The contribution of the lungs to thoracic impedance measurements: a simulation study based on a high resolution finite difference model

Wearable Bioimpedance Monitoring: Viewpoint for Application in Chronic Conditions

Currently, nearly 6 in 10 US adults are suffering from at least one chronic condition. Wearable technology could help in controlling the health care costs by remote monitoring and early detection of disease worsening. However, in recent years, there have been disappointments in wearable technology with respect to reliability, lack of feedback, or lack of user comfort. One of the promising sensor techniques for wearable monitoring of chronic disease is bioimpedance, which is a noninvasive, versatile sensing method that can be applied in different ways to extract a wide range of health care parameters. Due to the changes in impedance caused by either breathing or blood flow, time-varying signals such as respiration and cardiac output can be obtained with bioimpedance. A second application area is related to body composition and fluid status (eg, pulmonary congestion monitoring in patients with heart failure). Finally, bioimpedance can be used for continuous and real-time imaging (eg, during mechanical ventilation). In this viewpoint, we evaluate the use of wearable bioimpedance monitoring for application in chronic conditions, focusing on the current status, recent improvements, and challenges that still need to be tackled.

Chest Movement and Respiratory Volume both Contribute to Thoracic Bioimpedance during Loaded Breathing

Bioimpedance has been widely studied as alternative to respiratory monitoring methods because of its linear relationship with respiratory volume during normal breathing. However, other body tissues and fluids contribute to the bioimpedance measurement. The objective of this study is to investigate the relevance of chest movement in thoracic bioimpedance contributions to evaluate the applicability of bioimpedance for respiratory monitoring. We measured airflow, bioimpedance at four electrode configurations and thoracic accelerometer data in 10 healthy subjects during inspiratory loading. This protocol permitted us to study the contributions during different levels of inspiratory muscle activity. We used chest movement and volume signals to characterize the bioimpedance signal using linear mixed-effect models and neural networks for each subject and level of muscle activity. The performance was evaluated using the Mean Average Percentage Errors for each respiratory cycle. The lowest errors corresponded to the combination of chest movement and volume for both linear models and neural networks. Particularly, neural networks presented lower errors (median below 4.29%). At high levels of muscle activity, the differences in model performance indicated an increased contribution of chest movement to the bioimpedance signal. Accordingly, chest movement contributed substantially to bioimpedance measurement and more notably at high muscle activity levels.

Realistic forward and inverse model mesh generation for rapid three-dimensional thoracic electrical impedance imaging

One of the most promising clinical applications of Electrical Impedance Tomography (EIT) is real-time monitoring of lung function in ambulatory or ICU due to the rapid, non-invasive and non-ionizing nature of the measurements. However, to move this modality into routine clinical use will, as a minimum, require the development of realistic and computationally efficient forward and inverse meshes of the thorax and the lungs alongside mechanisms to extract quantitative information from the resulting reconstructed images. The latter will allow for reduction of artefacts and better localization of conductivity changes within the image domain. This research aims to contribute towards this goal, by introducing a pipeline for the generation of anatomically accurate meshes for EIT forward and inverse models. We achieve this by the segmentation of realistic volumetric data from thoracic CT volumes, and subsequent tessellation. Mesh quality is assessed in terms of aspect ratio, dihedral and face angles. Moreover, the generated meshes are fused with currently available EIT software, with a novel electrode placement method, to show the practical application of the generated meshes. Results show that anatomically constrained unstructured meshes can be generated, conforming to realistic anatomical geometry, and performing well in EIT numerical computations. Such realistic computational models will further enhance the performance of EIT reconstruction algorithms, thus offering significant benefits to clinical EIT lung imaging.

