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.

Recent advances in and limitations of cardiac output monitoring by means of electrical impedance tomography.

BACKGROUND: Currently, the monitoring of cardiac output (CO) and stroke volume (SV) is mainly performed using invasive techniques. Therefore, performing CO monitoring noninvasively by means of electrical impedance tomography (EIT) would be advantageous for intensive care. Our hypothesis was that, by means of EIT, it is possible to assess heart rate (HR) and to quantify changes in SV due to changes in ventilator settings. METHODS: CO (HR and SV) of 14 pigs (32-40 kg body weight) was changed by incremental increases in positive end-expiratory pressure levels (0, 5, 10, 15, and 20 cm·H2O; ramp maneuver). This ramp maneuver was applied 4 times in each animal, yielding 43 evaluable single experiments. At each positive end-expiratory pressure level, SV was assessed by transpulmonary thermodilution using a PiCCO device. EIT data were acquired using a Dräger EIT Evaluation Kit 2. RESULTS: The EIT-based SV-related signal, Z(SV) (in [AU]), showed only a weak correlation (after excluding 2 measurements) with SV(TTD) of r = 0.58 (95% confidence interval, 0.43-0.71). If Z(SV) is calibrated by the reference 1 time for each experiment (defined as SVEIT), the correlation is approximately 0.85 (95% confidence interval, 0.78-0.90). A possible reason for the moderate correlation is the unexpected scaling pattern, leading to amplification of the cardiac impedance signal, found in some animals. The scaling is probably due to the imperfect reconstruction (i.e., a change of sensitivity) of the EIT images or to a change in the position of the heart. CONCLUSIONS: The hypothesis that EIT can be used to monitor CO and SV was confirmed, but further studies are required before this technique can be applied in clinical practice. HR was determined robustly and accurately. For SV monitoring, promising results were obtained in 80% of the experiments. However, unexpected scaling of the cardiac EIT signal causing inaccurate estimation of SV remains an issue. Before robust assessment of SV by EIT is suitable for clinical practice, the cause of and compensation for undesired scaling effects need to be investigated.

Electrical impedance tomography: potentials and pitfalls.

PURPOSE OF REVIEW: Electrical impedance tomography (EIT) is a useful noninvasive tool for monitoring ventilation finding its way into the clinical setting. The focus of this review is to discuss the balance between the potential for EIT as a clinical monitor accepting a level of uncertainty and the scientific demand for absolute perfection. RECENT FINDINGS: The controversy concerning whether EIT impedance changes can be safely used to monitor lung volume changes now appears to be solved after recent elegant studies. It is now high time to display lung volume changes measured by EIT in clinical units, that is in millilitres following calibration versus tidal volume. A growing number of indices for regional ventilation distribution are emerging some of which should be further evaluated and developed for clinical decision support. SUMMARY: Already now EIT is a useful clinical monitor. Still more work is needed to develop and interpret indices which are simple enough to be used in the clinical setting to guide the clinician towards effective and safe ventilator management.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16.

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Gas distribution by EIT during PEEP inflation: PEEP response and optimal PEEP with lowest trans-pulmonary driving pressure can be determined without esophageal pressure during a rapid PEEP trial in patients with acute respiratory failure.

Objective. Protective ventilation should be based onlungmechanics and transpulmonary driving pressure (ΔPTP), as this 'hits' the lung directly.Approach. The change in end-expiratory lung volume (ΔEELV) is determined by the size of the PEEP step and the elastic properties of the lung (EL), ΔEELV/ΔPEEP. Consequently, EL can be determined as ΔPEEP/ΔEELV. By calibration of tidal inspiratory impedance change with ventilator inspiratory tidal volume, end-expiratory lung impedance changes were converted to volume changes and lung P/V curves were obtained during a PEEP trial in ten patients with acute respiratory failure. The PEEP level where ΔPTP was lowest (optimal PEEP) was determined as the steepest point of the lung P/V curve.Main results. Over-all EL ranged between 7.0-23.2 cmH2O/L. Optimal PEEP was 12.9 cmH2O (10-16) with ΔPTP of 4.1 cmH2O (2.8-7.6). Patients with highest EL were PEEP non-responders, where EL increased in non-dependent and dependent lung at high PEEP, indicating over-distension in all lung. Patients with lower EL were PEEP responders with decreasing EL in dependent lung when increasing PEEP.Significance. PEEP non-responders could be identified by regional lung P/V curves derived from ventilator calibrated EIT. Optimal PEEP could be determined from the equation for the lung P/V curve.

