Electrical impedance tomography compared to positron emission tomography for the measurement of regional lung ventilation: an experimental study

The application of electrical impedance tomography and surgical outcomes of thoracoscope-assisted surgical stabilization of rib fractures in severe chest trauma

Serious blunt chest trauma usually induces hemothorax, pneumothorax, and rib fractures. More studies have claimed that early video-assisted thoracoscopic surgery with surgical stabilization of rib fractures (SSRF) results in a good prognosis in patients with major trauma. This study aimed to verify the outcomes in patients with chest trauma whether SSRF was performed. Consecutive patients who were treated in a medical center in Taiwan, for traumatic events between January 2015 and June 2020, were retrospectively reviewed. This study focused on patients with major trauma and thoracic injuries, and they were divided into groups based on whether they received SSRF. We used electrical impedance tomography (EIT) to evaluate the change of ventilation conditions. Different scores used for the evaluation of trauma severity were also compared in this study. Among the 8396 patients who were included, 1529 (18.21%) had major trauma with injury severity score > 16 and were admitted to the intensive care unit initially. A total of 596 patients with chest trauma were admitted, of whom 519 (87%) survived. Younger age and a lower trauma score (including injury severity scale, new injury severity score, trauma and injury severity score, and revised trauma score) account for better survival rates. Moreover, 74 patients received SSRF. They had a shorter intensive care unit (ICU) stay (5.24, p = 0.045) and better performance in electrical impedance tomography (23.46, p < 0.001). In patients with major thoracic injury, older age and higher injury survival scale account for higher mortality rate. Effective surgical stabilization of rib fractures shortened the ICU stay and helped achieve better performance in EIT. Thoracoscope-assisted rib fixation is suggested in severe trauma cases.

Lung impedance changes during awake prone positioning in COVID-19. A non-randomized cross-over study

BACKGROUND

The effects of awake prone positioning (APP) on respiratory mechanics in patients with COVID-19 are not well characterized. The aim of this study was to investigate changes of global and regional lung volumes during APP compared with the supine position using electrical lung impedance tomography (EIT) in patients with hypoxemic respiratory failure due to COVID-19.

MATERIALS AND METHODS

This exploratory non-randomized cross-over study was conducted at two university hospitals in Sweden between January and May 2021. Patients admitted to the intensive care unit with confirmed COVID-19, an arterial cannula in place, a PaO2/FiO2 ratio <26.6 kPa (<200 mmHg) and high-flow nasal oxygen or non-invasive ventilation were eligible for inclusion. EIT-data were recorded at supine baseline, at 30 and 60 minutes after APP-initiation, and 30 minutes after supine repositioning. The primary outcomes were changes in global and regional tidal impedance variation (TIV), center of ventilation (CoV), global and regional delta end-expiratory lung-impedance (dEELI) and global inhomogeneity (GI) index at the end of APP compared with supine baseline. Data were reported as median (IQR).

RESULTS

All patients (n = 10) were male and age was 64 (47-73) years. There were no changes in global or regional TIV, CoV or GI-index during the intervention. dEELI increased from supine reference value 0 to 1.51 (0.32-3.62) 60 minutes after APP (median difference 1.51 (95% CI 0.19-5.16), p = 0.04) and returned to near baseline values after supine repositioning. Seven patients (70%) showed an increase >0.20 in dEELI during APP. The other EIT-variables did not change during APP compared with baseline.

CONCLUSION

Awake prone positioning was associated with a transient lung recruiting effect without changes in ventilation distribution measured with EIT in patients with hypoxemic respiratory failure due to COVID-19.

Effect of automated head-thorax elevation during chest compressions on lung ventilation: a model study

Our goal was to investigate the effects of head-thorax elevation (HUP) during chest compressions (CC) on lung ventilation. A prospective study was performed on seven human cadavers. Chest was automatically compressed-decompressed in flat position and during progressive HUP from 18 to 35°. Lung ventilation was measured with electrical impedance tomography. In each cadaver, 5 sequences were randomly performed: one without CC at positive end-expiratory pressure (PEEP) 0cmH2O, 3 s with CC at PEEP0, 5 or 10cmH2O and 1 with CC and an impedance threshold device at PEEP0cmH2O. The minimal-to-maximal change in impedance (VTEIT in arbitrary unit a.u.) and the minimal impedance in every breathing cycle (EELI) the) were compared between flat, 18°, and 35° in each sequence by a mixed-effects model. Values are expressed as median (1st-3rd quartiles). With CC, between flat, 18° and 35° VTEIT decreased at each level of PEEP. It was 12416a.u. (10,689; 14,442), 11,239 (7667; 13,292), and 6457 (4631; 9516), respectively, at PEEP0. The same was true with the impedance threshold device. EELI/VTEIT significantly decreased from - 0.30 (- 0.40; - 0.15) before to - 1.13 (- 1.70; - 0.61) after the CC (P = 0.009). With HUP lung ventilation decreased with CC as compared to flat position. CC are associated with decreased in EELI.

Effect of individualized positive end-expiratory pressure based on electrical impedance tomography guidance on pulmonary ventilation distribution in patients who receive abdominal thermal perfusion chemotherapy

Background

Electrical impedance tomography (EIT) has been shown to be useful in guiding individual positive end-expiratory pressure titration for patients with mechanical ventilation. However, the appropriate positive end-expiratory pressure (PEEP) level and whether the individualized PEEP needs to be adjusted during long-term surgery (>6 h) were unknown. Meanwhile, the effect of individualized PEEP on the distribution of pulmonary ventilation in patients who receive abdominal thermoperfusion chemotherapy is unknown. The primary aim of this study was to observe the effect of EIT-guided PEEP on the distribution of pulmonary ventilation in patients undergoing cytoreductive surgery (CRS) combined with hot intraperitoneal chemotherapy (HIPEC). The secondary aim was to analyze their effect on postoperative pulmonary complications.

Methods

A total of 48 patients were recruited and randomly divided into two groups, with 24 patients in each group. For the control group (group A), PEEP was set at 5 cm H2O, while in the EIT group (group B), individual PEEP was titrated and adjusted every 2 h with EIT guidance. Ventilation distribution, respiratory/circulation parameters, and PPC incidence were compared between the two groups.

Results

The average individualized PEEP was 10.3 ± 1.5 cm H2O, 10.2 ± 1.6 cm H2O, 10.1 ± 1.8 cm H2O, and 9.7 ± 2.1 cm H2O at 5 min, 2 h, 4 h, and 6 h after tracheal intubation during CRS + HIPEC. Individualized PEEP was correlated with ventilation distribution in the regions of interest (ROI) 1 and ROI 3 at 4 h mechanical ventilation and ROI 1 at 6 h mechanical ventilation. The ventilation distribution under individualized PEEP was back-shifted for 6 h but moved to the control group's ventral side under PEEP 5 cm H2O. The respiratory and circulatory function indicators were both acceptable either under individualized PEEP or PEEP 5 cm H2O. The incidence of total PPCs was significantly lower under individualized PEEP (66.7%) than PEEP 5 cm H2O (37.5%) for patients with CRS + HIPEC.

Conclusion

The appropriate individualized PEEP was stable at approximately 10 cm H2O during 6 h for patients with CRS + HIPEC, along with better ventilation distribution and a lower total PPC incidence than the fixed PEEP of 5 cm H2O.Clinical trial registration: identifier ChiCTR1900023897.

The influence of gravity on electrical impedance tomography measurements during upper body position change

Objective

The aim of the study was to examine the influence of gravity on regional ventilation measured by electrical impedance tomography (EIT) with the standard electrode belt position at the 5th intercostal space during tilting from supine to sitting positions.

