The college loves to use this topic for hot cases. Assessment for extubation and weaning from mechanical ventilation is a topic which has also appeared in numerous past paper SAQs:
- Question 24 from the second paper of 2014 (SBT, RSBI and extubation on to NIV)
- Question 15 from the first paper of 2009 (standard extubation criteria)
- Question 1d from the first paper of 2001 (causes of weaning failure)
- Question 1c from the second paper of 2000 (weaning a high C-spine quad)
- Question 1d from the first paper of 2000 (weaning a high C-spine quad)
In addition to these, there are also Question 11 from the second paper of 2011 and Question 2a from the second paper of 2004 which are more about extubating a "borderline" patient, somebody with ongoing airway problems but without any significant cardiopulmonary pathology. This is more an issue of difficult re-intubation and anatomically abnormal airways; as such, it is explored in the Airway Management section. This chapter is more concerned with the spontaneous breathing trial, RSBI and the various indices used in the assessment of readiness for liberation from the ventilator.
A list of standard criteria to assess readiness for extubation
This paragraph offers an extubation readiness assessment strategy which is very similar to the one offered in a 2007 Joint Task Force publication (Boles et al), Table 5. Modifications and additions were made on the basis of the author's own experience of (potentially highly idiosyncratic) local practice.
|Airway protection assessment||
|Gas exchange criteria||
|Lung mechanics criteria||
... in the model answer, one may also want to mention that 30% of patients who don't satisfy these criteria can still be successfully extubated. This checklist -if one were using Boles et al as a benchmark - is an assessment for readiness for weaning; after you pass all these criteria, in a protocol-driven unit you would qualify for a spontaneous breathing trial. The SBT is essentially the gold standard diagnostic test to firmly assure everybody that extubation will be a sustained success.
Now, in some detail: the following is a suggested process for extubation assessment which has so far yielded no tragic extubation failure, nor attracted excessive criticism from senior colleagues. It borrows heavily from the AARC statement, embracing their more user-friendly suggestions.
Optimisation of conditions prior to the attempt
- The condition which the patient was intubated for has resolved.
- Otherwise, this is a "one way extubation" and you don't plan to reintubate the patient if they deteriorate.
- The mode of breathing is patient-triggered; eg. PSV
- The patient is able to initiate breaths; ideally this mode has been going on for some time and has been well-tolerated.
- FiO2 is reasonable: 40% or lower
- Obviously, you cant always get what you want. Oxygenation is a poor predictor of weaning success anyway.
- PEEP is reasonable: 5-8 cmH2O
- There is some argument that extubating from a higher PEEP might cause pulmonary oedema. Furthermore, the higher PEEP suggests that the patient is relying on positive pressure to maintain patency of lung units which would otherwise become atelectatic.
- In other words, if you cant wean the PEEP to 5-8, extubating the patient would cause their respiratory function to deteriorate.
- A way to get around this is to extubate onto CPAP.
- There is no reason to anticipate increased respiratory effort or increased respiratory difficulty
- i.e. there is no severe abdominal or chest wall pain (the flail chest and the laparotomy wound are well covered by regional anaesthesia, for example), no metabolic acidosis, no bronchospasm, and the patients respiratory rate is not too high.
- The sedation is minimised (or...can be safely minimised)
- One ought to give their patients the best chance of a good neurological performance at the extubation assessment.
- If the patient is in the grip of some sort of wild thrashing delirium, one can anticipate a rocky periextubation course. One may defer such an extubation until somebody elses shift. One may wish to tinker with the sedation and optimise whatever concurrent encephalopathy.
- The nasogastric feeds have been turned off for 6 hours or so
- Given that this airway manipulation is an elective procedure, one ought to treat the fasting status in the same way one would treat any elective airway procedure.
Assessment of extubation readiness
- The level of consciousness is satisfactory and the patient is cooperative
- There needs to be some minimum of patient cooperation, or at least the promise of adequate airway patency at extubation. You are not going to extubate somebody whose GCS would mandate intubation if they were found on the sidewalk.
- The patient can raise their head off the pillow, and their arms off the bed.
- This is a crude rapid test for muscle weakness. If the patient is able to do this, they probably don't have critical illness neuromyopathy.
- There is an adequate cough.
- If the tracheal suctioning results in a vigorous cough, one can expect secretions dribbling into the carina should do the same. One can rely on such a patient to clear their own sputum, instead of letting it fester in their airway.
- There is an adequate gag.