Electrical Impedance Tomography-guided PEEP Titration in Patients Undergoing Laparoscopic Abdominal Surgery

The aim of the study is to utilize electrical impedance tomography (EIT) to guide positive end-expiratory pressure (PEEP) and to optimize oxygenation in patients undergoing laparoscopic abdominal surgery.Fifty patients were randomly assigned to the control (C) group and the EIT (E) group (n = 25 each). We set the fraction of inspired oxygen (FiO2) at 0.30. The PEEP was titrated and increased in a 2-cm H2O stepwise manner, from 6 to 14 cm H2O. Hemodynamic variables, respiratory mechanics, EIT images, analysis of blood gas, and regional cerebral oxygen saturation were recorded. The postoperative pulmonary complications within the first 5 days were also observed.We chose 10 cm H2O and 8 cm H2O as the "ideal" PEEP for the C and the E groups, respectively. EIT-guided PEEP titration led to a more dorsal shift of ventilation. The PaO2/FiO2 ratio in the E group was superior to that in the C group in the pneumoperitoneum period, though the difference was not significant (330 ± 10 vs 305.56 ± 4 mm Hg; P = 0.09). The C group patients experienced 8.7% postoperative pulmonary complications versus 5.3% among the E group patients (relative risk 1.27, 95% confidence interval 0.31-5.3, P = 0.75).Electrical impedance tomography represents a new promising technique that could enable anesthesiologists to assess regional ventilation of the lungs and optimize global oxygenation for patients undergoing laparoscopic abdominal surgery.

Physiologic effect of high-flow nasal cannula in infants with bronchiolitis

OBJECTIVE

To assess the effect of delivering high-flow nasal cannula flow on end-expiratory lung volume, continuous distending pressure, and regional ventilation distribution in infants less than 12 months old with bronchiolitis.

DESIGN

Prospective observational clinical study.

SETTING

Nineteen bed medical and surgical PICU.

PATIENTS

Thirteen infants with bronchiolitis on high-flow nasal therapy.

INTERVENTIONS

The study infants were measured on a flow rate applied at 2 and 8 L/min through the high-flow nasal cannula system.

MEASUREMENTS AND RESULTS

Ventilation distribution was measured with regional electrical impedance amplitudes and end-expiratory lung volume using electrical impedance tomography. Changes in continuous distending pressure were measured from the esophagus via the nasogastric tube. Physiological variables were also recorded. High-flow nasal cannula delivered at 8 L/min resulted in significant increases in global and anterior end-expiratory lung volume (p < 0.01) and improvements in the physiological variables of respiratory rate, SpO2, and FIO2 when compared with flows of 2 L/min.

CONCLUSION

In infants with bronchiolitis, high-flow nasal cannula oxygen/air delivered at 8 L/min resulted in increases in end-expiratory lung volume and improved respiratory rate, FIO2, and SpO2.

Development of an Anatomically Realistic Forward Solver for Thoracic Electrical Impedance Tomography

Electrical impedance tomography (EIT) has the potential to provide a low cost and safe imaging modality for clinically monitoring patients being treated with mechanical ventilation. Variations in reconstruction algorithms at different clinical settings, however, make interpretation of regional ventilation across institutions difficult, presenting the need for a unified algorithm for thoracic EIT reconstruction. Development of such a consensual reconstruction algorithm necessitates a forward model capable of predicting surface impedance measurements as well as electric fields in the interior of the modeled thoracic volume. In this paper, we present an anatomically realistic forward solver for thoracic EIT that was built based on high resolution MR image data of a representative adult. Accuracy assessment of the developed forward solver in predicting surface impedance measurements by comparing the predicted and observed impedance measurements shows that the relative error is within the order of 5%, demonstrating the ability of the presented forward solver in generating high-fidelity surface thoracic impedance data for thoracic EIT algorithm development and evaluation.

Effect of body position on ventilation distribution in ventilated preterm infants

RATIONALE

Positioning is considered vital to the maintenance of good lung ventilation by optimizing oxygen transport and gas exchange in ventilated premature infants. Previous studies suggest that the prone position is advantageous; however, no data exist on regional ventilation distribution for this age group.

OBJECTIVES

To investigate the effect of body position on regional ventilation distribution in ventilated and nonventilated preterm infants using electrical impedance tomography.

DESIGN

Randomized crossover study design.

SETTING

Neonatal ICU.

PATIENTS

A total of 24 ventilated preterm infants were compared with six spontaneously breathing preterm infants.

INTERVENTIONS

Random assignment of the order of the positions supine, prone, and quarter prone.