Methods

A total of 30 healthy volunteers were examined prospectively in supine position during quiet tidal breathing. Subsequently, the bed was tilted so that the upper body of the subjects achieved 30, 60 and 90° every 3 min. Regional ventilation distribution and end-expiratory lung impedance (EELI) were monitored with EIT throughout the whole experiment. Absolute tidal volumes were measured with spirometry and the volume-impedance ratio was calculated for each position.

Results

The volume-impedance ratio did not differ statistically between the studied body positions but 11 subjects exhibited a large change in ratio at one of the positions (outside 99.3% coverage). In general, ventilation distribution became more heterogeneous and moved towards dorsal regions as the upper body was tilted to 90-degree position. EELI increased and tidal volume decreased. The lung regions identified at various positions differed significantly.

Conclusion

Gravity has non-negligible influence on EIT data, as the upper body tilted from supine to sitting positions. The standard electrode belt position might be reconsidered if ventilation distribution is to be compared between supine and sitting positions.

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.

Electrical Impedance Tomography: From the Traditional Design to the Novel Frontier of Wearables

Electrical impedance tomography (EIT) is a medical imaging technique based on the injection of a current or voltage pattern through electrodes on the skin of the patient, and on the reconstruction of the internal conductivity distribution from the voltages collected by the electrodes. Compared to other imaging techniques, EIT shows significant advantages: it does not use ionizing radiation, is non-invasive and is characterized by high temporal resolution. Moreover, its low cost and high portability make it suitable for real-time, bedside monitoring. However, EIT is also characterized by some technical limitations that cause poor spatial resolution. The possibility to design wearable devices based on EIT has recently given a boost to this technology. In this paper we reviewed EIT physical principles, hardware design and major clinical applications, from the classical to a wearable setup. A wireless and wearable EIT system seems a promising frontier of this technology, as it can both facilitate making clinical measurements and open novel scenarios to EIT systems, such as home monitoring.

A review of bio-impedance devices

Bio-impedance measurement analysis primarily refers to a safe and a non-invasive technique to analyze the electrical changes in living tissues on the application of low-value alternating current. It finds applications both in the biomedical and the agricultural fields. This paper concisely reviews the origin and measurement approaches for concepts and fundamentals of bio-impedance followed by a critical review on bio-impedance portable devices with main emphasis on the embedded system approach which is in demand due to its miniature size and present lifestyle preference of monitoring health in real time. The paper also provides a comprehensive review of various bio-impedance circuits with emphasis on the measurement and calibration techniques.

Structural priors represented by discrete cosine transform improve EIT functional imaging

Structural prior information can improve electrical impedance tomography (EIT) reconstruction. In this contribution, we introduce a discrete cosine transformation-based (DCT-based) EIT reconstruction algorithm to demonstrate a way to incorporate the structural prior with the EIT reconstruction process. Structural prior information is obtained from other available imaging methods, e.g., thorax-CT. The DCT-based approach creates a functional EIT image of regional lung ventilation while preserving the introduced structural information. This leads to an easier interpretation in clinical settings while maintaining the advantages of EIT in terms of bedside monitoring during mechanical ventilation. Structural priors introduced in the DCT-based approach are of two categories in terms of different levels of information included: a contour prior only differentiates lung and non-lung region, while a detail prior includes information, such as atelectasis, within the lung area. To demonstrate the increased interpretability of the EIT image through structural prior in the DCT-based approach, the DCT-based reconstructions were compared with reconstructions from a widely applied one-step Gauss-Newton solver with background prior and from the advanced GREIT algorithm. The comparisons were conducted both on simulation data and retrospective patient data. In the simulation, we used two sets of forward models to simulate different lung conditions. A contour prior and a detail prior were derived from simulation ground truth. With these two structural priors, the reconstructions from the DCT-based approach were compared with the reconstructions from both the one-step Gauss-Newton solver and the GREIT. The difference between the reconstructions and the simulation ground truth is calculated by the ℓ2-norm image difference. In retrospective patient data analysis, datasets from six lung disease patients were included. For each patient, a detail prior was derived from the patient's CT, respectively. The detail prior was used for the reconstructions using the DCT-based approach, which was compared with the reconstructions from the GREIT. The reconstructions from the DCT-based approach are more comprehensive and interpretable in terms of preserving the structure specified by the priors, both in simulation and retrospective patient data analysis. In simulation analysis, the ℓ2-norm image difference of the DCT-based approach with a contour prior decreased on average by 34% from GREIT and 49% from the Gauss-Newton solver with background prior; for reconstructions of the DCT-based approach with detail prior, on average the ℓ2-norm image difference is 53% less than GREIT and 63% less than the reconstruction with background prior. In retrospective patient data analysis, the reconstructions from both the DCT-based approach and GREIT can indicate the current patient status, but the DCT-based approach yields more interpretable results. However, it is worth noting that the preserved structure in the DCT-based approach is derived from another imaging method, not from the EIT measurement. If the structural prior is outdated or wrong, the result might be misleadingly interpreted, which induces false clinical conclusions. Further research in terms of evaluating the validity of the structural prior and detecting the outdated prior is necessary.

The calculation of electrical impedance tomography based silent spaces requires individual thorax and lung contours

OBJECTIVE

The present study evaluates the influence of different thorax contours (generic vs individual) on the parameter "silent spaces" computed from electrical impedance tomography (EIT) measurements.

APPROACH

Six patients with acute respiratory distress syndrome were analyzed retrospectively. EIT measurements were performed and the silent spaces were calculated based on (1) patient-specific contours Sind, (2) generic adult male contours SEidorsA and (3) generic neonate contours SEidorsN.

MAIN RESULTS

The differences among all studied subjects were 5±6% and 8±7% for Sind vs. SEidorsA, Sind vs. SEidorsN, respectively (median ± interquartile range). Sind values were higher than the generic ones in two patients.

SIGNIFICANCE

In the present study, we demonstrated the differences in values when the silent spaces were calculated based on different body and organ contours. To our knowledge, this study was the first one showing explicitly that silent spaces calculated with generic thorax and lung contours might lead to results with different locations and values as compared to the calculation with subject-specific models. Interpretations of silent spaces should be proceeded with caution.

Imaging the acute respiratory distress syndrome: past, present and future

In patients with the acute respiratory distress syndrome (ARDS), lung imaging is a fundamental tool in the study of the morphological and mechanistic features of the lungs. Chest computed tomography studies led to major advances in the understanding of ARDS physiology. They allowed the in vivo study of the syndrome's lung features in relation with its impact on respiratory physiology and physiology, but also explored the lungs' response to mechanical ventilation, be it alveolar recruitment or ventilator-induced lung injuries. Coupled with positron emission tomography, morphological findings were put in relation with ventilation, perfusion or acute lung inflammation. Lung imaging has always been central in the care of patients with ARDS, with modern point-of-care tools such as electrical impedance tomography or lung ultrasounds guiding clinical reasoning beyond macro-respiratory mechanics. Finally, artificial intelligence and machine learning now assist imaging post-processing software, which allows real-time analysis of quantitative parameters that describe the syndrome's complexity. This narrative review aims to draw a didactic and comprehensive picture of how modern imaging techniques improved our understanding of the syndrome, and have the potential to help the clinician guide ventilatory treatment and refine patient prognostication.

Is there a need for individualized adjustment of electrode belt position during EIT-guided titration of positive end-expiratory pressure?

OBJECTIVE

The aim of the present study was to evaluate the variation of tidal volume-to-impedance ratio (VT/ZT) during positive end-expiratory pressure (PEEP) titration with electrical impedance tomography (EIT) measurement.