Inspiratory and end-expiratory effects of lung recruitment in the prone position on dorsal lung aeration - new physiological insights in a secondary analysis of a randomised controlled study in post-cardiac surgery patients.

Background: Cardiac surgery produces dorso-basal atelectasis and ventilation/perfusion mismatch, associated with infection and prolonged intensive care. A postoperative lung volume recruitment manoeuvre to decrease the degree of atelectasis is routine. In patients with severe respiratory failure, prone positioning and recruitment manoeuvres may increase survival, oxygenation, or both. We compared the effects of lung recruitment in prone vs supine positions on dorsal inspiratory and end-expiratory lung aeration. Methods: In a prospective RCT, 30 post-cardiac surgery patients were randomly allocated to recruitment manoeuvres in the prone (n=15) or supine position (n=15). The primary endpoints were late dorsal inspiratory volume (arbitrary units [a.u.]) and left/right dorsal end-expiratory lung volume change (a.u.), prone vs supine after extubation, measured using electrical impedance tomography. Secondary outcomes included left/right dorsal inspiratory volumes (a.u.) and left/right dorsal end-expiratory lung volume change (a.u.) after prone recruitment and extubation. Results: The last part of dorsal end-inspiratory volume after extubation was higher after prone (49.1 a.u.; 95% confidence interval [CI], 37.4-60.6) vs supine recruitment (24.2 a.u.; 95% CI, 18.4-29.6; P=0.024). Improvement in left dorsal end-expiratory lung volume after extubation was higher after prone (382 a.u.; 95% CI, 261-502) vs supine recruitment (-71 a.u., 95% CI, -140 to -2; n=15; P<0.001). After prone recruitment, left vs right predominant end-expiratory dorsal lung volume change disappeared after extubation. However, both left and right end-expiratory volumes were higher in the prone group, after extubation. Conclusions: Recruitment in the prone position improves dorsal inspiratory and end-expiratory lung volumes after cardiac surgery. Clinical trial registration: NCT03009331.

Electrical impedance tomography applied to assess matching of pulmonary ventilation and perfusion in a porcine experimental model.

INTRODUCTION: Electrical impedance tomography (EIT) can be used to measure impedance changes related to the thoracic content of air and blood. Few studies, however, have utilised EIT to make concurrent measurements of ventilation and perfusion. This experimental study was performed to investigate the feasibility of EIT to describe ventilation/perfusion (V/Q) matching after acute changes of pulmonary perfusion and aeration. METHODS: Six mechanically ventilated, anaesthetised pigs in the supine position were studied at baseline, after inflation of a balloon in the inferior caval vein (Binfl) to reduce cardiac output and after an increased positive end-expiratory pressure (PEEP) of 20 cmH2O (PEEP20) to increase pulmonary aeration. EIT measurements were performed at the mid-thoracic level to measure the amplitude of impedance changes related to ventilation (ZV) and perfusion (ZQ), both globally and in four defined regions of interest (ROI) extending from the ventral to dorsal distance. RESULTS: A largely parallel distribution of ZV and ZQ in all four ROIs during baseline conditions corresponded to a bell-shaped frequency distribution of ZV/ZQ ratios with only moderate scatter. Binfl and PEEP20 with unchanged tidal volumes significantly increased the mismatch of regional ZV and ZQ, the scatter of ZV/ZQ ratios and the heterogeneity of the ZV/ZQ frequency distribution. Significant positive and negative correlations were demonstrated between fractional alveolar dead space (r2 = 0.63 [regression coefficient]) and venous admixture (r2 = 0.48), respectively, and the global ZV/ZQ ratio. CONCLUSIONS: EIT may be used to monitor the distribution of pulmonary ventilation and perfusion making detailed studies of V/Q matching possible.

Improved response time with a new miniaturised main-stream multigas monitor.

