the sedation/analgesia, but on the other hand, the patient should not be over sedated.
SBT's should last from 30 minutes to 2 hours. At any point, if the patient is not tolerating the trial, they should be placed back on their last ventilator settings. Any longer than 2 hours will only serve to tire the patient.
During the SBT, the patient should be observed for signs of intolerance - desaturation, agitation, tachypnea are just a few signals that the trial should be discontinued. A measurement that we often quote is the Rapid Shallow Breathing Index (RSBI).
The RSBI is calculated by dividing the patients respiratory rate (breaths per minute) by their Tidal volume in Liters. For example, a patient with a respiratory rate of 25 and TV of 250 ml has an RSBI of
25
0.25 = 100 breaths/min/L
The threshold that is most often quoted is 105. A patient with an RSBI over 105 has "failed" the SBT, while one with an RSBI less than 105 has "passed". Essentially, an RSBI less than 105 predicts that the patient will do well off the ventilator.
Of course, prior to extubation, other factors should be considered, such as whether the patient can protect their airway (ie. due to decreased mental status or airway edema).
A patient who fails their SBT should be placed back on the ventilator. Any abnormalities should be addressed such as electrolytes repleted, pneumonia treated or hemothorax drained. The SBT may be repeated daily until the patient is able to be extubated.
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