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

First Real-Time Imaging of Acute Effects of Arteriovenous Fistula on Regional Distribution of Pulmonary Perfusion in a Novel Porcine Model

The effects of a large arteriovenous fistula (AVF) on pulmonary perfusion remains to be elucidated. We aimed to study, for the first time, the real-time acute effects of a large AVF on regional distribution of pulmonary perfusion in a novel porcine model. Ten healthy swine under general anesthesia were studied. AVF was created by the connection of femoral artery and femoral vein using high-diameter perfusion cannulas. The AVF was closed and after 30 min of stabilization the first values were recorded. The fistula was then opened, and new data were collected after reaching stable state. Continuous hemodynamic monitoring was performed throughout the protocol. The following functional images were analyzed by electrical impedance tomography (EIT): perfusion and ventilation distributions. We found an increased cardiac output and right ventricular work, which was strongly correlated to an increased pulmonary artery mean pressure (r=0.878, P=0.001). The ventral/dorsal ratio of pulmonary perfusion decreased from 1.9+/-1.0 to 1.5+/-0.7 (P=0.025). The percentage of total pulmonary blood flow through the dorsal lung region increased from 38.6+/-11.7 to 42.2+/-10.4 (P=0.016). In conclusion, we have used EIT for the first time for studying the acute effects of a large AVF on regional distribution of pulmonary perfusion in a novel porcine model. In this new experimental model of hyperkinetic circulation caused by AVF, we documented an increased percentage of total pulmonary blood flow through the dorsal lung region and a more homogeneous perfusion distribution. Key words Arteriovenous fistula, Hyperkinetic circulation, Tissue perfusion, Animal model, Pulmonary blood flow.

Calibration of ventilation/perfusion match in electrical impedance tomography: a novel method based on arterial blood pressure

Introduction

Electrical impedance tomography (EIT) enables non-invasive, continuous, bedside evaluation of ventilation/perfusion (V/Q) match. To avoid the presence of invasive monitoring for cardiac output in relative V/Q ratio calculation, we proposed a novel calibration method based on arterial blood pressure to optimize EIT V/Q match assessments.

Methods

We involved 12 mechanically ventilated piglets in three experimental phases: baseline, pulmonary embolism, and atelectasis. After a thorough measurement of EIT signals, arterial blood pressure, cardiac output, and additional physiological parameters, EIT V/Q match was evaluated using existing area limited method (ALM), cardiac output calibrated method (COCM), and our proposed novel blood pressure calibrated method (BPCM). Finally, VD/VT and P/F ratio were calculated and correlated with V/Q match indicators derived from COCM and BPCM.

Results

Arterial blood pressure waveform integration demonstrated strong correlation with cardiac output (R 2 = 0.80, p < 0.001), validating its utility for cardiac output estimation and V/Q match calibration. Both COCM and BPCM provided enhanced V/Q match region segmentation compared to ALM, yielding comprehensive diagnostic information with statistically significant differences across all three states (p < 0.05). COCM demonstrates a slightly higher correlation compared to BPCM (r = -0.63 vs. -0.52) between low ventilation index (LVI) and VD/VT, while BPCM demonstrates a slightly higher correlation compared to COCM (r = 0.49 vs. 0.44) between low perfusion index (LQI) and P/F ratio.

Conclusion

This study described a novel calibration method for calculating corrected EIT-based V/Q match that utilized arterial blood pressure. Our method exhibited comparable capability in distinguishing V/Q mismatch areas compared to conventional cardiac output-based calibration techniques. With clinical data to establish a linear regression model, our method will ultimately enable us to calculate calibrated EIT V/Q match without cardiac output monitoring.

Three-dimensional electrical impedance tomography to study regional ventilation/perfusion ratios in anesthetized pigs

This study aimed to develop a three-dimensional (3-D) method for assessing ventilation/perfusion (V/Q̇) ratios in a pig model of hemodynamic perturbations using electrical impedance tomography (EIT). To evaluate the physiological coherence of changes in EIT-derived V/Q̇ ratios, global EIT-derived V/Q̇ mismatches were compared with global gold standards. The study found regional heterogeneity in the distribution of V/Q̇ ratios in both the ventrodorsal and craniocaudal directions. Although global EIT-derived indices of V/Q̇ mismatch consistently underestimated both low and high V/Q̇ mismatch compared with global gold standards, the direction of the change was similar. We made the software available at no cost for other researchers to use. Future studies should compare regional V/Q̇ ratios determined by our method against other regional, high-resolution methods.NEW & NOTEWORTHY In this study, we introduce a novel 3-D method for assessing ventilation-perfusion (V/Q̇) ratios using electrical impedance tomography (EIT). Heterogeneity in V/Q̇ distribution showcases the significant potential for enhanced understanding of pulmonary conditions. This work signifies a substantial step forward in the application of EIT for monitoring and managing lung diseases.

