Thoracic electrical impedance tomographic measurements during volume controlled ventilation-effects of tidal volume and positive end-expiratory pressure

Regional ventilation distribution determined by electrical impedance tomography: reproducibility and effects of posture and chest plane

BACKGROUND AND OBJECTIVE

Reliable assessment of regional lung ventilation and good reproducibility of electrical impedance tomography (EIT) data are the prerequisites for the future application of EIT in a clinical setting. The aims of our study were to determine (i) the reproducibility of repeated EIT measurements and (ii) the effect of the studied transverse chest plane on ventilation distribution in different postures.

METHODS

Ten healthy adult subjects were studied in three postures on two separate days. EIT and spirometric data were obtained during tidal breathing and slow vital capacity (VC) manoeuvres. EIT data were acquired in two chest planes at 13 scans/s. Reproducibility of EIT findings was assessed by Bland-Altman analysis and Pearson correlation in 16 regions of interest in each plane. Regional ventilation distribution during tidal breathing and deep expiration was determined as fractional ventilation in four quadrants of the studied chest cross-sections.

RESULTS

Our study showed a good reproducibility of EIT measurements repeated after an average time interval of 8 days. Global tidal volumes and VCs determined by spirometry on separate days were not significantly different. Regional ventilation in chest quadrants assessed by EIT was also unaffected. Posture exerted a significant effect on ventilation distribution among the chest quadrants during spontaneous breathing and deep expiration in both planes. The spatial distribution patterns in the two planes were not identical.

CONCLUSIONS

We conclude that regional EIT ventilation findings are reproducible and recommend that the EIT examination location on the chest is carefully chosen especially during repeated measurements and follow-up.

Short term effect of adaptive servo-ventilation on muscle sympathetic nerve activity in patients with heart failure

Chronic heart failure (HF) is characterized by sympathetic overactivation and periodic breathing. We examined whether adaptive servo-ventilation (ASV) exerts a sympathoinhibitory effect in patients with HF via normalizing respiratory pattern. Muscle sympathetic nerve activity (MSNA), heart rate, blood pressure, respiratory pattern and oxygen saturation were examined in 29 HF patients without obstructive sleep apnea (age, 61±15years; ejection fraction, 0.32±0.09; obstructive apnea index, <5/h) before (10 min), during (30 min) and after (10 min) the application of ASV. Periodic breathing was defined as a repeated oscillation of tidal volume with regularly recurring hyperpnea and hypopnea with a variation in tidal volume of greater than 25%. The severity of respiratory instability was determined using the coefficient of variation of tidal volume (CV-TV). Of 29 patients with HF, 11 had periodic breathing and 18 did not. There was a modest positive correlation between MSNA and CV-TV (n=29, p<0.05). ASV reduced respiratory rate, CV-TV and MSNA only in the group with periodic breathing (p<0.01). Change in MSNA significantly correlated with changes in respiratory rate, CV-TV and presence of periodic breathing. However, multivariate analyses revealed that respiratory rate and CV-TV were independent predictors of change in MSNA. ASV reduces MSNA by slowing respiratory rates and stabilizing respiratory patterns in patients with HF.

A fisher information matrix interpretation of the NOSER algorithm in electrical impedance tomography

In this paper, we employ the concept of the Fisher information matrix (FIM) to reformulate and improve on the "Newton's One-Step Error Reconstructor" (NOSER) algorithm. FIM is a systematic approach for incorporating statistical properties of noise, modeling errors and multi-frequency data. The method is discussed in a maximum likelihood estimator (MLE) setting. The ill-posedness of the inverse problem is mitigated by means of a nonlinear regularization strategy. It is shown that the overall approach reduces to the maximum a posteriori estimator (MAP) with the prior (conductivity vector) described by a multivariate normal distribution. The covariance matrix of the prior is a diagonal matrix and is computed directly from the Fisher information matrix. An eigenvalue analysis is presented, revealing the advantages of using this prior to a Gaussian smoothness prior (Laplace). Reconstructions are shown using measured data obtained from a shallow breathing of an adult human subject. The reconstructions show that the FIM approach clearly improves on the original NOSER algorithm.

Effects of restricted thoracic movement on the regional distribution of ventilation

BACKGROUND

Restricted thoracic movement is often encountered in patients, necessitating mechanical ventilation during surgery or intensive care treatment. High intraabdominal pressure, obesity or thorax rigidity and deformity reduce the chest distensibility and deteriorate the lung function. They render the selection of proper ventilator settings difficult and complicate the weaning process. Electrical impedance tomography (EIT) is currently being proposed as a bedside imaging method for monitoring regional lung ventilation. The objective of our study was to establish whether the effects of decreased chest compliance on regional lung ventilation can be determined by EIT.