APPROACH

Forty-two patients with acute respiratory distress syndrome were retrospectively analyzed. An incremental and subsequently a decremental PEEP trial were performed with steps of 2 cmH2O and duration of 2 minutes per step during volume-controlled ventilation with decelerating flow. EIT measurement was conducted in the 5th intercostal space and VT was recorded simultaneously. The variation of VT/ZT (RatioV) was defined as the changes in percentage to average ratio per cmH2O PEEP change. A z-score>1 was considered as a significant variation and an implication that the measurement plane was inadequate.

MAIN RESULTS

The RatioV of 42 patients was 1.29±0.80 %∙cmH2O-1. A z-score of 1 corresponded to the variation of 2.09 %∙cmH2O-1. Seven patients (16.7%) had a z-score>1 and showed either positive or negative correlation between the volume-to-impedance ratio and PEEP.

SIGNIFICANCE

Electrode placement at 5th intercostal space might not be ideal for every individual during EIT measurement. Evaluation of volume-to-impedance ratio variation is necessary for patients undergoing maneuvers with wide alteration in absolute lung volume.

Global and Regional Tidal Volume Distribution in Spontaneously Breathing Mechanically Ventilated Children

BACKGROUND

Allowing the ventilated adult patient to breathe spontaneously may improve tidal volume (V_T) distribution toward the dependent lung regions, reduce shunt fraction, and decrease dead space. It has not been studied if these effects under various levels of ventilatory support also occur in children. We sought to explore the effect of level of ventilatory support on V_T distribution and end-expiratory lung volume (EELV) in spontaneously breathing ventilated children in the recovery phase of their acute respiratory failure.

METHODS

This is a secondary analysis of data from a prospective clinical trial comparing 2 different ventilator modes during weaning in mechanically ventilated children < 5 y: CPAP + pressure support ventilation (PSV) and pressure control (PC)/intermittent mandatory ventilation (IMV) + PSV with the mandatory breath rate set at 25% of baseline. Using electrical impedance tomography (EIT), we assessed V_T distribution by calculating the center of ventilation. Polynomial functions of the second degree were plotted to evaluate regional lung filling characteristics. Changes in end-expiratory impedance were calculated to assess changes in EELV. Baseline measurements were compared with measurements during CPAP/PSV, PC/IMV + PSV, and during a downward titration of the level of pressure support.

RESULTS

Thirty-five subjects with a median age 4.5 (2.1-12.9) months and a median ventilation time of 4.9 (3.3-6.9) d were studied. The overall median coefficient of variation was 50.1% and not different between CPAP/PSV or PC/synchronized IMV + PSV. Regional filling characteristics of the lung identified a homogeneous V_T distribution under all study conditions. Downtapering of the level of PSV resulted in a significant shift of the coefficient of variation toward the dependent lung regions.

CONCLUSIONS

Our data showed that allowing ventilated children in the recovery phase of respiratory failure to breathe spontaneously in a continuous spontaneous ventilation mode did not negatively affect V_T distribution or EELV.

Evaluation of atelectasis using electrical impedance tomography during procedural deep sedation for MRI in small children: A prospective observational trial

STUDY OBJECTIVE

To investigate the variation of poorly ventilated lung units (i.e., silent spaces) in children undergoing procedural sedation in a day-hospital setting, until discharge home from the Post-Anesthesia Care Unit (PACU).

DESIGN

Prospective, single-center, observational cohort trial.

SETTING

This study was conducted at the radiology department and in PACU at Bern University Hospital (Switzerland), a tertiary care hospital.

PATIENTS

We included 25 children (1-6 years, ASA I-III) scheduled for cerebral magnetic resonance imaging scan, spontaneously breathing under deep sedation. Children planned for tracheal intubation, supraglottic airway insertion, or with contraindication for propofol were excluded.

INTERVENTION

After intravenous or inhaled induction, deep sedation was performed with 10 mg/kg/h Propofol. All children received nasal oxygen 0.3 ml/kg/min.

MEASUREMENTS

The proportion of silent spaces and the global inhomogeneity index were determined at each of five procedural points, using electrical impedance tomography: before induction (T1); before (T2) and after (T3) magnetic resonance imaging; at the end of sedation before transport to the PACU (T4); and before hospital discharge (T5).

MAIN RESULTS

The median [interquartile range (IQR)] proportion of silent spaces at the five analysis points were: T1, 5% [2%-14%]; T2, 10% [7%-14%]; T3, 12% [5%-23%]; T4, 12% [7%-24%]; and T5, 3% [2%-11%]. These defined significant changes in silent spaces over the course of sedation (p = 0.009), but no differences in silent spaces from before induction to before discharge from the PACU (T1 vs. T5; p = 0.29). Median [IQR] global inhomogeneity indices were 0.57 [0.55-0.58], 0.56 [0.53-0.59], 0.56 [0.54-0.59], 0.57 [0.54-0.60] and 0.56 [0.54-0.57], respectively (p = 0.93). None of the children reported anesthesia-related complications.

CONCLUSION

Deep sedation results in significantly increased poorly ventilated lung units during sedation. However, this does not significantly affect ventilation homogeneity, which was fully resolved at discharge from the PACU.

TRIAL REGISTRATION

clinicaltrials.gov, identifier NCT04507581.

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.

Regional Lung Perfusion Analysis in Experimental ARDS by Electrical Impedance and Computed Tomography

Electrical impedance tomography is clinically used to trace ventilation related changes in electrical conductivity of lung tissue. Estimating regional pulmonary perfusion using electrical impedance tomography is still a matter of research. To support clinical decision making, reliable bedside information of pulmonary perfusion is needed. We introduce a method to robustly detect pulmonary perfusion based on indicator-enhanced electrical impedance tomography and validate it by dynamic multidetector computed tomography in two experimental models of acute respiratory distress syndrome. The acute injury was induced in a sublobar segment of the right lung by saline lavage or endotoxin instillation in eight anesthetized mechanically ventilated pigs. For electrical impedance tomography measurements, a conductive bolus (10% saline solution) was injected into the right ventricle during breath hold. Electrical impedance tomography perfusion images were reconstructed by linear and normalized Gauss-Newton reconstruction on a finite element mesh with subsequent element-wise signal and feature analysis. An iodinated contrast agent was used to compute pulmonary blood flow via dynamic multidetector computed tomography. Spatial perfusion was estimated based on first-pass indicator dilution for both electrical impedance and multidetector computed tomography and compared by Pearson correlation and Bland-Altman analysis. Strong correlation was found in dorsoventral (r = 0.92) and in right-to-left directions (r = 0.85) with good limits of agreement of 8.74% in eight lung segments. With a robust electrical impedance tomography perfusion estimation method, we found strong agreement between multidetector computed and electrical impedance tomography perfusion in healthy and regionally injured lungs and demonstrated feasibility of electrical impedance tomography perfusion imaging.

