The following questions relate to separation from invasive mechanical ventilation:
a) With reference to a spontaneous breathing trial (SBT):
i. What is an SBT?
ii. Over what duration should it occur?
iii. Why would you perform an SBT in a mechanically ventilated patient?
iv. List three methods of performing an SBT.
b) What is the rapid shallow breathing index (RSBI) and how should it ideally be measured?
c) Briefly outline the role of prophylactic (planned) non-invasive ventilation (NIV) immediately following extubation. Explain how this differs from therapeutically applied (rescue) NIV used in the same context.
The SBT is the most direct way to assess a patient’s performance or tolerance of unassisted
breathing without ventilatory support.
A procedure in which a mechanically ventilated patient is given a trial of spontaneous breathing
without ventilatory support for a limited time without extubation or formal liberation from the ventilator.
Optimal SBT duration has been examined and good evidence supports that 30minutes is
equivalent to 120 minutes with either T piece or PSV.
The SBT can be used to either assess the patient’s suitability for liberation from MV or used
daily as a weaning strategy. Multiple studies have found that patients tolerant of SBTs were found
to have successful discontinuations at least 77% of the time.
It can be performed using either: Low level Pressure Support (PSV < 7cm H2O), CPAP circuit, or
unassisted via a simple T-piece.
The RSBI is the ratio of frequency of breathing to tidal volume (f/Vt). Rapid shallow breathing as
reflected by f/Vt predicts weaning failure with a threshold of about 105 breaths per minute per litre
(Yang and Tobin). It is less predictive in those ventilated > 8d.It should be measured during the
first minute of a T piece trial using a spirometer to measure Vt. It is of limited value when
measured during trials of pressure support ventilation.
Note: references cited are not expected for marks
Prophylactic NIV: the use immediately after extubation in absence of respiratory failure-High
risk patients may benefit (CHF, COPD, high severity scores).
Ferrer et al , Am J Res and Crit Care Med, 2006 ~ Early NIV avoided respiratory failure and
decreased ICU mortality - in this study NIV appeared useful mainly in a subset of hypercapnic
patients with chronic respiratory disorders.
However, of no benefit if applied indiscriminately in unselected patients, see Su et al, Resp.
Therapeutic NIV: Used post extubation in the presence of established or evolving respiratory
failure- it has no proven benefit in the overall population of patients in this context- it may even
increase mortality by delaying re intubation, see Esteban, NEJM, 2004.
a) a Spontaneous Breathing Trial:
b) rapid shallow breathing index (RSBI):
Specific features of the classical RSBI:
You might not need to use a T-piece trial. A recent study by Zhang et al (2014) suggests that you can use pressure support ventilation (with PEEP = 5 and PS = 5-7); the failure threshold is a value of 75 breaths.min-1L-1
c) prophylactic post-extubation NIV:
The examiners quote several studies in their model answer:
Ferrer, Miquel, et al. "Early noninvasive ventilation averts extubation failure in patients at risk: a randomized trial." American journal of respiratory and critical care medicine 173.2 (2006): 164-170.
Su, Chien-Ling, et al. "Preventive use of noninvasive ventilation after extubation: a prospective, multicenter randomized controlled trial." Respiratory care 57.2 (2012): 204-210.
Esteban, Andrés, et al. "Noninvasive positive-pressure ventilation for respiratory failure after extubation." New England Journal of Medicine 350.24 (2004): 2452-2460.