METHODS

Ten healthy male volunteers were studied in our pilot study under three conditions: (1) unrestricted breathing and (2) restricted breathing by abdominal and (3) lower rib cage strapping. The subjects were followed during spontaneous tidal breathing in five postures (sitting, supine, prone, left and right side). EIT and spirometry data were acquired in each condition.

RESULTS

The distribution of ventilation in subjects with unrestricted breathing corresponded with the physiologically expected values. In the left and right lateral postures, abdominal and thoracic cage restrictions reduced the ventilation in the dependent lung areas; the non-dependent areas were unaffected. In the prone position, the ventilation of the dependent and non-dependent areas was reduced. The effects of strapping were least pronounced in the supine posture.

CONCLUSIONS

We conclude that EIT is able to measure changes in the regional distribution of ventilation induced by restricted chest movement and has the potential for optimising artificial ventilation in patients with limited chest compliance of different origins.

Monitoring retroperitoneal bleeding model of piglets by electrical impedance tomography

To investigate continuous monitoring capacity of electrical impedance tomography (EIT) for retroperitoneal bleeding, studies were carried out on six anesthetized piglet's bleeding model produced by injecting anticoagulated blood into renal region. For each subject, total blood of about 200 ml was injected within time periods ranging from tens of minutes to several hours. The simulated bleeding processes were detected and monitored by EIT system with sixteen electrodes at a rate of one image per second. EIT images were reconstructed by dynamic back-projection algorithm. The results showed that impedance changes caused by bleeding could be revealed by EIT images.

Performance of electrical impedance tomography in detecting regional tidal volumes during one-lung ventilation

BACKGROUND

Electrical impedance tomography (EIT) is becoming a new medical imaging modality for continuous monitoring of regional lung function in the intensive care unit or operating room. The aim of our study was to evaluate the performance of EIT in detecting regional tidal volumes in patients during volume-controlled mechanical ventilation of one or both lungs.

METHODS

Ten adult patients undergoing elective thoracic surgery were included. EIT measurements were performed with the Goe-MF II EIT system. Data were collected before surgery during ventilation of both, the right and left lungs. Tidal volumes of 800 and 400 ml were applied during bilateral and unilateral ventilation, respectively.

RESULTS

Ventilation-related impedance changes determined in the whole chest cross-section during the right and left lung ventilation did not significantly differ from each other and were equal to 47.6+/-5.6% and 48.5+/-7.8% (mean+/-SD) of the value determined during bilateral ventilation. During unilateral ventilation, EIT clearly separated the ventilated and non-ventilated lung regions; nevertheless, ventilation-related impedance changes were also detected at the non-ventilated sides in areas corresponding to 3.4+/-4.1% and 12.4+/-6.9% of the scan halves during ventilation of the left and right lung, respectively. Changes in global tidal volumes were adequately detected by EIT during both bilateral and unilateral lung ventilation.

CONCLUSION

Although good separation of the ventilated and non-ventilated sides of the chest was possible, the data indicate that reliable quantification of regional tidal volumes during asymmetric or inhomogeneous distribution patterns requires regions-of-interest analysis.

Bronchoscopic suctioning may cause lung collapse: a lung model and clinical evaluation

OBJECTIVE

To assess lung volume changes during and after bronchoscopic suctioning during volume or pressure-controlled ventilation (VCV or PCV).

DESIGN

Bench test and patient study.

PARTICIPANTS

Ventilator-treated acute lung injury (ALI) patients.

SETTING

University research laboratory and general adult intensive care unit of a university hospital.

INTERVENTIONS

Bronchoscopic suctioning with a 12 or 16 Fr bronchoscope during VCV or PCV.

MEASUREMENTS AND RESULTS

Suction flow at vacuum levels of -20 to -80 kPa was measured with a Timeter(trade mark) instrument. In a water-filled lung model, airway pressure, functional residual capacity (FRC) and tidal volume were measured during bronchoscopic suctioning. In 13 ICU patients, a 16 Fr bronchoscope was inserted into the left or the right main bronchus during VCV or PCV and suctioning was performed. Ventilation was monitored with electric impedance tomography (EIT) and FRC with a modified N(2) washout/in technique. Airway pressure was measured via a pressure line in the endotracheal tube. Suction flow through the 16 Fr bronchoscope was 5 l/min at a vacuum level of -20 kPa and 17 l/min at -80 kPa. Derecruitment was pronounced during suctioning and FRC decreased with -479+/-472 ml, P<0.001.

