Background
The major goal of lung-protective ventilation in the Acute Respiratory Distress Syndrome (ARDS) is to reduce ventilator/ventilation-induced lung injuries (VILI) by minimizing strain and stress applied to the lung by mechanical ventilation [
1,
2]. As ARDS patients are characterized by a dramatical decrease in aerated lung volume, lowering the tidal volume (
VT) down to 6 ml/kg of predicted body weight (PBW) has been shown more than 20 years ago to improve their survival [
3] and has become the cornerstone of lung protective ventilation [
4]. In such patients, the decrease in respiratory system compliance (
CRS) is correlated to the decrease in the lung volume available for ventilation [
5‐
7]. The driving pressure (ΔP), defined as the difference between the plateau pressure and the positive end expiratory pressure (PEEP), represents the ratio between the
VT and the
CRS [
8]. Therefore, the ΔP normalizes the
VT to a surrogate of the aerated lung, rather than to the theoretical lung size as is the case when
VT is related to PBW. In a post hoc analysis of nine randomized control trials, Amato et al
. found that high ΔP was a better predictor of mortality than high
VT [
9], with an increased risk of death when the ΔP exceeded 14 cm H
2O. These findings have been confirmed in a subsequent meta-analysis [
10] and reinforced by large-scale observational data [
11‐
13]. More recently, it has been shown that the mortality benefit of lowering
VT in ARDS varied according to the
CRS: the patients with lower
CRS were likely to have a greater mortality benefit, whereas patients with higher
CRS were likely to have a lower mortality benefit [
14]. All these data argue for targeting ΔP rather than
VT scaled to PBW in ARDS.
In this study, we hypothesized that a ΔP-guided tidal ventilation strategy (ΔP-Vent), targeting a ΔP between 12 and 14 cm H
2O, may reduce the risk of VILI as compared to a PBW-guided tidal ventilation strategy (PBW-Vent) in ARDS patients. Such ∆P-vent approach could protect against excessive driving pressure in patients with lower
CRS while permitting higher driving pressure in patients with higher
CRS, allowing a concomitant decrease in the respiratory rate. Interestingly, recent data suggest that both driving pressure and respiratory rate are predictors of mortality in ARDS [
15]. Their combined effect may depend on
CRS with potential value in increasing tidal volume while decreasing respiratory rate in patients with higher
CRS. The mechanical power represents the energy applied to the respiratory system by the ventilator and has been considered as a surrogate for the risk of VILI [
16‐
18]. It is associated with mortality during controlled mechanical ventilation in ARDS [
15,
19] and has the advantage of taking into account both an elastic component related to the driving pressure and the possible impact of respiratory rate. Thus, the aim of our study was to compare, in ARDS patients, the mechanical power resulting from a ΔP-Vent versus that resulting from a PBW-Vent.
Discussion
The main findings of our study are the followings: a ∆P-guided ventilation targeting a ∆P between 12 and 14 cm H2O represented a distinct strategy from a conventional PBW-guided ventilation as it required VT changes in 90% of the patients; the direction of the change in VT was an increase in the majority of cases (72%), accompanied by a concomitant decrease in respiratory rate; such ∆P-guided ventilation led to a significant decrease in mechanical power while PaO2/FiO2 and ventilatory ratio improved; the decrease in mechanical power was mainly driven by patients with higher respiratory system compliance, thus in whom the VT increased during ∆P-guided ventilation.
A ΔP-limited strategy (aiming at minimizing the ΔP) has been proposed and seems feasible [
28]. However, decreasing the ΔP at the price of an increase in respiratory rate may be at higher risk of unfavorable outcome in patients with higher compliance [
19]. In this study, we rather assessed a ΔP-Vent strategy targeting a ΔP range below the threshold identified as associated with an increased risk of death. If the ΔP was above the threshold during PBW-Vent, the
VT was decreased in order to avoid excessive strain. However, the ΔP at baseline during PBW-Vent in our series was below the predefined target range in a majority of patients, in accordance with values usually observed in ARDS patients when the
VT is set at 6–8 ml/kg of PBW [
11].
