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. 2020 Apr 15;15(4):e0230830. doi: 10.1371/journal.pone.0230830

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

Mascha O Fiedler 1, B Luise Deutsch 2, Emilis Simeliunas 1, Dovile Diktanaite 1, Alexander Harms 3, Maik Brune 4, Florian Uhle 1, Markus Weigand 1, Thorsten Brenner 1, Armin Kalenka 5,6,*
Editor: Linda L Maerz7
PMCID: PMC7159202  PMID: 32294090

Abstract

Introduction

Intra-abdominal hypertension (IAH) is a well-known phenomenon in critically ill patients. Effects of a moderately elevated intra-abdominal pressure (IAP) on lung mechanics are still not fully analyzed. Moreover, the optimal positive end-expiratory pressure (PEEP) in elevated IAP is unclear.

Methods

We investigated changes in lung mechanics and transformation in histological lung patterns using three different PEEP levels in eighteen deeply anesthetized pigs with an IAP of 10 mmHg. After establishing the intra-abdominal pressure, we randomized the animals into 3 groups. Each of n = 6 (Group A = PEEP 5, B = PEEP 10 and C = PEEP 15 cmH2O). End-expiratory lung volume (EELV/kg body weight (bw)), pulmonary compliance (Cstat), driving pressure (ΔP) and transpulmonary pressure (ΔPL) were measured for 6 hours. Additionally, the histological lung injury score was calculated.

Results

Comparing hours 0 and 6 in group A, there was a decrease of EELV/kg (27±2 vs. 16±1 ml/kg; p<0.05) and of Cstat (42±2 vs. 27±1 ml/cmH2O; p<0.05) and an increase of ΔP (11±0 vs. 17±1 cmH2O; p<0.05) and ΔPL (6±0 vs. 10±1 cmH2O; p<0.05). In group B, there was no significant change in EELV/kg (27±3 vs. 24±3 ml/kg), but a decrease in Cstat (42±3 vs. 32±1 ml/cmH20; p<0.05) and an increase in ΔP (11±1 vs. 15±1 cmH2O; p<0.05) and ΔPL (5±1 vs. 7±0 cmH2O; p<0.05). In group C, there were no significant changes in EELV/kg (27±2 vs. 29±3 ml/kg), ΔP (10±1 vs. 12±1 cmH2O) and ΔPL (5±1 vs. 7±1 cmH2O), but a significant decrease of Cstat (43±1 vs. 37±1 ml/cmH2O; p<0.05). Histological lung injury score was lowest in group B.

Conclusions

A moderate elevated IAP of 10 mmHg leads to relevant changes in lung mechanics during mechanical ventilation. In our study, a PEEP of 10 cmH2O was associated with a lower lung injury score and was able to overcome the IAP induced alterations of EELV.

Introduction

The interactions between the abdominal and the thoracic compartments represent a challenge for ICU physicians [1, 2]. Approximately 50% of intra-abdominal pressure (IAP) is transmitted to the intrathoracic compartment [3–6]. It therefore has a direct impact on functional residual capacity (FRC), end-expiratory lung volume (EELV), driving pressure (ΔP) and transpulmonary pressure (ΔPL). Almost half of the patients admitted to the ICUs worldwide develop intra-abdominal hypertension (IAH). Two-thirds of these cases were already present on the day of ICU admission [7]. An elevated IAP can be classified as follows: 1. intra-abdominal hypertension (IAH) with an IAP above 12 mmHg and 2. abdominal compartment syndrome with an IAP above 20 mmHg [8, 9]. Both are an independent risk factor for organ failure and mortality in the ICU [1, 7].

Clinical studies on critically ill patients identified an average IAP of 10 mmHg in supine position [1, 10, 11]. Moderate elevated IAP also occurs in obesity, pregnancy and during anesthesia [12–14]. The role of this moderate elevated IAP on lung mechanics and potential organ failure is yet not fully analysed.

We therefore analysed the effect of a moderate elevated IAP of 10 mmHg on lung mechanics in a porcine model up to 6 hours. To investigate the consequences of different PEEP levels, we used three levels of PEEP (5, 10, 15 cmH2O). The hypothesis in our study was that a PEEP of 10 cmH2O in moderate elevated IAP (10 mmHg) is protective by reducing lung injury and preserving the EELV during mechanical ventilation.

