Heterogeneity of regional inflection points from pressure-volume curves assessed by electrical impedance tomography

Determination of optimal positive end-expiratory pressure using electrical impedance tomography in infants under general anesthesia: Comparison between supine and prone positions

AIMS

This study determined the optimal positive end-expiratory pressure levels in infants in supine and prone positions under general anesthesia using electrical impedance tomography (EIT).

METHODS

This prospective observational single-centre study included infants scheduled for surgery in the prone position. An electrical impedance tomography sensor was applied after inducing general anesthesia. The optimal positive end-expiratory pressure in the supine position was determined in a decremental trial based on EIT and compliance. Subsequently, the patient's position was changed to prone. Electrical impedance tomography parameters, including global inhomogeneity index, regional ventilation delay, opening pressure, the centre of ventilation, and pendelluft volume, were continuously obtained up to 1 h after prone positioning. The optimal positive end-expiratory pressure in the prone position was similarly determined.

RESULTS

Data from 30 infants were analyzed. The mean value of electrical impedance tomography-based optimal positive end-expiratory pressure in the prone position was significantly higher than that in the supine position [10.9 (1.6) cmH2O and 6.1 (0.9) cmH2O, respectively (p < .001)]. Significant differences were observed between electrical impedance tomography- and compliance-based optimal positive end-expiratory pressure. Peak and mean airway, plateau, and driving pressures increased 1 h after prone positioning compared with those in the supine position. In addition, the centre of ventilation for balance in ventilation between the ventral and dorsal regions improved.

CONCLUSION

The prone position required higher positive end-expiratory pressure than the supine position in mechanically ventilated infants under general anesthesia. EIT is a promising tool to find the optimal positive end-expiratory pressure, which needs to be individualized.

Effects of individualised lung-protective ventilation with lung dynamic compliance-guided positive end-expiratory pressure titration on postoperative pulmonary complications of paediatric video-assisted thoracoscopic surgery: protocol for a randomised controlled trial

INTRODUCTION

Lung-protective ventilation strategies (LPVS) for one-lung ventilation (OLV) in paediatric patients pose greater challenges than in adults. Optimising LPVS for paediatric OLV to mitigate postoperative pulmonary complications (PPCs) has emerged as a current research focal point. However, there remains a divergence of opinions concerning the individualised setting and application of positive end-expiratory pressure (PEEP). Lung dynamic compliance (Cdyn) can serve as a reflection of the lung's physiological state in children during OLV and is a readily obtainable parameter. This study protocol is formulated to assess the effectiveness of Cdyn-guided PEEP titration on PPCs during paediatric OLV.

METHODS AND ANALYSIS

This study constitutes a single-centre, prospective, double-blind, randomised controlled trial. The trial aims to recruit 60 paediatric patients scheduled for video-assisted thoracoscopic surgery. These eligible patients will be randomly assigned to either the Cdyn-guided PEEP group or the conventional PEEP group during general anaesthesia for OLV. The primary outcome will involve assessing the incidence of PPCs at 7 days after surgery. Secondary outcomes will encompass the evaluation of the modified lung ultrasound score following surgery, as well as monitoring the oxygenation index, driving pressure and Cdyn during mechanical ventilation. Data collection will be performed by investigators who are kept blinded to the interventions.

ETHICS AND DISSEMINATION

The Clinical Trial Ethics Committee at Shenzhen Children's Hospital has conferred ethical approvals for this trial (approval number: 2022076). Results from this trial will be disseminated in peer-reviewed journals and presented at professional symposiums.

TRAIL REGISTRATION NUMBER

NCT05386901.

Correlation between normally aerated lung and respiratory system compliance at clinical high positive end-expiratory pressure in patients with COVID-19

Normally aerated lung tissue on computed tomography (CT) is correlated with static respiratory system compliance (Crs) at zero end-expiratory pressure. In clinical practice, however, patients with acute respiratory failure are often managed using elevated PEEP levels. No study has validated the relationship between lung volume and tissue and Crs at the applied positive end-expiratory pressure (PEEP). Therefore, this study aimed to demonstrate the relationship between lung volume and tissue on CT and Crs during the application of PEEP for the clinical management of patients with acute respiratory distress syndrome due to COVID-19. Additionally, as a secondary outcome, the study aimed to evaluate the relationship between CT characteristics and Crs, considering recruitability using the recruitment-to-inflation ratio (R/I ratio). We analyzed the CT and respiratory mechanics data of 30 patients with COVID-19 who were mechanically ventilated. The CT images were acquired during mechanical ventilation at PEEP level of 15 cmH2O and were quantitatively analyzed using Synapse Vincent system version 6.4 (Fujifilm Corporation, Tokyo, Japan). Recruitability was stratified into two groups, high and low recruitability, based on the median R/I ratio of our study population. Thirty patients were included in the analysis with the median R/I ratio of 0.71. A significant correlation was observed between Crs at the applied PEEP (median 15 [interquartile range (IQR) 12.2, 15.8]) and the normally aerated lung volume (r = 0.70 [95% CI 0.46-0.85], P < 0.001) and tissue (r = 0.70 [95% CI 0.46-0.85], P < 0.001). Multivariable linear regression revealed that recruitability (Coefficient = - 390.9 [95% CI - 725.0 to - 56.8], P = 0.024) and Crs (Coefficient = 48.9 [95% CI 32.6-65.2], P < 0.001) were significantly associated with normally aerated lung volume (R-squared: 0.58). In this study, Crs at the applied PEEP was significantly correlated with normally aerated lung volume and tissue on CT. Moreover, recruitability indicated by the R/I ratio and Crs were significantly associated with the normally aerated lung volume. This research underscores the significance of Crs at the applied PEEP as a bedside-measurable parameter and sheds new light on the link between recruitability and normally aerated lung.

