Weaning from mechanical ventilation is an essential part of the caring for critically ill, intubated patients, accounting for approximately 40% of the time spent on mechanical ventilation1,2. This is a percentage which some researchers believe may be significantly higher in patients with more slowly resolving lung disease processes3.
Unnecessary delays in the discontinuation of ventilation may result in increased costs, as well as higher rates of complications, such as airway trauma and pneumonia, as well as morbidity and mortality.3 Because of this, it is vital to ensure a patient is removed from ventilation at the earliest possible, safe point3. Yet, at the same time, premature discontinuation carries its own unique risks, including compromised gas exchange and difficulty in reestablishing the artificial airway3.
This makes optimal determination of when a patient is ready to be liberated from a ventilator vital to providing the highest level of care. Fortunately, hospitals and physicians have an important tool that can help them in the pursuit of that goal, a spontaneous breathing trial (SBT), which assesses a patient’s ability to breathe spontaneously with minimal or no support.
In this article, we’ll discuss the timing of SBT initiation. We’ll also take a look at the different types of trials that may be used, the appropriate duration of the trial and how to determine success or failure to eliminate common sources of confusion.
When an SBT should be considered
Because prolonged mechanical ventilation is associated with significant morbidity and mortality, liberation should be considered as early as possible, keeping in mind patient safety1. Criteria to assess readiness to wean are made up of both subjective and objective measurements1,3.
Subjective criteria include:
- Adequate cough
- Absence of excessive tracheobronchial secretions
- Resolution of disease acute phase for which the patient was intubated
- No neuromuscular blocking agents
- Adequate mentation on sedation or no continuous sedation infusion
Objective assessment includes:
- Stable cardiovascular state, including systolic BP 90–160 mmHg and heart rate ≤140 beat/minute
- No or minimal vasopressors or intropes (< 5µg/kg/minute dopamine or dobutamine)
- Afebrile (36° C < temperature < 38° C)
- No active myocardial ischemia
- Adequate hemoglobin level (≥8 g/dl)
Additionally, the patient should be assessed for adequate pulmonary function and oxygenation including:
- Tidal volume >5 mL/kg
- Vital capacity >10 mL/kilogram
- Proper inspiratory effort
- Respiratory rate ≤35/minute
- PaO2 ≥ and PaCO2 ≤ 60 mm Hg
- Positive End Expiratory Pressure (PEEP)≤8 cmH2O
- No significant respiratory acidosis (pH ≥7.30)
- Maximal inspiratory pressure(MIP) ≤ -20 – -25 cmH2O
- O2 saturation > 90% on Fi02 ≤ 0.4 (or PaO2/Fi02 ≥200)
Once a patient meets the above criteria, an SBT should be considered. However, these criteria are not rigid requirements and many patients, who do not meet them, are able to be successfully liberated from mechanical ventilation1.
Additionally, weaning readiness may be assessed using the Rapid Shallow Breathing Index (RSBI), which is performed in the absence of ventilatory support and calculated by comparing the frequency of breathing (respiratory rate) to tidal volume (RR/Vt)4. An RSBI of less than 105 is the traditional threshold for movement to an SBT and the first true weaning attempt1.
However, Goharani et al found that an RSBI of 85 best predicts extubation success in patients with chronic obstructive pulmonary disease (COPD) with hypercapnic respiratory failure, yielding a 95.5% probability of extubation success, independent of ventilation duration or hospital length of stay5.
A study by Huo et al found that using the above criteria, including a new parameter known as Weaning Index (WI), is a better predictor for weaning as compared to RSBI alone since the WI takes into account the compliance of the respiratory system, minute ventilation and RSBI.6. Their study randomly allocated patients with respiratory failure to either an assist/control (AC) group or a pressure support ventilation (PSV) group and screened to be weaned daily when the initial reason for being mechanically ventilated was improved or resolved. 6
Their results showed that:
- A WI of less than 50.44 led to a sensitivity and specificity of predicting weaning success of 72% and 98%, respectively.
- A WI in the A/C group of less than 61.45 equated to a sensitivity and specificity of predicting weaning success of 98% and 72%, respectively.
- A WI in the PSV group of less than 51.45 equated to a sensitivity and specificity of predicting weaning success of 74.6% and 100%, respectively.
SBT types
The ability of a patient to breathe spontaneously may be assessed via a number of different types of SBTs prior to determine weaning readiness. These include7:
- Synchronized Intermittent Mechanical Ventilation (SIMV)
- Continuous Positive Airway Pressure (CPAP)
- Automatic Tube Compensation (ATC)
- Pressure Support Ventilation (PSV)
- T-tube (T-piece)
Is there a preferred SBT method that should be used?
