Mechanical ventilation (MV) is often life-saving in critically ill patients with acute respiratory failure and/or acute respiratory distress syndrome (ARDS) (Bellani et al., 2016). However, MV may lead to ventilator-induced lung injury (VILI) (Dreyfuss and Saumon, 1998). Different mechanisms of VILI have been identified so far. High distending pressures may promote baro- and volutrauma, whereas repetitive aeration and collapse of alveoli may induce atelectrauma (Güldner et al., 2016). Inhomogeneous lung aeration can further aggravate mechanical stress and lung injury (Mead et al., 1970). Although certain measures to prevent VILI have been established (Acute Respiratory Distress Syndrome Network Brower et al., 2000; Amato et al., 2015), e.g., limitation of tidal volume (V_T) or airway plateau and driving pressures, discussion about adequate levels of positive end-expiratory pressure (PEEP) is ongoing, and the interplay among parameters is complex (Battaglini et al., 2021). Irrespectively of specific parameters, mechanical energy is inevitably transferred to the respiratory system in every single MV cycle, resulting in transferred mechanical power (MP) when multiplied with respiratory rate (RR). Although it has been known that the transferred energy, or power, is partly restored and dissipated in the respiratory system (Sassoon and Mahutte, 1998; Guttmann, 2010), the concept gained new attention recently when MP was proposed as the main determinant of VILI (Cressoni et al., 2016; Gattinoni et al., 2016), which is still under development (Huhle et al., 2018). Although recent research mainly focused on single ventilator parameters as VILI determinants, MP may summarize the potentially damaging effects of different parameters in one single variable. Yet, MP was shown to be associated with different characteristics of experimental lung injury, i.e., radiological signs of lung edema, lung wet/dry ratio, and histological features (Cressoni et al., 2016; Collino et al., 2019; Vassalli et al., 2020). In retrospective clinical trials, MP was associated with mortality in critically ill patients (Serpa Neto et al., 2018; Costa et al., 2021). However, its effects on the pulmonary neutrophilic inflammation as assessed by positron-emission tomography (PET)/computed tomography (CT) have not been determined yet, though neutrophilic inflammation is a mainstay in ARDS pathogenesis (Grommes and Soehnlein, 2011). In this study, we aimed to investigate the applied resulting MP and the neutrophilic pulmonary inflammation in a clinically relevant model of acute lung injury in pigs ventilated with fixed combinations of PEEP and inspiratory oxygen fractions (F_IO₂), as recommended by the ARDS network’s low PEEP table (Brower et al., 2004). We hypothesized that neutrophilic inflammation correlates with MP.

The Institutional Animal Care and Welfare Committee and the Government of the State of Saxony, Germany, approved the study protocol (file 25-5131/474/31; 27.09.2019; Dr. B. Langen, Landesdirektion Sachsen). Animals received humane care according to German law and the Principles of Laboratory Animal Care formulated by the National Society for Medical Research and the United States National Academy of Sciences Guide for the Care and Use of Laboratory Animals, and the Animal Research: Reporting of in vivo Experiments guidelines were followed.

The main findings of the present study are in pigs ventilated mechanically according to the low PEEP table of the ARDS network: (1) pulmonary neutrophilic inflammation, as assessed by PET/CT, increased significantly over time; (2) among all ventilatory variables investigated, only MP and its elastic and resistive components showed a positive correlation with neutrophilic inflammation; and (3) global pulmonary gas volume decreased whereas hyper-aerated relative lung mass increased over time.

To the best of our knowledge, this is the first experimental study that prospectively investigated the relationship between MP and pulmonary neutrophilic inflammation as assessed with PET/CT. Previous studies were limited to a less sensitive assessment of VILI, including radiographic evidence of pulmonary edema, cumulative histological scores, and lung wet/dry ratio (Cressoni et al., 2016; Collino et al., 2019; Vassalli et al., 2020). Neutrophilic infiltration and inflammation is deemed to be a main pathological mechanism in the development and course of ARDS (Zemans et al., 2009; Grommes and Soehnlein, 2011; Matthay and Zemans, 2011).