Imaging Biomarkers in Animal Models of Drug-Induced Lung Injury: A Systematic Review

For drug-induced interstitial lung disease (DIILD) translational imaging biomarkers are needed to improve detection and management of lung injury and drug-toxicity. Literature was reviewed on animal models in which in vivo imaging was used to detect and assess lung lesions that resembled pathological changes found in DIILD, such as inflammation and fibrosis. A systematic search was carried out using three databases with key words “Animal models”, “Imaging”, “Lung disease”, and “Drugs”. A total of 5749 articles were found, and, based on inclusion criteria, 284 papers were selected for final data extraction, resulting in 182 out of the 284 papers, based on eligibility. Twelve different animal species occurred and nine various imaging modalities were used, with two-thirds of the studies being longitudinal. The inducing agents and exposure (dose and duration) differed from non-physiological to clinically relevant doses. The majority of studies reported other biomarkers and/or histological confirmation of the imaging results. Summary of radiotracers and examples of imaging biomarkers were summarized, and the types of animal models and the most used imaging modalities and applications are discussed in this review. Pathologies resembling DIILD, such as inflammation and fibrosis, were described in many papers, but only a few explicitly addressed drug-induced toxicity experiments.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Optimal mean airway pressure during high-frequency oscillatory ventilation in an experimental model of acute respiratory distress syndrome: EIT-based method

Background

High-frequency oscillatory ventilation (HFOV) may theoretically provide lung protective ventilation. The negative clinical results may be due to inadequate mean airway pressure (mPaw) settings in HFOV. Our objective was to evaluate the air distribution, ventilatory and hemodynamic effects of individual mPaw titration during HFOV in ARDS animal based on oxygenation and electrical impedance tomography (EIT).

Methods

ARDS was introduced with repeated bronchoalveolar lavage followed by injurious mechanical ventilation in ten healthy male pigs (51.2 ± 1.9 kg). Settings of HFOV were 9 Hz (respiratory frequency), 33% (inspiratory time) and 70 cmH₂O (∆pressure). After lung recruitment, the mPaw was reduced in steps of 3 cmH₂O every 6 min. Hemodynamics and blood gases were obtained in each step. Regional ventilation distribution was determined with EIT.

Results

PaO₂/FiO₂ decreased significantly during the mPaw decremental phase (p < 0.001). Lung overdistended regions decreased, while recruitable regions increased as mPaw decreased. The optimal mPaw with respect to PaO₂/FiO₂ was 21 (18.0–21.0) cmH₂O, that is comparable to EIT-based center of ventilation (EIT-CoV) and EIT-collapse/over, 19.5 (15.0–21.0) and 19.5 (18.0–21.8), respectively (p = 0.07). EIT-CoV decreasing along with mPaw decrease revealed redistribution toward non-dependent regions. The individual mPaw titrated by EIT-based indices improved regional ventilation distribution with respect to overdistension and collapse (p = 0.035).

Conclusion

Our data suggested personalized optimal mPaw titration by EIT-based indices improves regional ventilation distribution and lung homogeneity during high-frequency oscillatory ventilation.

Low-pressure support vs automatic tube compensation during spontaneous breathing trial for weaning

BACKGROUND

During spontaneous breathing trial, low-pressure support is thought to compensate for endotracheal tube resistance, but it actually should provide overassistance. Automatic tube compensation is an option available in the ventilator to compensate for flow-resistance of endotracheal tube. Its effects on patient effort have been poorly investigated. We aimed to compare the effects of low-pressure support and automatic tube compensation during spontaneous breathing trial on breathing power and lung ventilation distribution.

RESULTS

We performed a randomized crossover study in 20 patients ready to wean. Each patient received both methods for 30 min separated by baseline ventilation: pressure support 0 cmH₂O and automatic tube compensation 100% in one period and pressure support 7 cmH₂O without automatic tube compensation in the other period, a 4 cmH₂O positive end-expiratory pressure being applied in each. Same ventilator brand (Evita XL, Draeger, Germany) was used. Breathing power was assessed from Campbell diagram with esophageal pressure, airway pressure, flow and volume recorded by a data logger. Lung ventilation distribution was assessed by using electrical impedance tomography (Pulmovista, Draeger, Germany). During the last 2 min of low-pressure support and automatic compensation period breathing power and lung ventilation distribution were measured on each breath. Breathing power generated by the patient's respiratory muscles was 7.2 (4.4-9.6) and 9.7 (5.7-21.9) J/min in low-pressure support and automatic tube compensation periods, respectively (P = 0.011). Lung ventilation distribution was not different between the two methods.

CONCLUSIONS

We found that ATC was associated with higher breathing power than low PS during SBT without altering the distribution of lung ventilation.

Noninvasive measurement of stroke volume changes in critically ill patients by means of electrical impedance tomography

Previous animal experiments have suggested that electrical impedance tomography (EIT) has the ability to noninvasively track changes in cardiac stroke volume (SV). The present study intended to reproduce these findings in patients during a fluid challenge. In a prospective observational study including critically ill patients on mechanical ventilation, SV was estimated via ECG-gated EIT before and after a fluid challenge and compared to transpulmonary thermodilution reference measurements. Relative changes in EIT-derived cardiosynchronous impedance changes in the heart ([Formula: see text]) and lung region ([Formula: see text]) were compared to changes in reference SV by assessing the concordance rate (CR) and Pearson's correlation coefficient (R). We compared 39 measurements of 20 patients. [Formula: see text] did not show to be a reliable estimate for tracking changes of SV (CR = 52.6% and R = 0.13 with P = 0.44). In contrast, [Formula: see text] showed an acceptable trending performance (CR = 94.4% and R = 0.72 with P < 0.0001). Our results indicate that ECG-gated EIT measurements of [Formula: see text] are able to noninvasively monitor changes in SV during a fluid challenge in critically ill patients. However, this was not possible using [Formula: see text]. The present approach is limited by the influences induced by ventilation, posture or changes in electrode-skin contact and requires further validation.

Lung monitoring with electrical impedance tomography: technical considerations and clinical applications

In recent years there has been substantial progress in the imaging evaluation of patients with lung disease requiring mechanical ventilatory assistance. This has been demonstrated by the inclusion of pulmonary ultrasound, positron emission tomography, electrical impedance tomography (EIT), and magnetic resonance imaging (MRI). The EIT uses electric current to evaluate the distribution of alternating current conductivity within the thoracic cavity. The advantage of the latter is that it is non-invasive, bedside radiation-free functional imaging modality for continuous monitoring of lung ventilation and perfusion. EIT can detect recruitment or derecruitment, overdistension, variation of poorly ventilated lung units (silent spaces), and pendelluft phenomenon in spontaneously breathing patients. In addition, the regional expiratory time constants have been recently explored.

Global and regional degree of obstruction determined by electrical impedance tomography in patients with obstructive ventilatory defect

Background

Electrical impedance tomography is a continuous imaging method capable of measuring lung volume changes. The purpose of this study was to examine whether EIT was capable of evaluating the degree of obstructive ventilatory defect (OVD) on the global and regional level.

Methods

41 healthy subjects with no lung diseases and 67 subjects suffering from obstructive lung diseases were examined using EIT and spirometry during forced vital capacity (FVC) maneuver. The subjects were divided into control group (n = 41), early airway obstruction group (n = 26), mild group (n = 17), moderate group (n = 16) and severe group (n = 8) according to the degree of obstruction. Forced expiratory volume in 1 second (FEV₁) and FEV₁/FVC were determined by EIT. The mode index (MI) was proposed to evaluate the degree of global and regional obstruction; the effectiveness of MI was validated by evaluating posture related change of lung emptying capacity in sitting and supine postures; the degree of regional obstruction was determined according to the cut-off values of MI obtained from receiver operating characteristic (ROC) analysis; regional obstruction was located in the four-quadrant region of interest (ROI) and the contour-map ROI with contour lines at the cut-off values of MI.

Results

Significant differences were found between different groups (P<0.05) and the global MI was 0.93±0.03, 0.86±0.05, 0.81±0.09, 0.73±0.09 and 0.60±0.11 (mean ±SD), respectively. The cut-off MI value was 0.90, 0.83, 0.77, and 0.65, respectively.

Conclusion

The results indicated the potential of EIT to evaluate the degree of obstruction in patients with obstructive ventilatory defect on the global and regional level.