BACKGROUND: For paediatric monitoring and demanding applications such as metabolic monitoring and measurements of functional residual capacity combining gas concentration with flow/volume measurements the performance of side-stream monitors (SSGM) is suboptimal. The objective was to evaluate the performance of a miniaturised mainstream multigas monitor (MSGM) alleged to offer fast response gas monitoring. The MSGM uses infrared technique for measurements of carbon dioxide, nitrous oxide and inhalation agents and fuel cell technique for oxygen monitoring. The MSGM performance was com- pared to a state of the art side-stream monitor in a bench study. METHODS: Response time was measured in two bench study set ups; a high flow oxygen flush to achieve one step change in gas concentrations and during continuous ventilation using a circuit with an oxygen consuming/carbon dioxide producing lung model connected to a ventilator. Averaged tracings from the tested monitors were used for calculation of the 90-10% decline of CO(2), the corresponding 10-90% incline of O(2) and N(2)O and of Isoflurane concentrations in the flush set up and at different inspired O(2) for the O(2) upslope and corresponding CO(2) down- slope during continuous ventilation at different breathing frequencies. Calibration gases with different concentrations of CO(2), O(2) and N(2)O were used for testing of accuracy. RESULTS: The MSGM response time for CO(2) was 96 (88-100) compared to 348 (340-352) ms for the SSGM (P < 0.001). Corresponding response times for O(2) was 108 (76-144), and 432 (360-448) ms (P < 0.001), respectively. At a respiratory rate of 60 BPM the SSGM trace was damped and sinusoidal whereas the MSGM displayed wider amplitude and a square waveform. The deviations from calibration gas values were within clinically acceptable range and linear for all gases over the concentration range studied for both monitors. CONCLUSIONS: The MSGM response time for CO(2) and O(2) was less than 1/3 of the SSGM. The performance of the MSGM was maintained at high breathing frequencies. The accuracy was within clinically acceptable limits for both monitors.

Can we estimate transpulmonary pressure without an esophageal balloon?-yes.

A protective ventilation strategy is based on separation of lung and chest wall mechanics and determination of transpulmonary pressure. So far, this has required esophageal pressure measurement, which is cumbersome, rarely used clinically and associated with lack of consensus on the interpretation of measurements. We have developed an alternative method based on a positive end expiratory pressure (PEEP) step procedure where the PEEP-induced change in end-expiratory lung volume is determined by the ventilator pneumotachograph. In pigs, lung healthy patients and acute lung injury (ALI) patients, it has been verified that the determinants of the change in end-expiratory lung volume following a PEEP change are the size of the PEEP step and the elastic properties of the lung, ∆PEEP × Clung. As a consequence, lung compliance can be calculated as the change in end-expiratory lung volume divided by the change in PEEP and esophageal pressure measurements are not needed. When lung compliance is determined in this way, transpulmonary driving pressure can be calculated on a breath-by-breath basis. As the end-expiratory transpulmonary pressure increases as much as PEEP is increased, it is also possible to determine the end-inspiratory transpulmonary pressure at any PEEP level. Thus, the most crucial factors of ventilator induced lung injury can be determined by a simple PEEP step procedure. The measurement procedure can be repeated with short intervals, which makes it possible to follow the course of the lung disease closely. By the PEEP step procedure we may also obtain information (decision support) on the mechanical consequences of changes in PEEP and tidal volume performed to improve oxygenation and/or carbon dioxide removal.

Regional lung derecruitment after endotracheal suction during volume- or pressure-controlled ventilation: a study using electric impedance tomography.

OBJECTIVE: To assess lung volume and compliance changes during open- and closed-system suctioning using electric impedance tomography (EIT) during volume- or pressure-controlled ventilation. DESIGN AND SETTING: Experimental study in a university research laboratory. SUBJECTS: Nine bronchoalveolar saline-lavaged pigs. INTERVENTIONS: Open and closed suctioning using a 14-F catheter in volume- or pressure-controlled ventilation at tidal volume 10 ml/kg, respiratory rate 20 breaths/min, and positive end-expiratory pressure 10 cmH2O. MEASUREMENTS AND RESULTS: Lung volume was monitored by EIT and a modified N2 washout/-in technique. Airway pressure was measured via a pressure line in the endotracheal tube. In four ventral-to-dorsal regions of interest regional ventilation and compliance were calculated at baseline and 30 s and 1, 2, and 10 min after suctioning. Blood gases were followed. At disconnection functional residual capacity (FRC) decreased by 58+/-24% of baseline and by a further 22+/-10% during open suctioning. Arterial oxygen tension decreased to 59+/-14% of baseline value 1 min after open suctioning. Regional compliance deteriorated most in the dorsal parts of the lung. Restitution of lung volume and compliance was significantly slower during pressure-controlled than volume-controlled ventilation. CONCLUSIONS: EIT can be used to monitor rapid lung volume changes. The two dorsal regions of the lavaged lungs are most affected by disconnection and suctioning with marked decreases in compliance. Volume-controlled ventilation can be used to rapidly restitute lung aeration and oxygenation after lung collapse induced by open suctioning.

Transpulmonary and pleural pressure in a respiratory system model with an elastic recoiling lung and an expanding chest wall.

