Marc Bodenstein

Principles of electrical impedance tomography and its clinical application.

Objective:This review outlines the basic principle, in addition to validated and upcoming clinical use of electrical impedance tomography (EIT). EIT generates functional tomograms of the thorax for detection of changes in regional lung aeration. These images allow an intraindividual comparison of changes in regional lung function. Specifically, EIT aims to optimize ventilation therapy in patients with acute lung failure. Data Sources:PubMed: National Library of Medicine and the National Institutes of Health. Study Selection:Studies with the key words “electrical impedance tomography” since 1983. Data Extraction:Qualitative and quantitative results of the studies. Data Synthesis:We summarize basic principles of the technique and subsequent analyzing methods, and give an overview of clinical and scientific questions that can be addressed by EIT. Conclusion:Potential applications in the future as well as limitations of EIT technology are described. In summary, EIT is a promising functional tomography technology on the verge of its clinical application.

Overview of the Pathology of Three Widely Used Animal Models of Acute Lung Injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are syndromes of acute diffuse damage to the pulmonary parenchyma by a variety of local or systemic insults. Increased alveolar capillary membrane permeability was recognized as the common end organ injury and a central feature in all forms of ALI/ARDS. Although great strides have been made in understanding the pathogenesis of ALI/ARDS and in intensive care medicine, the treatment approach to ARDS is still relying on ventilatory and cardiovascular support based on the recognition of the clinical picture. In the course of evaluating novel treatment approaches to ARDS, 3 models of ALI induced in different species, i.e. the surfactant washout lavage model, the oleic acid intravenous injection model and the endotoxin injection model, were widely used. This review gives an overview of the pathological characteristics of these models from studies in pigs, dogs or sheep. We believe that a good morphological description of these models, both spatially and temporally, will help us gain a better understanding of the real pathophysiological picture and apply these models more accurately and liberally in evaluating novel treatment approaches to ARDS.

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.

Impact of Model Shape Mismatch on Reconstruction Quality in Electrical Impedance Tomography

Electrical impedance tomography (EIT) is a low-cost, noninvasive and radiation free medical imaging modality for monitoring ventilation distribution in the lung. Although such information could be invaluable in preventing ventilator-induced lung injury in mechanically ventilated patients, clinical application of EIT is hindered by difficulties in interpreting the resulting images. One source of this difficulty is the frequent use of simple shapes which do not correspond to the anatomy to reconstruct EIT images. The mismatch between the true body shape and the one used for reconstruction is known to introduce errors, which to date have not been properly characterized. In the present study we, therefore, seek to 1) characterize and quantify the errors resulting from a reconstruction shape mismatch for a number of popular EIT reconstruction algorithms and 2) develop recommendations on the tolerated amount of mismatch for each algorithm. Using real and simulated data, we analyze the performance of four EIT reconstruction algorithms under different degrees of shape mismatch. Results suggest that while slight shape mismatch is well tolerated by all algorithms, using a circular shape severely degrades their performance.

Toward Morphological Thoracic EIT: Major Signal Sources Correspond to Respective Organ Locations in CT

Lung and cardiovascular monitoring applications of electrical impedance tomography (EIT) require localization of relevant functional structures or organs of interest within the reconstructed images. We describe an algorithm for automatic detection of heart and lung regions in a time series of EIT images. Using EIT reconstruction based on anatomical models, candidate regions are identified in the frequency domain and image-based classification techniques applied. The algorithm was validated on a set of simultaneously recorded EIT and CT data in pigs. In all cases, identified regions in EIT images corresponded to those manually segmented in the matched CT image. Results demonstrate the ability of EIT technology to reconstruct relevant impedance changes at their anatomical locations, provided that information about the thoracic boundary shape (and electrode positions) are used for reconstruction.

Ventilator-Associated Lung Injury Superposed to Oleic Acid Infusion or Surfactant Depletion: Histopathological Characteristics of Two Porcine Models of Acute Lung Injury

Background: The pathophysiological concept of acute lung injury (ALI) in combination with ventilator-associated lung injury (VALI) is still unclear. We characterized the histopathological features of intravenous injection of oleic acid (OAI) and lung lavage (LAV) combined with VALI. Methods: Pigs were randomized to the control, LAV or OAI group and ventilated by pressure-controlled ventilation. Measurements included: haemodynamics, spirometry, blood gas analysis, lung wet-to-dry weight ratio (W/D), total protein content in broncho-alveolar lavage fluid (BALF), and lung pathological description and scoring. Results: Five hours after lung injury induction, gas exchange was significantly impaired in both the OAI and the LAV groups. Compared to controls, we found an increase in W/D and histopathological total injury scores in both the LAV and OAI groups and an increase in BALF total protein content in the OAI group. In contrast to the LAV group, the OAI group showed septal necrosis and alveolar oedema. Both groups exhibited dorsal and caudal atelectasis and interstitial oedema. In addition, the OAI group demonstrated a propensity to dorsal necrosis and congestion whereas the LAV group tended to develop ventral overdistension and barotrauma. Conclusions: This study presents a comparison of porcine OAI and LAV models combined with VALI, providing information for study design in research on ALI.

Observation of ventilation-induced Spo(2) oscillations in pigs: first step to noninvasive detection of cyclic recruitment of atelectasis?