Assessment of changes of regional ventilation distribution in the lung tissue depending on the driving pressure applied during high frequency jet ventilation

Background

Electrical impedance tomography (EIT) is a tool to monitor regional ventilation distribution in patient’s lungs under general anesthesia. The objective of this study was to assess the regional ventilation distribution using different driving pressures (DP) during high frequency jet ventilation (HFJV).

Methods

Prospective, observational, cross-over study. Patients undergoing rigid bronchoscopy were ventilated HFJV with DP 1.5 and 2.5 atm. Hemodynamic and ventilation parameters, as well as ventilation in different regions of the lungs in percentage of total ventilation, assessed by EIT, were recorded.

Results

Thirty-six patients scheduled for elective rigid bronchoscopy. The final analysis included thirty patients. There was no significant difference in systolic, diastolic and mean arterial blood pressure, heart rate, and peripheral saturation between the two groups. Peak inspiratory pressure, mean inspiratory pressure, tidal volume, and minute volume significantly increased in the second, compared to the first intervention group. Furthermore, there were no statistically significant differences between each time profiles in all ROI regions in EIT.

Conclusions

In our study intraoperative EIT was an effective method of functional monitoring of the lungs during HFJV for rigid bronchoscopy procedure. Lower driving pressure was as effective in providing sufficient ventilation distribution through the lungs as the higher driving pressure but characterized by lower airway pressure.

Trial registration

The study was registered on ClinicalTrials.gov under no. NCT02997072.

Regional lung ventilation and perfusion by electrical impedance tomography compared to single-photon emission computed tomography

OBJECTIVE

Electrical impedance tomography (EIT) is a noninvasive imaging modality that allows real-time monitoring of regional lung ventilation ([Formula: see text]) in intensive care patients at bedside. However, for improved guidance of ventilation therapy it would be beneficial to obtain regional ventilation-to-perfusion ratio ([Formula: see text]) by EIT.

APPROACH

In order to further explore the feasibility, we first evaluate a model-based approach, based on semi-negative matrix factorization and a gamma-variate model, to extract regional lung perfusion ([Formula: see text]) from EIT measurements. Subsequently, a combined validation of both [Formula: see text] and [Formula: see text] measured by EIT against single-photon emission computed tomography (SPECT) is performed on data acquired as part of a porcine animal trial. Four pigs were ventilated at two different levels of positive end-expiratory pressure (PEEP 0 and 15 cm H₂O, respectively) in randomized order. Repeated injections of an EIT contrast agent (NaCl 10%) and simultaneous SPECT measurements of [Formula: see text] (81^mKr gas) and [Formula: see text] (99^mTc-labeled albumin) were performed.

MAIN RESULTS

Both [Formula: see text] and [Formula: see text] from EIT and SPECT were compared by correlation analysis. Very strong (r ²  =  0.94 to 0.95) correlations were found for [Formula: see text] and [Formula: see text] in the dorsal-ventral direction at both PEEP levels. Moderate (r ²  =  0.36 to 0.46) and moderate to strong (r ²  =  0.61 to 0.82) correlations resulted for [Formula: see text] and [Formula: see text] in the right-left direction, respectively.

SIGNIFICANCE

The results of combined validation indicate that monitoring of [Formula: see text] and [Formula: see text] by EIT is possible. However, care should be taken when trying to quantify [Formula: see text] by EIT, as imaging artefacts and model bias may void necessary spatial matching.

Effects of individualized electrical impedance tomography and image reconstruction settings upon the assessment of regional ventilation distribution: Comparison to 4-dimensional computed tomography in a porcine model

Electrical impedance tomography (EIT) is a promising imaging technique for bedside monitoring of lung function. It is easily applicable, cheap and requires no ionizing radiation, but clinical interpretation of EIT-images is still not standardized. One of the reasons for this is the ill-posed nature of EIT, allowing a range of possible images to be produced–rather than a single explicit solution. Thus, to further advance the EIT technology for clinical application, thorough examinations of EIT-image reconstruction settings–i.e., mathematical parameters and addition of a priori (e.g., anatomical) information–is essential. In the present work, regional ventilation distribution profiles derived from different EIT finite-element reconstruction models and settings (for GREIT and Gauss Newton) were compared to regional aeration profiles assessed by the gold-standard of 4-dimensional computed tomography (4DCT) by calculating the root mean squared error (RMSE). Specifically, non-individualized reconstruction models (based on circular and averaged thoracic contours) and individualized reconstruction models (based on true thoracic contours) were compared. Our results suggest that GREIT with noise figure of 0.15 and non-uniform background works best for the assessment of regional ventilation distribution by EIT, as verified versus 4DCT. Furthermore, the RMSE of anteroposterior ventilation profiles decreased from 2.53±0.62% to 1.67±0.49% while correlation increased from 0.77 to 0.89 after embedding anatomical information into the reconstruction models. In conclusion, the present work reveals that anatomically enhanced EIT-image reconstruction is superior to non-individualized reconstruction models, but further investigations in humans, so as to standardize reconstruction settings, is warranted.

Prone Positioning Improves Ventilation Homogeneity in Children With Acute Respiratory Distress Syndrome

OBJECTIVES

To determine the effect of prone positioning on ventilation distribution in children with acute respiratory distress syndrome.

DESIGN

Prospective observational study.

SETTING

Paediatric Intensive Care at Red Cross War Memorial Children's Hospital, Cape Town, South Africa.

PATIENTS

Mechanically ventilated children with acute respiratory distress syndrome.

INTERVENTIONS

Electrical impedance tomography measures were taken in the supine position, after which the child was turned into the prone position, and subsequent electrical impedance tomography measurements were taken.

MEASUREMENTS AND MAIN RESULTS

Thoracic electrical impedance tomography measures were taken at baseline and after 5, 20, and 60 minutes in the prone position. The proportion of ventilation, regional filling characteristics, and global inhomogeneity index were calculated for the ventral and dorsal lung regions. Arterial blood gas measurements were taken before and after the intervention. A responder was defined as having an improvement of more than 10% in the oxygenation index after 60 minutes in prone position. Twelve children (nine male, 65%) were studied. Four children were responders, three were nonresponders, and five showed no change to prone positioning. Ventilation in ventral and dorsal lung regions was no different in the supine or prone positions between response groups. The proportion of ventilation in the dorsal lung increased from 49% to 57% in responders, while it became more equal between ventral and dorsal lung regions in the prone position in nonresponders. Responders showed greater improvements in ventilation homogeneity with R improving from 0.86 ± 0.24 to 0.98 ± 0.02 in the ventral lung and 0.91 ± 0.15 to 0.99 ± 0.01 in the dorsal lung region with time in the prone position.

CONCLUSIONS

The response to prone position was variable in children with acute respiratory distress syndrome. Prone positioning improves homogeneity of ventilation and may result in recruitment of the dorsal lung regions.

Coupling of EIT with computational lung modeling for predicting patient-specific ventilatory responses

Providing optimal personalized mechanical ventilation for patients with acute or chronic respiratory failure is still a challenge within a clinical setting for each case anew. In this article, we integrate electrical impedance tomography (EIT) monitoring into a powerful patient-specific computational lung model to create an approach for personalizing protective ventilatory treatment. The underlying computational lung model is based on a single computed tomography scan and able to predict global airflow quantities, as well as local tissue aeration and strains for any ventilation maneuver. For validation, a novel “virtual EIT” module is added to our computational lung model, allowing to simulate EIT images based on the patient's thorax geometry and the results of our numerically predicted tissue aeration. Clinically measured EIT images are not used to calibrate the computational model. Thus they provide an independent method to validate the computational predictions at high temporal resolution. The performance of this coupling approach has been tested in an example patient with acute respiratory distress syndrome. The method shows good agreement between computationally predicted and clinically measured airflow data and EIT images. These results imply that the proposed framework can be used for numerical prediction of patient-specific responses to certain therapeutic measures before applying them to an actual patient. In the long run, definition of patient-specific optimal ventilation protocols might be assisted by computational modeling.

