Effects of positive end-expiratory pressure strategy in supine and prone position on lung and chest wall mechanics in acute respiratory distress syndrome

Dipartimento di Anestesiologia, Terapia Intensiva e Scienze Dermatologiche, Fondazione IRCCS–Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Università degli Studi di Milano, Milan, Italy, and Dipartimento di Anestesia, Rianimazione e Terapia del Dolore, Fondazione IRCCS–Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Milan, Italy. gattinon@policlinico.mi.itIN the current issue of Anesthesiology, Petersson et al. provides us with a physiologic study describing, in anesthetized human volunteers, the effects of prone positioning and the application of 10 cm H2O positive end-expiratory pressure (PEEP) on the regional distribution of pulmonary ventilation and perfusion.1This article creates a complete formulation of the pulmonary ventilation and perfusion in the prone position.In the supine position, at 0 cm H2O PEEP, the size of the alveolar units decreases exponentially from ventral (nondependent) to dorsal (dependent) lung regions.2This indicates that the distending forces of the lung (i.e. , the difference between the alveolar and the pleural pressure) decreases along the ventral-to-dorsal axis. The increase of pleural pressure close to the dependent lung regions is commonly considered the result of the push of the abdominal organs towards the lungs, which increases from the ventral to the dorsal regions.In spontaneously breathing subjects, the engine of ventilation is the diaphragm, which displaces a huge amount in its dorsal (dependent) portion. This action is associated with a more favorable position of the dependent alveolar units along their pressure-volume curves3and accounts for the greater ventilation observed in the most dependent lung regions. During anesthesia and paralysis, however, the diaphragm acts as a passive flaccid membrane. The insufflated gas is then preferentially distributed towards the ventral and nondependent lung areas.4Because the regional distribution of lung perfusion is greater in the dependent lung regions, the final result is that mechanical ventilation, at 0 cm H2O PEEP, is associated with some degree of ventilation-to-perfusion (VA/Q) mismatch. This result is consistent with both the gravitational (West et al .5) or fractal distribution (Glenny et al. 6) theories of lung ventilation/perfusion. The addition of PEEP partially corrects this mismatch because it progressively moves ventilation towards the dependent lung regions (as previously shown by computed tomography scanning7and in the current study1), whereas perfusion is further increased in the dependent lung regions.In the prone position, at 0 cm H2O PEEP, the size of alveolar units decreases with an exponential decay from dorsal (now nondependent) to ventral (now dependent) lung regions. This occurs to a much lower extent than that observed in the supine position. As a result, alveolar ventilation is more homogeneously distributed in the prone than in the supine position.2Because lung perfusion redistributes towards the dependent regions, this results in a more homogenous VA/Q matching at 0 cm H2O PEEP, such as shown by Petersson et al. 1and others.8,9Surprisingly, after the addition of PEEP, Petersson et al. found that perfusion increased in the ventral lung regions (now dependent), whereas the distribution of alveolar ventilation remained unchanged. Consequently, the authors claimed that VA/Q matching was decreased by the addition of PEEP in the prone position and suggested that lower PEEP levels might be preferred in the prone position compared with the levels of PEEP used in the supine position.This conclusion may be incorrect when a patient has underlying acute lung injury. Because patients with acute lung injury often have severe hypoxemia resistant to typical therapies, Bryan 10suggested that prone positioning might lead to improved oxygenation. His prediction was fully confirmed in most of the studies subsequently published, which undoubtedly showed that in approximately 70% of patients with acute respiratory distress syndrome (ARDS), prone position–always applied in association with some degree of PEEP–improves oxygenation. Therefore, there is clearly a difference between normal lungs; for example, a deterioration of VA/Q was observed by the current authors after 10 cm H2O PEEP was added to the patients in the prone position.1The explanation for the improvement of VA/Q in patients with ARDS in the prone position involves understanding the distribution of edema in the diseased lungs. In patients with ARDS, the mass of the lung with the edema may be increased to 300% of that of normal lungs.11Therefore, the dependent lung regions in ARDS patients are compressed from the abnormal weight of the lung tissue above (nondependent) in the supine position.12When the ARDS patient is prone, the mass of the dorsal lung, which reinflates (i.e. , dorsal becomes the nondependent lung regions), is greater than the potential mass of the ventral (now dependent) lung regions, which may collapse.13When lung perfusion is substantially unmodified, the overall VA/Q matching improves as new pulmonary units are recruited for more effective gas exchange.This is probably the primary mechanism for the improvement in oxygenation in the prone ARDS patient, although other mechanisms (including a different shape of the diaphragm, changes of hypoxic pulmonary vasoconstriction, and a differential production of nitric oxide in different lung regions) may play a role. Sadly, there can be negative consequences to prone positioning, including a possible increase in chest wall stiffness.2The reduced chest wall compliance leads, in the case of pressure-controlled ventilation, to an initial reduction in transpulmonary pressure (i.e. , decreased tidal volume) or, in the case of volume-controlled ventilation, to an increase in plateau airway pressure. The overall balance of the positive and negative effects of the prone position can be observed by looking at the variation in arterial carbon dioxide. Independent of oxygenation changes, a decrease in arterial carbon dioxide indicates a recruitment of lung parenchyma, whereas an increase in arterial carbon dioxide may indicate a large increase in chest wall stiffness.We believe that the most recent clinical trial of prone ARDS patients may provide some insights about the relationship between PEEP and the prone position.14In that study, the patients that had been randomized to the prone-arm were allowed to undergo a variation in the ventilator settings aimed towards a less dangerous ventilation, if the oxygenation improved. Two maneuvers were allowed: first, a reduction of inspired oxygen fraction, and second, a reduction of PEEP, with a target arterial partial pressure of oxygen between 70–90 mmHg. The results clearly showed an identical level of PEEP between the two arms, suggesting that a decrease in PEEP was not possible in the prone ARDS patients.These data from prone ARDS patients, contrast with the findings observed by Petersson et al. in normal patients.1The comparison of the results suggest that in ARDS patients, reductions of PEEP are inappropriate, at least when VA/Q matching and systemic oxygenation are being evaluated.Finally, although the article by Petersson et al. , as well as our comments, have focused on gas exchange, there may be an effect from prone positioning in ARDS patients on their survival. The survival benefit of prone positioning during ARDS is probably a result of a decrease in the harmful effects of mechanical ventilation. The prone position leads to more homogeneous lung inflation and more homogeneous alveolar ventilation, suggesting that the strain applied to the lung parenchyma and its associated stress are more homogeneously distributed than in the supine position.15This should decrease ventilator-induced lung injury. As a matter of fact, all the meta-analyses performed on prone positioning of ARDS patients, so far, agree with two major points: (1) In all patients, a systemic oxygenation improvement is observed, and this is obviously greater in the most hypoxemic patients; and (2) in the most severe ARDS patients, when lung dishomogeneity is the greatest, prone positioning appears to provide about a 10% survival benefit.16,17Dipartimento di Anestesiologia, Terapia Intensiva e Scienze Dermatologiche, Fondazione IRCCS–Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Università degli Studi di Milano, Milan, Italy, and Dipartimento di Anestesia, Rianimazione e Terapia del Dolore, Fondazione IRCCS–Ospedale Maggiore Policlinico, Mangiagalli, Regina Elena di Milano, Milan, Italy. gattinon@policlinico.mi.it