Materials and methods

Animal preparation and instrumentation

The protocol was approved by the responsible committee for animal research (Regierungspräsidium Karlsruhe, No. 35–9185.81/G-161/17). The animals were kept within the interfacultary biomedical faculty of the university of Heidelberg and were provided by a local pig breeder. All proceedings were in accordance with animal welfare notes regulated by German law. After overnight fasting with free access to water, 18 female domestic pigs were anaesthetized intramuscularly in combination with 7 mg/kg Azaperon (Stresnil®, Lilly, Bad Homburg, Germany), 8 mg/kg Ketaminhydrochlorid 10% (Ketamin10%®, Bremer Pharma, Warburg, Germany) and 0.3 mg/kg Midazolam (Midazolam, Hameln Pharma, Hameln, Germany). Anaesthesia was maintained by continuous infusion of 6 mg/kg/h Ketanest S (Pfizer Pharma, Berlin, Germany), 3.6 mg/kg/h Midazolam and 10–30 mg/kg/h Propofol 2% (Propofol, Fresenius Kabi, Bad Homburg, Germany). There was no use of neuromuscular blockers. Adequacy of the depth of anaesthesia was regularly assessed by absence of spontaneous breathing efforts and lack of muscle tone.

After induction of anaesthesia the pigs were tracheotomised and ventilated with an intensive care ventilator (Carescape R860, GE Healthcare, Madison, USA) using an inspiratory oxygen concentration (FiO2) of 0.4 in a pressure-controlled mode with volume guaranty. Also, a tidal volume of 8 ml/kg body weight, an inspiration/expiration ratio of 1:2 and a PEEP of 5 cmH2O was provided.

A 5F thermistor-tipped catheter (PiCCO®, Pulsion Medical systems, Feldkirchen, Germany) and a central venous catheter (Logicath, Smiths Medical, Grasbrunn, Germany) were inserted with ultrasound guidance. Crystalloid solution (Sterofundin ISO, Braun, Melsungen, Germany) was infused to keep the study population hemodynamically stable during the experiment. A polyethylene catheter (Nutrivent multifunction nasogastric catheter, Sidam, San Glacomo Roncole, Italy) was used to measure esophageal pressure. Appropriate catheter position was confirmed as previously described [15].

After a midline laparotomy a large intra-abdominal balloon (200-litre weather balloon, Stratoflight, Blomberg, Germany) was placed in the peritoneal cavity. Correct position in all abdominal quadrants was ensured by visual inspection and partial inflation. The abdomen was carefully closed. A urine catheter was placed in the bladder.

Measurements and calculations

Peak inspiratory airway pressure (PInsp), PEEP, inspiratory esophageal pressure (PEsInsp) and end-expiratory esophageal pressure (PEsExp) were recorded from the ventilator. ΔP and ΔPL were calculated as previously described [16]. Transpulmonary inspiratory pressure (TPPInsp) was calculated as TTPInsp = PInsp—PEsInsp and transpulmonary expiratory pressure (TPPExp) as TPPExp = PEEP—PEsExp. CStat was measured by the ventilator during an inspiratory hold. Elastance of the respiratory system (ERS) was calculated as ERS = (PInsp—PEEP) / VT, chest wall elastance (ECW) as ECW = (PEsInsp—PEsExp) / VT and elastance of the lung (EL) as EL = ERS—ECW.

We measured EELV bedside as previously described [17] without interrupting mechanical ventilation on the designated PEEP level. CI (cardiac index) was calculated with the PiCCO® System. End-expiratory IAP (IAPEndex) was measured as recommended [18, 19] and zeroed at midaxillary level [20]. P/F ratio was calculated based on the ratio of partial arterial pressure of oxygen to FiO2.

Experimental protocol

Data were assessed after a 30 minutes stabilization period (H0). The abdominal balloon was then filled with water up to an IAPEndex of 10 mmHg. We randomized into group A (n = 6) with a PEEP of 5, group B (n = 6) = PEEP 10 and group C (n = 6) = PEEP 15 cmH2O for 6 hours (H6) (Fig 1).

Fig 1. Experimental model study protocol.

