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.

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.

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.

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.

The assessment of transpulmonary pressure in mechanically ventilated ARDS patients.

PURPOSE: The optimal method for estimating transpulmonary pressure (i.e. the fraction of the airway pressure transmitted to the lung) has not yet been established. METHODS: In this study on 44 patients with acute respiratory distress syndrome (ARDS), we computed the end-inspiratory transpulmonary pressure as the change in airway and esophageal pressure from end-inspiration to atmospheric pressure (i.e. release derived) and as the product of the end-inspiratory airway pressure and the ratio of lung to respiratory system elastance (i.e. elastance derived). The end-expiratory transpulmonary pressure was estimated as the product of positive end-expiratory pressure (PEEP) minus the direct measurement of esophageal pressure and by the release method. RESULTS: The mean elastance- and release-derived transpulmonary pressure were 14.4 ± 3.7 and 14.4 ± 3.8 cmH2O at 5 cmH2O of PEEP and 21.8 ± 5.1 and 21.8 ± 4.9 cmH2O at 15 cmH2O of PEEP, respectively (P = 0.32, P = 0.98, respectively), indicating that these parameters were significantly related (r(2) = 0.98, P < 0.001 at 5 cmH2O of PEEP; r(2) = 0.93, P < 0.001 at 15 cmH2O of PEEP). The percentage error was 5.6 and 12.0 %, respectively. The mean directly measured and release-derived transpulmonary pressure were -8.0 ± 3.8 and 3.9 ± 0.9 cmH2O at 5 cmH2O of PEEP and -1.2 ± 3.2 and 10.6 ± 2.2 cmH2O at 15 cmH2O of PEEP, respectively, indicating that these parameters were not related (r(2) = 0.07, P = 0.08 at 5 cmH2O of PEEP; r (2) = 0.10, P = 0.53 at 15 cmH2O of PEEP). CONCLUSIONS: Based on our observations, elastance-derived transpulmonary pressure can be considered to be an adequate surrogate of the release-derived transpulmonary pressure, while the release-derived and directly measured end-expiratory transpulmonary pressure are not related.

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.

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.

Whither lung EIT: where are we, where do we want to go and what do we need to get there?

Breathing moves volumes of electrically insulating air into and out of the lungs, producing conductivity changes which can be seen by electrical impedance tomography (EIT). It has thus been apparent, since the early days of EIT research, that imaging of ventilation could become a key clinical application of EIT. In this paper, we review the current state and future prospects for lung EIT, by a synthesis of the presentations of the authors at the 'special lung sessions' of the annual biomedical EIT conferences in 2009-2011. We argue that lung EIT research has arrived at an important transition. It is now clear that valid and reproducible physiological information is available from EIT lung images. We must now ask the question: How can these data be used to help improve patient outcomes? To answer this question, we develop a classification of possible clinical scenarios in which EIT could play an important role, and we identify clinical and experimental research programmes and engineering developments required to turn EIT into a clinically useful tool for lung monitoring.

Electrical Impedance Tomography for hemodynamic monitoring.

Electrical Impedance Tomography (EIT) is a known technique to monitor impedance changes in a cross-section of a body segment, which recently gained increasing interest for regional ventilation monitoring. In this paper, we focus on hemodynamic monitoring using EIT. Past and ongoing research activities to obtain cardiac related signals and regional perfusion information from EIT image streams are summarized. Finally, we present some preliminary results on stroke volume estimation using EIT.

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.

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.

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.