Electrical Impedance Tomography to Monitor Hypoxemic Respiratory Failure

Hypoxemic respiratory failure is one of the leading causes of mortality in intensive care. Frequent assessment of individual physiological characteristics and delivery of personalized mechanical ventilation (MV) settings is a constant challenge for clinicians caring for these patients. Electrical impedance tomography (EIT) is a radiation-free bedside monitoring device that is able to assess regional lung ventilation and changes in aeration. With real-time tomographic functional images of the lungs obtained through a thoracic belt, clinicians can visualize and estimate the distribution of ventilation at different ventilation settings or following procedures such as prone positioning. Several studies have evaluated the performance of EIT to monitor the effects of different MV settings in patients with acute respiratory distress syndrome, allowing more personalized MV. For instance, EIT could help clinicians find the positive end-expiratory pressure that represents a compromise between recruitment and overdistension and assess the effect of prone positioning on ventilation distribution. The clinical impact of the personalization of MV remains to be explored. Despite inherent limitations such as limited spatial resolution, EIT also offers a unique noninvasive bedside assessment of regional ventilation changes in the ICU. This technology offers the possibility of a continuous, operator-free diagnosis and real-time detection of common problems during MV. This review provides an overview of the functioning of EIT, its main indices, and its performance in monitoring patients with acute respiratory failure. Future perspectives for use in intensive care are also addressed.

Setting positive end-expiratory pressure by using electrical impedance tomography

PURPOSE OF REVIEW

This review presents the principles and possibilities of setting positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT). It summarizes the major findings of recent studies where EIT was applied to monitor the effects of PEEP on regional lung function and to guide the selection of individualized PEEP setting.

RECENT FINDINGS

The most frequent approach of utilizing EIT for the assessment of PEEP effects and the PEEP setting during the time period from January 2022 till June 2023 was based on the analysis of pixel tidal impedance variation, typically acquired during stepwise incremental and/or decremental PEEP variation. The most common EIT parameters were the fraction of ventilation in various regions of interest, global inhomogeneity index, center of ventilation, silent spaces, and regional compliance of the respiratory system. The studies focused mainly on the spatial and less on the temporal distribution of ventilation. Contrast-enhanced EIT was applied in a few studies for the estimation of ventilation/perfusion matching.

SUMMARY

The availability of commercial EIT devices resulted in an increase in clinical studies using this bedside imaging technology in neonatal, pediatric and adult critically ill patients. The clinical interest in EIT became evident but the potential of this method in clinical decision-making still needs to be fully exploited.

Prone Positioning and Molecular Biomarkers in COVID and Non-COVID ARDS: A Narrative Review

Prone positioning (PP) represents a therapeutic intervention with the proven capacity of ameliorating gas exchanges and ventilatory mechanics indicated in acute respiratory distress syndrome (ARDS). When PP is selectively applied to moderate-severe cases of ARDS, it sensitively affects clinical outcomes, including mortality. After the COVID-19 outbreak, clinical application of PP peaked worldwide and was applied in 60% of treated cases, according to large reports. Research on this topic has revealed many physiological underpinnings of PP, focusing on regional ventilation redistribution and the reduction of parenchymal stress and strain. However, there is a lack of evidence on biomarkers behavior in different phases and phenotypes of ARDS. Patients response to PP are, to date, decided on PaO2/FiO2 ratio improvement, whereas scarce data exist on biomarker tracking during PP. The purpose of this review is to explore current evidence on the clinical relevance of biomarkers in the setting of moderate-severe ARDS of different etiologies (i.e., COVID and non-COVID-related ARDS). Moreover, this review focuses on how PP may modulate biomarkers and which biomarkers may have a role in outcome prediction in ARDS patients.

The effect of driving pressure-guided versus conventional mechanical ventilation strategy on pulmonary complications following on-pump cardiac surgery: A randomized clinical trial

STUDY OBJECTIVE

Postoperative pulmonary complications occur frequently and are associated with worse postoperative outcomes in cardiac surgical patients. The advantage of driving pressure-guided ventilation strategy in decreasing pulmonary complications remains to be definitively established. We aimed to investigate the effect of intraoperative driving pressure-guided ventilation strategy compared with conventional lung-protective ventilation on pulmonary complications following on-pump cardiac surgery.

DESIGN

Prospective, two-arm, randomized controlled trial.

SETTING

The West China university hospital in Sichuan, China.

PATIENTS

Adult patients who were scheduled for elective on-pump cardiac surgery were enrolled in the study.

INTERVENTIONS

Patients undergoing on-pump cardiac surgery were randomized to receive driving pressure-guided ventilation strategy based on positive end-expiratory pressure (PEEP) titration or conventional lung-protective ventilation strategy with fixed 5 cmH2O of PEEP.

