Henning Luepschen

Protective ventilation using electrical impedance tomography

Dynamic thoracic EIT is capable of detecting changes of the ventilation distribution in the lung. Nevertheless, it has yet to become an established clinical tool. Therefore, it is necessary to consider application scenarios wherein fast and distinct changes of the tissue conductivities are to be found and also have a clear diagnostic significance. One such a scenario is the artificial ventilation of patients suffering from the acute respiratory distress syndrome (ARDS). New protective ventilation strategies involving recruitment manoeuvres are associated with noticeable shifts of body fluids and regional ventilation, which can quite easily be detected by EIT. The bedside assessment of these recruitment manoeuvres will help the attending physician to optimize treatment. Hence, we performed an animal study of lavage-induced lung failure and investigated if EIT is capable of qualitatively as well as quantitatively monitoring lung recruitment during a stepwise PEEP trial. Additionally, we integrated EIT into a fuzzy controller-based ventilation system which allows one to perform automated recruitment manoeuvres (open lung concept) based on online PaO2 measurements. We found that EIT is a useful tool to titrate the proper PEEP level after fully recruiting the lung. Furthermore, EIT seems to be able to determine the status of recruitment when combining it with other physiological parameters. These results suggest that EIT may play an important role in the individualization of protective ventilation strategies.

Dynamic separation of pulmonary and cardiac changes in electrical impedance tomography

In spontaneously breathing or ventilated subjects, it is difficult to image cardiac-related conductivity changes using electrical impedance tomography (EIT) due to the high amplitude of the ventilation component. Previous attempts to separate these components included either electrocardiogram-gated averaging, frequency domain filtering or holding the breath while performing the measurements. However, such methods are either not able to produce continuous real-time images or to fully separate cardiac and pulmonary changes. The aim of this work was to develop a new dynamic filtering method for the online separation of pulmonary and cardiac changes avoiding the drawbacks of the previous attempts. The approach is based on estimating template functions for the pulmonary and cardiac components by means of principal component analysis and frequency domain filtering. Then, these templates are fitted into the input signals. The new method enables an observer to examine the variation of the cardiac signal beat-by-beat after a one-time setup period of 20 s. Preliminary in vivo results of two healthy subjects are presented. The results are superior to frequency domain filtering and in good agreement with signals averaged over several cardiac cycles. The method does not depend on ECG or other a priori knowledge. The apparent validity of the methods ability to separate cardiac and pulmonary changes in EIT images was shown and has to be confirmed in future studies. The algorithm opens up new possibilities for future clinical trials on continuous monitoring by means of EIT and for the examination of the relation between the cardiac component and lung perfusion.

Tidal recruitment assessed by electrical impedance tomography and computed tomography in a porcine model of lung injury

Objectives:To determine the validity of electrical impedance tomography to detect and quantify the amount of tidal recruitment caused by different positive end-expiratory pressure levels in a porcine acute lung injury model. Design:Randomized, controlled, prospective experimental study. Setting:Academic research laboratory. Subjects:Twelve anesthetized and mechanically ventilated pigs. Interventions:Acute lung injury was induced by central venous oleic acid injection and abdominal hypertension in seven animals. Five healthy pigs served as control group. Animals were ventilated with positive end-expiratory pressure of 0, 5, 10, 15, 20, and 25 cm H2O, respectively, in a randomized order. Measurements and Main Results:At any positive end-expiratory pressure level, electrical impedance tomography was obtained during a slow inflation of 12 mL/kg of body weight. Regional-ventilation-delay indices quantifying the time until a lung region reaches a certain amount of impedance change were calculated for lung quadrants and for every single electrical impedance tomography pixel, respectively. Pixel-wise calculated regional-ventilation-delay indices were plotted in a color-coded regional-ventilation-delay map. Regional-ventilation-delay inhomogeneity that quantifies heterogeneity of ventilation time courses was evaluated by calculating the scatter of all pixel-wise calculated regional-ventilation-delay indices. End-expiratory and end-inspiratory computed tomography scans were performed at each positive end-expiratory pressure level to quantify tidal recruitment of the lung. Tidal recruitment showed a moderate inter-individual (r = .54; p < .05) and intra-individual linear correlation (r = .46 up to r = .73 and p < .05, respectively) with regional-ventilation-delay obtained from lung quadrants. Regional-ventilation-delay inhomogeneity was excellently correlated with tidal recruitment intra- (r = .90 up to r = .99 and p < .05, respectively) and inter-individually (r = .90; p < .001). Conclusions:Regional-ventilation-delay can be noninvasively measured by electrical impedance tomography during a slow inflation of 12 mL/kg of body weight and visualized using ventilation delay maps. Our experimental data suggest that the impedance tomography-based analysis of regional-ventilation-delay inhomogeneity provides a good estimate of the amount of tidal recruitment and may be useful to individualize ventilatory settings.