NEW & NOTEWORTHY In this work, we present a patient-specific computational lung model that is able to predict global and local ventilatory quantities for a given patient and any selected ventilation protocol. For the first time, such a predictive lung model is equipped with a virtual electrical impedance tomography module allowing real-time validation of the computed results with the patient measurements. First promising results obtained in an acute respiratory distress syndrome patient show the potential of this approach for personalized computationally guided optimization of mechanical ventilation in future.

An Official American Thoracic Society/European Respiratory Society Workshop Report: Evaluation of Respiratory Mechanics and Function in the Pediatric and Neonatal Intensive Care Units

Ready access to physiologic measures, including respiratory mechanics, lung volumes, and ventilation/perfusion inhomogeneity, could optimize the clinical management of the critically ill pediatric or neonatal patient and minimize lung injury. There are many techniques for measuring respiratory function in infants and children but very limited information on the technical ease and applicability of these tests in the pediatric and neonatal intensive care unit (PICU, NICU) environments. This report summarizes the proceedings of a 2011 American Thoracic Society Workshop critically reviewing techniques available for ventilated and spontaneously breathing infants and children in the ICU. It outlines for each test how readily it is performed at the bedside and how it may impact patient management as well as indicating future areas of potential research collaboration. From expert panel discussions and literature reviews, we conclude that many of the techniques can aid in optimizing respiratory support in the PICU and NICU, quantifying the effect of therapeutic interventions, and guiding ventilator weaning and extubation. Most techniques now have commercially available equipment for the PICU and NICU, and many can generate continuous data points to help with ventilator weaning and other interventions. Technical and validation studies in the PICU and NICU are published for the majority of techniques; some have been used as outcome measures in clinical trials, but few have been assessed specifically for their ability to improve clinical outcomes. Although they show considerable promise, these techniques still require further study in the PICU and NICU together with increased availability of commercial equipment before wider incorporation into daily clinical practice.

Chest electrical impedance tomography examination, data analysis, terminology, clinical use and recommendations: consensus statement of the TRanslational EIT developmeNt stuDy group

Electrical impedance tomography (EIT) has undergone 30 years of development. Functional chest examinations with this technology are considered clinically relevant, especially for monitoring regional lung ventilation in mechanically ventilated patients and for regional pulmonary function testing in patients with chronic lung diseases. As EIT becomes an established medical technology, it requires consensus examination, nomenclature, data analysis and interpretation schemes. Such consensus is needed to compare, understand and reproduce study findings from and among different research groups, to enable large clinical trials and, ultimately, routine clinical use. Recommendations of how EIT findings can be applied to generate diagnoses and impact clinical decision-making and therapy planning are required. This consensus paper was prepared by an international working group, collaborating on the clinical promotion of EIT called TRanslational EIT developmeNt stuDy group. It addresses the stated needs by providing (1) a new classification of core processes involved in chest EIT examinations and data analysis, (2) focus on clinical applications with structured reviews and outlooks (separately for adult and neonatal/paediatric patients), (3) a structured framework to categorise and understand the relationships among analysis approaches and their clinical roles, (4) consensus, unified terminology with clinical user-friendly definitions and explanations, (5) a review of all major work in thoracic EIT and (6) recommendations for future development (193 pages of online supplements systematically linked with the chief sections of the main document). We expect this information to be useful for clinicians and researchers working with EIT, as well as for industry producers of this technology.

Regional lung response to bronchodilator reversibility testing determined by electrical impedance tomography in chronic obstructive pulmonary disease

Patients with obstructive lung diseases commonly undergo bronchodilator reversibility testing during examination of their pulmonary function by spirometry. A positive response is defined by an increase in forced expiratory volume in 1 s (FEV1). FEV1 is a rather nonspecific criterion not allowing the regional effects of bronchodilator to be assessed. We employed the imaging technique of electrical impedance tomography (EIT) to visualize the spatial and temporal ventilation distribution in 35 patients with chronic obstructive pulmonary disease at baseline and 5, 10, and 20 min after bronchodilator inhalation. EIT scanning was performed during tidal breathing and forced full expiration maneuver in parallel with spirometry. Ventilation distribution was determined by EIT by calculating the image pixel values of FEV1, forced vital capacity (FVC), tidal volume, peak flow, and mean forced expiratory flow between 25 and 75% of FVC. The global inhomogeneity indexes of each measure and histograms of pixel FEV1/FVC values were then determined to assess the bronchodilator effect on spatial ventilation distribution. Temporal ventilation distribution was analyzed from pixel values of times needed to exhale 75 and 90% of pixel FVC. Based on spirometric FEV1, significant bronchodilator response was found in 17 patients. These patients exhibited higher postbronchodilator values of all regional EIT-derived lung function measures in contrast to nonresponders. Ventilation distribution was inhomogeneous in both groups. Significant improvements were noted for spatial distribution of pixel FEV1 and tidal volume and temporal distribution in responders. By providing regional data, EIT might increase the diagnostic and prognostic information derived from reversibility testing.

Regional lung function determined by electrical impedance tomography during bronchodilator reversibility testing in patients with asthma

The measurement of rapid regional lung volume changes by electrical impedance tomography (EIT) could determine regional lung function in patients with obstructive lung diseases during pulmonary function testing (PFT). EIT examinations carried out before and after bronchodilator reversibility testing could detect the presence of spatial and temporal ventilation heterogeneities and analyse their changes in response to inhaled bronchodilator on the regional level. We examined seven patients suffering from chronic asthma (49  ±  19 years, mean age  ±  SD) using EIT at a scan rate of 33 images s(-1) during tidal breathing and PFT with forced full expiration. The patients were studied before and 5, 10 and 20 min after bronchodilator inhalation. Seven age- and sex-matched human subjects with no lung disease history served as a control study group. The spatial heterogeneity of lung function measures was quantified by the global inhomogeneity indices calculated from the pixel values of tidal volume, forced expiratory volume in one second (FEV1), forced vital capacity (FVC), peak flow and forced expiratory flow between 25% and 75% of FVC as well as histograms of pixel FEV1/FVC values. Temporal heterogeneity was assessed using the pixel values of expiration times needed to exhale 75% and 90% of pixel FVC. Regional lung function was more homogeneous in the healthy subjects than in the patients with asthma. Spatial and temporal ventilation distribution improved in the patients with asthma after the bronchodilator administration as evidenced mainly by the histograms of pixel FEV1/FVC values and pixel expiration times. The examination of regional lung function using EIT enables the assessment of spatial and temporal heterogeneity of ventilation distribution during bronchodilator reversibility testing. EIT may become a new tool in PFT, allowing the estimation of the natural disease progression and therapy effects on the regional and not only global level.