CONCLUSIONS

Suction flow through the bronchoscope at the vacuum levels commonly used is well above minute ventilation in most ALI patients. The ventilator was unable to deliver enough volume in either VCV or PCV to maintain FRC and tracheal pressure decreased below atmospheric pressure.

Regional lung volume changes in children with acute respiratory distress syndrome during a derecruitment maneuver

OBJECTIVE

Regional differences in lung volume have been described in adults with acute respiratory distress syndrome, but it remains unclear to what extent they occur in children. To quantify regional alveolar collapse that occurred during mechanical ventilation during a standardized suctioning maneuver, we evaluated regional and global relative impedance changes (relative DeltaZ) in children with acute respiratory distress syndrome using electrical impedance tomography.

DESIGN

Prospective observational trial.

SETTING

A 30-bed pediatric intensive care unit.

PATIENTS

Six children with acute respiratory distress syndrome.

INTERVENTIONS

Standardized suctioning maneuver.

MEASUREMENTS AND MAIN RESULTS

By comparing layers from nondependent (layers 1 and 2) to dependent lung areas (layers 3 and 4), it was demonstrated that the middle layers (2 and 3) had the greatest ventilation-induced change in relative DeltaZ; layer 4 showed the least ventilation-induced change in relative DeltaZ. During suctioning, layers 1, 2, and 3 showed a negative change in relative DeltaZ, whereas layer 4 showed no significant change in relative DeltaZ. The derecruitment-induced change in relative DeltaZ representing the lung-volume loss was -9.8 (-3.0 mL/kg) during the first suctioning maneuver, -16.1 (-5.4 mL/kg) during the second, and -21.7 (-7.4 mL/kg) during the third. The ventilation-induced change in relative DeltaZ during mechanical ventilation remained unchanged after suctioning (mean change in relative DeltaZ before vs. after suctioning, 40.1 +/- 9.1 vs. 41.4 +/- 10.8; p = .30). Dynamic compliance was 11.8 +/- 6.1 mL.cm H2O before and 11.8 +/- 6.9 mL.cm H2O after the suctioning sequence (p = .90).

CONCLUSIONS

Considerable regional heterogeneity was present during ventilation and a derecruitment maneuver. Significantly lower change in relative DeltaZ in the most dependent lung regions suggests alveolar collapse during ventilation before suctioning.

Reproducibility of regional lung ventilation distribution determined by electrical impedance tomography during mechanical ventilation

Electrical impedance tomography (EIT) has the potential to become a new tool for bedside monitoring of regional lung ventilation. The aim of our study was to assess the reproducibility of regional lung ventilation distribution determined by EIT during mechanical ventilation under identical ventilator settings. The experiments were performed on 10 anaesthetized supine pigs ventilated in a volume-controlled mode. EIT measurements were performed with the Goe-MF II device (Viasys Healthcare, Höchberg, Germany) during repeated changes in positive end-expiratory pressure (PEEP) from 0 to 10 cm H2O. Regional lung ventilation was determined in the right and left hemithorax as well as in 64 regions of interest evenly distributed over each chest side in the ventrodorsal direction. Ventilation distributions in both lungs were visualized as ventrodorsal ventilation profiles and shifts in ventilation distribution quantified in terms of centres of ventilation in relation to the chest diameter. The proportion of the right lung on total ventilation in the chest cross-section was 0.54+/-0.04 and remained unaffected by repetitive PEEP changes. Initial PEEP increase resulted in a redistribution of ventilation towards dorsal lung regions with a shift of the centre of ventilation from 45+/-3% to 49+/-3% of the chest diameter in the right and from 47+/-2% to 50+/-2% in the left hemithorax. Excellent reproducibility of the results in the individual regions of interest with almost identical patterns of ventilation distribution was found during repeated PEEP changes.

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.

Available ventilation monitoring methods during pre-hospital cardiopulmonary resuscitation

High quality cardiopulmonary resuscitation (CPR) in the pre-hospital setting has been associated with improved survival rates during cardiopulmonary arrest (CPA). Recent documentation of hyperventilation associated deterioration in hemodynamics during CPR, suggests that guided or controlled ventilation strategies may contribute to improved hemodynamics and increased survival. This article briefly reviews the mechanical methods, advantages, and disadvantages of the available ventilation monitoring methods currently available for clinical use, with an emphasis on pre-hospital implementation. We recommend that more objective measurement of ventilation during CPR be performed, with emphasis on a strategy for measuring both attempted ventilation frequency (f) and delivered tidal volume (VT). The use of improved thoracic impedance pneumography and capnography are appealing for such monitoring because of the widespread availability, but modifications to existing software and clinical data compared to a clinical standard would be required before general acceptance is possible. Other methods listed may offer advantages over these in select circumstances.