The optimal strategy for positive end-expiratory pressure (PEEP) titration in the management of severe acute respiratory distress syndrome (ARDS) patients remains unclear. Current guidelines emphasize the importance of a careful risk–benefit assessment for PEEP titration in terms of cardiopulmonary function in these patients. Over the last few decades, the primary goal of PEEP usage has shifted from merely improving oxygenation to emphasizing lung protection, with a growing focus on the individual pattern of lung injury, lung and chest wall mechanics, and the hemodynamic consequences of PEEP. In moderate-to-severe ARDS patients, prone positioning (PP) is recommended as part of a lung protective ventilation strategy to reduce mortality. However, the physiologic changes in respiratory mechanics and hemodynamics during PP may require careful re-assessment of the ventilation strategy, including PEEP. For the most severe ARDS patients with refractory gas exchange impairment, where lung protective ventilation is not possible, veno-venous extracorporeal membrane oxygenation (V-V ECMO) facilitates gas exchange and allows for a “lung rest” strategy using “ultraprotective” ventilation. Consequently, the importance of lung recruitment to improve oxygenation and homogenize ventilation with adequate PEEP may differ in severe ARDS patients treated with V-V ECMO compared to those managed conservatively. This review discusses PEEP management in severe ARDS patients and the implications of management with PP or V-V ECMO with respect to respiratory mechanics and hemodynamic function.