BACKGROUND: We have shown in acute lung injury patients that lung elastance can be determined by a positive end-expiratory pressure (PEEP) step procedure and proposed that this is explained by the spring-out force of the rib cage off-loading the chest wall from the lung at end-expiration. The aim of this study was to investigate the effect of the expanding chest wall on pleural pressure during PEEP inflation by building a model with an elastic recoiling lung and an expanding chest wall complex. METHODS: Test lungs with a compliance of 19, 38, or 57 ml/cmH2O were placed in a box connected to a plastic container, 3/4 filled with water, connected to a water sack of 10 l, representing the abdomen. The space above the water surface and in the lung box constituted the pleural space. The contra-directional forces of the recoiling lung and the expanding chest wall were obtained by evacuating the pleural space to a negative pressure of 5 cmH2O. Chest wall elastance was increased by strapping the plastic container. Pressure was measured in the airway and pleura. Changes in end-expiratory lung volume (ΔEELV), during PEEP steps of 4, 8, and 12 cmH2O, were determined in the isolated lung, where airway equals transpulmonary pressure and in the complete model as the cumulative inspiratory-expiratory tidal volume difference. Transpulmonary pressure was calculated as airway minus pleural pressure. RESULTS: Lung pressure/volume curves of an isolated lung coincided with lung P/V curves in the complete model irrespective of chest wall stiffness. ΔEELV was equal to the size of the PEEP step divided by lung elastance (EL), ΔEELV = ΔPEEP/EL. The end-expiratory "pleural" pressure did not increase after PEEP inflation, and consequently, transpulmonary pressure increased as much as PEEP was increased. CONCLUSIONS: The rib cage spring-out force causes off-loading of the chest wall from the lung and maintains a negative end-expiratory "pleural" pressure after PEEP inflation. The behavior of the respiratory system model confirms that lung elastance can be determined by a simple PEEP step without using esophageal pressure measurements.

Acute hemodynamic changes during lung recruitment in lavage and endotoxin-induced ALI.

OBJECTIVE: To assess acute cardiorespiratory effects of recruitment manoeuvres in experimental acute lung injury. DESIGN: Experimental study in animal models of acute lung injury. SETTING: Experimental laboratory at a University Medical Centre. ANIMALS: Ten pigs with bronchoalveolar lavage and eight pigs with endotoxin-induced ALI. INTERVENTIONS: Two kinds of recruitment manoeuvres during 1 min; a) vital capacity manoeuvres (ViCM) consisting in a sustained inflation at 30 cmH(2)O and 40 cmH(2)O; b) manoeuvres obtained during ongoing pressure-controlled ventilation (PCRM) with peak airway pressure 30 cmH(2)O, positive end-expiratory pressure (PEEP) 15 and peak airway pressure 40, PEEP 20. Recruitment manoeuvres were repeated after volume expansion (dextran 8 ml/kg). Oxygenation, mean arterial, and pulmonary artery pressures, aortic, mesenteric, and renal blood flow were monitored. MEASUREMENTS AND RESULTS: Lower pressure recruitment manoeuvres (ViCM30 and PCRM30/15) did not significantly improve oxygenation. With ViCM and PCRM at peak airway pressure 40 cmH(2)O, PaO(2) increased to similar levels in both lavage and endotoxin groups. Aortic blood flow was reduced from baseline during PCRM40/20 and ViCM40 by 57+/-3% and 61+/-6% in the lavage group and by 57+/-8% and 82+/-7% (P<0.05 vs PCRM40/20) in endotoxin group. The decrease in blood pressure was less pronounced. Prior volume expansion attenuated circulatory impairment. After cessation of recruitment hemodynamic parameters were restored within 3 min. CONCLUSION: Effective recruitment resulted in systemic hypotension, pulmonary hypertension, and decrease in aortic blood flow especially in endotoxinemic animals. Circulatory depression may be attenuated using recruitment manoeuvres during ongoing pressure-controlled ventilation and by prior volume expansion.

Slow moderate pressure recruitment maneuver minimizes negative circulatory and lung mechanic side effects: evaluation of recruitment maneuvers using electric impedance tomography.