Quantification of exhaled propofol is not feasible during single-lung ventilation using double-lumen tubes: A multicenter prospective observational trial

BACKGROUND

Volatile propofol can be measured in exhaled air and correlates to plasma concentrations with a time delay. However, the effect of single-lung ventilation on exhaled propofol is unclear. Therefore, our goal was to evaluate exhaled propofol concentrations during single-lung compared to double-lung ventilation using double-lumen tubes.

METHODS

In a first step, we quantified adhesion of volatile propofol to the inner surface of double-lumen tubes during double- and single-lumen ventilation in vitro. In a second step, we enrolled 30 patients scheduled for lung surgery in two study centers. Anesthesia was provided with propofol and remifentanil. We utilized left-sided double-lumen tubes to separately ventilate each lung. Exhaled propofol concentrations were measured at 1-min intervals and plasma for propofol analyses was sampled every 20 min. To eliminate the influence of dosing on volatile propofol concentration, exhalation rate was normalized to plasma concentration.

RESULTS

In-vitro ventilation of double-lumen tubes resulted in increasing propofol concentrations at the distal end of the tube over time. In vitro clamping the bronchial lumen led to an even more pronounced increase (Δ AUC +62%) in propofol gas concentration over time. Normalized propofol exhalation during lung surgery was 31% higher during single-lung compared to double-lung ventilation.

CONCLUSION

During single-lung ventilation, propofol concentration in exhaled air, in contrast to our expectations, increased by approximately one third. However, this observation might not be affected by change in perfusion-ventilation during single-lung ventilation but rather arises from reduced propofol absorption on the inner surface area of the double-lumen tube. Thus, it is only possible to utilize exhaled propofol concentration to a limited extent during single-lung ventilation.

REGISTRATION OF CLINICAL TRIAL

DRKS-ID DRKS00014788 (www.drks.de).

Real-time effects of lateral positioning on regional ventilation and perfusion in an experimental model of acute respiratory distress syndrome

Low-volume lung injury encompasses local concentration of stresses in the vicinity of collapsed regions in heterogeneously ventilated lungs. We aimed to study the effects on ventilation and perfusion distributions of a sequential lateral positioning (30°) strategy using electrical impedance tomography imaging in a porcine experimental model of early acute respiratory distress syndrome (ARDS). We hypothesized that such strategy, including a real-time individualization of positive end-expiratory pressure (PEEP) whenever in lateral positioning, would provide attenuation of collapse in the dependent lung regions. A two-hit injury acute respiratory distress syndrome experimental model was established by lung lavages followed by injurious mechanical ventilation. Then, all animals were studied in five body positions in a sequential order, 15 min each: Supine 1; Lateral Left; Supine 2; Lateral Right; Supine 3. The following functional images were analyzed by electrical impedance tomography: ventilation distributions and regional lung volumes, and perfusion distributions. The induction of the acute respiratory distress syndrome model resulted in a marked fall in oxygenation along with low regional ventilation and compliance of the dorsal half of the lung (gravitational-dependent in supine position). Both the regional ventilation and compliance of the dorsal half of the lung greatly increased along of the sequential lateral positioning strategy, and maximally at its end. In addition, a corresponding improvement of oxygenation occurred. In conclusion, our sequential lateral positioning strategy, with sufficient positive end-expiratory pressure to prevent collapse of the dependent lung units during lateral positioning, provided a relevant diminution of collapse in the dorsal lung in a porcine experimental model of early acute respiratory distress syndrome.

Electrical impedance tomography to aid in the identification of hypoxemia etiology: Massive atelectasis or pneumothorax? A case report

Background

Bedside ultrasound is often used to determine the etiology of hypoxaemia, but not always with definitive results. This case reports the application of electrical impedance tomography (EIT) and saline injection to determine the etiology of hypoxaemia in a complex case that could not be identified by bedside ultrasound. The determination of the etiology of hypoxaemia by EIT and saline injection, regional ventilation and perfusion information can be used as a new clinical diagnostic method.

Case presentation

A post-cardiac surgery patient under prolonged mechanical ventilation for lung emphysema developed sudden hypoxemia in the intensive care unit (ICU). A line pattern and lung sliding sign abolishment were found in the left lung, but there was no evidence of a lung point sign on bedside ultrasound. Hence, the initial diagnosis was considered to be a massive pneumothorax. To further define the etiology, EIT and saline bolus were used to assess regional ventilation and perfusion. A massive ventilation defect was found in the left lung, in which regional perfusion was maintained, resulting in an intrapulmonary shunt in the left lung. Finally, the conjecture of a pneumothorax was ruled out considering the massive atelectasis. After the diagnosis was clarified, hypoxaemia was corrected by restorative ventilation of the left lung after changing the patient's posture and enhancing sputum drainage with chest physiotherapy.

Conclusions

This was the clinical case involving EIT and saline bolus to establish the differential diagnosis and guide clinical decisions for patients with acute hypoxemia. This study highlighted that combination regional ventilation, EIT perfusion, and saline bolus provided helpful information for determining the etiology of hypoxemia. The results of this study contribute to the development of emergency patient management.