MEASUREMENTS

The primary outcome of pulmonary complications (including acute respiratory distress syndrome, atelectasis, pneumonia, pleural effusion, and pneumothorax) within the first 7 postoperative days were prospectively identified. Secondary outcomes included pulmonary complication severity, ICU length of stay, and in-hospital and 30-day mortality.

MAIN RESULTS

Between August 2020 and July 2021, we enrolled 694 eligible patients who were included in the final analysis. Postoperative pulmonary complications occurred in 140 (40.3%) patients in the driving pressure group and 142 (40.9%) in the conventional group (relative risk, 0.99; 95% confidence interval, 0.82-1.18; P = 0.877). Intention-to-treat analysis showed no significant difference between study groups regarding the incidence of primary outcome. The driving pressure group had less atelectasis than the conventional group (11.5% vs 17.0%; relative risk, 0.68; 95% confidence interval, 0.47-0.98; P = 0.039). Secondary outcomes did not differ between groups.

CONCLUSION

Among patients who underwent on-pump cardiac surgery, the use of driving pressure-guided ventilation strategy did not reduce the risk of postoperative pulmonary complications when compared with conventional lung-protective ventilation strategy.

Respiratory Mechanics: Revisiting the Appraisement of Lung Recruitment

Mechanical ventilation has long been recognized as the most vital therapy for patients with ARDS. Compared with lung-protective ventilation, debates that involve the open lung strategy, which consists primarily of the lung recruitment maneuver and higher PEEP, have never been resolved. In terms of the beneficial and detrimental effects of this aggressive maneuver, appraisal of lung recruitment is essential for intensivists to make clinical decisions. This review aimed to clarify how to assess the potential for lung recruitment based on respiratory mechanics when using the pressure-volume curve or loop method and end-expiratory lung volume-static compliance of the respiratory system method. However, their limitations related to excessive generalization, accuracy, and identification of cutoff values cannot be omitted. Finally, future studies are warranted to combine these classic methods with newly invented techniques to achieve safer and more effective lung recruitment.

Clinical Features and Outcomes of Acute Kidney Injury in Critically Ill COVID-19 Patients: A Retrospective Observational Study

BACKGROUND

An alarming number of COVID-19 patients, especially in severe cases, have developed acute kidney injury (AKI).

AIM

The study aimed to assess the frequency, risk factors, and impact of AKI on mortality in critically ill COVID-19 patients.

METHODS

The study was a retrospective observational study conducted in the MICU. Univariate and multivariate analyses were performed to identify risk factors for AKI and clinical outcomes.

RESULTS

During the study period, 465 consecutive COVID-19 patients were admitted to the MICU. The patients' characteristics were median age, 64 [54-71] years; median SAPSII, 31 [24-38]; and invasive mechanical ventilation (IMV), 244 (52.5%). The overall ICU mortality rate was 49%. Two hundred twenty-nine (49.2%) patients developed AKI. The factors independently associated with AKI were positive fluid balance (OR, 2.78; 95%CI [1.88-4.11]; p < 0.001), right heart failure (OR, 2.15; 95%CI [1.25-3.67]; p = 0.005), and IMV use (OR, 1.55; 95%CI [1.01-2.40]; p = 0.044). Among the AKI patients, multivariate analysis identified the following factors as independently associated with ICU mortality: age (OR, 1.05; 95%CI [1.02-1.09]; p = 0.012), IMV use (OR, 48.23; 95%CI [18.05-128.89]; p < 0.001), and septic shock (OR, 3.65; 95%CI [1.32-10.10]; p = 0.012).

CONCLUSION

The present study revealed a high proportion of AKI among critically ill COVID-19 patients. This complication seems to be linked to a severe cardiopulmonary interaction and fluid balance management, thus accounting for a poor outcome.

Advanced Point-of-care Bedside Monitoring for Acute Respiratory Failure

Advanced respiratory monitoring involves several mini- or noninvasive tools, applicable at bedside, focused on assessing lung aeration and morphology, lung recruitment and overdistention, ventilation-perfusion distribution, inspiratory effort, respiratory drive, respiratory muscle contraction, and patient-ventilator asynchrony, in dealing with acute respiratory failure. Compared to a conventional approach, advanced respiratory monitoring has the potential to provide more insights into the pathologic modifications of lung aeration induced by the underlying disease, follow the response to therapies, and support clinicians in setting up a respiratory support strategy aimed at protecting the lung and respiratory muscles. Thus, in the clinical management of the acute respiratory failure, advanced respiratory monitoring could play a key role when a therapeutic strategy, relying on individualization of the treatments, is adopted.

Long-term dyspnea, regional ventilation distribution and peripheral lung function in COVID-19 survivors: a 1 year follow up study

Background

Dyspnea is common after COVID-19 pneumonia and can be characterized by a defective CO2 diffusion (DLCO) despite normal pulmonary function tests (PFT). Nevertheless, DLCO impairment tends to normalize at 1 year, with no dyspnea regression. The altered regional distribution of ventilation and a dysfunction of the peripheral lung may characterize dyspnea at 1 year after COVID-19 pneumonia. We aimed at assessing the pattern of airway resistance and inflammation and the regional ventilation inhomogeneity in COVID-19 pneumonia survivors at 12-months after hospital discharge.