Positive end-expiratory pressure optimization using electric impedance tomography in morbidly obese patients during laparoscopic gastric bypass surgery

BACKGROUND

Morbidly obese patients have an increased risk for peri-operative lung complications and develop a decrease in functional residual capacity (FRC). Electric impedance tomography (EIT) can be used for continuous, fast-response measurement of lung volume changes. This method was used to optimize positive end-expiratory pressure (PEEP) to maintain FRC.

METHODS

Fifteen patients with a body mass index of 49 +/- 8 kg/m(2) were studied during anaesthesia for laparoscopic gastric bypass surgery. Before induction, 16 electrodes were placed around the thorax to monitor ventilation-induced impedance changes. Calibration of the electric impedance tomograph against lung volume changes was made by increasing the tidal volume in steps of 200 ml. PEEP was titrated stepwise to maintain a horizontal baseline of the EIT curve, corresponding to a stable FRC. Absolute FRC was measured with a nitrogen wash-out/wash-in technique. Cardiac output was measured with an oesophageal Doppler method. Volume expanders, 1 +/- 0.5 l, were given to prevent PEEP-induced haemodynamic impairment.

RESULTS

Impedance changes closely followed tidal volume changes (R(2) > 0.95). The optimal PEEP level was 15 +/- 1 cmH(2)O, and FRC at this PEEP level was 1706 +/- 447 ml before and 2210 +/- 540 ml after surgery (P < 0.01). The cardiac index increased significantly from 2.6 +/- 0.5 before to 3.1 +/- 0.8 l/min/m(2) after surgery, and the alveolar dead space decreased. P(a)O2/F(i)O2, shunt and compliance remained unchanged.

CONCLUSION

EIT enables rapid assessment of lung volume changes in morbidly obese patients, and optimization of PEEP. High PEEP levels need to be used to maintain a normal FRC and to minimize shunt. Volume loading prevents circulatory depression in spite of a high PEEP level.

Comparison of different methods to define regions of interest for evaluation of regional lung ventilation by EIT

The measurement of regional lung ventilation by electrical impedance tomography (EIT) has been evaluated in many experimental studies. However, EIT is not routinely used in a clinical setting, which is attributable to the fact that a convenient concept for how to quantify the EIT data is missing. The definition of region of interest (ROI) is an essential point in the data analysis. To date, there are only limited data available on the different approaches to ROI definition to evaluate regional lung ventilation by EIT. For this survey we examined ten patients (mean age +/- SD: 60 +/- 10 years) under controlled ventilation. Regional tidal volumes were quantified as pixel values of inspiratory-to-expiratory impedance differences and four types of ROIs were subsequently applied. The definition of ROI contours was based on the calculation of the pixel values of (1) standard deviation from each pixel set of impedance data and (2) the regression coefficient from linear regression equations between the individual local (pixel) and average (whole scan) impedance signals. Additionally, arbitrary ROIs (four quadrants and four anteroposterior segments of equal height) were used. Our results indicate that both approaches to ROI definition using statistical parameters are suitable when impedance signals with high sensitivity to ventilation-related phenomena are to be analyzed. The definition of the ROI contour as 20-35% of the maximum standard deviation or regression coefficient is recommended. Simple segmental ROIs are less convenient because of the low ventilation-related signal component in the dorsal region.

Electrical impedance tomography: changes in distribution of pulmonary ventilation during laparoscopic surgery in a porcine model

BACKGROUND

Because of the creation of a pneumoperitoneum, impairment of ventilation is a common side-effect during laparoscopic surgery. Electrical impedance tomography (EIT) is a method with the potential for becoming a tool to quantify these alterations during surgery. We have studied the change of regional ventilation during and after laparoscopic surgery with EIT and compared the diagnostic findings with computed tomography (CT) scans in a porcine study.

MATERIALS AND METHODS

After approval by the local animal ethics committee, six pigs were included in the study. Two laparoscopic operations were performed [colon resection (n=3) and fundoplicatio (n=3)]. The EIT measurements (6th parasternal intercostal space) were continuously recorded by an EIT prototype (EIT Evaluation Kit, Dräger Medical, Lübeck, Germany). To verify ventilatory alterations detected by EIT, a CT scan was performed postoperatively.