Thoracic Electrical Impedance Tomography-The 2022 Veterinary Consensus Statement

Electrical impedance tomography (EIT) is a non-invasive real-time non-ionising imaging modality that has many applications. Since the first recorded use in 1978, the technology has become more widely used especially in human adult and neonatal critical care monitoring. Recently, there has been an increase in research on thoracic EIT in veterinary medicine. Real-time imaging of the thorax allows evaluation of ventilation distribution in anesthetised and conscious animals. As the technology becomes recognised in the veterinary community there is a need to standardize approaches to data collection, analysis, interpretation and nomenclature, ensuring comparison and repeatability between researchers and studies. A group of nineteen veterinarians and two biomedical engineers experienced in veterinary EIT were consulted and contributed to the preparation of this statement. The aim of this consensus is to provide an introduction to this imaging modality, to highlight clinical relevance and to include recommendations on how to effectively use thoracic EIT in veterinary species. Based on this, the consensus statement aims to address the need for a streamlined approach to veterinary thoracic EIT and includes: an introduction to the use of EIT in veterinary species, the technical background to creation of the functional images, a consensus from all contributing authors on the practical application and use of the technology, descriptions and interpretation of current available variables including appropriate statistical analysis, nomenclature recommended for consistency and future developments in thoracic EIT. The information provided in this consensus statement may benefit researchers and clinicians working within the field of veterinary thoracic EIT. We endeavor to inform future users of the benefits of this imaging modality and provide opportunities to further explore applications of this technology with regards to perfusion imaging and pathology diagnosis.

Lung Perfusion Assessment by Bedside Electrical Impedance Tomography in Critically Ill Patients

As a portable, radiation-free imaging modality, electrical impedance tomography (EIT) technology has shown promise in the bedside visual assessment of lung perfusion distribution in critically ill patients. The two main methods of EIT for assessing lung perfusion are the pulsatility and conductivity contrast (saline) bolus method. Increasing attention is being paid to the saline bolus EIT method in the evaluation of regional pulmonary perfusion in clinical practice. This study seeks to provide an overview of experimental and clinical studies with the aim of clarifying the progress made in the use of the saline bolus EIT method. Animal studies revealed that the saline bolus EIT method presented good consistency with single-photon emission CT (SPECT) in the evaluation of lung regional perfusion changes in various pathological conditions. Moreover, the saline bolus EIT method has been applied to assess the lung perfusion in a pulmonary embolism and the effect of positive end-expiratory pressure (PEEP) on regional ventilation/perfusion ratio (V/Q) and acute respiratory distress syndrome (ARDS) in several clinical studies. The implementation of saline boluses, data analyses, precision, and cutoff values varied among different studies, and a consensus must be reached regarding the clinical application of the saline bolus EIT method. Further study is required to validate the impact of the described saline bolus EIT method on decision-making, therapeutic management, and outcomes in critically ill patients.

Three broad classifications of acute respiratory failure etiologies based on regional ventilation and perfusion by electrical impedance tomography: a hypothesis-generating study

BACKGROUND

The aim of this study was to validate whether regional ventilation and perfusion data measured by electrical impedance tomography (EIT) with saline bolus could discriminate three broad acute respiratory failure (ARF) etiologies.

METHODS

Perfusion image was generated from EIT-based impedance-time curves caused by 10 ml 10% NaCl injection during a respiratory hold. Ventilation image was captured before the breath holding period under regular mechanical ventilation. DeadSpace%, Shunt% and VQMatch% were calculated based on lung perfusion and ventilation images. Ventilation and perfusion maps were divided into four cross-quadrants (lower left and right, upper left and right). Regional distribution defects of each quadrant were scored as 0 (distribution% ≥ 15%), 1 (15% > distribution% ≥ 10%) and 2 (distribution% < 10%). Data percentile distributions in the control group and clinical simplicity were taken into consideration when defining the scores. Overall defect scores (DefectV, DefectQ and DefectV+Q) were the sum of four cross-quadrants of the corresponding images.

RESULTS

A total of 108 ICU patients were prospectively included: 93 with ARF and 15 without as a control. PaO2/FiO2 was significantly correlated with VQMatch% (r = 0.324, P = 0.001). Three broad etiologies of ARF were identified based on clinical judgment: pulmonary embolism-related disease (PED, n = 14); diffuse lung involvement disease (DLD, n = 21) and focal lung involvement disease (FLD, n = 58). The PED group had a significantly higher DeadSpace% [40(24)% vs. 14(15)%, PED group vs. the rest of the subjects; median(interquartile range); P < 0.0001] and DefectQ score than the other groups [1(1) vs. 0(1), PED vs. the rest; P < 0.0001]. The DLD group had a significantly lower DefectV+Q score than the PED and FLD groups [0(1) vs. 2.5(2) vs. 3(3), DLD vs. PED vs. FLD; P < 0.0001]. The FLD group had a significantly higher DefectV score than the other groups [2(2) vs. 0(1), FLD vs. the rest; P < 0.0001]. The area under the receiver operating characteristic (AUC) for using DeadSpace% to identify PED was 0.894 in all ARF patients. The AUC for using the DefectV+Q score to identify DLD was 0.893. The AUC for using the DefectV score to identify FLD was 0.832.