Methods

We followed up at 1-year patients previously admitted to the respiratory units (intensive care or sub-intensive care unit) for COVID-19 acute respiratory failure at 1-year after hospital discharge. PFT (spirometry, DLCO), impulse oscillometry (IOS), measurements of the exhaled nitric oxide (FENO) and Electrical Impedance Tomography (EIT) were used to evaluate lung volumes, CO2 diffusion capacity, peripheral lung inflammation/resistances and the regional inhomogeneity of ventilation distribution. A full medical examination was conducted, and symptoms of new onset (not present before COVID-19) were recorded. Patients were therefore divided into two groups based on the presence/absence of dyspnea (defined as mMRC ≥1) compared to evaluate differences in the respiratory function derived parameters.

Results

Sixty-seven patients were admitted between October and December 2020. Of them, 42/67 (63%) patients were discharged alive and 33 were evaluated during the follow up. Their mean age was 64 ± 11 years and 24/33 (73%) were males. Their maximum respiratory support was in 7/33 (21%) oxygen, in 4/33 (12%) HFNC, in 14/33 (42%) NIV/CPAP and in 8/33 (24%) invasive mechanical ventilation. During the clinical examination, 15/33 (45%) reported dyspnea. When comparing the two groups, no significant differences were found in PFT, in the peripheral airway inflammation (FENO) or mechanical properties (IOS). However, EIT showed a significantly higher regional inhomogeneity in patients with dyspnea both during resting breathing (0.98[0.96–1] vs 1.1[1–1.1], p = 0.012) and during forced expiration (0.96[0.94–1] vs 1 [0.98–1.1], p = 0.045).

Conclusions

New onset dyspnea characterizes 45% of patients 1 year after COVID-19 pneumonia. In these patients, despite pulmonary function test may be normal, EIT shows a higher regional inhomogeneity both during quiet and forced breathing which may contribute to dyspnea.

Clinical trial registration

Clinicaltrials.gov NCT04343053, registration date 13/04/2020.

Roles of electrical impedance tomography in lung transplantation

Lung transplantation is the preferred treatment method for patients with end-stage pulmonary disease. However, several factors hinder the progress of lung transplantation, including donor shortages, candidate selection, and various postoperative complications. Electrical impedance tomography (EIT) is a functional imaging tool that can be used to evaluate pulmonary ventilation and perfusion at the bedside. Among patients after lung transplantation, monitoring the graft’s pulmonary function is one of the most concerning issues. The feasible application of EIT in lung transplantation has been reported over the past few years, and this technique has gained increasing interest from multidisciplinary researchers. Nevertheless, physicians still lack knowledge concerning the potential applications of EIT in lung transplantation. We present an updated review of EIT in lung transplantation donors and recipients over the past few years, and discuss the potential use of ventilation- and perfusion-monitoring-based EIT in lung transplantation.

PEEP titration by EIT strategies for patients with ARDS: A systematic review and meta-analysis

OBJECTIVE

To determine which method of Positive End-expiratory Pressure (PEEP) titration is more useful, and to establish an evidence base for the clinical impact of Electrical Impedance Tomography (EIT) based individual PEEP setting which appears to be a promising method to optimize PEEP in Acute Respiratory Distress Syndrome (ARDS) patients.

DESIGN

A systematic review and meta-analysis.

SETTING

4 databases (PUBMED, EMBASE, Web Of Science, and the Cochrane Library) from 1980 to December 2020 were performed.

PARTICIPANTS

Randomized clinical trials patients with ARDS.

MAIN VARIABLES

PaO₂/FiO₂-ratio and respiratory system compliance.

INTERVENSION

The quality of the studies was assessed with the Cochrane risk and bias tool.

RESULTS

8 trials, including a total of 222 participants, were eligible for analysis. Meta-analysis demonstrates a significantly EIT-based individual PEEP setting for patients receiving higher PaO₂/FiO₂ ratio as compared to other PEEP titration strategies [5 trials, 202 patients, SMD 0.636, (95% CI 0.364-0.908)]. EIT-drived PEEP titration strategy did not significantly increase respiratory system compliance when compared to other peep titration strategies, [7 trials, 202 patients, SMD -0.085, (95% CI -0.342 to 0.172)].

CONCLUSIONS

The benefits of PEEP titration with EIT on clinical outcomes of ARDS in placebo-controlled trials probably result from the visible regional ventilation of EIT. These findings offer clinicians and stakeholders a comprehensive assessment and high-quality evidence for the safety and efficacy of the EIT-based individual PEEP setting as a superior option for patients who undergo ARDS.

Prone positioning in ARDS patients supported with VV ECMO, what we should explore?

Background

Acute respiratory distress syndrome (ARDS), a prevalent cause of admittance to intensive care units, is associated with high mortality. Prone positioning has been proven to improve the outcomes of moderate to severe ARDS patients owing to its physiological effects. Venovenous extracorporeal membrane oxygenation (VV ECMO) will be considered in patients with severe hypoxemia. However, for patients with severe hypoxemia supported with VV ECMO, the potential effects and optimal strategies of prone positioning remain unclear. This review aimed to present these controversial questions and highlight directions for future research.