Fig 1

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After instrumentation and a 30-min stabilization period animals underwent measurements at H0 with PEEP 5 and no elevated intraabdominal pressure (IAP). Thereafter the intraabdominal balloon was filled with water up to an IAP of 10 mmHg. Animals were randomized afterwards to the three different groups with group A: PEEP 5 cmH2O, group B: PEEP 10 cmH2O and group C: PEEP 15 cmH2O. The IAP was held at 10 mmHg for 6 hours.

At the end of the experimental protocol, the pigs were euthanized with an intravenous bolus of 200 mg Propofol followed by 40mmol potassium chloride. We exposed the complete right lung and regional lung samples were extracted to evaluate wet-dry weight ratio and to perform histological examination.

Histology

Samples from the anterior, medial and dorsal position of the medial lobe were selected and immediately fixed in formalin. After fixation, the tissue samples were dehydrated and embedded. The sections were stained with hematoxylin and eosin. A pathologist, blinded to the study variables, evaluated each sample histologically to determine a lung injury score. To quantify the extent of histologic lung injury the pathologist used a lung injury scoring system [21] (S1 Table). Five independent variables were scored to generate the lung injury score. The sum of each of the five independent variables were weighted according to the relevance for acute lung injury [21]. The resulting lung injury score ranges from 0 to 1. Zero represents minimal to no damage and 1 the worst damage possible.

Wet-dry weight ratio

Wet-dry weight ratio was measured in samples of the medial lobe. Samples were weighed, dried and then weighted again.

Statistical analysis

Sample size was calculated based on the expected alterations in EELV using data from previous studies performed in our lab.

Statistical analysis was performed using SPSS (Version 25). H0 values and lung injury score were analyzed using a one-way ANOVA. In case of significance a post hoc analyze with a correction for multiple tests were performed. We used ANCOVA for comparing H0 with H6 data with post hoc analysis. Data are expressed as mean ± SEM (standard error of the mean). For all tests, P ≤ 0.05 was considered statistically significant.

Results

We included 18 animals in this study with a weight of 47 ± 1 kg. There were no significant differences at H0 among the experimental groups with the exeption of the significant higher heart rate of group B compared to group A.

Lung mechanics, hemodynamic parameters and oxygenation

When compared with data at H0 we observed several alterations after 6 hours of mechanical ventilation (H6) (Table 1). In group A EELV (1323±95 vs. 774±67; p<0.05), EELV/kg bw (27±2 vs. 16±1 ml/kg; p<0.05) (Figs 2 and 3) and Cstat (42±2 vs. 27±1 ml/cmH2O; p<0.05) decreased (S1 Fig). The ΔP (11±0 vs. 17±1 cmH2O; p<0.05) and ΔPL (6±0 vs. 10±1 cmH2O; p<0.05) showed an increase (Figs 2 and 3). In group B EELV (1337±168 vs. 1159±140; p>0.05) and EELV/kg (27±3 vs. 24±3 ml/kg; p>0.05) were not influenced. Nonetheless a significant decrease in Cstat (42±3 vs. 32±1 ml/cmH2O; p<0.05) and an increase in ΔP (11±1 vs. 15±1 cmH2O; p<0.05) and ΔPL (5±1 vs. 7±0 cmH2O; p<0.05) could be observed. In group C EELV (1231±89 vs. 1310±135; p>0.05), EELV/kg (27±2 vs. 29±3 ml/kg; p>0.05), ΔP (10±1 vs. 12±1 cmH2O; p>0.05) and ΔPL (5±1 vs. 7±1 cmH2O; p>0.05) where not changed but a significant decrease of Cstat (43±1 vs. 37±1 ml/cmH2O; p<0.05) was found. The TPPInsp were different from H0 in group A and C (A: 6 ± 1 vs. 10 ± 1 cmH20, C: 6 ± 1 vs. 12 ± 2 cmH2O, p < 0.05) but not in group B (6 ± 1 vs. 7 ± 1 cmH2O, p > 0.05). The TTPEXP stayed unaffected between H0 and H6 (Table 1) (S2 Fig).

