Driving Pressure & Elastance – New Drivers in ARDS

Driving Pressure and Elastance – New Drivers in ARDS

The respiratory system driving pressure or distending pressure is a crucial respiratory mechanical concept that represents how much pressure is transmitted in the respiratory system during tidal ventilation.

The elastance is the change is pressure over the change in volume. The normal value being < 1cmH2O/ml/kg.

It remains unknown whether the benefit of lowering VT differs between patients with high elastance (and high driving pressure) and patients with low elastance (and low driving pressure).

Gallagher et al undertook the study to establish whether the causal effect of lowering the VT of ventilation on mortality in randomized trials varied according to elastance.

Effect of Lowering VT on Mortality in Acute Respiratory Distress Syndrome Varies with Respiratory System Elastance

Am J Respir Crit Care Med Vol 203, Iss 11, pp 1378–1385, June 1, 2021

Methods

This study is a secondary analysis from previous randomized controlled trials comparing low vs. higher tidal volume ventilation. The study evaluated the association between elastance and the low tidal volume-related reduction in mortality. Data were included from 5 previous RCTs that compared low vs. higher Vt in ARDS (the same studies were used in the Amato et al. study). The following data were extracted:

• Treatment assignment (low vs. higher Vt)
• Day 60 mortality
• SAPS, APACHE scores
• Elastance (driving pressure/Vt) – normalized to body weight and expressed as cm H2O/ (ml/kg) (normal elastance: 1 cm H2O/ (ml/kg)

Statistical analysis

Hypothesis: The mortality benefit of low tidal volume depends on the elastance. The mortality benefit of a low tidal volume is greater when the elastance is lower. The mortality benefit of a low tidal volume is less when the elastance is higher.

Based on a Bayesian logistic regression model to calculate the posterior probability of an interaction tidal volume and elastance on 60-day mortality. Adjustments were made for the severity of illness including the PaO2/FIO2 ratio, APACHE or Simplified Acute Physiology Score, and the mortality rate of the control group in each study.

A Subpopulation Treatment Effect Pattern Plot (STEPP) technique was used for testing the frequentist hypothesis for interaction between tidal volumes and elastance regarding the difference in the absolute risk of mortality.

Results

1202 patients enrolled from 5 RCTs.

Data were available from 1096 patients.

416 patients (38%) died on or before Day 60.

The mortality benefit of low tidal volume ventilation depended on the elastance. As the elastance increased, lower tidal volumes progressively reduced the risk of death. The posterior probability that the mortality benefit of ventilation with lower VT varied according to elastance was 93% (posterior median interaction OR, 0.80 per cm H2O/[ml/kg]; 90% CrI, 0.63 to 1.02) (Figure E4). The absolute risk reduction (ARR) associated with a lower-VT ventilation strategy increased progressively with increasing elastance (Figure 2).

Elastance: cm H2O/ (ml/kg)

Elastance Posterior probability of ARR of at least 1% More than 5%

Less than 2 55% 29%

2–3 82% 58%

More than 3 92% 82%

The mortality benefit of low tidal volume ventilation did not vary with the P/F ratio

The investigators used a Skeptical prior expressing skepticism that the effect of VT is unlikely to vary meaningfully with Ers (This is similar to the null hypothesis in the frequentist method). The Bayes factor for the one-sided null-hypothesis test of no interaction under the skeptical prior was 3.6, indicating substantial evidence against the null hypothesis (equivalent to P , 0.01 under conventional frequentist testing)

Subpopulation Treatment Effect Pattern Plot (STEPP) technique was employed for a frequentist hypothesis test of the interaction between VT strategy and Ers. lower-VT ventilation on mortality also varied according to elastance (interaction, P=0.02)

Limitations –

1. Pre-randomisation baseline elastance is not available. The elastance might have increased due to de-recruitment or might have improved due to lowering the tidal volume.
2. Patients with low elastance are likely to benefit from low tidal volume as the underlying ARDS might have resolved quickly.
3. These study results are based on the post-hoc analysis performed on old RCTs published prior to 2000.

4. not all published randomized trials of lower versus higher VT were included in this study.

5. Data on physiological variables such as hypercapnia, mechanical power and respiratory rate is not available.

Author’s Conclusions  

• The mortality benefit of low tidal volume ventilation depends on the elastance
• The mortality benefit of low tidal volume ventilation is greater at higher levels of elastance
• The mortality benefit of low tidal volume ventilation is lower when the elastance is lower
• Lung protective ventilation should primarily target driving pressure (not tidal volume)

Our Recommendations  

It’s time to move away from the dogmatic practice of applying 6-8 ml/kg Vt to all patients with severe ARDS.

Plateau pressure <30 cm H2O alone is not a reliable marker in the management of patients with ARDS.

Driving pressure and elastance measurement should be a routine part of the ventilatory adjustment strategy.

The time spent > 15 cm H2O of driving pressure is more important than one absolute value.