Structural-functional lung imaging using a combined CT-EIT and a Discrete Cosine Transformation reconstruction method

Lung EIT is a functional imaging method that utilizes electrical currents to reconstruct images of conductivity changes inside the thorax. This technique is radiation free and applicable at the bedside, but lacks of spatial resolution compared to morphological imaging methods such as X-ray computed tomography (CT). In this article we describe an approach for EIT image reconstruction using morphologic information obtained from other structural imaging modalities. This leads to recon- structed images of lung ventilation that can easily be superimposed with structural CT or MRI images, which facilitates image interpretation. The approach is based on a Discrete Cosine Transformation (DCT) of an image of the considered transversal thorax slice. The use of DCT enables reduction of the dimensionality of the reconstruction and ensures that only conductivity changes of the lungs are reconstructed and displayed. The DCT based approach is well suited to fuse morphological image information with functional lung imaging at low computational costs. Results on simulated data indicate that this approach preserves the morphological structures of the lungs and avoids blurring of the solution. Images from patient measurements reveal the capabilities of the method and demonstrate benefits in possible applications.

Identification of regional overdistension, recruitment and cyclic alveolar collapse with electrical impedance tomography in an experimental ARDS model

Background

Information on regional ventilation distribution in mechanically ventilated patients is important to develop lung protective ventilation strategies. In the present prospective animal study, we introduce an electrical impedance tomography (EIT)-based method to classify lungs into normally ventilated, overinflated, tidally recruited/derecruited and recruited regions.

Methods

Acute respiratory distress syndrome (ARDS) was introduced with repeated bronchoalveolar lavage in ten healthy male pigs until the ratio of arterial partial pressure of oxygen and fraction of inspired oxygen (PaO₂/FiO₂) decreased to less than 100 mmHg and remained stable for 30 minutes. Stepwise positive end-expiratory pressure (PEEP) increments were performed from 0 cmH₂O to 30 cmH₂O with 3 cmH₂O increase every 5 minutes. Respiratory system compliance (Crs), blood gases and hemodynamics were measured at the same time. Lung regions at end-expiration and during tidal breathing were identified in EIT images.

Results

Overinflated regions contain air at end-expiration but they are not or are only minimally ventilated. Recruited regions compared to reference PEEP level contain air at end-expiration of arbitrary PEEP level but not at that of reference PEEP level. Tidally recruited/derecruited regions are not represented in lung regions at end-expiration but are ventilated during tidal breathing. The results coincided with measurements of blood gases. The coefficient for correlation between the number of recruited pixels and PaO₂/FiO₂ was 0.89 ± 0.12 (p = 0.02).

Conclusion

The proposed novel EIT-based method provides information on overinflation, recruitment and cyclic alveolar collapse at the bedside, which may improve the ventilation strategies used.

Assessment of distribution of ventilation by electrical impedance tomography in standing horses

The aim was to evaluate the feasibility of using electrical impedance tomography (EIT) in horses. Thoracic EIT was used in nine horses. Thoracic and abdominal circumference changes were also measured with respiratory ultrasound plethysmography (RUP). Data were recorded during baseline, rebreathing of CO2 and sedation. Three breaths were selected for analysis from each recording. During baseline breathing, horses regularly took single large breaths (sighs), which were also analysed. Functional EIT images were created using standard deviations (SD) of pixel signals and correlation coefficients (R) of each pixel signal with a reference respiratory signal. Left-to-right ratio, centre-of-ventilation and global-inhomogeneity-index were calculated. RM-ANOVA and Bonferroni tests were used (P < 0.05). Distribution of ventilation shifted towards right during sighs and towards dependent regions during sighs, rebreathing and sedation. Global-inhomogeneity-index did not change for SD but increased for R images during sedation. The sum of SDs for the respiratory EIT signals correlated well with thoracic (r(2) = 0.78) and abdominal (r(2) = 0.82) tidal circumferential changes. Inverse respiratory signals were identified on the images at sternal location and based on reviewing CT images, seemed to correspond to location of gas filled intestines. Application of EIT in standing non-sedated horses is feasible. EIT images may provide physiologically useful information even in situations, such as sighs, that cannot easily be tested by other methods.

Imaging of acute respiratory distress syndrome

IMPORTANCE OF THE FIELD

Acute respiratory distress syndrome (ARDS) describes a relatively common and frequently lethal syndrome at the severe end of the spectrum of acute lung injury. Onset of symptoms is usually within 72 h of the inciting event and complicates a wide variety of clinical disorders, ranging from infection to trauma. It may be defined as resistant hypoxaemia in the clinical setting of one of the group of recognised causes, in association with bilateral pulmonary infiltrates and in the absence of left atrial hypertension. Accurate diagnosis and differentiation from other treatable conditions is crucial.

AREAS COVERED IN THIS REVIEW

This publication addresses the clinical and radiological features of ARDS, a review of the imaging technology with illustrations and differential diagnosis.

WHAT THE READER WILL GAIN

This paper will give insight into the strengths and weaknesses of imaging modalities used in the management of patients with ARDS.

TAKE HOME MESSAGE

Imaging plays a vital role in the assessment of acute respiratory syndromes. Computed tomography is much more sensitive compared with chest radiography, and relatively under-utilised. Other methods, such as bedside ultrasound and impedance tomography, may have roles to play in the future.

Effect of Trigger Sensitivity on Redistribution of Ventilation During Pressure Support Ventilation Detected by Electrical Impedance Tomography

Background:

In supine position, pressure support ventilation causes a redistribution of ventilation towards the ventral regions of the lung. Theoretically, a less sensitive support trigger would cause the patient to breathe more actively, potentially attenuating the effect of positive pressure ventilation.

Objectives:

To quantify the effect of trigger setting, we assessed redistribution of ventilation during pressure support ventilation (PSV) using electrical impedance tomography (EIT).

Patients and Methods:

With approval from the local ethics committee, six orthopedic patients were enrolled. All patients had general anesthesia with a laryngeal mask airway and a standardized anesthetic regimen (sufentanil, propofol and sevoflurane). Pressure support trigger settings varied between 2 and 15 L/minute and compared to unassisted spontaneous breathing. From EIT data, the center of ventilation (COV), the fraction of the total ventilation per region of interest (ROI) and intratidal gas distribution were calculated.

Results:

At all trigger settings, pressure support ventilation caused a significant ventral shift of the center of ventilation compared with during spontaneous breathing, confirmed by the analysis by regions of interest. During spontaneous breathing, COV was not different from baseline values obtained before induction of anesthesia. During PSV, the intratidal regional gas distribution (ITV-analysis) revealed subtle changes during the early inspiratory phase not detected by the COV-analysis.

Conclusions:

Pressure support ventilation, but not spontaneous breathing, induces a significant redistribution of ventilation towards the ventral region. The sensitivity of the support trigger appears to influence the distribution of ventilation only during the early phase of inspiration.

Real-time ventilation and perfusion distributions by electrical impedance tomography during one-lung ventilation with capnothorax

BACKGROUND

Carbon dioxide insufflation into the pleural cavity, capnothorax, with one-lung ventilation (OLV) may entail respiratory and hemodynamic impairments. We investigated the online physiological effects of OLV/capnothorax by electrical impedance tomography (EIT) in a porcine model mimicking the clinical setting.

METHODS

Five anesthetized, muscle-relaxed piglets were subjected to first right and then left capnothorax with an intra-pleural pressure of 19 cm H2 O. The contra-lateral lung was mechanically ventilated with a double-lumen tube at positive end-expiratory pressure 5 and subsequently 10 cm H2 O. Regional lung perfusion and ventilation were assessed by EIT. Hemodynamics, cerebral tissue oxygenation and lung gas exchange were also measured.