Table 1. Pulmonary and hemodynamic parameters in the setting of 6 hours mechanical ventilation with an intraabdominal pressure of 10 mmHg.

group A group B group C
Weight (kg) HO 48±1 49±1 45±1
IAP (mmHg) HO 3±0 2±1 2±0
IAP (mmHg) H6 10±0* 10±0* 10±0*
EELV H0 1323±95 1337±168 1231±89
EELV H6 774±67* 1159±140# 1310±135§
EELV‎/kg H0 27±2 27±3 28±2
EELV/kg H6 16±1* 24±3# 29±3§$
ΔP H0 11±0 11±1 10±1
ΔP H6 17±1* 15±1*# 12±1§$
ΔPL H0 6±0 5±0 5±1
ΔPL H6 10±1* 7±0* 7±1
TPPInsp H0 7±1 6±1 6±1
TPPInsp H6 11±2* 7±1 12±2*
TPPExp H0 1±1 1±0 1±1
TPPExp H6 1±1 1±1 5±1
CStat H0 42±2 42±3 45±2
CStat H6 27±1* 32±1*# 37±1*§$
ERS H0 28±1 29±1 28±1
ERS H6 44±1* 37±2*# 32±1*§$
ECW H0 14±1 15±2 14±1
ECW H6 18±3 20±2 13±1
EL H0 14±1 13±1 13±2
EL H6 26±3* 17±1* 19±2
HR H0 79±9 113±9# 85±9
HR H6 77±12 99±6 70±4
MAP H0 78±6 98±5 88±7
MAP H6 100±6* 102±6 104±6
P/F ratio H0 452±25 425±20 505±24
P/F ratio H6 439±19 396±18 501±20$
CI H0 4.9±0.5 5.7±0.3 4.4±0.4
CI H6 4.3±0.4 4.9±0.3 3.6±0.2
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EELV = end-expiratory lung volume (ml), EELV/kg = end-expiratory lung volume per kg bodyweight (ml/kg), ΔP = driving pressure (cmH2O), ΔPL = transpulmonary pressure (cmH2O), TPPInsp = inspiratory transpulmonary pressure (cmH2O), TPPExp = expiratory transpulmonary pressure (cmH2O), CStat = static pulmonary compliance (ml‎/cmH2O), ERS = Elastance of the respiratory system (cmH2O/ml), ECW = Elastance of the chest wall (cmH2O/ml), EL = lung elastance (cmH2O/ml), HR = heart rate (beats/min), MAP = mean arterial pressure (mmHg), P/F ratio = ratio between arterial pressure of oxygen and inspired oxygen concentration (mmHg), CI = cardiac index (l/min/m2)

* = P<0.05 HO vs. H6

# = p<0.05 group A vs. group B

§ = P<0.05 group A vs. group C

$ = p<0.05 group B vs. group C.

Fig 2. Alterations of end-expiratory lung volume in absolute values and in relation to body weight in response to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

Fig 2

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Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6, # = p<0.05 group A vs. group B, § = p<0.05 group A vs. group C. Mean values with SEM are illustrated.

Fig 3. Alterations of driving pressure and transpulmonary pressure in response to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

Fig 3

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Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6, # = p<0.05 group A vs. group B, § = p<0.05 group A vs. group C, $ = p<0.05 group B vs. group C. Mean values with SEM are illustrated.

Mean values with SEM are illustrated.

At H6 we found a lower EELV/kg in group A (16±1 ml/kg) compared to group B (24±3 ml/kg) and C (29±3 ml/kg) (P<0.05) (Fig 2). Driving pressure decreased significantly with increasing PEEP (A: 17±1 cmH2O, B: 15±1 cmH2O, C: 12±1 cmH2O, p<0.05) (Fig 3).

Hemodynamic parameters and oxygenation are summarised in Table 1. We could not find a relevant alteration of cardiac index with higher PEEP nor was the P/F ratio significantly influenced.

Lung injury score and wet-dry weight ratio

The global lung injury score was lowest in group B (0.17±0.02) compared to group A (0.30±0.04) and group C (0.32±0.02) (p<0.05) (Fig 4). The wet-dry weight ratios were not different (S3 Fig).

Fig 4. Histologic assessment of lung injury.

Fig 4

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Quantitative score for lung injury (from 0 = no damage to 1 = maximal alteration) calculated by the averaging score for five independent variables: neutrophils in the alveolar space, neutrophils in the interstitial space, hyaline membranes, proteinaceous debris filling the airspaces and alveolar septal thickening. Apical, medial, dorsal and global score (mean of scores for apical, medial and dorsal) are illustrated. # = p<0.05 group A vs. group B, § = p<0.05 group A vs. group C, $ = p<0.05 group B vs. group C. Mean values with SEM are illustrated.