- If the oropharyngeal suctioning results in a vigorous gag, one can expect oral secretions will also be detected, and the patient will protect their lower airway from their upper airway secretions.
- The patient can generate a good tidal volume with zero pressure support.
- We are talking 10ml per kg of ideal body weight, at least.
- It means they probably have enough available lung tissue to support good ventilation post-extubation
- The patient can overcome a -20 cmH2O pressure trigger
- This is the pressure trigger of a patient-triggered mode of ventilation
- If they are able to generate such a negative pressure, one can make some assumption about the strength of their respiratory muscles.
- There is a satisfactory audible and/or measured cuff leak
- This is controversial. The audible cuff leak is the faint gurgle of escaping air one hears with the cuff down during mechanical inspiration; the measured cuff leak is the difference between the administered gas volume and the end-tidal gas volume (the end-tidal should be at least 15% less than the administered)
- Fisher and Raper from RNSH published a study which demonstrated that a failed cuff leak test does not predict a failed extubation. The presence of a cuff leak was still predictive of extubation success.
- Not everybody believes in cuff leaks. However, the coroner will be interested to know why you did not perform this simple bedside test.
A list of unrealistic criteria
The following is an exhausting monologue of extubation criteria, which in many ways appears ridiculous. Measuring the work of breathing in joules per litre at the bedside may not result in an improvement of your extubation failure rate. However, the AARC has put these together for a reason. And this was a monumental task. I feel the need to pay homage to their effort by summarising their list of criteria. Their statement also mentions that this already massive list is far from complete, and that other criteria exist which for practical reasons have not been included.
- Adequate oxygenation: FiO2 40%
- Normal acid base status (pH >7.25)
- Patient-triggered mode of ventilation
- A respiratory rate < 35 breaths per minute
- Adequate respiratory muscle strength
- Patient able to generate a negative respiratory pressure of -30 cmH2O (though -20cmH2O is probably also adequate)
- Vital capacity over 10ml/kg of ideal body weight
- Pressure measured across the diaphragm < 15% of maximum
- Minute ventilation less than 10 L/min
- Rapid Shallow Breathing Index (RSBI) - respiratory rate to tidal volume ratio of less than 105 breaths per minute
- Modified CROP index (Compliance, Resistance, Oxygenation, Ventilation pressure) more than 0.1 to 0.15 ml x mmHg/breaths/minute/kg
- Thoracic compliance over 25ml/cm H2O
- Work of breathing less than 0.8 J/L
- Oxygen cost of breathing <15% of total
- Dead space to tidal volume ratio (VD/VT) less than 0.6
- Maximum voluntary minute ventilation is twice the value for resting minute volume ventilation
- Peak expiratory flow > 60L/min after 3 attempts
- Sustained maximal inspiratory pressures (SMIP) > 57.5 pressure time units
- Respiratory frequency to tidal volume ratio (f/VT) = 88 breaths/min/L
- Maximal expiratory pressure > 28 cmH2O
- Appropriate level of consciousness
- Adequate airway protection reflexes
- Adequate cough strength
- Easily managed secretions
- Cuff leak percentage ( difference between inspired volume and end-tidal measured volume) is > 15.5%
- The nasogastric feeds have been stopped for > 6 hours
- The process which required intubation in the first place has resolved.
- No immediate need for reintubation is anticipated.
- Issues regarding airway difficulty have been considered.
- There is hemodynamic stability
- There is stable nonrespiratory organ function
- Electrolytes are well corrected
- The patient is not enfeebled by malnutrition, and respiratory muscle power is adequate.
The Rapid Shallow Breathing Index
The RSBI is the ratio of frequency of breathing to tidal volume (fR/VT). Yang and Tobin discovered this relationship in 1991; their NEJM paper is mentioned in the model college answer to Question 24 from the second paper of 2014. Yang and Tobin looked at several indices, and found that RSBI had the best sensitivity and specificity (1.00 and 0.64, respectively).
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
The Spontaneous Breathing Trial
The glorious Task Force (Boles et al, 2007) define a spontaneous breathing trial as "assessment of the patient’s ability to breathe spontaneously" which is a ridiculously unhelpful definition. Essentially, an SBT is the process of taking a still-intubated patient and then simulating the workload of spontaneous extubated breathing with the tube still in situ.
The followuing issues were raised in the model college answer to Question 24 from the second paper of 2014:
- 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.
How does one know that the SBT has failed? The following is a list of features suggesting that the patient is not coping with the work of spontaneous breathing:
- 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