Spontaneous breathing trial and post-extubation work of breathing in morbidly obese critically ill patients

Extubation is a critical decision in the Intensive Care Unit (ICU). Extubation failure may occur in up to 20 % [1] of patients and is associated with morbidity. Excessive and non-sustainable work of breathing (WOB) is likely a major reason for extubation failure [2‐5]. Evaluation of how the critically ill patient is breathing with no assistance or a minimal level of assistance (the period known as the weaning test or the spontaneous breathing trial) [4] is therefore recommended before extubation [3, 4, 6, 7]. Different weaning tests are suggested for non-selected adult patients: a T-piece trial (oxygen supply without positive pressure), continuous positive airway pressure (CPAP) and low pressure support ventilation (PSV), with a low level of PSV, from 5 to 8 cmH₂O, to compensate for the imposed workload due to the ventilator circuit [3, 4, 6, 7]. Although these weaning tests are not equivalent in term of the WOB [8, 9] and studies are underpowered to assess the risk of extubation failure, they are recommended to assess whether a patient is ready to be extubated [3, 6].

Predicting whether an obese critically ill patient can be successfully extubated may be specially challenging. Obesity decreases respiratory system compliance, inspiratory and expiratory lung volumes, functional residual capacity, upper airway mechanical function and neuromuscular strength [10]. Moreover, in obese patients, oxygen consumption is increased, with a high proportion of this consumption spent in the WOB [11‐13]. Although the T piece, CPAP and low PSV levels have been used to reproduce post-extubation conditions in non-selected critically ill patients, the weaning test modality that would best reproduce post-extubation inspiratory effort (WOB and pressure time product indexes) in obese critically ill patients has never been evaluated and many clinicians are worried about using no support during the test [14, 15].

The aim of our study was thus to assess which weaning test would best reproduce post-extubation inspiratory effort in obese critically ill patients. We compared a T-piece trial to weaning tests with PSV 7 and positive end-expiratory pressure (PEEP) 7 cmH₂O; PSV 0 and PEEP 7 cmH₂O; PSV 7 and PEEP 0 cmH₂O; PSV 0 and PEEP 0 cmH₂O, in this particular population. We hypothesized that the T-tube or PSV 0 and PEEP 0 cmH₂O would best approximate the post-extubation WOB.

To our knowledge, this is the first physiological study that specifically investigates the inspiratory effort during weaning of mechanical ventilation in a population of critically ill morbidly obese patients. The main result of this study is that for obese patients, the T piece and PSV 0 + PEEP 0 cmH₂O weaning tests are the tests that best predict post-extubation inspiratory effort and WOB.

Because of a lack of consensus on the best test to use before extubation in this population, we aimed to determine which one reflects the breathing effort after extubation. Some authors described extubation of obese patients after a 30-minute period of CPAP 5 cmH₂O [14], others after a trial of FiO₂ 100 % combined with a CPAP of 10 cmH₂O. [15] An ongoing multicenter observational study in France (FREEREA study), will provide some epidemiological data about weaning and extubation in this particular population. The preliminary results (unpublished) show that among 64 critically ill morbidly obese patients extubated, 22 (34 %) were extubated after a T tube, 28 (44 %) after a low PSV trial, 12 (19 %) with no spontaneous breathing trial and 2 (3 %) after a different weaning trial. These data justify our study as there is wide heterogeneity of extubation practice in this population, with a high proportion of patients being extubated from a substantial level of support.

Our study presents limitations. First, we investigated the inspiratory effort indexes twenty minutes after extubation and the study was not designed to explore long-term consequences of several weaning tests on oxygenation, end-expiratory lung volume or outcome. Because outcome was not a study endpoint, we cannot make any final recommendation about which weaning test is associated with the highest rate of weaning success. Ideally, a weaning test would perfectly predict the ability of the patient to breathe alone and without being ventilatory assisted by simulating the post-extubation respiratory constraint [26]. Second, post-extubation intermittent non-invasive ventilation is routinely used in our unit for high-risk patients [14, 21] to rest the inspiratory muscles and improve lung aeration. It may have contributed to our low rate of re-intubation (6 %).

The present study focused on morbidly obese patients and found results consistent with the studies published by Straus et al. [25] and Cabello et al. [8], which included non-obese patients. We report that the T piece and PSV 0 + PEEP 0 cmH₂O weaning tests were the two tests that best approximated the WOB after extubation. We also found that the PSV 7 + PEEP 0 cmH₂O test leads to a major underestimation of the WOB after extubation in obese patients with significantly less inspiratory effort in comparison with both the T piece test and 20 minutes after extubation. Straus et al demonstrated that post-extubation WOB was well-approximated by the WOB during a T-piece test and that the endotracheal tube was responsible for about 11 % of the total work of breathing. [25] More recently, Cabello et al. compared a spontaneous breathing trial on a T-piece with low PSV (7 cm H₂O) with or without PEEP in a subpopulation of patients with heart failure who were difficult to wean. [8] The authors concluded that performing the weaning test while maintaining a positive pressure in the circuit underestimates the post-extubation WOB and unmasks a possible effect on left ventricular function, and suggested the T piece as the weaning test of choice in these patients.

In a landmark physiological study, Brochard et al. demonstrated that breathing through the T piece overestimates the WOB by 27 ± 18 % compared to the post-extubation period [26]. Contrary to the present study, Brochard et al. included a high proportion of patients with chronic obstructive pulmonary disease, and used ventilators with higher ventilatory circuit-resistive load [28] and lower pressurization performance, especially in terms of inspiratory-trigger-imposed WOB [29, 30].

As compared to the literature on non-obese patients, WOB values evaluated in the present study were higher [26, 31]. In morbidly obese patients, an elevation of pharyngeal collapsibility and upper airway resistance related to fatty deposits on pharynx and oral soft tissue and associated with local inflammation can increase the WOB [32]. Weaning trials performed with positive pressure underestimated post-extubation WOB by 33 % (0.5 J/L) up to 50 % (0.8 J/L) according to the ventilator setting. An increase of 0.5–0.8 J/L represents a significant additional workload, as WOB in healthy subjects during quiet breathing is about 0.35–0.5 J/L [33, 34]. Furthermore, WOB ≥0.8 J/L has been reported as being associated with weaning failure [35]. Extubating an obese patient after having performed a weaning test without positive pressure could lead to early onset atelectasis if the patient was unable to control for end-expiratory lung volume without PEEP.

For the first time the present study reports new insights into respiratory physiology in morbidly obese critically ill candidates to be weaned from the ventilator. These data may be useful for clinicians managing these challenging patients and help make difficult decisions about extubation. We report that either a T piece or a PSV 0 and PEEP 0 cmH₂O test are the trials that predict post-extubation work of breathing in morbidly obese patients. The consequences on mid-term oxygenation and lung aeration, and on the weaning success rate of such weaning tests were, however, not studied.