RESULTS

During right-sided capnothorax, mixed venous oxygen saturation (P = 0.018), as well as a tissue oxygenation index (P = 0.038) decreased. There was also an increase in central venous pressure (P = 0.006), and a decrease in mean arterial pressure (P = 0.045) and cardiac output (P = 0.017). During the left-sided capnothorax, the hemodynamic impairment was less than during the right side. EIT revealed that during the first period of OLV/capnothorax, no or very minor ventilation on the right side could be seen (3 ± 3% vs. 97 ± 3%, right vs. left, P = 0.007), perfusion decreased in the non-ventilated and increased in the ventilated lung (18 ± 2% vs. 82 ± 2%, right vs. left, P = 0.03). During the second OLV/capnothorax period, a similar distribution of perfusion was seen in the animals with successful separation (84 ± 4% vs. 16 ± 4%, right vs. left).

CONCLUSION

EIT detected in real-time dynamic changes in pulmonary ventilation and perfusion distributions. OLV to the left lung with right-sided capnothorax caused a decrease in cardiac output, arterial oxygenation and mixed venous saturation.

Electrical impedance tomography imaging of the cardiopulmonary system

PURPOSE OF REVIEW

This review article summarizes the recent advances in electrical impedance tomography (EIT) related to cardiopulmonary imaging and monitoring on the background of the 30-year development of this technology.

RECENT FINDINGS

EIT is expected to become a bedside tool for monitoring and guiding ventilator therapy. In this context, several studies applied EIT to determine spatial ventilation distribution during different ventilation modes and settings. EIT was increasingly combined with other signals, such as airway pressure, enabling the assessment of regional respiratory system mechanics. EIT was for the first time used prospectively to define ventilator settings in an experimental and a clinical study. Increased neonatal and paediatric use of EIT was noted. Only few studies focused on cardiac function and lung perfusion. Advanced radiological imaging techniques were applied to assess EIT performance in detecting regional lung ventilation. New approaches to improve the quality of thoracic EIT images were proposed.

SUMMARY

EIT is not routinely used in a clinical setting, but the interest in EIT is evident. The major task for EIT research is to provide the clinicians with guidelines how to conduct, analyse and interpret EIT examinations and combine them with other medical techniques so as to meaningfully impact the clinical decision-making.

Effect of body position on ventilation distribution during PEEP titration in a porcine model of acute lung injury using advanced respiratory monitoring and electrical impedance tomography

Background

Lung failure after acute lung injury remains a challenge in different clinical settings. Various interventions for restoration of gas exchange have been investigated. Recruitment of collapsed alveoli by positive end expiratory pressure (PEEP) titration and optimization of ventilation-perfusion ratio by prone positioning have been extensively described in animal and clinical trials. This animal study was conducted to investigate the effects of PEEP and positioning by means of advanced respiratory monitoring including gas exchange, respiratory mechanics, volumetric capnography and electrical impedance tomography.

Methods

After induction of acute lung injury by oleic acid and lung lavage, 12 domestic pigs were studied in randomly assigned supine or prone position during a PEEP titration trial with maximal PEEP of 30 mbar.

Results

Induction of lung injury resulted in significant deterioration of oxygenation [partial pressure of arterial oxygen/inspiratory fraction of oxygen (PaO₂/FiO₂): p = 0.002] and ventilation [partial pressure of arterial carbon dioxide (PaCO₂): p = 0.002] and elevated alveolar dead-space ratios (Valv/Vte: p = 0.003) in both groups. Differences in the prone and the supine group were significant for PaCO₂ at incremental PEEP 10 and 20 and at decremental PEEP 20 (20d) and 10 (10d), for PaO₂/FiO₂ at PEEP 10 and 10d and for alveolar dead space at PEEP 10d. Electrical impedance tomography revealed homogenous ventilation distribution in prone position during PEEP 20, 30 and 20d.

Conclusions

Prone position leads to improved oxygenation and ventilation parameters in a lung injury model. Respiratory monitoring with measurement of alveolar dead space and electrical impedance tomography may visualize optimized ventilation in a PEEP titration trial.

Monitoring Lung Volumes During Mechanical Ventilation

Respiratory inductive plethysmography (RIP) is a non-invasive method of measuring change in lung volume which is well-established as a monitor of tidal ventilation and thus respiratory patterns in sleep medicine. As RIP is leak independent, can measure end-expiratory lung volume as well as tidal volume and is applicable to both the ventilated and spontaneously breathing patient, there has been a recent interest in its use as a bedside tool in the intensive care unit.

Advanced imaging in COPD: insights into pulmonary pathophysiology

Chronic obstructive pulmonary disease (COPD) involves a complex interaction of structural and functional abnormalities. The two have long been studied in isolation. However, advanced imaging techniques allow us to simultaneously assess pathological processes and their physiological consequences. This review gives a comprehensive account of the various advanced imaging modalities used to study COPD, including computed tomography (CT), magnetic resonance imaging (MRI), and the nuclear medicine techniques positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Some more recent developments in imaging technology, including micro-CT, synchrotron imaging, optical coherence tomography (OCT) and electrical impedance tomography (EIT), are also described. The authors identify the pathophysiological insights gained from these techniques, and speculate on the future role of advanced imaging in both clinical and research settings.

First-time imaging of effects of inspired oxygen concentration on regional lung volumes and breathing pattern during hypergravity

PURPOSE

Aeroatelectasis can develop in aircrew flying the latest generation high-performance aircraft. Causes alleged are relative hyperoxia, increased gravity in the head-to-foot direction (+Gz), and compression of legs and stomach by anti-G trousers (AGT). We aimed to assess, in real time, the effects of hyperoxia, +Gz accelerations and AGT inflation on changes in regional lung volumes and breathing pattern evaluated in an axial plane by electrical impedance tomography (EIT).

METHODS

The protocol mimicked a routine peacetime flight in combat aircraft. Eight subjects wearing AGT were studied in a human centrifuge during 1 h 15 min exposure of +1 to +3.5Gz. They performed this sequence three times, breathing AIR, 44.5 % O2 or 100 % O2. Continuous recording of functional EIT enabled uninterrupted assessment of regional lung volumes at the 5th intercostal level. Breathing pattern was also monitored.

RESULTS

EIT data showed that +3.5Gz, compared with any moment without hypergravity, caused an abrupt decrease in regional tidal volume (VT) and regional end-expiratory lung volume (EELV) measured in the EIT slice, independently of inspired oxygen concentration. Breathing AIR or 44.5 % O2, sub-regional EELV measured in the EIT slice decreased similarly in dorsal and ventral regions, but sub-regional VT measured in the EIT slice decreased significantly more dorsally than ventrally. Breathing 100 % O2, EELV and VT decreased similarly in both regions. Inspired tidal volume increased in hyperoxia, whereas breathing frequency increased in hypergravity and hyperoxia.

CONCLUSIONS

Our findings suggest that hypergravity and AGT inflation cause airway closure and air trapping in gravity-dependent lung regions, facilitating absorption atelectasis formation, in particular during hyperoxia.

Effect of different breathing aids on ventilation distribution in adults with cystic fibrosis

Background and objectives

We investigated the effect of different breathing aids on ventilation distribution in healthy adults and subjects with cystic fibrosis (CF).

Methods

In 11 healthy adults and 9 adults with CF electrical impedance tomography measurements were performed during spontaneous breathing, continuous positive airway pressure (CPAP) and positive expiratory pressure (PEP) therapy randomly applied in upright and lateral position. Spatial and temporal ventilation distribution was assessed.

Results

The proportion of ventilation directed to the dependent lung significantly increased in lateral position compared to upright in healthy and CF. This effect was enhanced with CPAP but neutralised with PEP, whereas the effect of PEP was larger in the healthy group. Temporal ventilation distribution showed exactly the opposite with homogenisation during CPAP and increased inhomogeneity with PEP.

Conclusions

PEP shows distinct differences to CPAP with respect to its impact on ventilation distribution in healthy adults and CF subjects EIT might be used to individualise respiratory physiotherapy.