Impedance tomography as a new monitoring technique.

Purpose of reviewElectrical impedance tomography (EIT) noninvasively creates images of the local ventilation and arguably lung perfusion distribution at bedside. Methodological and clinical aspects of EIT when used as a monitoring tool in the intensive care unit are reviewed and discussed. Recent findingsWhereas former investigations addressed the issue of validating EIT to measure regional ventilation, current studies focus on clinical applications such as detection of pneumothorax. Furthermore, EIT has been used to quantify lung collapse and tidal recruitment in order to titrate positive end-expiratory pressure. Indicator-free EIT measurements might be sufficient for the continuous measurement of cardiac stroke volume, but assessment of regional lung perfusion presumably requires the use of a contrast agent such as hypertonic saline. SummaryGrowing evidence suggests that EIT may play an important role in individually optimizing ventilator settings in critically ill patients.

Experimental case report : development of a pneumothorax monitored by electrical impedance tomography

Electrical impedance tomography (EIT) is a non‐invasive, radiation‐free functional imaging technique, which allows continuous bedside measurement of regional lung ventilation. Pneumothorax is an uncommon but nevertheless potentially dangerous incident that may arise unexpectedly. We report an incident of an accidental tension pneumothorax during an experimental ventilation study in a pig that was continuously monitored by EIT. The early sign of the occurring pneumothorax, prior to all clinical signs, was a fast increase of end‐expiratory impedance in the ventral part of the right lung indicating that non‐ventilated air entered this part, followed by a disappearance of ventilation in this region. At the same time the ventilation‐related impedance changes of the left lung remained almost unchanged. The pneumothorax onset was localized using a newly introduced pneumothorax dynamics map directly derived from dynamic EIT data. We conclude that non‐invasive EIT may be helpful as a tool to detect the development of a pneumothorax, which could be of particular interest during invasive procedures such as insertion of a central venous catheter.

Robust Closed-Loop Control of the Inspired Fraction of Oxygen for the Online Assessment of Recruitment Maneuvers

Recruitment maneuvers are commonly used in patients suffering from acute respiratory failure. A continuous measurement of PaO2 would help to assess the proper execution of such maneuvers. Unfortunately, there are only static offline measurement devices available. However, if the oxygen saturation would be held close to a fixed set-point of 90-92% by automatically adjusting the FIO2 parameter of the mechanical ventilator, the latter would be inversely related to PaO2. In this work, a new robust PID control system is presented, which accounts for model uncertainty in a modified Smith predictor approach. The controller was tested using a computer model of respiratory failure. The model parameters were identified in an animal experiment with pigs.

The effect of pumpless extracorporeal CO2 removal on regional perfusion of the brain in experimental acute lung injury.

Background: Lung-protective mechanical ventilation with low tidal volumes (VT) is often associated with hypercapnia (HC), which may be unacceptable in patients with brain injury. CO2 removal using a percutaneous extracorporeal lung assist (pECLA) enables normocapnia despite low VT, but its effects on regional cerebral blood flow (rCBF) remain ambiguous. We hypothesized that reversal of HC by pECLA impairs rCBF in a porcine lung injury model. Methods: Lung injury was induced in 9 anesthetized pigs by hydrochloric acid aspiration. rCBF and systemic hemodynamics were measured by colored microsphere technique and transpulmonary-thermodilution during a randomized sequence of 4 experimental situations: pECLA shunt-on (1) with HC and (2) without HC, pECLA shunt-off (3) with HC and (4) without HC. Results: HC increased rCBF (P<0.05). CO2 removal with pECLA resulting in normocapnia, decreased rCBF to levels comparable to those without pECLA and normocapnia. HC resulted in increased cardiac output (+25.5%). Cardiac output was highest during HC with pECLA shunt (+44.9%). During pECLA with CO2 removal, cardiac output (+38.1%) decreased compared with pECLA without CO2 removal, but stayed higher than during normocapnia/no pECLA shunt (P<0.05). Conclusions: In this animal model, mechanical ventilation with low VT was associated with HC and increased rCBF. CO2 removal by pECLA restored normocapnia, reduced rCBF to levels of normocapnia, but required a higher systemic blood flow for the perfusion of the pECLA device. If these results could be transferred to patients, extracorporeal CO2 removal might be an option for treatment of combined lung and brain injury in condition of a sufficient cardiac flow reserve.