BackgroundProne positioning in combination with the application of low tidal volume and adequate positive end-expiratory pressure (PEEP) improves survival in patients with moderate to severe acute respiratory distress syndrome (ARDS). However, the effects of PEEP on end-expiratory transpulmonary pressure (Ptpexp) during prone positioning require clarification. For this purpose, the effects of three different PEEP titration strategies on Ptpexp, respiratory mechanics, mechanical power, gas exchange, and hemodynamics were evaluated comparing supine and prone positioning.MethodsIn forty consecutive patients with moderate to severe ARDS protective ventilation with PEEP titrated according to three different titration strategies was evaluated during supine and prone positioning: (A) ARDS Network recommendations (PEEPARDSNetwork), (B) the lowest static elastance of the respiratory system (PEEPEstat,RS), and (C) targeting a positive Ptpexp (PEEPPtpexp). The primary endpoint was to analyze whether Ptpexp differed significantly according to PEEP titration strategy during supine and prone positioning.ResultsPtpexp increased progressively with prone positioning compared with supine positioning as well as with PEEPEstat,RS and PEEPPtpexp compared with PEEPARDSNetwork (positioning effect p < 0.001, PEEP strategy effect p < 0.001). PEEP was lower during prone positioning with PEEPEstat,RS and PEEPPtpexp (positioning effect p < 0.001, PEEP strategy effect p < 0.001). During supine positioning, mechanical power increased progressively with PEEPEstat,RS and PEEPPtpexp compared with PEEPARDSNetwork, and prone positioning attenuated this effect (positioning effect p < 0.001, PEEP strategy effect p < 0.001). Prone compared with supine positioning significantly improved oxygenation (positioning effect p < 0.001, PEEP strategy effect p < 0.001) while hemodynamics remained stable in both positions.ConclusionsProne positioning increased transpulmonary pressures while improving oxygenation and hemodynamics in patients with moderate to severe ARDS when PEEP was titrated according to the ARDS Network lower PEEP table. This PEEP titration strategy minimized parameters associated with ventilator-induced lung injury induction, such as transpulmonary driving pressure and mechanical power. We propose that a lower PEEP strategy (PEEPARDSNetwork) in combination with prone positioning may be part of a lung protective ventilation strategy in patients with moderate to severe ARDS.Trial registrationGerman Clinical Trials Register (DRKS00017449). Registered June 27, 2019. https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00017449

Experimental models suggest that prone position and positive end-expiratory pressure (PEEP) homogenize ventral-dorsal ventilation distribution and regional respiratory compliance. However, this response still needs confirmation on humans. Therefore, this study aimed to assess the changes in global and regional respiratory mechanics in supine and prone positions over a range of PEEP levels in acute respiratory distress syndrome (ARDS) patients. A prospective cohort study. Twenty-two intubated patients with ARDS caused by COVID-19 pneumonia. Electrical impedance tomography and esophageal manometry were applied during PEEP titrations from 20 cm H2O to 6 cm H2O in supine and prone positions. Global respiratory system compliance (Crs), chest wall compliance, regional lung compliance, ventilation distribution in supine and prone positions. Compared with supine position, the maximum level of Crs changed after prone position in 59% of ARDS patients (n = 13), of which the Crs decreased in 32% (n = 7) and increased in 27% (n = 6). To reach maximum Crs after pronation, PEEP was changed in 45% of the patients by at least 4 cm H2O. After pronation, the ventilation and compliance of the dorsal region did not consistently change in the entire sample of patients, increasing specifically in a subgroup of patients who showed a positive change in Crs when transitioning from supine to prone position. These combined changes in ventilation and compliance suggest dorsal recruitment postpronation. In addition, the subgroup with increased Crs postpronation demonstrated the most pronounced difference between dorsal and ventral ventilation distribution from supine to prone position (p = 0.01), indicating heterogeneous ventilation distribution in prone position. Prone position modifies global respiratory compliance in most patients with ARDS. Only a subgroup of patients with a positive change in Crs postpronation presented a consistent improvement in dorsal ventilation and compliance. These data suggest that the response to pronation on global and regional mechanics can vary among ARDS patients, with some patients presenting more dorsal lung recruitment than others.