Reviews by Alia, Butler, Cook, Caroleo and Meade between 1991 and 2007 found SIMV to be the least-preferred method, making up just 1.6% of SBTs performed7.
While some SBT techniques deliver pressure during inspiration to overcome endotracheal tube resistance (such as PS and ATC), other techniques may offer improvements in respiratory mechanics or cardiac function. Because of this, the latter types, such as CPAP, may overestimate the patient’s ability to breathe on their own post-extubation2.
On the other hand, use of the T-piece for an SBT, which is performed by disconnecting the patient from the ventilator to allow them to breathe spontaneously through the simple T-piece, provides no support. This is perceived to increase the work of breathing and therefore may underestimate the patient’s spontaneous breathing ability2.
However, according to Boles et al in “Weaning from mechanical ventilation”, little difference has been found in either the percentage of patients who pass the SBT or the percentage of patients successfully extubated when comparing T-tube trials to the use of PS or CPAP1.
Additionally, the authors concluded that ATC, which adjusts for the assumed resistance of the endotracheal tube, is at least as successful as either PS or T-tube1.
Yet, the debate has not been settled for a final time, as other studies have found differences in success depending on the type of SBT chosen to evaluate weaning readiness.
A review of 31 trials by Burns et al found that2:
- Patients undergoing PS compared to T-piece SBTs were more likely to be extubated successfully
- ATC plus CPAP versus CPAP alone or PS increase passing rate of SBT, but not extubation success
Additionally, while Ladeira et al found three studies which suggested a shorter weaning duration in patients undergoing PS SBT, they also point to a fourth study, which found a shorter weaning duration with a T-tube7.
SBT plus SAT, is there a better way?
Finally, we can look to the Coordinated Spontaneous Awakening and Breathing Trials Protocol8. This protocol is based on the model Girard et al developed in 2008. The model uses coordinated spontaneous awakening trial (SAT) and SBT, which was found to significantly reduce the number of days patients are on mechanical ventilation (3.1-day reduction, 95% confidence interval 0.7–5.6; p=0.02), as well as reduce the length of hospital stay (4.0 days difference) when compared with SBT alone.
The protocol begins with the SAT which consists of two distinct parts, a safety screen and the trial itself. For a patient to pass the safety screen, the following factors must not be present8:
- Receiving sedative infusion for active seizures or alcohol withdrawal
- Receiving increasing doses of sedative for agitation
- Receiving neuromuscular blockers
- Evidence of active myocardial ischemia in prior 24 hours
- Evidence of increased intracranial pressure
Upon passing the safety screen the patient is removed from all sedatives, and analgesics other than analgesics used to control pain. Then the patient is assessed for their ability to complete three out of four simple tasks, when requested, including8:
- Opening their eyes
- Looking at their caregiver
- Squeezing the hand
- Sticking out their tongue
The patient may also be considered to pass the trial if they are capable of going without sedation for four or more hours without experiencing8:
- Sustained anxiety, agitation or pain
- Respiratory rate of 35 breaths/minute for at least 5 minutes
- Oxygen saturation (SpO2) of less than 88% for at least 5 minutes
- Acute cardiac dysrhythmia
- Two or more signs of respiratory distress including:
- Tachycardia
- Bradycardia
- Use of accessory muscles
- Abdominal paradox
- Diaphoresis
- Marked dyspnea
Upon passing the SAT, the patient is then assessed for SBT safety for the subjective and objective criteria, as well as adequate oxygenation measurements discussed above8.
SBT duration
While the debate on which type of SBT is best is on-going, the issue of SBT duration may help to settle the issue. SBTs ranging from 30 minutes to two hours in duration for simple and difficult weaning cases have been found to offer similar outcomes in many cases7. However, important insight on the value of a shorter duration, using PSV rather than a longer duration T-tube SBT comes from Subirà and colleagues.
The team conducted a randomized clinical trial from January 2016 to April 2017 among 1,153 adults deemed ready for weaning after at least 24 hours of mechanical ventilation at 18 intensive care units in Spain9,10.
The results demonstrated that a 30-minute PS SBT resulted in a significantly higher 72-hour successful extubation rate than a 2-hour T-piece SBT, with similar extubation failure rates in both groups9,10. Additionally in comparing the PS to T-tube group, hospital mortality was 10.4% vs. 14.9%, and 90-day mortality was 13.2% vs. 17.3%.
