Spontaneous Breathing Trial

OVERVIEW

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

PREDICTORS OF FAILURE TO WEAN

See Indices that predict difficulty weaning

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

REASONS FOR REINTUBATION FOLLOWING SUCCESSFUL SBT

A successful SBT does not guarantee that the patient will avoid reintubation:

• Upper airway resistance (supraglottic edema)
• poor cough and excessive secretions
• poor airway reflexes leading to aspiration
• Respiratory weakness masked by pressure support
• Increased cardiac load induced by removal of CPAP
• Onset of new pathology

MINIMAL VENTILATOR SETTINGS

The concept of ‘minimal ventilator settings’ is controversial:

• Martin Tobin has argued that adding either 5 cm H2O as “physiologic” PEEP or pressure support of 7 cm H2O to overcome the resistance in an endotracheal tube (or both, as is usually done) may actually reduce the “spontaneously” breathing patient’s workload by >40%
• It has been shown experimentally that the work of breathing through an endotracheal tube, compared to the work of breathing following extubation, is almost identical due to upper airway edema resulting from an ETT being in place for several days
• Tobin argues for wider use of true T-piece spontaneous breathing trials, especially in those at high risk of failed extubation and when the consequences of failed extubation may be catastrophic
• An alternative is to have the ventilator set on “flow-by,” with pressure support and PEEP set at zero
• There is no strong evidence in favour of any of these approaches

References and Links

LITFL

• CCC — Weaning from mechanical ventilation
• CCC — Difficulty weaning from mechanical ventilation
• CCC — Weaning from mechanical ventilation (Hot Case)
• CCC — Extubation assessment in the ICU
• CCC — Extubation assessment in the ED

Journal articles and textbooks

• Esteban A, Frutos F, Tobin MJ, Alía I, Solsona JF, Valverdú I, Fernández R, de la Cal MA, Benito S, Tomás R, et al. A comparison of four methods of weaning patients from mechanical ventilation. Spanish Lung Failure Collaborative Group. N Engl J Med. 1995 Feb 9;332(6):345-50. PubMed PMID: 7823995. [Free Fulltext]
• Macintyre NR. Evidence-based assessments in the ventilator discontinuation process. Respir Care. 2012 Oct;57(10):1611-8. Review. PubMed PMID: 23013898. [Free Fulltext]
• Sassoon CS, Light RW, Lodia R, Sieck GC, Mahutte CK. Pressure-time product during continuous positive airway pressure, pressure support ventilation, and T-piece during weaning from mechanical ventilation. Am Rev Respir Dis. 1991 Mar;143(3):469-75. PubMed PMID: 2001053.
• Tobin MJ. Extubation and the myth of “minimal ventilator settings”. Am J Respir Crit Care Med. 2012 Feb 15;185(4):349-50. doi: 10.1164/rccm.201201-0050ED. PubMed PMID: 22336673. [Reply to Letters to the Editor]

Social media and web resources

• PulmCCM.org — Tobin: “Minimal” PEEP and pressure support during SBT kills some patients (AJRCCM)