VT was therefore most often increased to achieve ΔP-Vent. One may assume that allowing some increase in
VT in patients with higher compliance during the ΔP-Vent strategy could be associated with favorable physiological effects as promotion of recruitment and decrease in alveolar dead space [
29], improvement of oxygenation [
30], unloading of respiratory muscles, attenuation of respiratory drive [
31,
32], relief of dyspnea [
14], decreased risk of occurrence of breath stacking [
33,
34] and decreased need for sedative drugs [
35]. Of note, in our study, ΔP-Vent was accompanied by an increase in PaO
2/FiO
2 ratio, and a decrease in ventilatory ratio. Whether such physiological effects could be associated with improvement in clinical outcome warrants further research.
In our study, ΔP-Vent was associated with a significant decrease in both the resistive and elastic component of the mechanical power, as compared to PBW-Vent. Mechanical power, which represents the energy delivered to the respiratory system, could be considered as a target for VILI prevention [
18]. Cressoni et al
. conducted an experimental study on piglets suggesting that neither the
VT alone nor the respiratory rate could generate VILI, which instead was induced by their combination when the mechanical power produced was higher than a certain threshold [
17]. Therefore, paying attention to mechanical power might help broaden our focus on VILI, taking into account not only
VT and ΔP, but also respiratory rate and their combination. In an analysis of more than 8000 critically ill patients from the MIMIC-III and eICU databases, Serpa Neto et al
. retrieved that high mechanical power was independently associated with high in-hospital mortality, even at low tidal volumes [
19]. More recently, a retrospective analysis of 4549 patients included in six randomized clinical trials of protective ventilation showed that mechanical power was a significant predictor of mortality at 28 or 60 days [
15]. In our study, the ΔP-Vent induced a relative decrease in mechanical power of 7% [0–16]. In a prospective cohort study involving 13,408 patients, Urner et al
. reported a significant increase in the hazard of death with each daily increment in mechanical power over the whole duration of mechanical ventilation, suggesting that even a small decrease in mechanical power could be relevant if maintained over time. However, a causal relationship between mechanical power and clinical outcome has not been demonstrated to date. High mechanical power may be a marker of lung injury rather than inappropriate ventilator settings. Indeed, during conventional protective ventilation, the mechanical power increases in case of
CRS impairment and decreases during resolution of the lung disease. Thus, the clinical impact of a ventilation strategy that is accompanied by a decrease in mechanical power remains unclear and deserves future clinical trials.
Our study has some limitations. First, this was a single-center study with a significant proportion of COVID-19-related ARDS among the included patients. Conflicting data have been reported about potential differences in the respiratory system compliance in the early phase of COVID-19-related ARDS compared to ARDS of other origins [
36‐
40]. However, the ΔP at baseline during PBW-Vent in our population was consistent with reports prior to COVID-19 pandemics [
11], suggesting external validity. Second, PEEP management in our unit may have influenced the value of mechanical power. In fact, mechanical power is modeled with a positive linear relationship with PEEP. However, we did not modify PEEP between PBW-Vent and ΔP-Vent. The difference in mechanical power was therefore exclusively related to variations in
VT and respiratory rate, and should be reproducible as long as the PEEP level remains of the same order of magnitude. Third, our PEEP management may also have influenced the plateau pressure values. Excessive plateau pressure may be associated with higher mortality even with limited driving pressure [
41]. In our study however, the plateau pressure remained below 30 cm H
2O during both ventilation strategies in all patients. Lastly, the ΔP value is influenced by the chest wall compliance and a high ΔP may be related to a low chest wall compliance rather than an excessive lung strain. We did not record the esophageal pressure and were thus unable to measure the chest wall compliance. However, we aimed at assessing a pragmatic approach that could be easily translated into clinical practice. As the ΔP-Vent was feasible and different from PBW-Vent in 90% of our patients, designing clinical trials comparing the two strategies is attainable.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.