CONCLUSIONS

Our study showed that it was feasible to characterize three broad etiologies of ARF with EIT-based regional ventilation and perfusion. Further study is required to validate clinical applicability of this method. Trial registration clinicaltrials, NCT04081142. Registered 9 September 2019-retrospectively registered, https://clinicaltrials.gov/show/NCT04081142 .

A model-based source separation algorithm for lung perfusion imaging using electrical impedance tomography

Objective. Electrical impedance tomography (EIT) for lung perfusion imaging is attracting considerable interest in intensive care, as it might open up entirely new ways to adjust ventilation therapy. A promising technique is bolus injection of a conductive indicator to the central venous catheter, which yields the indicator-based signal (IBS). Lung perfusion images are then typically obtained from the IBS using the maximum slope technique. However, the low spatial resolution of EIT results in a partial volume effect (PVE), which requires further processing to avoid regional bias.Approach. In this work, we repose the extraction of lung perfusion images from the IBS as a source separation problem to account for the PVE. We then propose a model-based algorithm, called gamma decomposition (GD), to derive an efficient solution. The GD algorithm uses a signal model to transform the IBS into a parameter space where the source signals of heart and lung are separable by clustering in space and time. Subsequently, it reconstructs lung model signals from which lung perfusion images are unambiguously extracted.Main results. We evaluate the GD algorithm on EIT data of a prospective animal trial with eight pigs. The results show that it enables lung perfusion imaging using EIT at different stages of regional impairment. Furthermore, parameters of the source signals seem to represent physiological properties of the cardio-pulmonary system.Significance. This work represents an important advance in IBS processing that will likely reduce bias of EIT perfusion images and thus eventually enable imaging of regional ventilation/perfusion (V/Q) ratio.

Calderón's Method with a Spatial Prior for 2-D EIT Imaging of Ventilation and Perfusion

Bedside imaging of ventilation and perfusion is a leading application of 2-D medical electrical impedance tomography (EIT), in which dynamic cross-sectional images of the torso are created by numerically solving the inverse problem of computing the conductivity from voltage measurements arising on electrodes due to currents applied on electrodes on the surface. Methods of reconstruction may be direct or iterative. Calderón’s method is a direct reconstruction method based on complex geometrical optics solutions to Laplace’s equation capable of providing real-time reconstructions in a region of interest. In this paper, the importance of accurate modeling of the electrode location on the body is demonstrated on simulated and experimental data, and a method of including a priori spatial information in dynamic human subject data is presented. The results of accurate electrode modeling and a spatial prior are shown to improve detection of inhomogeneities not included in the prior and to improve the resolution of ventilation and perfusion images in a human subject.

Electrical impedance tomography detects changes in ventilation after airway clearance in spinal muscular atrophy type I

The effect of mechanical insufflation-exsufflation (MIE) for airway clearance in patients with spinal muscular atrophy type I (SMA-I) on the distribution of ventilation in the lung is unknown, as is the duration of its beneficial effects. A pilot study to investigate the feasibility of using three dimensional (3-D) electrical impedance tomography (EIT) images to estimate lung volumes pre- and post-MIE for assessing the effectiveness of mechanical insufflation-exsufflation (MIE) was conducted in 6 pediatric patients with SMA-I in the neuromuscular clinic at Children's Hospital Colorado. EIT data were collected before, during, and after the MIE procedure on two rows of 16 electrodes placed around the chest. Lung volumes were computed from the images and compared before, during, and after the MIE procedure to assess the ability of EIT to estimate changes in lung volume during insufflation and exsufflation. Images of pulsatile pulmonary perfusion were computed in subjects able to perform breath-holding. In four of the six subjects, lung volumes during tidal breathing increased after MIE (average change from pre to post MIE was 58.8±55.1 mL). The time-dependent plots of lung volume computed from the EIT data clearly show when the MIE device insufflates and exsufflates air and the rest periods between mechanical coughs. Images of pulmonary pulsatile perfusion were computed from data collected during breathing pauses. The results suggest that EIT holds promise for estimating lung volumes and ventilation/perfusion mismatch, both of which are useful for assessing the effectiveness of MIE in clearing mucus plugs.

Evaluation of regional ventilation by electric impedance tomography during percutaneous dilatational tracheostomy in neurocritical care: a pilot study

Background

Percutaneous dilatational tracheostomy (PDT) has become a widely performed technique in neurocritical care, which is however known to be accompanied by some risks to the patient. The aim of this pilot study was to assess the derecruitment effects of PDT with the electric impedance tomography (EIT) during the PDT procedure in neurocritical care.