Main body

The clinically significant benefit of prone positioning and early VV ECMO alone was confirmed in patients with severe ARDS. However, a number of questions regarding the combination of VV ECMO and prone positioning remain unanswered. We discussed the potential effects of prone positioning on gas exchange, respiratory mechanics, hemodynamics, and outcomes. Strategies to achieve optimal outcomes, including indications, timing, duration, and frequency of prone positioning, as well as the management of respiratory drive during prone positioning sessions in ARDS patients receiving VV ECMO, are challenging and controversial. Additionally, whether and how to implement prone positioning according to ARDS phenotypes should be evaluated. Lung morphology monitored by computed tomography, lung ultrasound, or electrical impedance tomography might be a potential indication to make an individualized plan for prone positioning therapy in patients supported with VV ECMO.

Conclusion

For patients with ARDS supported with VV ECMO, the potential effects of prone positioning have yet to be clarified. Ensuring an optimal strategy, especially an individualized plan for prone positioning therapy during VV ECMO, is particularly challenging and requires further research.

Asymmetrical Lung Injury: Management and Outcome

Among mechanically ventilated patients, asymmetrical lung injury is probably extremely frequent in the intensive care unit but the lack of standardized measurements does not allow to describe any prevalence among mechanically ventilated patients. Many past studies have focused only on unilateral injury and have mostly described the effect of lateral positioning. The good lung put downward might receive more perfusion while the sick lung placed upward receive more ventilation than supine. This usually results in better oxygenation but can also promote atelectasis in the healthy lung and no consensus has emerged on the clinical indication of this posture. Recently, electrical impedance tomography (EIT) has allowed for the first time to precisely describe the distribution of ventilation in each lung and to better study asymmetrical lung injury. At low positive-end-expiratory pressure (PEEP), a very heterogeneous ventilation exists between the two lungs and the initial increase in PEEP first helps to recruit the sick lung and protect the healthier lung. However, further increasing PEEP distends the less injured lung and must be avoided. The right level can be found using EIT and transpulmonary pressure. In addition, EIT can show that in the two lungs, airway closure is present but with very different airway opening pressures (AOPs) which cannot be identified on a global assessment. This may suggest a very different PEEP level than on a global assessment. Lastly, epidemiological studies suggest that in hypoxemic patients, the number of quadrants involved has a strong prognostic value. The number of quadrants is more important than the location of the unilateral or bilateral nature of the involvement for the prognosis, and hypoxemic patients with unilateral lung injury should probably be considered as requiring lung protective ventilation as classical acute respiratory distress syndrome.

Expert opinion document: "Electrical impedance tomography: applications from the intensive care unit and beyond"

Mechanical ventilation is a life-saving technology, but it can also inadvertently induce lung injury and increase morbidity and mortality. Currently, there is no easy method of assessing the impact that ventilator settings have on the degree of lung inssflation. Computed tomography (CT), the gold standard for visually monitoring lung function, can provide detailed regional information of the lung. Unfortunately, it necessitates moving critically ill patients to a special diagnostic room and involves exposure to radiation. A technique introduced in the 1980s, electrical impedance tomography (EIT) can non-invasively provide similar monitoring of lung function. However, while CT provides information on the air content, EIT monitors ventilation-related changes of lung volume and changes of end expiratory lung volume (EELV). Over the past several decades, EIT has moved from the research lab to commercially available devices that are used at the bedside. Being complementary to well-established radiological techniques and conventional pulmonary monitoring, EIT can be used to continuously visualize the lung function at the bedside and to instantly assess the effects of therapeutic maneuvers on regional ventilation distribution. EIT provides a means of visualizing the regional distribution of ventilation and changes of lung volume. This ability is particularly useful when therapy changes are intended to achieve a more homogenous gas distribution in mechanically ventilated patients. Besides the unique information provided by EIT, its convenience and safety contribute to the increasing perception expressed by various authors that EIT has the potential to be used as a valuable tool for optimizing PEEP and other ventilator settings, either in the operative room and in the intensive care unit. The effects of various therapeutic interventions and applications on ventilation distribution have already been assessed with the help of EIT, and this document gives an overview of the literature that has been published in this context.

Management of Intraoperative Mechanical Ventilation to Prevent Postoperative Complications after General Anesthesia: A Narrative Review

Mechanical ventilation (MV) is still necessary in many surgical procedures; nonetheless, intraoperative MV is not free from harmful effects. Protective ventilation strategies, which include the combination of low tidal volume and adequate positive end expiratory pressure (PEEP) levels, are usually adopted to minimize the ventilation-induced lung injury and to avoid post-operative pulmonary complications (PPCs). Even so, volutrauma and atelectrauma may co-exist at different levels of tidal volume and PEEP, and therefore, the physiological response to the MV settings should be monitored in each patient. A personalized perioperative approach is gaining relevance in the field of intraoperative MV; in particular, many efforts have been made to individualize PEEP, giving more emphasis on physiological and functional status to the whole body. In this review, we summarized the latest findings about the optimization of PEEP and intraoperative MV in different surgical settings. Starting from a physiological point of view, we described how to approach the individualized MV and monitor the effects of MV on lung function.

Derecruitment volume assessment derived from pressure-impedance curves with electrical impedance tomography in experimental acute lung injury

Objective

To investigate the accuracy of derecruitment volume (V_DER) assessed by pressure–impedance (P-I) curves derived from electrical impedance tomography (EIT).