BackgroundHigh positive end-expiratory pressure (PEEP) may improve ventilation-perfusion (V/Q) mismatch in acute respiratory distress syndrome (ARDS) patients with high recruitability in supine position. However, impacts of PEEP on V/Q mismatch in prone position relative to supine position remain poorly understood. We aimed to compare PEEP effects between positions, and evaluate its relationship with lung recruitability.MethodsA total of 50 patients with moderate-to-severe ARDS were consecutively enrolled. Recruitment-to-inflation (R/I) ratio was used to measure baseline lung recruitability. V/Q mismatch, gas exchange, and respiratory mechanics were evaluated at PEEP 5 cm H2O and PEEP 15 cm H2O in supine position, followed by PEEP 5 cm H2O and PEEP 15 cm H2O in prone position.ResultsMedian R/I ratio was 0.60 [0.38, 0.72], separating patients with higher and lower R/I ratio. In patients with higher R/I ratio, PEEP 15 cm H2O (compared to PEEP 5 cm H2O) reduced shunt without significantly increasing dead space in both supine and prone positions, thereby improving V/Q mismatch (supine: 30.7 [28.6, 36.8]% at PEEP 15 cm H2O vs.38.0 [34.9, 45.4]% at PEEP 5 cm H2O, P < 0.001; prone: 25.7 [21.4, 30.4]% at PEEP 15 cm H2O vs. 32.8 [27.5, 36.7]% at PEEP 5 cm H2O, P < 0.001). However, in patients with lower R/I ratio, PEEP 15 cm H2O (compared to PEEP 5 cm H2O) in supine position did not improve V/Q mismatch (37.8 [34.7, 42.1]% at PEEP 15 cm H2O vs. 41.3 [34.8, 45.4]% at PEEP 5 cm H2O, P = 0.078), as the reduction in shunt was accompanied by a significant increase in dead space; in prone position, PEEP 15 cm H2O significantly worsened V/Q mismatch (35.9 [28.3, 43.4]% at PEEP 15 cm H2O vs. 31.7 [24.3, 37.6]% at PEEP 5 cm H2O, P < 0.001), as it failed to reduce shunt while significantly increasing dead space.ConclusionsIn patients with higher R/I ratio, PEEP 15 cm H2O improved V/Q mismatch in both supine and positions. In patients with lower R/I ratio, PEEP 15 cm H2O did not impact V/Q mismatch in supine position but significantly worsened it in prone position.

Positive end-expiratory pressure titrated according to respiratory system mechanics or to ARDSNetwork table did not guarantee positive end-expiratory transpulmonary pressure in acute respiratory distress syndrome