Methods

The prospective observational pilot study investigated 11 adult, intubated, mechanically ventilated patients with acute brain disease. We recorded EIT data to determine regional ventilation delay standard deviation (RVD SD), compliance win (CW) and loss (CL), end-expiratory lung impedance (EELI), with the EIT belt placed at the level of Th 4 before, during and after the PDT, performed in the standard PDT position ensuring hyperextension of the neck.

Results

From 11 patients, we finally analyzed EIT data in 6 patients - EIT data of 5 patients have been excluded due to the insufficient EIT recordings. The mean RVD SD post-PDT decreased to 7.00 ± 1.29% from 7.33 ± 1.89%. The mean post-PDT CW was 27.33 ± 15.81 and PDT CL 6.33 ± 6.55. Only in one patient, where the trachea was open for 170 s, was a massive dorsal collapse (∆EELI − 25%) detected. In other patients, the trachea was open from 15 to 50 s.

Conclusions

This pilot study demonstrated the feasibility of EIT to detect early lung derecruitment occurring due to the PDT procedure. The ability to detect regional changes in ventilation could be helpful in predicting further progression of ventilation impairment and subsequent hypoxemia, to consider optimal ventilation regimes or time-schedule and type of recruitment maneuvres required after the PDT.

Reconstruction of Organ Boundaries With Deep Learning in the D-Bar Method for Electrical Impedance Tomography

OBJECTIVE

Medical electrical impedance tomography is a non-ionizing imaging modality in which low-amplitude, low-frequency currents are applied on electrodes on the body, the resulting voltages are measured, and an inverse problem is solved to determine the conductivity distribution in the region of interest. Due the ill-posedness of the inverse problem, the boundaries of internal organs are typically blurred in the reconstructed image.

METHODS

A deep learning approach is introduced in the D-bar method for reconstructing a 2-D slice of the thorax to recover the boundaries of organs. This is accomplished by training a deep neural network on labeled pairs of scattering transforms and the boundaries of the organs in the data from which the transforms were computed. This allows the network to "learn" the nonlinear mapping between them by minimizing the error between the output of the network and known actual boundaries. Further, a "sparse" reconstruction is computed by fusing the results of the standard D-bar reconstruction with reconstructed organ boundaries from the neural network.

RESULTS

Results are shown on simulated and experimental data collected on a saline-filled tank with agar targets simulating the conductivity of the heart and lungs.

CONCLUSIONS AND SIGNIFICANCE

The results demonstrate that deep neural networks can successfully learn the mapping between scattering transforms and the internal boundaries of structures.

Increasing veno-arterial extracorporeal membrane oxygenation flow reduces electrical impedance of the lung regions in porcine acute heart failure

Veno-arterial extracorporeal membrane oxygenation (VA ECMO) is a technique used in patients with severe heart failure. The aim of this study was to evaluate its effects on left ventricular afterload and fluid accumulation in lungs with electrical impedance tomography (EIT). In eight swine, incremental increases of extracorporeal blood flow (EBF) were applied before and after the induction of ischemic heart failure. Hemodynamic parameters were continuously recorded and computational analysis of EIT was used to determine lung fluid accumulation. With an increase in EBF from 1 to 4 l/min in acute heart failure the associated increase of arterial pressure (raised by 44 %) was accompanied with significant decrease of electrical impedance of lung regions. Increasing EBF in healthy circulation did not cause lung impedance changes. Our findings indicate that in severe heart failure EIT may reflect fluid accumulation in lungs due to increasing EBF.

Influence of different PEEP levels on electrical impedance tomography findings in patients under general anesthesia ventilated in the lateral decubitus position

To determine the effect of various PEEP levels on electrical impedance tomography (EIT) measured differences in regional ventilation, hemodynamics, lung mechanics and parameters of alveolar gas exchange. Thirty three patients scheduled for elective urologic surgery in general anesthesia in lateral decubitus position were randomized into three groups-PEEP 0, 5 and 10 mbar. EIT recording, arterial blood gas analysis and hemodynamic parameters were captured at three timepoints-before induction (T0), 5 min after lateral positioning (T1) and 90 min after positioning (T2). Dynamic compliance (Cdyn) was measured at T1 and T2. Offline EIT data analysis was performed to calculate EIT derived parameters of ventilation distribution. Patients ventilated with PEEP of 10 mbar had a significantly lower A-a (alveolo arterial) gradient over measurements and symmetrical distribution of ventilation measured by EIT. There was no significant difference in Cdyn, center of ventilation indices and inhomogeneity index between groups. There was no difference of mean arterial pressure, cardiac index and heart rate between groups. Patients with 5 mbar of PEEP had higher stroke volume index compared to 0 and 10 mbar at baseline and over measurements. Nondependent/dependent TV ratio as well as global inhomogeneity index were correlated with A-a gradient. Dynamic compliance showed no correlation to A-a gradient. In our study, a PEEP level of 10 mbar improved alveolar gas exchange without compromising hemodynamic stability in patients mechanically ventilated in the lateral decubitus position. EIT measured parameters may be used to determine optimal ventilation parameters in these patients with inhomogeneous lung mechanics. Further studies are needed in patients with various lung pathologies.

