In a recent post, we discussed the process of determining whether a patient is safe for ventilator weaning. Once a patient is determined safe, a spontaneous breathing trial (SBT) can be done. SBT is usually conducted in one of two ways — minimal ventilator support (pressure support ventilation) and T-piece trial. This article will discuss these two different modalities and the data supporting each. Keep in mind that daily SBTs are still one of the main components of vent weaning as they shorten the duration of mechanical ventilation, but the type of SBT is what has remained in debate.
What is pressure support ventilation?
Pressure support ventilation (PSV) is a patient triggered, pressure limited mode where the generated airway pressure at inhalation, and pressure rise time are set by the operator. This mode augments the volume of a spontaneous breath by selecting an inspiratory pressure that overcomes the impedance of the endotracheal tube. The flow given to the patient is stopped when it decreases to a set percentage (usually 25%) of the peak flow. The tidal volume delivered depends on the flow and duration of the inspiratory phase. The system is patient-cycled, which means that the duration of inspiration, expiration, and the total cycle are determined by the patient. The flow delivered to the patient depends on the set pressure, airway resistance, lung compliance, and inspiratory effort of the patient. Dyssynchrony can occur with the ventilator if the flow where the ventilator is cycling does not match with the patient’s intrinsic respiratory cycle.
If the patient becomes apneic or has a respiratory rate lower than the preset number on the ventilator, the ventilator will initiate a backup ventilation and alarm to alert the clinician. It would be safe to say that the patient has likely failed the SBT when this occurs. It should also behoove the physician to check for lingering analgosedation or examine what the patient’s “normal” CO2 is. For example, in a COPD patient who chronically has a high CO2 and higher tidal volumes, may not adjust to the backup rate that is preset on the ventilator. Adjusting for these circumstances and trying an SBT again may be beneficial.
Initial settings for this mode are an inspiratory pressure of 5-8 cm H2O, PEEP 5-8 cm H2O and an FiO2 less than or equal to 40%.
A major benefit of this mode is that it may overcome the resistance offered by the endotracheal tube to get a good idea of how the patient would do post extubation. The trial on PSV is anywhere from 30 minutes to 2 hours, with a shorter duration being better tolerated.
What is a T-piece trial?
A T-piece trial is done by disconnecting the patient from the ventilator and attaching a T-piece to the endotracheal tube. There is no ventilator support in this modality, but there is oxygen supplementation.
The thought behind this type of SBT is that a T piece requires the same amount of work as breathing without a mechanical ventilator. What it does not take into account is the resistance of the endotracheal tube that the patient has to overcome to attain a breath. Supporters of this mode of SBT also argue that if a patient does well with a T piece, they are strong enough to overcome the resistance of the endotracheal tube, and therefore, will do well with extubation. However, a study done by Stratus et al showed that the mean resistance of the supraglottic airway post extubation was actually similar to the resistance that a 7.0mm endotracheal tube applies. If your patient has a smaller ETT, this may increase the resistance.
Another part of a T piece SBT is there is no PEEP. Patients with heart failure and pulmonary edema, who require this positive pressure may falsely do well on a PSV because of the minimal PEEP that is given. Supporters of the T piece trial argue that PEEP is taken away in this mode of SBT, which is a more accurate marker of how this type of patient will do when extubated.
Which SBT is better? Let’s look at the evidence behind the two types of SBT.
In 2014, a Cochrane systematic review concluded that there is no difference between T-piece trials and pressure support trials regarding extubation failure and mortality albeit with a low quality of evidence. However, PSV was found to be superior to T-piece in the proportion of successful SBT in patients considered to have a “simple weaning process.” They also mention that the T-piece approach can be used if there is “borderline performance” with other techniques or there are concerns that loss of PEEP that is received from PSV could cause cardiac or pulmonary issues. At this point of examining the literature, decisions regarding the type of SBT are left up to the clinician to decide without clear cut guidelines.
Since then, there have been several studies that have examined these two modalities.
In 2019, Subira et al published an RCT in JAMA examining these two types of SBTs. In this randomized clinical trial, out of 1153 adults receiving mechanical ventilation, the proportion of patients successfully extubated was 82.3% in those who received 30 minutes of PSV compared with 74% in those who received 2 hours of T-piece (P value of 0.001).