The objective of the hypertension programme of the Coronary Risk Factor Study (CORIS) was to evaluate the effectiveness of the first 4 years of community-based intervention. The hypertension intervention model comprised a blood pressure station where the whole population was screened for hypertension, non-drug management was provided and hypertensives were monitored after referral to general practitioners for drug therapy. Two levels of intervention were maintained: in the high-intensity intervention town (N = 2,278) hypertensives were actively followed up, and in the low-intensity intervention town (N = 2,620) no active follow-up procedure existed. A third town acted as control (N = 2,290). In the cohort which was hypertensive at baseline, the net decreases in systolic blood pressure (mean +/- SE) after correction for changes in the control town were 0.5 +/- 2.2 mmHg (men) and 4.5 +/- 2.2 mmHg (women) in the low-intensity intervention town, and 5.6 +/- 2.3 mmHg (men) and 7.5 +/- 2.2 mmHg (women) in the high-intensity intervention town. The net decrease in diastolic blood pressure was 3.4 +/- 1.2 mmHg (men) and 4.4 +/- 1.1 mmHg (women) in the low-intensity intervention town, and 6.1 +/- 1.2 mmHg (men) and 5.9 +/- 1.1 mmHg (women) in the high-intensity intervention town. These reductions were statistically significant with one exception. The changes in the total population in the 3 communities after 4 years of intervention were similar to those found in the hypertensive cohort.(ABSTRACT TRUNCATED AT 250 WORDS)

Real-time effects of changing body position and positive end-expiratory pressure (PEEP) on regional lung overdistension and collapse in individual patients remain largely unknown and not timely monitored. The aim of this study was to individualize PEEP in supine and prone body positions seeking to reduce lung collapse and overdistension in mechanically ventilated patients with coronavirus disease (COVID-19)-induced acute respiratory distress syndrome (ARDS). We hypothesized that prone positioning with bedside titrated PEEP would provide attenuation of both overdistension and collapse. In this prospective observational study, patients with COVID-19-induced ARDS under mechanical ventilation were included. We used electrical impedance tomography (EIT) with decremental PEEP titration algorithm (PEEPEIT-titration), which provides information on regional lung overdistension and collapse, along with global respiratory system compliance, to individualize PEEP and body position. PEEPEIT-titration in supine position followed by PEEPEIT-titration in prone position were performed. Immediately before each PEEPEIT-titration, the same lung recruitment maneuver was performed: 2 min of PEEP 24 cmH2O and driving pressure of 15 cmH2O. Forty-two PEEPEIT-titration were performed in ten patients (21 pairs supine and prone positions). We have found larger % of overdistension along the PEEP titration in prone than supine position (P = 0.042). A larger % of collapse along the PEEP titration was found in supine than prone position (P = 0.037). A smaller respiratory system compliance was found in prone than supine position (P < 0.0005). In patients with COVID-19-induced ARDS, prone body position, when compared with supine body position, decreased lung collapse at low PEEP levels, but increased lung overdistension at PEEP levels greater than 10 cm H2O.Trial registration number: NCT04460859.

RATIONALE: Impaired ventilation-perfusion matching is an inherent nature and prognostic marker of acute respiratory distress syndrome (ARDS). Higher positive end-expiratory pressure (PEEP) or prone position or both could improve ventilation-perfusion matching. However, the efficacy of these interventions is modulated by lung recruitability, which varies among ARDS patients. We aim to compare the effects of position and PEEP on ventilation-perfusion matching between high and low-recruiters and propose a personalized ventilation strategy based on lung recruitability. METHOD: 30 patients with moderate-to-severe ARDS were enrolled. Lung recruitability (assessed by recruitment-to-inflation ratio) and ventilation-perfusion matching (through EIT by saline bonus injection) were measured with a combination of body position (supine or prone) and PEEP (5 cmH2O or 15 cmH2O) in sequential. The utility of recruitment-to-inflation ratio to identify the optimal ventilation strategy was examined by global ventilation-perfusion matching. RESULTS: Median recruitment-to-inflation ratio was 0.62 [IQR, 0.42-0.73], separating high and low-recruiters. In high recruiters, a combination of high PEEP and prone position achieved the highest ventilation-perfusion matching (high PEEP + prone vs. low PEEP + supine: mean difference, 16.31% [95% CI, 9.66% to 22.95%], p&amp;lt;0.001; high PEEP + prone vs. low PEEP + prone: mean difference, 6.93% [95% CI, 0.29% to 13.58%], p=0.037), although no significant difference was detected when high PEEP + prone was compared with high PEEP + supine (mean difference, 6.04% [95% CI, -0.60% to 12.67%], p= 0.087). In low recruiters, only the combination of low PEEP and prone improved ventilation-perfusion matching (low PEEP + prone vs. low PEEP + supine: mean difference, 12.71% [95% CI, 4.55% to 20.87%], p&amp;lt;0.001). Recruitment-to-inflation ratio showed a positive correlation with the improvement of global lung ventilation-perfusion matching after applying higher PEEP both in supine and prone position (supine: Pearson's r = 0.512, p=0.004; prone: Pearson's r = 0.742, p&amp;lt;0.001). CONCLUSION: Recruitment-to-inflation ratio may be helpful to personalize ventilation strategy in ARDS patients. Higher PEEP in prone position and lower PEEP in prone position achieve the optimal ventilation-perfusion matching in high recruiters and low recruiters, respectively.