DYNAMIC OPTIMIZED PRIORS FOR D-BAR RECONSTRUCTIONS OF HUMAN VENTILATION USING ELECTRICAL IMPEDANCE TOMOGRAPHY

A method of including dynamic spatial priors in the 2-D D-bar reconstruction algorithm is presented for use on time-difference reconstructions of human subject thoracic data. The conductivity values for the prior are updated at each frame in the reconstruction using an optimization method applied to the scattering transform. The updates of the dynamic spatial priors are guided by a principle component analysis of the data to determine the timepoint in the ventilatory (or cardiac) cycle. The effectiveness of the method is demonstrated on human subject ventilatory data.

Electrical impedance tomography in perioperative medicine: careful respiratory monitoring for tailored interventions

BACKGROUND

Electrical impedance tomography (EIT) is a non-invasive radiation-free monitoring technique that provides images based on tissue electrical conductivity of the chest. Several investigations applied EIT in the context of perioperative medicine, which is not confined to the intraoperative period but begins with the preoperative assessment and extends to postoperative follow-up.

MAIN BODY

EIT could provide careful respiratory monitoring in the preoperative assessment to improve preparation for surgery, during anaesthesia to guide optimal ventilation strategies and to monitor the hemodynamic status and in the postoperative period for early detection of respiratory complications. Moreover, EIT could further enhance care of patients undergoing perioperative diagnostic procedures. This narrative review summarizes the latest evidence on the application of this technique to the surgical patient, focusing also on possible future perspectives.

CONCLUSIONS

EIT is a promising technique for the perioperative assessment of surgical patients, providing tailored adaptive respiratory and haemodynamic monitoring. Further studies are needed to address the current technological limitations, confirm the findings and evaluate which patients can benefit more from this technology.

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).

Alveolar dynamics during mechanical ventilation in the healthy and injured lung

Mechanical ventilation is a life-saving therapy in patients with acute respiratory distress syndrome (ARDS). However, mechanical ventilation itself causes severe co-morbidities in that it can trigger ventilator-associated lung injury (VALI) in humans or ventilator-induced lung injury (VILI) in experimental animal models. Therefore, optimization of ventilation strategies is paramount for the effective therapy of critical care patients. A major problem in the stratification of critical care patients for personalized ventilation settings, but even more so for our overall understanding of VILI, lies in our limited insight into the effects of mechanical ventilation at the actual site of injury, i.e., the alveolar unit. Unfortunately, global lung mechanics provide for a poor surrogate of alveolar dynamics and methods for the in-depth analysis of alveolar dynamics on the level of individual alveoli are sparse and afflicted by important limitations. With alveolar dynamics in the intact lung remaining largely a "black box," our insight into the mechanisms of VALI and VILI and the effectiveness of optimized ventilation strategies is confined to indirect parameters and endpoints of lung injury and mortality.In the present review, we discuss emerging concepts of alveolar dynamics including alveolar expansion/contraction, stability/instability, and opening/collapse. Many of these concepts remain still controversial, in part due to limitations of the different methodologies applied. We therefore preface our review with an overview of existing technologies and approaches for the analysis of alveolar dynamics, highlighting their individual strengths and limitations which may provide for a better appreciation of the sometimes diverging findings and interpretations. Joint efforts combining key technologies in identical models to overcome the limitations inherent to individual methodologies are needed not only to provide conclusive insights into lung physiology and alveolar dynamics, but ultimately to guide critical care patient therapy.

Optimal PEEP during one-lung ventilation with capnothorax: An experimental study

BACKGROUND

One-lung ventilation (OLV) with induced capnothorax carries the risk of severely impaired ventilation and circulation. Optimal PEEP may mitigate the physiological perturbations during these conditions.

METHODS

Right-sided OLV with capnothorax (16 cm H₂ O) on the left side was initiated in eight anesthetized, muscle-relaxed piglets. A recruitment maneuver and a decremental PEEP titration from PEEP 20 cm H₂ O to zero end-expiratory pressure (ZEEP) was performed. Regional ventilation and perfusion were studied with electrical impedance tomography and computer tomography of the chest was used. End-expiratory lung volume and hemodynamics were recorded and.

RESULTS

PaO₂ peaked at PEEP 12 cm H₂ O (49 ± 14 kPa) and decreased to 11 ± 5 kPa at ZEEP (P < 0.001). PaCO₂ was 9.5 ± 1.3 kPa at 20 cm H₂ O PEEP and did not change when PEEP step-wise was reduced to 12 cm H₂ O PaCO_2. At lower PEEP, PaCO₂ increased markedly. The ventilatory driving pressure was lowest at PEEP 14 cm H₂ O (19.6 ± 5.8 cm H₂ O) and increased to 38.3 ± 6.1 cm H₂ O at ZEEP (P < 0.001). When reducing PEEP below 12-14 cm H₂ O ventilation shifted from the dependent to the nondependent regions of the ventilated lung (P = 0.003), and perfusion shifted from the ventilated to the nonventilated lung (P = 0.02).