A particular strength of our study is that we used MV settings of a clinically common MV concept in a model of moderate acute lung injury, rather than intentionally provoking lung injury by unusual or artificial settings or investigating the roles of certain ventilator parameters. Instead, we applied fixed PEEP/F_IO₂ combinations according to the ARDS network recommendations (low PEEP table) (Brower et al., 2004). Although large clinical trials did not reveal an outcome advantage of high over low PEEP in patients with ARDS (Brower et al., 2004; Meade et al., 2008; Mercat et al., 2008), MV concepts tolerating formation of atelectasis and moderate hypercapnia in order to avoid aggressive ventilation in terms of high distending pressures are under debate (Pelosi et al., 2018) and clinically favored. Use of the low PEEP table was recently shown to increase survival in patients with ARDS as compared with individualized but higher PEEP (Cavalcanti et al., 2017). In addition, titrating PEEP according to lung morphology did not improve outcome and even increased mortality if morphology was misclassified, respectively. Thus, as proposed earlier (Brower et al., 2004), fixed PEEP/F_IO₂ combinations should still be used for patients with ARDS (Chiumello et al., 2014; Battaglini et al., 2021), which justified our selected experimental setting.

The finding that MV at relatively low PEEP increased neutrophilic inflammation is in line with a previous experimental study by our group (Kiss et al., 2019). Therein, PEEP was titrated according to end-expiratory transpulmonary pressure allowing end-expiratory alveolar collapse. This approach was shown to increase neutrophilic inflammation as compared with higher PEEP under controlled MV (Kiss et al., 2019). In the present study, PEEP was titrated according to clinically common oxygenation thresholds, and according to the low PEEP/F_IO₂ table from the ARDS Net, yielding PEEP levels even lower than the titrated PEEP in the mentioned previous trial. Thus, it is not surprising that pulmonary neutrophilic inflammation was detected and even increased over time herein. Furthermore, median ΔP was higher than 15 cmH₂O throughout the intervention time, which was postulated as a threshold for increased risk of mortality in a retrospective clinical analysis of patients with ARDS (Amato et al., 2015). Although existing meta-analyses are ambiguous, our finding of increased pulmonary neutrophilic inflammation might be recognized as contradictory to the observation that survival in ARDS may be higher with lower PEEP (Cavalcanti et al., 2017). However, in general, clinical outcomes are influenced by many effects, e.g., disease severity, comorbidities, ventilation settings potentially inducing VILI, and hemodynamics. One could hypothesize that there is a similar inflammatory response in patients with ARDS undergoing similar ventilator settings, but that other effects in the recent clinical trials, e.g., impaired hemodynamics, affect outcomes majorly as well. Our study cannot assess how clinically relevant our observed pulmonary inflammation is and direct comparison with the recent large clinical trials must be done cautiously. However, we focused on an established mechanism in ARDS pathophysiology, the neutrophilic inflammation, which served as VILI surrogate in many high-quality experimental studies.

Despite the time point directly after starting the intervention time (0 h), median MP was always lower than the initially postulated VILI threshold of 12 J/min (Cressoni et al., 2016). However, it was high at Injury and the start of intervention time (0 h). Thus, it is not surprising that there is neutrophilic inflammation, though MP was relatively low during intervention time. This is also in line with the findings from another experimental study, where the lowest power was also associated with histological evidence of VILI (Collino et al., 2019). In a third experimental investigation, cumulative histological lung injury did not differ between MP around 14 and 29 J/min (Vassalli et al., 2020). In this regard, two major considerations have to be made. First, absolute MP values depend on the way of calculation. We used the respiratory tracings to assess MP, whereas different MP formulas have been postulated since the seminal publication in 2016 (Giosa et al., 2019). The major criticism regarding the initial formula was that there is incorrect mathematical modeling of the role of PEEP (Huhle et al., 2018), whereas others tried to reduce the formula’s complexity (Marini and Jaber, 2016; Giosa et al., 2019). This issue needs further investigation. Second, different experimental models and settings preclude from direct comparison of certain MP values and their respective associated results.

