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:
- "Assessment of the patient’s ability to breathe spontaneously" - Boles et al (2007)
- The simulation of extubated respiratory workload in a still-intubated patient
- Support during the SBT: Low level Pressure Support (PSV < 7cm H2O), CPAP circuit, or
- unassisted via a simple T-piece- all of these seem to be equivalent.
- Duration of the SBT: the college says 30 minutes is equivalent to 120 minutes. This is probably based on a 1999 study by Esteban et al, who compared two 30- and 120-min trial groups (about 250 patients in each) and found no difference between them in terms of outcome or reintubation.
- SBT failure is identified by the following features:
- Agitation and anxiety
- Evidence of increasing respiratory effort
- Hypoxia (eg. SpO2 <90%)
- Hypercapnea (eg. PaCO2 >50mmHg)
- Unsatisfactory RSBI: an fR/VT more than 105 breaths.min-1L-1
- Resp rate over 35/min, or increased by more than 50%
- Hypotension, hypertension, or tachycardia
- Cardiac arrhythmia
b) rapid shallow breathing index (RSBI):
- The RSBI is the ratio of frequency of breathing to tidal volume (fR/VT).
- The seminal Yang and Tobin (1991) paper is mentioned: they found that RSBI had the best sensitivity and specificity (1.00 and 0.64, respectively) for extubation failure, among all tested predicitive indices.
Specific features of the classical RSBI:
- You have to be on a T-piece (that's what Yang and Tobin used)
- You have to measure it during the first minute of the SBT
- You are supposed to use a spirometer
- The calculation is: resp rate divided by VT in litres
- Thus, if you have a resp rate of 30 and your VT is 300ml (0.3L), your RSBI is 100 and you have almost but not quite completely failed your SBT.
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 et al (2006): RCT; 162 patients who tolerated a spontaneous breathing trial (i.e. not in respiratory failure) who had at least one risk factor for post-extubation respiratory failure (eg. age over 65, APACHE-II score over 12 on the day of extubation, or cardiac failure as the cause of intubation). The investigators found reduced rates of respiratory failure in the electively NIVed group, but the 90 day mortality was only improved in the hypercapneic group.
- Su et al (2012): RCT, 406 patients who tolerated a spontaneous breathing trial randomised to NIV or standard care, looking at their rate of reintubation at 72 hours. There were no differences in extubation failure (13.2% in control and 14.9% in NIV), intensive care unit or hospital mortality. The conclusion reached by these investigators was that routine use of elective NIV will not prevent extubation failure.
- Esteban et al (2004): RCT, 221 unselected patients being extubated - randomised to NIV or standard therapy. There no difference in reintubation rate. However, recruitment was terminated early when interim analysis discovered an increased mortality rate in the NIV group, and a delay to intubation (i.e. more time was wasted on NIV, from the diagnosis of respiratory failure until the pointlessly delayed reintubation). However, only 10% of the patients in this study had COPD- the outcome would probably have been different if all of them had COPD, as in this highly positie Cochrane review of immediate elective NIV in extubated COPD patients.
- Cooperative hypercapneic high-risk patients may benefit from elective NIV immediately following extubation.
- COPD patients probabyl benefit the most
- Normocapneic patients may still benefit, but their mortality may not be affected.
- Patients without risk factors for post-extubation respiratory failure will not benefit.
- There is no point in doing this routinely, and it may actually be dangerous, as it may delay re-intubation, and promote aspiration.
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.