Most parameters of respiratory mechanics are useful only for predicting successful SBT and perform moderately or poorly with extubation prediction. We therefore present a brief discussion of these parameters.
Rapid shallow breathing index (RSBI - f/V T )
Capdevila [16] found significantly lower values of f/V T (50 ± 23 vs. 69 ± 25 breaths/min/l) in successfully extubated patients. Recently a cut off value of RSBI ≥ 57 was described [22] to separate patients who could not be extubated successfully. Epstein, [27] however, found that with f/V T < 100, 14 (of 84) patients failed extubation, 13 due to other organ system problems and suggested that f/V T was not physiologically suited to predict extubation failure. Other studies [2],[20],[29] have also reported inability of f/V T for predicting extubation outcome.
Airway occlusion pressure at 1 s (P 0.1 ) and Ratio of occlusion pressure to maximum inspiratory pressure (MIP) [P 0.1 /MIP]
P 0.1 /MIP is an index of balance between respiratory reserve and demand and reflects neuromuscular drive for breathing and it is unaffected by respiratory compliance or resistance. Capdevila and colleagues [16] reported that patients with low P 0.1 and P 0.1 /MIP failed extubation, Mergoni and colleagues [30] reported excellent prediction of success in weaning using P 0.1 /MIP, while Del Rosario [31] found similar P 0.1 /MIP values in patients with weaning success and failure. In a metaanalysis, [26] P 0.1 /MIP ratio of >0.3 had a pooled likelihood ratio of 2.3, indicating increased chances of successful extubation. Despite excellent predictive accuracy, the role of P 0.1 /MIP ratio may be limited in most ICUs due to need for a special device.
Minute ventilation recovery time (V E RT)
Minute ventilation recovery time (V E RT) allows physiologic assessment of work imposed after SBT. Thus V E RT may identify patients with better respiratory muscle reserve, capable of sustaining spontaneous breathing following extubation. Martinez and colleagues, [2] after a 2-h SBT, placed patients back on their pre-SBT ventilator settings for up to 25 min and measured various respiratory parameters including minute ventilation (V E ) at three intervals: baseline over preceding 24 h (pre-SBT), post-trial (after SBT) and recovery (return to baseline). Patients were assumed to recover when V E decreased to 110% of the predetermined baseline. V E RT of patients with successful extubation was significantly shorter than those who failed extubation (3.6 ± 2.7 min vs. 9.6 ± 5.8 min, P < 0.011). On multivariate analysis, V E RT was an independent predictor of extubation outcome and correlated with ICU LOS ( P < 0.01). Prolonged V E RT may reflect either a limited respiratory reserve or an unrecognized, underlying disease process. Seymour and colleagues [32] evaluated a more practical method. For pre-SBT V E they collected data in three ways, a 24-h nadir (as in previous study), an 8-h average and the last V E measurement prior to SBT. They also collected data on V E RT with threshold of 100% and 110%. They found that both, the 8-h average V E and immediate pre-SBT value of V E, were in close agreement with the original method. Similarly 100% threshold for V E RT also correlated well with 110% threshold. The same group later demonstrated [33] the utility of the new method in predicting extubation success. Recently Hernandez and colleagues [34] evaluated the utility of close observation of V E during the recovery phase after the SBT. Both V E RT RT50% ∆V E (recovery time needed to reduce V E to half the difference between End-SBT- V E and basal V E ) were significantly lower in successfully extubated patients. They found that a threshold of RT50% ∆V E of seven minutes was useful to discriminate between extubation failures and successes. V E RT and derivatives appear to be promising tools, the drawback being their inability to identify patients with possible upper airway compromise.
Work of breathing (WOB)
Kirton and others [35] found that patients who fail SBT due to increased imposed work of breathing (WOB) secondary to ventilatory apparatus and endotracheal tube, but have normal physiological WOB, can be successfully extubated. The same group later showed [36] that when physiological WOB ≤0.8 J/l, patients can be successfully extubated. Automatic Tube Compensation (ATC), a form of variable pressure support, was shown [37] to improve extubation success by reducing imposed work of breathing. WOB, a promising predictor; remains confined to the research setting due to technical difficulties.
Displacement of liver/spleen
Diaphragm fatigue results in slower movement and reduced excursion. Jiang and colleagues [38] hypothesized that liver and spleen displacement during SBT can be a surrogate of diaphragmatic endurance. In 55 ICU patients, two separate observers measured the displacement of liver and spleen by ultrasonography. Patients were extubated by clinicians blinded to the study results. Patients who were successfully extubated had higher mean values. With a cutoff value of 1.1 cm, the sensitivity and specificity to predict successful extubation was 84.4% and 82.6%. This is a noninvasive test and can be done bedside, but needs expertise.extu
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