OBJECTIVE: To evaluate the efficacy of different lung recruitment maneuvers using electric impedance tomography. DESIGN AND SETTING: Experimental study in animal model of acute lung injury in an animal research laboratory. SUBJECTS: Fourteen pigs with saline lavage induced lung injury. INTERVENTIONS: Lung volume, regional ventilation distribution, gas exchange, and hemodynamics were monitored during three different recruitment procedures: (a) vital capacity maneuver to an inspiratory pressure of 40 cmH2O (ViCM), (b) pressure-controlled recruitment maneuver with peak pressure 40 and PEEP 20 cmH2O, both maneuvers repeated three times for 30 s (PCRM), and (c) a slow recruitment with PEEP elevation to 15 cmH2O with end inspiratory pauses for 7 s twice per minute over 15 min (SLRM). MEASUREMENTS AND RESULTS: Improvement in lung volume, compliance, and gas exchange were similar in all three procedures 15 min after recruitment. Ventilation in dorsal regions of the lungs increased by 60% as a result of increased regional compliance. During PCRM compliance decreased by 50% in the ventral region. Cardiac output decreased by 63+/-4% during ViCM, 44+/-2% during PCRM, and 21+/-3% during SLRM. CONCLUSIONS: In a lavage model of acute lung injury alveolar recruitment can be achieved with a slow lower pressure recruitment maneuver with less circulatory depression and negative lung mechanic side effects than with higher pressure recruitment maneuvers. With electric impedance tomography it was possible to monitor lung volume changes continuously.

Tracheal double-lumen ventilation attenuates hypercapnia and respiratory acidosis in lung injured pigs.

OBJECTIVE: Evaluation of ventilatory and circulatory effects with coaxial double-lumen tube ventilation for dead-space reduction as compared with standard endotracheal tube ventilation. DESIGN: Experimental study in a pig model of lung lavage induced acute lung injury. SETTING: University research laboratory. MEASUREMENTS AND RESULTS: Tidal volumes of 6, 8 and 10 ml/kg body weight with a set respiratory rate of 20 breaths per minute were used in a random order with both double-lumen ventilation and standard endotracheal tube ventilation. Measurements of ventilatory and circulatory parameters were obtained after steady state at each experimental stage. With a tidal volume of 6 ml/kg, PaCO(2) was reduced from 10.9 kPa (95% CI 9.0-12.9) with a standard endotracheal tube to 8.2 kPa (95% CI 7.0-9.4) with double-lumen ventilation. This corresponds to a reduction in carbon dioxide levels by 25%. At 6 ml/kg, pH increased from 7.17 (95% CI 7.09-7.24) with a standard endotracheal tube to 7.27 (95% CI 7.21-7.32) with double-lumen ventilation. Tracheal pressure was monitored continuously and no difference between single- or double-lumen ventilation was noted at corresponding levels of ventilation. There was no formation of auto-PEEP. Partial tube obstruction due to secretions was not observed during the experiments. CONCLUSIONS: Coaxial double-lumen tube ventilation is an effective adjunct to reduce technical dead space. It attenuates hypercapnia and respiratory acidosis in a lung injury pig model.

Effectiveness and side effects of closed and open suctioning: an experimental evaluation.

OBJECTIVE: To compare the effectiveness of closed system suctioning (CSS) and open system suctioning (OSS) and the side effects on gas exchange and haemodynamics, during pressure-controlled ventilation (PCV) or continuous positive airway pressure (CPAP). DESIGN: Bench test and porcine lung injury model. PARTICIPANTS: Twelve bronchoalveolar saline-lavaged pigs. SETTING: Research laboratory in a university hospital. INTERVENTIONS: In a mechanical lung, the efficacy of OSS and CSS with 12 and 14 Fr catheters were compared during volume-control ventilation, PCV, CPAP 0 or 10 cmH(2)O by weighing the suction system before and after aspirating gel in a transparent trachea. Side effects were evaluated in the animals with the same ventilator settings during suctioning of 5, 10 or 20 s duration. MEASUREMENTS AND RESULTS: Suctioning with 12 and 14 Fr catheters was significantly more efficient with OSS (1.9+/-0.1, 2.8+/-0.9 g) and with CSS during CPAP 0 cmH(2)O (1.8+/-0.2, 4.2+/-0.5 g) as compared to CSS during PCV (0.2+/-0.2, 0.8+/-0.3 g) or CPAP 10 cmH(2)O (0.0+/-0.1, 0.7+/-0.4 g), p<0.01 (means +/- SD). OSS and CSS at CPAP 0 cmH(2)O resulted in a marked decrease in SpO(2), mixed venous oxygen saturation and tracheal pressure, p<0.001, but the side effects were considerably fewer during CSS with PCV and CPAP 10 cmH(2)O, p<0.05. CONCLUSIONS: Irrespective of catheter size, OSS and CSS during CPAP 0 cmH(2)O were markedly more effective than CSS during PCV and CPAP 10 cmH(2)O but had worse side effects. However, the side effects lasted less than 5 min in this animal model. Suctioning should be performed effectively when absolutely indicated and the side effects handled adequately.