Methods

Six pigs with acute lung injury received decremental positive end-expiratory pressure (PEEP) from 15 to 0 in steps of 5 cmH₂O. At the end of each PEEP level, the pressure–volume (P-V) curves were plotted using the low constant flow method and release maneuvers to calculate the V_DER between the PEEP of setting levels and 0 cmH₂O (V_DER-PV). The V_DER derived from P-I curves that were recorded simultaneously using EIT was the difference in impedance at the same pressure multiplied by the ratio of tidal volume and corresponding tidal impedance (V_DER-PI). The regional P-I curves obtained by EIT were used to estimate V_DER in the dependent and nondependent lung.

Results

The global lung V_DER-PV and V_DER-PI showed close correlations (r = 0.948, P<0.001); the mean difference was 48 mL with limits of agreement of −133 to 229 mL. Lung derecruitment extended into the whole process of decremental PEEP levels but was unevenly distributed in different lung regions.

Conclusions

P-I curves derived from EIT can assess V_DER and provide a promising method to estimate regional lung derecruitment at the bedside.

Estimation of changes in cyclic lung strain by electrical impedance tomography: Proof-of-concept study

RATIONALE

Cyclic strain may be a determinant of ventilator-induced lung injury. The standard for strain assessment is the computed tomography (CT), which does not allow continuous monitoring and exposes to radiation. Electrical impedance tomography (EIT) is able to monitor changes in regional lung ventilation. In addition, there is a correlation between mechanical deformation of materials and detectable changes in its electrical impedance, making EIT a potential surrogate for cyclic lung strain measured by CT (Strain_CT ).

OBJECTIVES

To compare the global Strain_CT with the change in electrical impedance (ΔZ).

METHODS

Acute respiratory distress syndrome patients under mechanical ventilation (V_T 6 mL/kg ideal body weight with positive end-expiratory pressure 5 [PEEP 5] and best PEEP according to EIT) underwent whole-lung CT at end-inspiration and end-expiration. Biomechanical analysis was used to construct 3D maps and determine Strain_CT at different levels of PEEP. CT and EIT acquisitions were performed simultaneously. Multilevel analysis was employed to determine the causal association between Strain_CT and ΔZ. Linear regression models were used to predict the change in lung Strain_CT between different PEEP levels based on the change in ΔZ.

MAIN RESULTS

Strain_CT was positively and independently associated with ΔZ at global level (P < .01). Furthermore, the change in Strain_CT (between PEEP 5 and Best PEEP) was accurately predicted by the change in ΔZ (R² 0.855, P < .001 at global level) with a high agreement between predicted and measured Strain_CT .

CONCLUSIONS

The change in electrical impedance may provide a noninvasive assessment of global cyclic strain, without radiation at bedside.

Gravitational distribution of regional opening and closing pressures, hysteresis and atelectrauma in ARDS evaluated by electrical impedance tomography

Background

The physiological behavior of lungs affected by the acute respiratory distress syndrome (ARDS) differs between inspiration and expiration and presents heterogeneous gravity-dependent distribution. This phenomenon, highlighted by the different distribution of opening/closing pressure and by the hysteresis of the pressure–volume curve, can be studied by CT scan, but the technique expose the patient to radiations, cannot track changes during time and is not feasible at the bedside. Electrical impedance tomography (EIT) could help in assessing at the bedside regional inspiratory and expiratory mechanical properties. We evaluated regional opening/closing pressures, hysteresis and atelectrauma during inspiratory and expiratory low-flow pressure–volume curves in ARDS using electrical impedance tomography.

Methods

Pixel-level inspiratory and expiratory PV curves (PV_pixel) between 5 and 40 cmH₂O were constructed integrating EIT images and airway opening pressure signal from 8 ARDS patients. The lower inflection point in the inspiratory and expiratory PV_pixel were used to find opening (OP_pixel) and closing (CP_pixel) pressures. A novel atelectrauma index (AtI) was calculated as the percentage of pixels opening during the inspiratory and closing during the expiratory PV curves. The maximal hysteresis (HysMax) was calculated as the maximal difference between normalized expiratory and inspiratory PV curves. Analyses were conducted in the global, dependent and non-dependent lung regions.

Results

Gaussian distribution was confirmed for both global OP_pixel (r² = 0.90) and global CP_pixel (r² = 0.94). The two distributions were significantly different with higher values for OP_pixel (p < 0.0001). Regional OP_pixel and CP_pixel distributions were Gaussian, and in the dependent lung regions, both were significantly higher than in the non-dependent ones (p < 0.001). Both AtI and the HysMax were significantly higher in the dependent regions compared to the non-dependent ones (p < 0.05 for both).

Conclusions

Gravity impacts the regional distribution of opening and closing pressure, hysteresis and atelectrauma, with higher values in the dorsal lung. Regional differences between inspiratory and expiratory lung physiology are detectable at the bedside using EIT and could allow in-depth characterization of ARDS phenotypes and guide personalized ventilation settings.

Graphic abstract

Personalized Positive End-Expiratory Pressure in Acute Respiratory Distress Syndrome: Comparison Between Optimal Distribution of Regional Ventilation and Positive Transpulmonary Pressure

OBJECTIVES

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.

DESIGN

Cross-over prospective physiologic study.

SETTING

Two academic ICUs.

PATIENTS

Twenty ICU patients affected by acute respiratory distress syndrome undergoing mechanical ventilation.

INTERVENTION

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.