Although ΔP, E, and V_T did not change and R increased during the intervention time, both RR and MP decreased, i.e., changed in the same direction. Of course, RR contributes linearly to MP; thus, RR and MP are closely linked mathematically. Still, one could hypothesize that it could be mainly RR contributing to VILI in our study. In fact, RR was associated with lung injury under certain circumstances (Rich et al., 2003). However, only MP correlated significantly with ΔK_iS in our study, whereas RR did not. The finding that MP but not is determinants, i.e., ΔP and RR, were associated with pulmonary neutrophilic inflammation may strengthen the potential role of MP regarding VILI as a unifying variable, summarizing the burden resulting from the clinically selected ventilator settings. For the first time, our study combined concepts of pulmonary neutrophilic inflammation and MP. Although the first is generally considered a cellular quantity indicative of VILI, the latter is discussed as a VILI determinant. MP depends on a number of factors including ventilator settings (ΔP, V_T, RR, etc.) and respiratory mechanics (R and E). Although driving pressure may reflect the mechanical burden, i.e., stress per breath alone, it does not reflect how often this mechanical stress is applied per unit of time. RR quantifies mechanical stress applied per unit of time. In a gross approximation, MP is the product of both RR and driving pressure. Consequently, although neither driving pressure nor RR are associated with neutrophilic inflammation, MP may still do because both do not necessarily covariate. In this investigation, we found an association of MP with neutrophilic inflammation supporting the notion that MP may be a determinant of VILI superior to the single respiratory components. Of note, this statistical association does not prove causality. During our intervention time, PEEP decreased significantly. However, its median difference between 0 and 18 h was only 0.2 cmH₂O, which we would not consider as clinically relevant. Thus, its contribution to the shown decrease of MP may have been negligible.

The CT scans revealed that both the total pulmonary gas volume decreased during the intervention time, whereas the relative mass of lung aeration compartments did not change substantially, despite the significantly increased hyper-aerated compartment. The decreased lung mass may be explained by a decrease of intrapulmonary fluid content, because lung lavage fluid partly remained in the airways and alveoli after induction of lung injury, but was removed by positive pressure ventilation and capillary/lymphatic resorption within the intervention time and before second CT. This is in line with our experiences with the lavage model. The total gas volume decreased most likely because first CT scan was performed at PEEP of 10 cmH₂O, whereas median PEEP was substantially lower at the moment of second CT (∼5 cmH₂O). Interestingly, net relative lung aeration (defined as aerated relative lung mass in relation to total lung mass at respective CT scans) did not differ; thus, the atelectatic compartment remained stable. Accordingly, median end-expiratory transpulmonary pressure was negative throughout the intervention time, which promotes end-expiratory lung collapse and atelectasis formation. Furthermore, elastance, ΔP, and transpulmonary driving pressure did not differ significantly between start and end of the intervention time. The significant increase of PaCO₂ over time could suggest decreasing lung aeration, but venous admixture did not differ between 0 and 18 h. More likely, PaCO₂ increased because RR was decreased to keep arterial pH in a desired range. Within the aerated lung compartments, only relative hyper-aeration increased significantly. Although this was not accompanied by significant changes of %E₂, it fits to the finding of increased K_iS in the second PET/CT.

As the only hemodynamic variable, SV significantly differed between 0 and 18 h. Most likely, hemodynamics were still impaired at 0 h related to the recent induction of lung injury. Already 6 h later, SV increased and remained stable or even increased until 12 and 18 h, respectively. The latter may reflect further hemodynamic stabilization or recovery, and may be related to the increasing cumulative intravenous fluid administration. In contrast, increased SV could be interpreted as a reaction to the increasing inflammatory status. However, HR, CO, MAP, and the need for norepinephrine did not change significantly, rendering this explanation less likely.

In experimental acute lung injury in pigs, MV according to the recommendations of the ARDS network and using the low PEEP/F_IO₂ table resulted in increased PET/CT-derived pulmonary neutrophilic inflammation, which correlated with MP.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Landesdirektion Sachsen, Referat 25, Chemnitz.

MS, JW, RH, JK, PP, MJS, PR, and MG planned and designed the study. MS, JW, RH, XR, YZ, AB, RT, LM, and GB performed the experiments. MS, JW, MG, AB, RH, and JK planned, performed and analyzed the PET/CT image acquisition. MS, JW, RH, XR, YZ, RT, LM, GB, TB, TK, and MG were involved in the data analyses. YZ, RT, LM, and RH performed the lung segmentations (semi-automatic followed by manual correction), image data preprocessing, and co-registration. MS, JW, RH, and MG wrote the manuscript draft. All authors read and approved the submitted manuscript, and agreed to be accountable for the content of the work and its publication.

MG received consultation fees from Dräger, Ambu, GE Healthcare, and ZOLL. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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