Discussion

Main findings

The present animal study proved that even a moderate IAP of 10 mmHg caused changes in lung mechanics and a histological lung injury after 6 hours of ventilation. A PEEP of 10 cmH2O overcame the intra-abdominal pressure induced alterations of EELV and revealed the lowest lung injury score.

Alterations in lung mechanics

The main aim of our study was to describe the alterations in EELV, ΔP and ΔPL in a porcine model of moderately elevated IAP. These crucial parameters have an impact on the development or prevention of ventilator induced lung injury (VILI) [22, 23]. By implementing a water-filled balloon with an IAP of 10 mmHg for up to 6 hours, we detected a drop of EELV and EELV/kg bw with a PEEP of 5 cmH2O over time. In the two groups with a PEEP of 10 and 15 cmH2O, these reductions were not observed. PInsp did not rise proportionally to PEEP, which strongly suggests a regional recruitment due to an increasing PEEP. TPPInsp did not change significantly from H0 with a PEEP of 10 cmH2O in contrast to the groups with PEEP 5 and 15 cmH2O. The PEEP of 10 cmH2O did not affect TPPExp in significant manner. We interpret these results that the best compliance and lowest EL in this setting occurred with a PEEP of 10 cmH2O. These results extend those obtained by Cortes-Puentes et al. [24], which were able to reveal an unresponsive TPPExp with an increase of IAP. The authors argued that a rising IAP stiffens the chest wall, whereas aerated lung volume decreases, either due to derecruitment or a reduced stretch of air spaces that remain patent. Cortes-Puentes already showed that a PEEP of 10 cmH2O is able to restore FRC, which appears to be reduced above an IAP of over 10 mmHg. Compared to our study design the researchers used intra-abdominal air insufflation and modified IAP levels between zero and 35 cmH2O [24].

In our model with an IAP of 10 mmHg, ERS decreased with increasing PEEP, but ECW was the same in all groups. These findings are in line with studies in patients with ARDS. Krebs et al. applied different PEEP levels (up to 20 cmH2O) in 20 patients with ARDS; one half of the study population had IAH (with a mean IAP of 8 and 16 mmHg, respectively) [25]. PEEP was found to decrease ERS by decreasing EL without influencing ECW in both groups.

PEEP in elevated IAP

In the context of elevated IAP, the management of PEEP is still a contentious issue [26]. The TPPExp seems to be an important parameter. A negative TPPExp should be avoided in order to prevent lung collapse and to overcome intratidal recruitment/derecruitment [27]. By inflating an intra-abdominal balloon, Regli et al. analyzed different PEEP levels (5, 8, 12 and 15 cmH2O) unmatched to the level of IAP [28]. PEEP levels below the IAP were not able to prevent the decline of EELV. In a second study the same group now matched PEEP and IAP levels [29]. It must be noted that the authors found that EELV was preserved without any improvement in the P/F ratio. They argued that a reduction of cardiac output with higher PEEP levels is the main cause for the lack of improvement.

A human study in mechanically ventilated patients recently matched the PEEP to IAP. Only 10 of 18 patients tolerated this matched PEEP. In these 10 patients, the matched PEEP improved oxygenation but a PEEP matched 0.5 x IAP in cmH2O did not [30]. In our study, TPPExp was positive at all times and we were not able to find a relevant change in hemodynamic parameters. Neither could we find relevant alterations in P/F ratio between the groups or over time.

Histological lung injury

Acute lung injury (ALI) in humans is characterized by disruption of the alveolar-capillary membrane barrier, proteinaceous alveolar exudate and pulmonary edema. We used the recommended scoring system for ALI in animal studies [21]. The lung injury score (LIS) was significantly lower in the group with a PEEP of 10 cmH2O. PEEP of 10 cmH2O was probably more lung protective during the ventilation than a lower or higher PEEP. As mentioned previously neither P/F ratio nor cardiac index were relevantly different between the groups.

Alteration in the wet-dry weight ratio is a typical feature of VILI, caused by high tidal volumes, endotoxin or bacterial induced ALI [31]. As expected, the groups did not differ regarding the wet-dry weight ratio. We therefore presume that the observed differences in LIS were not caused by lung edema.

