A
daily spontaneous breathing trial (SBT) should be performed on each intubated
patient unless a contraindication to do so is present.
Contraindications
for SBT. Patients should be on minimal ventilator settings prior to
initiation of daily SBT trials. A
patient should not receive a sedation awakening trial (SAT) during or prior to
placing the patient on a SBT. Doing so may cause the patient to fail the SBT
secondary to agitation and a true assessment of lung mechanics is not
accomplished. Rather, a patient’s sedation should be titrated so that 1) they
are comfortable and show no signs of agitation and 2) they are over-breathing
the ventilator. Failure to lighten a patient to over-breath the ventilator will
result in failure due to immediate apnea.
The patient should be placed on the
spontaneous breathing ventilator mode with tube compensation. The tube
compensation setting will provide the appropriate amount of support to overcome
the resistance of the circuit.
The widely accepted parameter to monitor pulmonary
mechanics during the SBT is the Rapid Shallow Breathing Index (RSBI). The RSBI
is calculated as the respiratory rate (RR) / tidal volume (TV) measured in liters.
A RSBI of less than or equal to 105 has shown to predict successful extubation.
A study by SK Epstein demonstrated a 20% false positive/reintubation rate in
patients with a RSBI less than 105, however, when the data was more closely
evaluated, almost all the failures were for non-pulmonary indications. They
concluded, while the RSBI is an excellent parameter to predict pulmonary
readiness for extubation, it fails to identify non-cardiac causes of extubation
failure. This should be kept in mind.
The SBT should be performed over 30
minutes if the patient tolerates. There is no advantage to performing an SBT
for longer than 30 minutes. Perren et al. demonstrated that a 30 minute SBT is
as effective as identifying a patient ready for extubation as a 2 hour trial. If
the patient passes the SBT, the next step prior to extubation is to proceed in
performing a SAT.
from Dr. Shaun Daly
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