CONCLUSION

Optimal PEEP was 12-18 cm H₂ O and probably relates to capnothorax insufflation pressure. With suboptimal PEEP, ventilation/perfusion mismatch in the ventilated lung and redistribution of blood flow to the nonventilated lung occurred.

Deep D-Bar: Real-Time Electrical Impedance Tomography Imaging With Deep Neural Networks

The mathematical problem for electrical impedance tomography (EIT) is a highly nonlinear ill-posed inverse problem requiring carefully designed reconstruction procedures to ensure reliable image generation. D-bar methods are based on a rigorous mathematical analysis and provide robust direct reconstructions by using a low-pass filtering of the associated nonlinear Fourier data. Similarly to low-pass filtering of linear Fourier data, only using low frequencies in the image recovery process results in blurred images lacking sharp features, such as clear organ boundaries. Convolutional neural networks provide a powerful framework for post-processing such convolved direct reconstructions. In this paper, we demonstrate that these CNN techniques lead to sharp and reliable reconstructions even for the highly nonlinear inverse problem of EIT. The network is trained on data sets of simulated examples and then applied to experimental data without the need to perform an additional transfer training. Results for absolute EIT images are presented using experimental EIT data from the ACT4 and KIT4 EIT systems.

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.

Evaluation of surrogate measures of pulmonary function derived from electrical impedance tomography data in children with cystic fibrosis

OBJECTIVE

Lung function monitoring by spirometry plays a critical role in the clinical care of pediatric cystic fibrosis (CF) patients, but many young children are unable to perform spirometry, and the outputs are often normal even in the presence of lung disease. Measures derived from electrical impedance tomography (EIT) images were studied for their utility as potential surrogates for spirometry in CF patients and to assess response to intravenous antibiotic treatment for acute pulmonary exacerbations (PEx) in a subset of patients.

APPROACH

EIT data were collected on 35 subjects (21 with CF, 14 healthy controls, 8 CF patients pre- and post-treatment for an acute PEx) ages 2 to 20 years during tidal breathing and also concurrently with spirometry on subjects over age 8. EIT-derived measures of FEV1, FVC, and FEV1/FVC were computed globally and regionally from dynamic EIT images.

MAIN RESULTS

Global EIT-derived FEV1/FVC showed good correlation with spirometry FEV1/FVC values (r  =  0.54, p  =  0.01), and were able to distinguish between the groups (p  =  0.01). Lung heterogeneity was assessed through the spatial coefficient of variation (CV) of EIT difference images between key time points, and the CVs for EIT-derived FEV1 and FVC showed significant correlation with the CV for tidal breathing (r  =  0.47, p  =  0.01 and r  =  0.50, p  =  0.01, respectively). Global EIT-derived FEV1/FVC was better able to distinguish between groups than spirometry FEV1 (F-values 776.5 and 146.3, respectively, p  <  0.01.) The same held true for the CVs for EIT-derived FEV1, FVC, and tidal breathing (F-values 215.93, 193.89, 204.57, respectively, p  <  0.01).

SIGNIFICANCE

The strong correlation between the CVs for tidal breathing, FEV1, and FVC, and the statistically significant ability of CV for tidal breathing to distinguish between healthy subjects and CF patients, and between the studied CF disease states suggests that the CV may be useful for measuring the extent and severity of structural lung disease.

Real-Time Implementation of Calderón's Method on Subject-Specific Domains

A real-time implementation of Calderón's method for the reconstruction of a 2-D conductivity from electrical impedance tomography data is presented, in which domain-specific modeling is taken into account. This is the first implementation of Calderón's method that accounts for correct modeling of non-symmetric domain boundaries in image reconstruction. The domain-specific Calderón's method is derived and reconstructions from experimental tank data are presented, quantifying the distortion when correct modeling is not included in the reconstruction algorithm. Reconstructions from human subject volunteers are presented, demonstrating the method's effectiveness for imaging changes due to ventilation and perfusion in the human thorax.

Electrical impedance tomography for predicting failure of spontaneous breathing trials in patients with prolonged weaning

Background

Spontaneous breathing trials (SBTs) on a T-piece can be difficult in patients with prolonged weaning because of remaining de-recruitment phenomena and/or insufficient ventilation. There is no clinically established method existent other than experience for estimating whether an SBT is most probably beneficial. Electrical impedance tomography (EIT) is a clinical useful online monitoring technique during mechanical ventilation, particularly because it enables analysis of effects of regional ventilation distribution. The aim of our observational study was to examine if EIT can predict whether patients with prolonged weaning will benefit from a planned SBT.

