Accuracy of Tidal Volume-Diaphragmatic Contraction Velocity for Predicting Weaning Outcome

The rapid shallow breathing Index (RSBI) has been in use for more than 34 years and is the most commonly used objective parameter to predict extubation success.¹ The RSBI is highly sensitive. A 2022 meta-analysis showed a sensitivity of 83%, although its specificity is poor at 58%, and it has led to numerous failed extubation attempts.² The placement of esophageal balloon catheters to measure both esophageal pressure swings (ΔPes) and pressure-time product of esophageal pressure (PTPes) as surrogates for transpulmonary pressure has increased sensitivity for extubation success to 89%.³

However, an esophageal balloon catheter is invasive and position dependent, requires specific training, and the interpretation of resultant data is not straightforward. So, experts note that use of either parameter alone is not superior to RSBI.⁴ One restraint of these current objective measurements is the failure to incorporate an assessment of breathing effort, loss of which will slowly lead to progressive failure to wean.⁵

Weaning difficulties can occur with any interval of mechanical ventilation and are difficult to predict. Failure to wean is often caused by an increase in respiratory workload in correlation to neuromuscular limitations of the thoracic wall and diaphragm.⁵ Menis et al suggest incorporating sonography to assess ventilatory capacity to tolerate extubation, focusing on comparing the diaphragmatic muscle contraction velocity (diaphragm excursion in relation to inspiratory contraction time) to the tidal volume generated on each breath, termed volume-velocity index (VVI).⁵

In their prospective study, the authors evaluated the accuracy of VVI to predict weaning success in comparison to ΔPes, PTPes, and maximal inspiratory pressures (MIP). The primary hypothesis is that VVI can discern weaning success and failure, and a secondary hypothesis is that VVI would correlate soundly with ΔPes, PTPes, and MIP.

An inception cohort was enrolled to establish the power needed for hypothesis testing. Adult patients (> age 18 years) who were on on mechanical ventilation for more than 48 hours, met criteria for readiness to wean, and had undergone their first weaning trial were enrolled. Each patient was placed in a semi-recumbent position and MIP was assessed prior to a 30-minute spontaneous breathing trial (SBT) through a T-piece. After a 5-minute stabilization period, esophageal pressure measurements and sonographic evaluation of diaphragmatic contraction velocity were collected. VVI was assessed using ultrasound on the right hemidiaphragm and calculated as the product of tidal volume and the slope of diaphragmatic contraction velocity (ratio of diaphragmatic excursion [Dex] to inspiratory time) in mL × m/s. RSBI, ΔPes, PTPes, and MIP were calculated via standard methods. Ultrasonographers were blinded to the pressure measurements and weaning outcomes, while treating physicians were blinded to the ultrasound measurements and pressure assessments.

The inception cohort included 30 patients, with 15 patients successfully weaned off the ventilator, three failing an SBT, and 12 reintubated. The patients who were effectively weaned from mechanical ventilation had higher VVI values (764.76 [± 432.61] vs. 278 [± 183.66], P < 0.001). In addition, Dex, slope, ΔPes, PTPes, and MIP were all higher in those weaned successfully. VVI exhibited an area under the receiver operating characteristic curve (AUROC) of 0.84 and correlated with ΔPes (r = 0.74, P < 0.001), PTPes (r = 0.76, P < 0.001), and MIP (r = 0.75, P < 0.001).

Using inception data, the validation cohort was powered to include 40 patients, with 23 successfully weaned from mechanical ventilation, five failing an SBT, and 12 reintubated. Successfully weaned patients had higher VVI, Dex, diaphragmatic slope, and tidal volume, and lower RSBI. VVI had the highest diagnostic accuracy (AUROC 0.92) with a cutoff value of 660 mL × m/s.

The authors concluded that VVI of less than 660 mL × m/s is correlated with failure to wean when a T-piece is used. The rationale is that a low VVI represents poor diaphragmatic contraction with low tidal volumes, causing an imbalance in respiratory demand compared with the neuromuscular competency of the chest wall, leading to shallow breathing and eventual ventilatory collapse. VVI was shown to be a superior predictor of weaning success when compared with Dex, diaphragmatic slope, and RSBI.

The limitations of this study are evident. First, a T-piece trial is a requirement but is not always the standard of care since most ICUs protocolize pressure support trials. Second, no patients with chronic obstructive pulmonary disease were enrolled. Third, all measurements were collected in the initial stages of the SBT, which studies have shown to be too early to detect differences in ΔPes or PTPes. Fourth, the rate of SBT failure was 42.5% to 50%, which is much higher than the standard acceptable rate of 10% to 15% in the literature and which could indicate that the parameters for SBT screening were too lenient. Finally, because patients were on the ventilator for an average of one to two weeks, the SBT was performed too late in the clinical course. A larger, multicenter, randomized controlled trial would further substantiate VVI as a surrogate for ability to tolerate ventilator weaning.

References

1. Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991 May 23;324(21):1445-50.
2. Trivedi V, Chaudhuri D, Jinah R, et al. The usefulness of the rapid shallow breathing index in predicting successful extubation: a systematic review and meta-analysis. Chest. 2022 Jan;161(1):97-111. 
3. Jubran A, Grant BJB, Laghi F, Parthasarathy S, Tobin MJ. Weaning prediction: esophageal pressure monitoring complements readiness testing. Am J Respir Crit Care Med. 2005 Jun 1;171(11):1252-1259. 
4. Ball L, Talmor D, Pelosi P. Transpulmonary pressure monitoring in critically ill patients: pros and cons. Crit Care. 2024 May 25;28(1):177. 
5. Menis AA, Tsolaki V, Papadonta ME, Vazgiourakis V, Zakynthinos E, Makris D. A study on the diagnostic accuracy of tidal volume-diaphragmatic contraction velocity: a novel index for weaning outcome prediction. Crit Care Med. 2025 Jun 1;53(6):e1214-e1223.