Although included studies suggest that prone positioning may offer some advantage, there was little evidence to make definitive recommendations. There appears to be low certainty evidence that positioning improves oxygenation in mechanically ventilated children with ARDS. Due to the increased risk of SIDS with prone positioning and lung injury with artificial ventilation, it is recommended that hospitalised infants and children should only be placed in this position while under continuous cardiorespiratory monitoring.

Background: Different positive end-expiratory pressure (PEEP) strategies are available for subjects with coronavirus disease 2019 (COVID-19)–induced acute respiratory distress syndrome (ARDS) requiring invasive mechanical ventilation. We aimed to evaluate three conventional PEEP strategies on their effects on respiratory mechanics, gas exchanges, and hemodynamics.Methods: This is a prospective, physiologic, multicenter study conducted in China. We recruited 20 intubated subjects with ARDS and confirmed COVID-19. We first set PEEP by the ARDSnet low PEEP–fraction of inspired oxygen (FIO2) table. After a recruitment maneuver, PEEP was set at 15, 10, and 5 cm H2O for 10 min, respectively. Among these three PEEP levels, best-compliance PEEP was the one providing the highest respiratory system compliance; best-oxygenation PEEP was the one providing the highest PaO2 (partial pressure of arterial oxygen)/FIO2.Results: At each PEEP level, we assessed respiratory mechanics, arterial blood gas, and hemodynamics. Among three PEEP levels, plateau pressure, driving pressure, mechanical power, and blood pressure improved with lower PEEP. The ARDSnet low PEEP–FIO2 table and the best-oxygenation strategies provided higher PEEP than the best-compliance strategy (11 ± 6 cm H2O vs. 11 ± 3 cm H2O vs. 6 ± 2 cm H2O, p = 0.001), leading to higher plateau pressure, driving pressure, and mechanical power. The three PEEP strategies were not significantly different in gas exchange. The subgroup analysis showed that three PEEP strategies generated different effects in subjects with moderate or severe ARDS (n = 12) but not in subjects with mild ARDS (n = 8).Conclusions: In our cohort with COVID-19–induced ARDS, the ARDSnet low PEEP/FIO2 table and the best-oxygenation strategies led to higher PEEP and potentially higher risk of ventilator-induced lung injury than the best-compliance strategy.Clinical Trial Registration: www.ClinicalTrials.gov, identifier: NCT04359251.