Methods

Thirty-one patients were examined. Blood gas analysis, vital parameter measurements, and EIT recordings were performed at three time points: (1) baseline with pressure support ventilation (PSV) (t0), (2) during a T-piece trial (t1), and (3) after resumption of PSV (t2). Calculation of EIT parameters was performed, including the impedance ratio (IR), the tidal variation of impedance (TIV), the changes in end-expiratory lung impedance (ΔEELI), the global inhomogeneity index (GI), and the regional ventilation delay (RVD) index with use of different thresholds of the percentage inspiration time (RVD40, RVD60, RVD80). The predictive power of the baseline GI with regard to clinical impairment of an SBT was analyzed by means of ROC curves. Clinical deterioration was assumed when tidal volume was decreased by at least 20 ml after the T-piece trial, measured at t2.

Results

Partial pressure of arterial oxygen significantly decreased at t1 (71 ± 15 mmHg) compared with t0 (85 ± 17 mmHg, p < 0.05) and t2 (82 ± 18 mmHg, p < 0.05). The IR trended toward higher values during t1. At t1, TIV and ΔEELI significantly decreased. The GI was significantly increased at t1 (t0 59.3 ± 46.1 vs t1 81.5 ± 62.5, p = 0.001), as were all RVD indexes. Assuming a GI cutoff value of >40, sensitivity of 85% and specificity of 50% were reached for predicting an increased future tidal volume.

Conclusions

EIT enables monitoring of regional ventilation distribution during SBTs and is suitable to estimate whether an SBT probably will be beneficial for an individual patient. Therefore, the application of EIT can support clinical decisions regarding patients in the phase of prolonged weaning.

Incorporating a Spatial Prior into Nonlinear D-Bar EIT Imaging for Complex Admittivities

Electrical Impedance Tomography (EIT) aims to recover the internal conductivity and permittivity distributions of a body from electrical measurements taken on electrodes on the surface of the body. The reconstruction task is a severely ill-posed nonlinear inverse problem that is highly sensitive to measurement noise and modeling errors. Regularized D-bar methods have shown great promise in producing noise-robust algorithms by employing a low-pass filtering of nonlinear (nonphysical) Fourier transform data specific to the EIT problem. Including prior data with the approximate locations of major organ boundaries in the scattering transform provides a means of extending the radius of the low-pass filter to include higher frequency components in the reconstruction, in particular, features that are known with high confidence. This information is additionally included in the system of D-bar equations with an independent regularization parameter from that of the extended scattering transform. In this paper, this approach is used in the 2-D D-bar method for admittivity (conductivity as well as permittivity) EIT imaging. Noise-robust reconstructions are presented for simulated EIT data on chest-shaped phantoms with a simulated pneumothorax and pleural effusion. No assumption of the pathology is used in the construction of the prior, yet the method still produces significant enhancements of the underlying pathology (pneumothorax or pleural effusion) even in the presence of strong noise.

Adult patient with pulmonary agenesis: focusing on one-lung ventilation during general anesthesia

Congenital pulmonary agenesis is a rare condition with high mortality. Mechanical ventilation in these patients is challenging and there has no such case been reported in the literature. We reported a 61-year-old female with lung agenesis who presented to our hospital with pneumonia and pelvic mass. In the past, she had undergone repairing of atrial septal defect and mitral valve forming surgery at 6-year-old. Thereafter she had remained asymptomatic until this time of hospital admission. The patient underwent operation for the pelvic mass with one-lung ventilation (OLV) under general anesthesia. We highlighted the use of protective ventilation (PV) strategy during OLV.

Acute respiratory distress syndrome: we can't miss regional lung perfusion!

In adult respiratory distress syndrome (ARDS), life-threatening hypoxemia may occur, dictating the need for differentiated ventilator strategies. Pronounced consolidation and/or atelectasis have been well documented in ARDS, but the contribution of regional perfusion to oxygenation has been poorly addressed. Evidence has accumulated that, in ARDS, regional perfusion is extremely variable and may affect oxygenation, independently from the amount of atelectatic-consolidated lung regions. Thus, the response in oxygenation to different ventilatory settings, both during controlled and assisted mechanical ventilation, should be interpreted with caution. In fact, gas exchange may be not determined solely by changes in aeration, but also redistribution of perfusion. Furthermore, regional perfusion can play an important role in worsening of lung injury due to increased transmural pressures. In addition, distribution of perfusion in lungs might affect the delivery of drugs through the pulmonary circulation, including antibiotics. In recent years, several techniques have been developed to determine pulmonary blood flow with increasing level of spatial resolution, allowing a better understanding of normal physiology and various pathophysiological conditions, but most of them are restricted to experimental or clinical research. Lung ultrasound and novel algorithms for electrical impedance tomography represent new promising techniques that could enable physicians to assess the distribution of pulmonary blood flow at the bedside. In ARDS, we cannot afford missing regional lung perfusion! Please see related article: http://dx.doi.org/10.1186/s12871-015-0013-0.