MEASUREMENTS AND MAIN RESULTS

Ventilation distribution was measured with regional impedance amplitudes and global inhomogeneity indices using electrical impedance tomography. In the spontaneously breathing infants, regional impedance amplitudes were increased in the posterior compared with the anterior lung (p < 0.01) and in the right compared with the left lung (p = 0.03). No differences were found in the ventilated infants. Ventilation was more inhomogeneous in the ventilated compared with the healthy infants (p < 0.01). Assessment of temporal regional lung filling showed that the posterior lung filled earlier than the anterior lung in the spontaneously breathing infants (p < 0.02) whereas in the in the ventilated infants the right lung filled before the left lung (p < 0.01).

CONCLUSIONS

In contrast to previous studies showing that ventilation is distributed to the nondependent lung in infants and children, this study shows that gravity has little effect on regional ventilation distribution.

Interaction of dependent and non-dependent regions of the acutely injured lung during a stepwise recruitment manoeuvre

The benefit of treating acute lung injury with recruitment manoeuvres is controversial. An impediment to settling this debate is the difficulty in visualizing how distinct lung regions respond to the manoeuvre. Here, regional lung mechanics were studied by electrical impedance tomography (EIT) during a stepwise recruitment manoeuvre in a porcine model with acute lung injury. The following interaction between dependent and non-dependent regions consistently occurred: atelectasis in the most dependent region was reversed only after the non-dependent region became overdistended. EIT estimates of overdistension and atelectasis were validated by histological examination of lung tissue, confirming that the dependent region was primarily atelectatic and the non-dependent region was primarily overdistended. The pulmonary pressure-volume equation, originally designed for modelling measurements at the airway opening, was adapted for EIT-based regional estimates of overdistension and atelectasis. The adaptation accurately modelled the regional EIT data from dependent and non-dependent regions (R(2) > 0.93, P < 0.0001) and predicted their interaction during recruitment. In conclusion, EIT imaging of regional lung mechanics reveals that overdistension in the non-dependent region precedes atelectasis reversal in the dependent region during a stepwise recruitment manoeuvre.

Effect of body position on ventilation distribution in preterm infants on continuous positive airway pressure

RATIONALE

Although continuous positive airway pressure is used extensively in neonatal intensive care units, and despite the belief that positioning is considered vital to the maintenance of good lung ventilation, no data exist on regional ventilation distribution in infants on continuous positive airway pressure ventilatory support.

OBJECTIVES

To investigate the effect of body position on regional ventilation in preterm infants on continuous positive airway pressure ventilatory support using electrical impedance tomography.

DESIGN

Randomized crossover study design.

SETTING

Neonatal intensive care unit.

PATIENTS

Twenty-four preterm infants on continuous positive airway pressure were compared to six spontaneously breathing preterm infants.

INTERVENTIONS

Random assignment of the order of the positions supine, prone, and quarter prone.

MEASUREMENTS AND RESULTS

Changes in global and regional lung volume were measured with electrical impedance tomography. Although there were no differences between positions, regional tidal volume was increased in the posterior compared with the anterior lung (p < .01) and in the right compared with the left lung (p < .03) in both the spontaneously breathing infants and in the infants on continuous positive airway pressure. The posterior lung filled earlier than the anterior lung in the spontaneously breathing infants (p < .02), whereas in the infants on continuous positive airway pressure the right lung filled before the left lung (p < .01). There was more ventilation inhomogeneity in the infants on continuous positive airway pressure than in the healthy infants (p < .01).

CONCLUSIONS

This study presents the first results on regional ventilation distribution in preterm infants on continuous positive airway pressure using electrical impedance tomography. Gravity had little impact on regional ventilation distribution in preterm infants on continuous positive airway pressure or in spontaneously breathing infants in the supine or prone position, indicating that ventilation distribution in preterm infants is not gravity-dependent but follows an anatomical pattern. AUSTRALIA NEW ZEALAND CLINICAL TRIALS REGISTRY:: ACTRN12606000210572.

Distribution of tidal ventilation during volume-targeted ventilation is variable and influenced by age in the preterm lung

PURPOSE

Synchronised volume-targeted ventilation (SIPPV + VTV) attempts to reduce lung injury by standardising volume delivery to the preterm lung. The aim of this study is to describe the regional distribution and variability of ventilation within the preterm lung during SIPPV + VTV.