ABSTRACT High arterial partial oxygen pressure (Pao2) oscillations within the respiratory cycle were described recently in experimental acute lung injury. This phenomenon has been related to cyclic recruitment of atelectasis and varying pulmonary shunt fractions. Noninvasive detection of Spo2 (oxygen saturation measured by pulse oximetry) as an indicator of cyclic collapse of atelectasis, instead of recording Pao2 oscillations, could be of clinical interest in critical care. Spo2 oscillations were recorded continuously in three different cases of lung damage to demonstrate the technical feasibility of this approach. To deduce Pao2 from Spo2, a mathematical model of the hemoglobin dissociation curve including left and right shifts was derived from the literature and adapted to the dynamic changes of oxygenation. Calculated Pao2 amplitudes (derived from Spo2 measurements) were compared to simultaneously measured fast changes of Pao2, using a current standard method (fluorescence quenching of ruthenium). Peripheral hemoglobin saturation was capable to capture changes of Spo2 within each respiratory cycle. For the first time, Spo2 oscillations due to cyclic recruitment of atelectasis within a respiratory cycle were determined by photoplethysmography, a technology that can be readily applied noninvasively in clinical routine. A mathematic model to calculate the respective Pao2 changes was developed and its applicability tested.

Effect of lower body negative pressure and gravity on regional lung ventilation determined by EIT.

The aim of our study was to check the effect of varying blood volume in the chest and gravity on the distribution of ventilation and aeration in the lungs. The change in intrathoracic blood volume was elicited by application of lower body negative pressure (LBNP) of -50 cmH2O. The variation of gravity in terms of hypogravity (approximately 0g) and hypergravity (approximately 2g) was induced by changes in vertical acceleration achieved during parabolic flights. Local ventilation magnitude and end-expiratory lung volume were determined in eight human subjects in the ventral and dorsal lung regions within a transverse cross-section of the lower chest by electrical impedance tomography. The subjects were studied in a 20 degrees head-down tilted supine body position during tidal breathing and full forced expirations. During tidal breathing, a significant effect of gravity on local magnitude of ventilation and end-expiratory lung volume was detected in the dorsal lung regions both with and without LBNP. In the ventral regions, this gravity dependency was only observed during LBNP. During forced expiration, LBNP had almost no effect on local ventilation and end-expiratory lung volume in either lung region. Gravity significantly influenced the end-expiratory lung volumes in dorsal lung regions. The results indicate that exposure to LBNP exerts a less appreciable effect on regional lung ventilation than the acute changes in gravity.

A Comparison of Micropore Membrane Inlet Mass Spectrometry-Derived Pulmonary Shunt Measurement with Riley Shunt in a Porcine Model

BACKGROUND: The multiple inert gas elimination technique was developed to measure shunt and the ratio of alveolar ventilation to simultaneous alveolar capillary blood flow in any part of the lung (VA′/Q′) distributions. Micropore membrane inlet mass spectrometry (MMIMS), instead of gas chromatography, has been introduced for inert gas measurement and shunt determination in a rabbit lung model. However, agreement with a frequently used and accepted method for quantifying deficits in arterial oxygenation has not been established. We compared MMIMS-derived shunt (M-S) as a fraction of total cardiac output (CO) with Riley shunt (R-S) derived from the R-S formula in a porcine lung injury model. METHODS: To allow a broad variance of atelectasis and therefore shunt fraction, 8 sham animals did not receive lavage, and 8 animals were treated by lung lavages with 30 mL/kg warmed lactated Ringers solution as follows: 2 animals were lavaged once, 5 animals twice, and 1 animal 3 times. Variables were recorded at baseline and twice after induction of lung injury (T1 and T2). Retention data of sulfur hexafluoride, krypton, desflurane, enflurane, diethyl ether, and acetone were analyzed by MMIMS, and M-S was derived using a known algorithm for the multiple inert gas elimination technique. Standard formulas were used for the calculation of R-S. RESULTS: Forty-four pairs of M-S and R-S were recorded. M-S ranged from 0.1% to 35.4% and R-S from 3.7% to 62.1%. M-S showed a correlation with R-S described by linear regression: M-S = −4.26 + 0.59 × R-S (r2 = 0.83). M-S was on average lower than R-S (mean = −15.0% CO, sd = 6.5% CO, and median = −15.1), with lower and upper limits of agreement of −28.0% and −2.0%, respectively. The lower and upper limits of the 95% confidence intervals were −17.0 and −13.1 (P < 0.001, Students t-test). CONCLUSIONS: Shunt derived from MMIMS inert gas retention data correlated well with R-S during breathing of oxygen. Shunt as derived by MMIMS was generally less than R-S.

Low tidal volume pressure support versus controlled ventilation in early experimental sepsis in pigs

BackgroundIn moderate acute respiratory distress syndrome (ARDS) several studies support the usage of assisted spontaneous breathing modes. Only limited data, however, focus on the application in systemic sepsis and developing lung injury. The present study examines the effects of immediate initiation of pressure support ventilation (PSV) in a model of sepsis-induced ARDS.Methods18 anesthetized pigs received a two-staged continuous lipopolysaccharide infusion to induce lung injury. The animals were randomly assigned to PSV or volume controlled (VCV) lung protective ventilation (tidal volume each 6 ml kg-1, n = 2x9) over six hours. Gas exchange parameters, hemodynamics, systemic inflammation, and ventilation distribution by multiple inert gas elimination and electrical impedance tomography were assessed. The post mortem analysis included histopathological scoring, wet to dry ratio, and alveolar protein content.ResultsWithin six hours both groups developed a mild to moderate ARDS with comparable systemic inflammatory response and without signs of improving gas exchange parameters during PSV. The PSV group showed signs of more homogenous ventilation distribution by electrical impedance tomography, but only slightly less hyperinflated lung compartments by multiple inert gas elimination. Post mortem and histopathological assessment yielded no significant intergroup differences.ConclusionsIn a porcine model of sepsis-induced mild ARDS immediate PSV was not superior to VCV. This contrasts with several experimental studies from non-septic mild to moderate ARDS. The present study therefore assumes that not only severity, but also etiology of lung injury considerably influences the response to early initiation of PSV.