This study aims to explore whether positive end-expiratory pressure (PEEP) guided by esophageal pressure is better than the acute respiratory distress syndrome network (ARDSNet) during the treatment of traumatic acute respiratory distress syndrome (ARDS) patients. The use of the oxygenation method of inhaled oxygen concentration titration PEEP is suggested. This study takes traumatic ARDS patients as the research object. The data of 23 patients were included in this study. The patients were randomly divided into two groups: the esophageal pressure titration PEEP group (n= 12), and the ARDSNet (PEEP-FiO2 table) titration PEEP group (n= 11). All patients were given mechanical ventilation, and changes in oxygenation index, respiratory mechanics, hemodynamics and inflammatory reaction index were recorded when titrating the best PEEP with the two methods on the current day of grouping and after grouping for 24, 48 and 72 hours. The PEEP titration value in the esophageal pressure group was 12 ± 4 cm H2O, and this value was significantly higher than the PEEP titration value in the ARDSNet group (8 ± 3 cm H2O) (P< 0.05). The end-expiratory transpulmonary pressure of titrating the best PEEP with the esophageal pressure method and ARDSNet method is 0.5 ± 0.7 cm H2O vs.-1.1 ± 3.3 cm H2O (P< 0.05). When titrating the best PEEP with the esophageal pressure method, lung tissue compliance, end-expiratory transpulmonary pressure and the oxygenation index are higher than those obtained through the ARDSNet method (P< 0.05). (2) In the esophageal pressure group, with the extension of treatment time, high-sensitivity C reactive protein (hs-CRP) and procalcitonin (PCT) exhibited a trend of significant decrease (P< 0.05). In the ARDSNet group, with the extension of treatment time, PCT also exhibited a significant decrease (P< 0.05), while the decrease in hs-CRP was not significant (P> 0.05). After comparing these two treatment groups at each monitoring time point, we found that the difference in hs-CRP and PCT was not statistically significant (P> 0.05). During the 72-hour treatment of interleukin-6 (IL-6) and interleukin-8 (IL-8), we found that these two were significantly lower in the esophageal pressure group than in the ARDSNet group (P< 0.05). The PEEP selection of mechanical ventilation of patients with traumatic ARDS guided by transpulmonary pressure and calculated by measuring intrapleural pressure can realize the individual adjustment of PEEP, identify ARDS patients benefiting from high PEEP, and provide a PEEP setting that can better meet the needs of traumatic patients.

BackgroundThe use of positive end-expiratory pressure (PEEP) and prone position (PP) is common in the management of severe acute respiratory distress syndrome patients (ARDS). We conducted this study to analyze the variation in lung volumes and PEEP-induced lung volume recruitment with the change from supine position (SP) to PP in ARDS patients.MethodsThe investigation was conducted in a multidisciplinary intensive care unit. Patients who met the clinical criteria of the Berlin definition for ARDS were included. The responsible physician set basal PEEP. To avoid hypoxemia, FiO2 was increased to 0.8 1 h before starting the protocol. End-expiratory lung volume (EELV) and functional residual capacity (FRC) were measured using the nitrogen washout/washin technique. After the procedures in SP, the patients were turned to PP and 1 h later the same procedures were made in PP.ResultsTwenty-three patients were included in the study, and twenty were analyzed. The change from SP to PP significantly increased FRC (from 965 ± 397 to 1140 ± 490 ml, p = 0.008) and EELV (from 1566 ± 476 to 1832 ± 719 ml, p = 0.008), but PEEP-induced lung volume recruitment did not significantly change (269 ± 186 ml in SP to 324 ± 188 ml in PP, p = 0.263). Dynamic strain at PEEP decreased with the change from SP to PP (0.38 ± 0.14 to 0.33 ± 0.13, p = 0.040).ConclusionsAs compared to supine, prone position increases resting lung volumes and decreases dynamic lung strain.