METHODS

Twenty-seven stable, supine, preterm infants with <32 weeks gestation receiving SIPPV + VTV were studied. From each infant, the anterior-to-posterior impedance change due to tidal ventilation (∆Z (VT); countless units) was determined during every breath from three, 30-s, electrical impedance tomography recordings. ∆Z (VT) within the anterior, middle and posterior thirds of the chest were compared using area under the curve analysis. The coefficient of variation (CV) of ∆Z (VT) in the anterior and posterior hemithoraces, inflation pressure and, where available, V (T) at airway opening were compared. Infants were sub-grouped by age (≤7 and >7 days), supplemental oxygen requirement and set tidal volume.

RESULTS

In all sub-groups, the middle third of the chest accounted for the greatest ∆Z (VT) [p < 0.0001, repeated-measures analysis of variance (ANOVA)]. The middle third of the chest constituted a greater relative ∆Z (VT) in infants aged >7 days compared with ≤7 days (p < 0.0001, repeated-measures ANOVA). Set tidal volume and oxygen requirement did not significantly influence the regional distribution of ∆Z (VT). The mean (standard deviation, SD) CV of ∆Z (VTANT) and ∆Z (VTPOST) were 30.6% (14.0%) and 31.9% (12.7%). ∆Z (VTANT) and ∆Z (VTPOST) expressed greater breath-to-breath variability than the variation in inflation pressure and V (T) at airway opening (p = 0.012 and p < 0.0001, respectively, paired t-tests).

CONCLUSION

During SIPPV + VTV the preterm infant exhibits marked breath-to-breath variability in regional ventilation which is influenced by age.

MEFS - MIND electrical impedance tomography forward solver

Electrical impedance tomography (EIT) offers a possibility of realizing of a low cost and safe modality for clinically monitoring patients being treated with mechanical ventilation. However, image reconstruction algorithm employed at different clinical or research settings varies from one to another, which in turn makes interpretation of regional ventilation across institutions difficult. Seeing the lack of a standardized algorithm, GREIT (Graz consensus Reconstruction algorithm for EIT) was proposed lately in an attempt to develop a unified EIT image reconstruction algorithm. To assess GREIT, an anatomically-detailed electrical model of the thorax is indispensable. In view of this need, we describe a high resolution, image-based electrical thoracic modeling software environment, named MEFS (MIND EIT Forward Solver). The software environment utilizes anatomically realistic geometry of the thorax, allows placing electrode of any shape interactively, and yields the detailed field information in the simulated volume. The goals of the development of MEFS are: 1) to generate electrical measurements needed for EIT image reconstruction, 2) to provide a platform to compare various EIT reconstruction algorithms.

ZMIND - an interactive environment for electrical modeling of the thorax

Interest in the electrical modeling of the thorax is motivated by various desires ranging from determining cardiac function, optimizing defibrillation efficacy, monitoring pulmonary edema, etc. However, existing models represent the thorax with rather coarse anatomical details, limiting their utilizations for accurately simulating small electrodes which typically occurs in pacing and defibrillation clinical practices. In this paper, we describe an anatomically realistic finite difference modeling software environment, referred as ZMIND. Segmented image-based finite difference models of a male adult at the end of systole and the end of diastole were constructed based on ECG-gated MRI scans. Up to 36 types of tissues were identified and included in the model, providing fine anatomical details in the heart and lung regions. The environment enables placing electrodes interactively and also provides a library of clinically-based, user-configurable electrodes. The analysis module of this environment allows performing sensitivity analysis and visualizing the computed electric fields, current density, and sensitivity distribution.

Modeling current pathways for therapeutic electrical applications

In determining how various devices will provide therapeutic currents it is necessary to know the current pathways along with the current densities and voltage gradients in the tissues of interest and in other tissues that may be influenced. In order to obtain this information a high resolution, computer based model (3.8 million elements) was created using gated ECG MRI images from an adult male. The model used the finite difference approach to obtain the needed data. A user friendly, graphical interface was developed for a PC program. The features of the program along with examples from studying the electrical therapies of pacing and defibrillation will be given. It will be show that the conductivity of the entire thorax is important in determining the current pathways within the heart.