Experimental protocol

Some important aspects of our study differ from recent examinations and show the strength of the actual study.

1. We did not use an air inflated balloon to increase IAP. Instead we modified previously described methods to induce IAP [32, 33]. By installing water in a large 200-litre weather balloon, we aimed to simulate the leading cause of elevated IAP due to liquid ascites or oedematous tissue.

2. We only used a moderately elevated IAP of 10 mmHg and kept it for 6 hours. Since IAP is well known to be around 10 mmHg in critically ill patients [7, 10, 11] the investigation we present here therefore simulated a common scenario.

3. We tried to find the best PEEP in this setting. Therefore we matched PEEP and IAP levels based on the assumption that abdominal-thoracic transmission (ATT) is between 17 and 62% [3–6]. The ATT describes the percentage increase in thoracic pressures for each incremental increase of IAP.

Limitations

This is an animal study. The results therefore cannot be transferred to human patients without any restrictions. We only used an IAP of 10mmHg. Hence, it is unclear to what extent the above-mentioned PEEP matching to IAP is useful and would be tolerated at higher IAP values. Clinical scenarios rarely end at 6 hours in human subjects and extending IAP and the study for a prolonged period may also reveal a difference in optimal PEEP recommendations.

Conclusions

A moderately elevated IAP of 10 mmHg has already relevant effects on lung mechanics and on histological lung injury. Measuring bladder pressure should encourage the clinician to find the best PEEP in order to realize a safer ventilation strategy. In healthy porcine lungs with an IAP of 10 mmHg the application of a PEEP of 10 cmH2O overcame the intra-abdominal pressure induced alterations of EELV. A PEEP of 10 cmH2O revealed the lowest lung injury score.

Supporting information

S1 Checklist

(PDF)

Click here for additional data file. (207.6KB, pdf)
S1 Table. The lung injury scoring system.

Quantitative assessment for lung injury calculated by the averaging score for five independent variables and the resulting score (from 0 = no damage to 1 = maximal alteration).

(DOCX)

Click here for additional data file. (14.4KB, docx)
S1 Fig. Alterations of static lung compliance to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6, # = p<0.05 group A vs. group B, § = p<0.05 group A vs. group C, $ = p<0.05 group B vs. group C. Mean values with SEM are illustrated.

(DOCX)

Click here for additional data file. (49.8KB, docx)
S2 Fig. Alterations of inspiratory transpulmonary pressure and expiratory transpulmonary pressure to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6. Mean values with SEM are illustrated.

(DOCX)

Click here for additional data file. (59.7KB, docx)
S3 Fig. Wet-dry weight ratio of areas of the right lung.

Mean values with SEM are illustrated.

(DOCX)

Click here for additional data file. (29KB, docx)
S1 Data

(XLSX)

Click here for additional data file. (37.6KB, xlsx)

Acknowledgments

We thank Daniel Kazdal, Mark Kriegsmann and Katarina Kriegsmann, Institute of Pathology, Heidelberg University Hospital, Heidelberg, Germany for the contribution in dyeing and scanning of the histological lung slices and Christopher Buesch, Institute for medical biometry and informatic, Heidelberg University Hospital, Heidelberg, Germany, for statistical advices.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

The author(s) received no specific funding for this work.

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Decision Letter 0

Linda L Maerz

3 Jan 2020

PONE-D-19-29054

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

PLOS ONE

Dear Dr. Kalenka,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

The authors are to be commended for the design and reporting of a study relevant to clinical practice in the critically ill patient population.  Although the manuscript is generally well written and well presented, there are several errors of English language usage and a few typographical errors that should be corrected.  These errors were identified during a careful line-by-line review of the manuscript.  The assistance of a copy editor with expertise in English publication may prove useful. 

More significantly, there are several errors in the references.  First, and most importantly, the references are to be cited in order of mention in the manuscript.  For example, the first references mentioned in the manuscript are Reference 5 and Reference 6.  Therefore, Reference 5 should actually be Reference 1.  Accordingly, all of the references will have to be re-ordered and re-numbered.  Additionally, several errors in the formatting of the references must be corrected.  Some of the titles of the references are capitalized when they should be listed in lower case; some of the terminal page numbers are missing; and there are discrepancies of years of publication in a few of the references (two different years of publication listed within the same reference).  The authors are directed to the PLOS ONE submission guidelines with respect to formatting of references:  “Because all references will be linked electronically as much as possible to the papers they cite, proper formatting of the references is crucial.”