Different techniques exist to select personalized positive end-expiratory pressure in patients affected by the acute respiratory distress syndrome. The positive end-expiratory transpulmonary pressure strategy aims to counteract dorsal lung collapse, whereas electrical impedance tomography could guide positive end-expiratory pressure selection based on optimal homogeneity of ventilation distribution. We compared the physiologic effects of positive end-expiratory pressure guided by electrical impedance tomography versus transpulmonary pressure in patients affected by acute respiratory distress syndrome. Cross-over prospective physiologic study. Two academic ICUs. Twenty ICU patients affected by acute respiratory distress syndrome undergoing mechanical ventilation. Patients monitored by an esophageal catheter and a 32-electrode electrical impedance tomography monitor underwent two positive end-expiratory pressure titration trials by randomized cross-over design to find the level of positive end-expiratory pressure associated with: 1) positive end-expiratory transpulmonary pressure (PEEPPL) and 2) proportion of poorly or nonventilated lung units (Silent Spaces) less than or equal to 15% (PEEPEIT). Each positive end-expiratory pressure level was maintained for 20 minutes, and afterward, lung mechanics, gas exchange, and electrical impedance tomography data were collected. PEEPEIT and PEEPPL differed in all patients, and there was no correlation between the levels identified by the two methods (Rs = 0.25; p = 0.29). PEEPEIT determined a more homogeneous distribution of ventilation with a lower percentage of dependent Silent Spaces (p = 0.02), whereas PEEPPL was characterized by lower airway-but not transpulmonary-driving pressure (p = 0.04). PEEPEIT was significantly higher than PEEPPL in subjects with extrapulmonary acute respiratory distress syndrome (p = 0.006), whereas the opposite was true for pulmonary acute respiratory distress syndrome (p = 0.03). Personalized positive end-expiratory pressure levels selected by electrical impedance tomography- and transpulmonary pressure-based methods are not correlated at the individual patient level. PEEPPL is associated with lower dynamic stress, whereas PEEPEIT may help to optimize lung recruitment and homogeneity of ventilation. The underlying etiology of acute respiratory distress syndrome could deeply influence results from each method.

Sir, We read with interest the article written by Sehgal et al[1] and want to raise certain issues pertaining to the important topic: In Case 1, an end-expiratory transpulmonary pressure of 0–10 cm of H2O and end-inspiratory transpulmonary pressure of <25 cm of H2O were targeted. As a result, the patient received positive end expiratory pressure (PEEP) of 13 [transpulmonary pressure (Ptp) PEEP of 3 or 4] though his FiO2 was 1 throughout the hospital course. Should a higher PEEP and hence a higher Ptp PEEP been targeted in this case (though the patient had hypotension)? Talmor et al.[2] had studied application of esophageal pressure values for titration of ventilator strategies in acute respiratory distress syndrome (ARDS) patients and had used an algorithm to determine the Plexp (end-expiratory pleural pressure) and PEEP (targeting a minimum PaO2 of 55 mm of Hg) which is provided in Table 1. Trials such as ALVEOLI have also suggested a higher PEEP at a FiO2 of 1. Table 2 portrays the PEEP–FIO2 combination used in the said trial (in the lower PEEP group). Thus, it seems that the authors have measured the esophageal pressures but have not used them for appropriate PEEP titration Table 1 FiO2 and Plexp combination used by Talmor et al2 Table 2 FiO2 and PEEP combination used in ARDSNet trial The authors have attempted to tailor the ventilator strategy based on the esophageal pressure monitoring, but they have not indicated how the strategy should be altered based on the subtype, namely, ARDSp versus ARDSexp. It is seen that application of PEEP results in increased recruitment and decreased elastance of the respiratory system in ARDSexp as compared to the ARDSp as suggested by Gattinoni et al.[3] However, whether the above findings should dissuade intensivists to apply PEEP in cases of ARDSp can be ardently debated due to lack of evidence in its favor. ARDSNet and ALVEOLI had consistently used PEEP in all ARDS patients (both ARDSp and ARDSexp) based on Table 2. Talmor et al.[2] had around 23% patients having ARDSp in the esophageal pressure group for which they had used the same strategy as in ARDSexp Of the two cases described by the authors, poor chest wall compliance in Case 2 appears to be predominantly due to increased abdominal pressures (as a result of hemoperitoneum). This, however, might not be true for all extrapulmonary ARDS patients, as also highlighted in a study by Pelosi et al,[4] indicating that ARDSexp may not be a homogenous group by itself Finally, a few caveats about using esophageal pressure for titrating PEEP – the esophageal pressure may not represent the pleural pressures in normal individuals as also in critically ill patients. Arbitrarily, correction factors (Talmor et al. had subtracted 5 cm of H2O from the esophageal pressure value to correct for the effects of mediastinal weight and balloon air volume on the observed pressures) have been used which can be variable and can make the interpretation difficult.[5] Furthermore, esophageal pressures can lead to increased PEEP administration and better oxygenation parameters in subjects but its effect on mortality, ventilator-free days, etc., is still unclear.[5] Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.