MEASUREMENTS AND MAIN RESULTS

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).

CONCLUSIONS

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.

Tidal Volume Lowering by Instrumental Dead Space Reduction in Brain-Injured ARDS Patients: Effects on Respiratory Mechanics, Gas Exchange, and Cerebral Hemodynamics

Background

Limiting tidal volume (V_T), plateau pressure, and driving pressure is essential during the acute respiratory distress syndrome (ARDS), but may be challenging when brain injury coexists due to the risk of hypercapnia. Because lowering dead space enhances CO₂ clearance, we conducted a study to determine whether and to what extent replacing heat and moisture exchangers (HME) with heated humidifiers (HH) facilitate safe V_T lowering in brain-injured patients with ARDS.

Methods

Brain-injured patients (head trauma or spontaneous cerebral hemorrhage with Glasgow Coma Scale at admission < 9) with mild and moderate ARDS received three ventilatory strategies in a sequential order during continuous paralysis: (1) HME with V_T to obtain a PaCO₂ within 30–35 mmHg (HME1); (2) HH with V_T titrated to obtain the same PaCO₂ (HH); and (3) HME1 settings resumed (HME2). Arterial blood gases, static and quasi-static respiratory mechanics, alveolar recruitment by multiple pressure–volume curves, intracranial pressure, cerebral perfusion pressure, mean arterial pressure, and mean flow velocity in the middle cerebral artery by transcranial Doppler were recorded. Dead space was measured and partitioned by volumetric capnography.

Results

Eighteen brain-injured patients were studied: 7 (39%) had mild and 11 (61%) had moderate ARDS. At inclusion, median [interquartile range] PaO₂/FiO₂ was 173 [146–213] and median PEEP was 8 cmH₂O [5–9]. HH allowed to reduce V_T by 120 ml [95% CI: 98–144], V_T/kg predicted body weight by 1.8 ml/kg [95% CI: 1.5–2.1], plateau pressure and driving pressure by 3.7 cmH₂O [2.9–4.3], without affecting PaCO₂, alveolar recruitment, and oxygenation. This was permitted by lower airway (− 84 ml [95% CI: − 79 to − 89]) and total dead space (− 86 ml [95% CI: − 73 to − 98]). Sixteen patients (89%) showed driving pressure equal or lower than 14 cmH₂O while on HH, as compared to 7 (39%) and 8 (44%) during HME1 and HME2 (p < 0.001). No changes in mean arterial pressure, cerebral perfusion pressure, intracranial pressure, and middle cerebral artery mean flow velocity were documented during HH.

Conclusion

The dead space reduction provided by HH allows to safely reduce V_T without modifying PaCO₂ nor cerebral perfusion. This permits to provide a wider proportion of brain-injured ARDS patients with less injurious ventilation.

Use of Electrical Impedance Tomography (EIT) to Estimate Global and Regional Lung Recruitment Volume (VREC) Induced by Positive End-Expiratory Pressure (PEEP): An Experiment in Pigs with Lung Injury

Background

Electrical impedance tomography (EIT) is a real-time tool used to monitor lung volume change at the bedside, which could be used to measure lung recruitment volume (V_REC) for setting positive end-expiratory pressure (PEEP). We assessed and compared the agreement in V_REC measurement with the EIT method versus the flow-derived method.

Material/Methods

In 12 Bama pigs, lung injury was induced by tracheal instillation of hydrochloric acid and verified by an arterial partial pressure of oxygen to inspired oxygen fraction ratio below 200 mmHg. During the end-expiratory occlusion, an airway release maneuver was conduct at 5 and 15 cmH₂O of PEEP. V_REC was measured by flow-integrated PEEP-induced lung volume change (flow-derived method) and end-expiratory lung impedance change (EIT-derived method). Linear regression and Bland-Altman analysis were used to test the correlation and agreement between these 2 measures.

Results

Lung injury was successfully induced in all the animals. EIT-derived V_REC was significantly correlated with flow-derived V_REC (R²=0.650, p=0.002). The bias (the lower and upper limits of agreement) was −19 (−182 to 144) ml. The median (interquartile range) of EIT-derived V_REC was 322 (218–469) ml, with 110 (59–142) ml and 194 (157–307) ml in dependent and nondependent lung regions, respectively. Global and regional respiratory system compliance increased significantly at high PEEP compared to those at low PEEP.

Conclusions

Close correlation and agreement were found between EIT-derived and flow-derived V_REC measurements. The advantages of EIT-derived recruitability assessment included the avoidance of ventilation interruption and the ability to provide regional recruitment information.

Driving pressure guided ventilation

Protective ventilation is a prevailing ventilatory strategy these days and is comprised of small tidal volume, limited inspiratory pressure, and application of positive end-expiratory pressure (PEEP). However, several retrospective studies recently suggested that tidal volume, inspiratory pressure, and PEEP are not related to patient outcomes, or only related when they influence the driving pressure. Therefore, this review introduces the concept of driving pressure and looks into the possibility of driving pressure-guided ventilation as a new ventilatory strategy, especially in thoracic surgery where postoperative pulmonary complications are common, and thus, lung protection is of utmost importance.