A revision that focuses on attention to these details and correction of the errors is required for the manuscript to be considered for publication in PLOS ONE.   

We would appreciate receiving your revised manuscript within 45 days of the date of this letter. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Linda L. Maerz, MD

Academic Editor

PLOS ONE

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In addition please complete and submit a copy of the ARRIVE Guidelines checklist, a document that aims to improve experimental reporting and reproducibility of animal studies for purposes of post-publication data analysis and reproducibility: https://www.nc3rs.org.uk/arrive-guidelines. Please include your completed checklist as a Supporting Information file. Note that if your paper is accepted for publication, this checklist will be published as part of your article.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Important question to answer and appropriate porcine model. Technically sound with reasonable statistical analysis and available data. It seems as though selection of an intra-abdominal study pressure of 10 mm Hg was thoughtful. Could consider follow up study at pressures of 15 mm Hg and perhaps 20 mm Hg to evaluate optimal PEEP in those instances. Clinical scenarios rarely end at 6 hrs in human subjects and extending IAP and the study for a prolonged period may also reveal a difference in optimal PEEP recommendations for short term vs. long term intubation.

Reviewer #2: This manuscript is a well-written summary and discussion of a well-designed study. It is a small study and an animal study, and so further study will be needed prior to any significant change in practice. However, the basics of the lung mechanics are relevant and have not been well-established in patients with mildly elevated intra-abdominal pressure. Importantly, this is not meant to comment on patients with abdominal compartment syndrome or larger increases in IAP.

I have no specific concerns about the ethics or publication of this study.

I recommend this study for publication.

**********

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Decision Letter 1

Linda L Maerz

22 Jan 2020

PONE-D-19-29054R1

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

PLOS ONE

Dear Dr. Kalenka,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

The authors are to be commended for beginning to address some of the technical concerns raised regarding their manuscript.  However, some of the English usage and typographical errors persist, and some new errors are identified in the revised manuscript.  For example:

  • In the templated abstract (at the very beginning of the submission) formatting irregularities that were not present previously occur with some of the abbreviations.

  • Also in the templated abstract, the words “Methods,” “Results,” and “Conclusions” should appear at the beginning of the paragraphs to which they refer.

  • Page 5, line 12:Is “0,3 mg/kg” correct, or should it be “0.3” or “0-3” or something else?

  • Page 5, line 14:Is “3,6 mg/kg” correct, or should it be something else (similar to the prior item)?

  • Page 6, line 1:“hemodynamic” should be “hemodynamically”

  • Page 7, line 24: “weighted” should be “weighed”

  • Page 8, lines 4-7:The two sentences beginning with “In case of significant results . . . “ are awkwardly worded and difficult to understand.

  • Page 9, line 19: “to” should be “from”

  • Page 10, line 2:“ration” should be “ratio”

  • Page 11, lines 21-24:The two sentences beginning with “The authors argued that. . . “ should be one sentence with a comma after “decreases” in line 23, or should be reworded altogether.

  • Page 13, line 3:“exsudate” should be “exudate”

  • Page 13, line 4:Use “ALI” instead of “acute lung injury” since the acronym has already been introduced.

  • Page 13, line 9:“Alterations” should be “Alteration”

  • Page 21, line 9:“hold” should be “held”

  • Page 21, line 12:“bodyweight” should be “body weight”

  • Figure 1:“FiO2 0,4” should be “FiO2 0.4” if the 0.4 parameter is correct.However, this is contradicted in the manuscript:Page 5, line 20 states “(FiO2) of 0.3”

  • Figure 1 legend, last line:“hold” should be “held”

Additionally, new errors are noted in the citing and reporting of the references.  Specifically:

  • References 25 and 26 are cited on page 11, line 11.However, there is no antecedent citing of references 22, 23, or 24.Reference 24 is cited out of order (after the aforementioned citing of References 25 and 26).Reference 22 and Reference 23 are not cited at all.

  • The last reference cited in the manuscript is Reference 30.However, 33 references are listed at the end of the manuscript.

  • The authors did not provide tracked changes for their revisions to the list of references at the end of the manuscript.