The results were backed up by work by Boles et al who concluded that patients, who fail an SBT, will do so within the initial 20 minutes of the trial1. Therefore, whether a trial is 30 minutes or 120 minutes, they may be expected to have similar success rates. The only difference is the time and strain experienced by the patient, therefore, a trial of 30 minutes may be indicated.
Determining SBT success or failure
No matter which SBT is utilized, the following suggest a failed test4:
- Increases in respiratory rate
- Decreases in oxygen saturation
- Changes in tidal volume, heart rate or blood pressure
- Patient anxiety or discomfort
On the other hand, a consistent respiratory pattern, adequate gas exchange, hemodynamic stability and subjective comfort combine to form the criteria for a successful SBT1. These include11:
- Respiratory rate <35 breaths/minute
- Good tolerance to spontaneous breathing trials
- Heart rate <140 / minute or heart rate variability of >20%
- Arterial oxygen saturation >90% or PA02 >60 mmHg on FiO2 <0.4
- 80 < systolic blood pressure < 180 mmHg or <20% change from baseline
- No signs of increased work of breathing or distress including:
- Accessory muscle use
- Paradoxical or asynchronous rib cage – abdominal movements
- Intercostal retractions
- Nasal flaring
- Profuse diaphoresis
- Agitation
Summary
- Prolonged mechanical ventilation is associated with increased morbidity and mortality
- To ensure readiness to liberate from ventilation, physicians may utilize an SBT
- There are a number of SBT types, including PS, T-tube and ATC
- A 30-minute PS SBT may be preferred over a 2-hour T-tube SBT for improving extubation success
- Determining SBT success or failure must take into account respiratory and hemodynamic stability and gas exchange, as well as subjective patient measures
References
1: Boles, JM et al. “Weaning from mechanical ventilation.” European Respiratory Journal. 2007 29: 1033-1056;
2: Burns KEA et al. “Trials directly comparing alternative spontaneous breathing trial techniques: a systematic review and meta-analysis.” Crit Care. 2017 Jun 1; 21(1):127.
3: MacIntyre, Neil et al. “Evidence-Based Guidelines for Weaning and Discontinuing Ventilatory Support.” Chest. 2001, Vol. 120, No. 6.
4: Lewis, Kimberley A. et al. “Comparison of Ventilatory Modes to Facilitate Liberation from Mechanical Ventilation: Protocol for a Systematic Review and Network Meta-Analysis.” BMJ Open 1 Sept. 2019. BMJ Open. Web.
5: Goharani, Reza et al. “A rapid shallow breathing index threshold of 85 best predicts extubation success in chronic obstructive pulmonary disease patients with hypercapnic respiratory failure.” Journal of Thoracic Disease. 2019 April. Vol 11, No 4.
6: Huo, Ysan et al. “Predictive efficacy of weaning index on mechanical ventilation evacuation.” Annals of Palliative Medicine. 2021 Jan. Vol 10, No 1.
7: Ladeira MT et al. “Pressure support versus T-tube for weaning from mechanical ventilation in adults.” Cochrane Database Syst Rev. 2014 May 27; 2014(5):CD006056. doi: 10.1002/14651858.CD006056.pub2. PMID: 24865303; PMCID: PMC6492521.
8: “Coordinated Spontaneous Awakening and Breathing Trials Protocol.” Agency for Healthcare Research and Quality, https://www.ahrq.gov/hai/tools/mvp/modules/technical/sat-sbt-protocol.html. Accessed 25 April 2022.
9: Perren, Andreas and Brochard, Laurent. “The importance of timing for the spontaneous breathing trial.” Annals of Translational Medicine. Sept. 2019. Vol. 7, Supplement 6.
10: Subirà, C et al. “Effect of Pressure Support vs T-Piece Ventilation Strategies During Spontaneous Breathing Trials on Successful Extubation Among Patients Receiving Mechanical Ventilation: A Randomized Clinical Trial.” JAMA. 2019 Jun 11; 321(22):2175-2182. doi: 10.1001/jama.2019.7234. Erratum in: JAMA. 2019 Aug 20; 322(7):696. PMID: 31184740; PMCID: PMC6563557.
11: Zein, Hossam & Baratloo, Alireza & Negida, Ahmed & Saeed, Safari. (2016). Ventilator Weaning and Spontaneous Breathing Trials; an Educational Review. EMERGENCY. 4.
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