Regional Pulmonary Time-Constant Maps based on EIT-Measurements

This article introduces a promising approach of assessing a patient’s lung status non-invasively by analyzing regional time constants from electrical impedance tomography (EIT) sequences. The main idea of this approach depends on the fact that the lung can be modeled using gas-flow resistivity Rrs and lung compliance Crs, which describes the elasticity of the lung tissue. Thus, global lung mechanics may be approximated by a first order system in which the global time constant τrs = Rrs x Crs is a very important parameter. Based on EIT images, a noninvasive method that provides images of impedance distribution in the thorax allowing to quantify regional ventilation, we extend the concept of a first order mechanical lung model to regional time constant maps. We expect monitoring of regional time constants to be an important tool to assess lung status and to evaluate protective ventilation schemes.

Enhancement of Protective Ventilation Strategies Using Electrical Impedance Tomography

The individualization of protective ventilation strategies for patients suffering from acute respiratory failure requires permanent bedside monitoring of pulmonary parameters and an immediate response to changing lung conditions. This can be achieved using the electrical impedance tomography (EIT), a fast, non-invasive technique to observe global and regional lung properties corresponding to changes in the conductivity distribution of the thorax. In this work, EIT has been evaluated in terms of its potential to assess different conditions of the lung during lung recruitment maneuvers (RM) which are frequently used for the treatment of respiratory failure. Therefore, an EIT system was integrated into a fuzzy-controller based medical expert system which is capable of automatically performing RM while recording physiological parameters. The setup additionally triggers a CT scanner to obtain reference images. Six anesthetized, artificially ventilated pigs received repetitive lung lavages with NaCl-solution to induce respiratory failure. In a first trial, PEEP was stepwise increased and subsequently decreased to examine the influence of PEEP on the patient. In a second trial, the expert system performed an automatic RM. Differential and functional EIT images were continuously recorded and compared to CT images made at the end of inspiration and expiration. Global impedance and mean lung density as well as tidal volume and tidal variation demonstrated a high linear correlation. However, the proportionality factor between the respective quantities changed with the state of lung recruitment. This was probably due to the grossly changing geometry of the conductivity distribution. During the automated RM, a clear shift of ventilation from dorsal to ventral regions of the lung could be observed in both, EIT and CT images. The experiments showed that EIT is capable of identifying dynamic changes in the lung such as global and regional recruitment and may provide important control variables for closed-loop protective ventilation controllers.

Automation of Protective Ventilation in Acute Lung Injury

Mechanical ventilation with positive pressure is the supportive therapy for patients with acute lung failure. To minimize pulmonary stress by prevention of end-expiratory alveolar collapse and over-distension of pulmonary areas, lung protective ventilation strategy has become standard therapy. Low tidal volume ventilation (VT ≤ 6ml, per kg predicated body weight) proved to reduce mortality rates in patients with lung failure notably. Recent surveys on intensive care units showed that the transfer of this evidence-based knowledge to ventilation therapy has not been realized in the current care of ventilated patients. Automated execution of protective ventilation protocols would help to optimize the individual setting in mechanical ventilated patients. To test the ability to automate protective ventilation protocols, the adjustment of positive pressure ventilation was realized in a saline lavage induced lung injury study in pigs. The implemented controllers were programmed to meet the therapeutic goals (tidal volume, oxygenation, plateau pressure, pH, inspiratory to expiratory ratio) of the ARDSnet-protocol. During the trial, all measurements were made using an online blood gas monitor (TrendCare Satellite, Diametrics Medical Inc., England), a monitor for hemodynamic parameters (Sirecust 1281, Siemens, Germany), a capnograph (CO2SMO+, Respironics, Inc., USA), and an electrical impedance tomography (EIT) prototype system (EIT evaluation Kit, Draeger Medical, Germany). After successful automated therapy, PaCO2 and FiO2 levels could be significantly reduced. Thus, the execution of automated protective ventilation protocols with an electronically controlled ventilator was possible and led to pulmonary stabilization in saline lavage induced acute lung injury.