  • Reference 5:The acronym “acs” should be “ACS”

  • Reference 21:“american thoracic society” should be “American Thoracic Society”

  • Reference 27:The title of the manuscript should be lower case.

The above notations are examples of errors.  Again, the assistance of a copy editor with expertise in English publication may prove useful.  An additional revision that focuses attention on correction of these and any additional errors is required for the manuscript to be considered for publication in PLOS ONE.  Of note, the errors in reference order and citation are particularly important.

We would appreciate receiving your revised manuscript by 45 days from the date of this letter. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Linda L. Maerz, MD

Academic Editor

PLOS ONE

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Decision Letter 2

Linda L Maerz

18 Feb 2020

PONE-D-19-29054R2

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

Dear Dr. Kalenka, 

Thank you for submitting your revised manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to again submit a revised version of the manuscript that addresses the points raised during the review process.

The authors are to be commended for addressing the majority of the requested revisions.  In particular, the re-numbering of the references is appreciated.  However, a few items remain to be addressed:

  • In the templated abstract (at the very beginning of the submission) formatting irregularities that were not present in the original submission occur with some of the abbreviations.  Specifically, this pertains to the abbreviations H2O, Cstat, and ΔPL.

  • Figure 1:  "FiO2 0,4" should be "FiO2 0.4"

  • Figure 1 legend, last line: “hold” should be “held”

  • Reference 27:The title of the manuscript should be lower case.

The authors’ attention to these remaining details will be appreciated.

==============================

We would appreciate receiving your revised manuscript within 45 days of the date of this letter. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Linda L. Maerz, MD

Academic Editor

PLOS ONE

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Decision Letter 3

Linda L Maerz

10 Mar 2020

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

PONE-D-19-29054R3

Dear Dr. Kalenka,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

Shortly after the formal acceptance letter is sent, an invoice for payment will follow. To ensure an efficient production and billing process, please log into Editorial Manager at https://www.editorialmanager.com/pone/, click the "Update My Information" link at the top of the page, and update your user information. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, you must inform our press team as soon as possible and no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

With kind regards,

Linda L. Maerz, MD

Academic Editor

PLOS ONE

Acceptance letter

Linda L Maerz

30 Mar 2020

PONE-D-19-29054R3

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

Dear Dr. Kalenka:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Linda L. Maerz

Academic Editor

PLOS ONE

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Checklist

    (PDF)

    Click here for additional data file. (207.6KB, pdf)
    S1 Table. The lung injury scoring system.

    Quantitative assessment for lung injury calculated by the averaging score for five independent variables and the resulting score (from 0 = no damage to 1 = maximal alteration).

    (DOCX)

    Click here for additional data file. (14.4KB, docx)
    S1 Fig. Alterations of static lung compliance to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

    Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6, # = p<0.05 group A vs. group B, § = p<0.05 group A vs. group C, $ = p<0.05 group B vs. group C. Mean values with SEM are illustrated.

    (DOCX)

    Click here for additional data file. (49.8KB, docx)
    S2 Fig. Alterations of inspiratory transpulmonary pressure and expiratory transpulmonary pressure to an intraabdominal pressure of 10 mmHg over 6 hours mechanical ventilation.

    Inline graphic = group A with PEEP 5 cmH2O, Inline graphic = group B with PEEP 10 cmH2O Inline graphic = group C with PEEP 15 cmH2O * = p<0.05 HO vs. H6. Mean values with SEM are illustrated.

    (DOCX)

    Click here for additional data file. (59.7KB, docx)
    S3 Fig. Wet-dry weight ratio of areas of the right lung.

    Mean values with SEM are illustrated.

    (DOCX)

    Click here for additional data file. (29KB, docx)
    S1 Data

    (XLSX)

    Click here for additional data file. (37.6KB, xlsx)
    Attachment

    Submitted filename: Response to reviewer PLOS_one.docx

    Click here for additional data file. (16.8KB, docx)
    Attachment

    Submitted filename: Response to reviewer PLOS_one_second_revision.docx

    Click here for additional data file. (18.2KB, docx)
    Attachment

    Submitted filename: Response to reviewer PLOS_one_second_revision.docx

    Click here for additional data file. (18.2KB, docx)

    Data Availability Statement

    All relevant data are within the manuscript and its Supporting Information files.

    Articles from PLoS ONE are provided here courtesy of PLOS

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