Unfortunately, this study examined the length of time in an SBT with type of SBT without direct head to head comparisons. These findings support the use of a shorter, less demanding strategy of 30 minutes of pressure support ventilation for spontaneous breathing trials overall, but it does not particularly comment on the type of SBT that is preferred. Among secondary outcomes, for the pressure support ventilation group vs the T-piece group, respectively, reintubation, length of stay in the ICU, median hospital length of stay did not statistically differ. Hospital mortality was 10.4% vs 14.9% (difference, −4.4%; P = .02), and 90-day mortality was 13.2% vs 17.3% (difference, −4.1%, P = .04].
One of the critiques of this study is that in this trial, clinicians, who were not blinded to treatment assignment, made greater use of prophylactic respiratory interventions in the PSV group (24.7%), such as noninvasive ventilation and high flow nasal cannula use than the T-piece group. It is unclear what the exact circumstances were and if the use of these prophylactic measures helped save the patient from a re-intubation or made no difference. Another critique is that the study only examined patients at low risk for re-intubation to begin with.
In October 2020, CHEST published a post hoc analysis of a multicenter trial examining 243 high risk patients that underwent PSV at 7cm H2O with 0 PEEP and 398 patients who underwent a T-piece SBT. The primary outcome was the proportion of patients who were extubated successfully 72 hours after the initial SBT and not reintubated. High-risk patients were identified as patients older than 65 years of age or with underlying cardiac or respiratory disease. The proportion of patients successfully extubated 72 hours after initial SBT in the PSV group was 67% (162/243 patients) and in the T-piece group, it was 56% (223/398 patients) with p value at 0.0076. The proportion of patients extubated after initial SBT in the PSV group was 77% (186/283 patients) and the T-piece group was 63% (249/398 patients) for a p value of 0.0002.. Reintubation rates within 72 hours did not differ. While this study looked at high risk patients (one of the high risk categories being patients with COPD), it did not specify how severe the disease was in terms of amount of emphysema or the FEV1. This can alter the lung mechanics and change how a patient will perform with SBTs.
In February 2020, Critical Care published a meta-analysis looking at 10 RCTs with a total of 3165 patients. The results of this meta-analysis showed that there was no significant difference in the successful extubation rate or rate of reintubation between the T-piece group and PSV group. Furthermore, there was no difference in the ICU mortality, hospital mortality, ICU length of stay, and hospital length of stay.
The pressure used in these studies varied from 5-18cm H2O and the PEEP varied from 0-8cm H2O, which is highly variable.
The latest American Thoracic Society guidelines for weaning recommend PSV spontaneous breathing trials instead of T-piece with moderate-quality evidence.
This simply means that more studies need to be done that examine these two methods of doing SBTs.
While no study has shown a clear cut benefit between PSV and T-piece trials, it has been shown that the length of time of an SBT should not be longer than 2 hours, and should ideally be at least 30 minutes. The shorter the time of SBT, the better tolerated the extubation will be.
As with all things in medicine, each patient should be thought of as a unique case and these studies should be used as guidelines, but not hard evidence and rules for how an SBT should be done. Remember the etiology of the respiratory failure of your patient, his or her current respiratory and cardiac situation, along with all the points we discussed in the last article before making an educated and thoughtful decision on which type of SBT you will do. Remember that the landing (and how the landing is done) is just as important as the take off when it comes to extubation!
- Friedrich JO, Burns KEA. Spontaneous Breathing Trials and Successful Extubation. JAMA. 2019;322(17):1716.
- Li, Y et al. Comparison of T-piece and pressure support ventilation as spontaneous breathing trials in critically ill patients: a systematic review and meta-analysis. Crit Care 24, 67 (2020).Thille AW et al. Pressure-Support Ventilation vs T-Piece During Spontaneous Breathing Trials Before Extubation Among Patients at High Risk of Extubation Failure: A Post-Hoc Analysis of a Clinical Trial. Chest. 2020 Oct;158(4):1446-1455.
- Straus C et al. Contribution of the endotracheal tube and the upper airway to breathing workload. Am J Respir Crit Care Med. 1998 Jan;157(1):23-30.
- Subirà C, Hernández G, Vázquez A, 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;321(22):2175–2182.
- Zein H, Baratloo A, Negida A, Safari S. Ventilator Weaning and Spontaneous Breathing Trials; an Educational Review. Emerg (Tehran). 2016;4(2):65-71.