Regional air trapping in acute exacerbation of obstructive lung diseases measured with electrical impedance tomography: a feasibility study

BACKGROUND

Since bronchial abnormalities often exhibit spatial non-uniformity which may be not correctly assessed by conventional global lung function measures, regional information may help to characterize the disease progress. We hypothesized that regional air trapping during mechanical ventilation could be characterized by regional end-expiratory flow (EEF) derived from electrical impedance tomography (EIT).

METHODS

Twenty-five patients suffering from chronic obstructive pulmonary disease (COPD grade 3 or 4) or severe asthma with acute exacerbation were examined prospectively. Patients were ventilated under assist-control mode. EIT measurements were conducted before and one hour after inhaled combined corticosteroid and long-acting β2 agonist, on two consecutive days. Regional EEF was calculated as derivative of relative impedance for every image pixel in the lung regions. The results were normalized to global flow values measured by the ventilator.

RESULTS

Regional and global EEF were highly correlated (P<0.00001) and regional effects of medication and disease progression were visible in the regional EEF maps. The sums of regional EEF in lung regions were 3.8 [2.0, 5.1] and 3.6 [1.9, 4.5] L/min in COPD patients before and after medication (median [lower, upper quartiles]; P=0.37). The corresponding values in asthma patients were 3.0 [2.5, 4.2] and 2.2 [1.7, 3.2] L/min (P<0.05). Histograms of regional EEF showed high spatial heterogeneity of EEF before medication. After one day of treatment, the histograms exhibited less heterogeneous and a decrease in EEF level.

CONCLUSIONS

Regional EEF characterizes air trapping and intrinsic PEEP, which could provide diagnostic information for monitoring the disease progress during treatment.

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Electrical impedance tomography (EIT) is a bedside monitoring tool that noninvasively visualizes local ventilation and arguably lung perfusion distribution. This article reviews and discusses both methodological and clinical aspects of thoracic EIT. Initially, investigators addressed the validation of EIT to measure regional ventilation. Current studies focus mainly on its clinical applications to quantify lung collapse, tidal recruitment, and lung overdistension to titrate positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a tool to measure regional lung perfusion. Indicator-free EIT measurements might be sufficient to continuously measure cardiac stroke volume. The use of a contrast agent such as saline might be required to assess regional lung perfusion. As a result, EIT-based monitoring of regional ventilation and lung perfusion may visualize local ventilation and perfusion matching, which can be helpful in the treatment of patients with acute respiratory distress syndrome (ARDS).

A Modified Method to Assess Tidal Recruitment by Electrical Impedance Tomography

Avoiding tidal recruitment and collapse during mechanical ventilation should reduce the risk of lung injury. Electrical impedance tomography (EIT) enables detection of tidal recruitment by measuring regional ventilation delay inhomogeneity (RVDI) during a slow inflation breath with a tidal volume (VT) of 12 mL/kg body weight (BW). Clinical applicability might be limited by such high VTs resulting in high end-inspiratory pressures (PEI) during positive end-expiratory pressure (PEEP) titration. We hypothesized that RVDI can be obtained with acceptable accuracy from reduced slow inflation VTs. In seven ventilated pigs with experimental lung injury, tidal recruitment was quantified by computed tomography at PEEP levels changed stepwise between 0 and 25 cmH2O. RVDI was measured by EIT during slow inflation VTs of 12, 9, 7.5, and 6 mL/kg BW. Linear correlation of tidal recruitment and RVDI was excellent for VTs of 12 (R2 = 0.83, p < 0.001) and 9 mL/kg BW (R2 = 0.83, p < 0.001) but decreased for VTs of 7.5 (R2 = 0.76, p < 0.001) and 6 mL/kg BW (R2 = 0.71, p < 0.001). With any reduction in slow inflation VT, PEI decreased at all PEEP levels. Receiver-Operator-Characteristic curve analyses revealed that RVDI-thresholds to predict distinct amounts of tidal recruitment differ when obtained from different slow inflation VTs. In conclusion, tidal recruitment can sufficiently be monitored by EIT-based RVDI-calculation with a slow inflation of 9 mL/kg BW.

The Effect of Positive End-Expiratory Pressure on Lung Micromechanics Assessed by Synchrotron Radiation Computed Tomography in an Animal Model of ARDS

Modern ventilatory strategies are based on the assumption that lung terminal airspaces act as isotropic balloons that progressively accommodate gas. Phase contrast synchrotron radiation computed tomography (PCSRCT) has recently challenged this concept, showing that in healthy lungs, deflation mechanisms are based on the sequential de-recruitment of airspaces. Using PCSRCT scans in an animal model of acute respiratory distress syndrome (ARDS), this study examined whether the numerosity (ASnum) and dimension (ASdim) of lung airspaces change during a deflation maneuver at decreasing levels of positive end-expiratory pressure (PEEP) at 12, 9, 6, 3, and 0 cmH2O. Deflation was associated with significant reduction of ASdim both in the whole lung section (passing from from 13.1 ± 2.0 at PEEP 12 to 7.6 ± 4.2 voxels at PEEP 0) and in single concentric regions of interest (ROIs). However, the regression between applied PEEP and ASnum was significant in the whole slice (ranging from 188 ± 52 at PEEP 12 to 146.4 ± 96.7 at PEEP 0) but not in the single ROIs. This mechanism of deflation in which reduction of ASdim is predominant, differs from the one observed in healthy conditions, suggesting that the peculiar alveolar micromechanics of ARDS might play a role in the deflation process.