RESULTS

Ventilation with defined tidal volumes was significantly correlated to EIT measurements (r2=0.99). After creation of the pneumoperitoneum, lung compliance typically decreased, which agreed well with an alteration of the distribution of pulmonary ventilation measured by EIT. Elevation of positive end-inspiratory pressure reopened non-aerated lung areas and showed a recovery of the regional ventilation measured by EIT. Additionally, we could detect pulmonary complications by EIT monitoring as verified by CT scans postoperatively.

CONCLUSION

EIT monitoring can be used as a continuous non-invasive intraoperative monitor of ventilation to detect regional changes of ventilation and pulmonary complications during laparoscopic surgery. These EIT findings indicate that surgeons and anesthetists may eventually be able to optimize ventilation directly in the operating theatre.

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.

A parametric model of the relationship between EIT and total lung volume

Spirometry and electrical impedance tomography (EIT) data from 26 healthy subjects (14 males, 12 females) were used to develop a model linking contrast variations in EIT difference images to lung volume changes. Eight recordings, each 64 s long, were made for each subject in four postures (standing, sitting, reclining at 45 degrees, supine) and two breathing modes (quiet tidal and deep breathing). Age, gender and five anthropometric variables were recorded. The database was divided into four subsets. The first subset, data from 22 subjects (12 males, 10 females) recorded in deep breathing mode, was used to create the model. Validation was done with the other subsets: data recorded during quiet tidal breathing in the same 22 subjects, and data recorded in both breathing modes for the other four subjects. A quadratic equation in DeltaV(P) (lung volume changes recorded by the spirometer) provided a very good fit to total contrast changes in the EIT images. The model coefficients were found to depend on posture, gender, thoracic circumference and scapular skin fold. To validate the model, the quadratic equation was inverted to estimate lung volume changes from the EIT images. The estimated changes were then compared to the measured volume changes. Validations with each data subset yielded mean standard errors ranging from 9.3% to 12.4%. The proposed model is a first step in enabling inter individual comparisons of EIT images since: (1) it provides a framework for incorporating the effects of anthropometric variables, gender and posture, and (2) it references the images to a physical quantity (volume) verifiable by spirometry.

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.

Regional ventilation by electrical impedance tomography: a comparison with ventilation scintigraphy in pigs

STUDY OBJECTIVE

The validation of electrical impedance tomography (EIT) for measuring regional ventilation distribution by comparing it with single photon emission CT (SPECT) scanning.

DESIGN

Randomized, prospective animal study.

SETTINGS

Animal laboratories and nuclear medicine laboratories at a university hospital.

PARTICIPANTS

Twelve anesthetized and mechanically ventilated pigs.

INTERVENTIONS

Lung injury was induced by central venous injection of oleic acid. Then pigs were randomized to pressure-controlled mechanical ventilation, airway pressure-release ventilation, or spontaneous breathing.

MEASUREMENTS AND RESULTS

Ventilation distribution was assessed by EIT using cross-sectional electrotomographic measurements of the thorax, and simultaneously by single SPECT scanning with the inhalation of (99m)Tc-labeled carbon particles. For both methods, the evaluation of ventilation distribution was performed in the same transverse slice that was approximately 4 cm in thickness. The transverse slice then was divided into 20 coronal segments (going from the sternum to the spine). We compared the percentage of ventilation in each segment, normalized to the entire ventilation in the observed slice. Our data showed an excellent linear correlation between the ventilation distribution measured by SPECT scanning and EIT according to the following equation: y = 0.82x + 0.7 (R(2) = 0.92; range, 0.86 to 0.97).

CONCLUSION

Based on these data, EIT seems to allow, at least in comparable states of lung injury, real-time monitoring of regional ventilation distribution at the bedside.

Distribution of lung ventilation in spontaneously breathing neonates lying in different body positions

OBJECTIVE

The aim of our study was to determine the effect of the irregular spontaneous breathing pattern and posture on the spatial distribution of ventilation in neonates free from respiratory disease by the non-invasive imaging method of electrical impedance tomography (EIT). Scanning of spontaneously breathing neonates is the prerequisite for later routine application of EIT in babies with lung pathology undergoing ventilator therapy.

DESIGN

Prospective study.

SETTING

Neonatal intensive care unit at a university hospital.

PATIENTS

Twelve pre-term and term neonates (mean age: 23 days; mean body weight: 2,465 g; mean gestational age: 34 weeks; mean birth weight: 2,040 g).

INTERVENTIONS

Change in body position in the sequence: supine, right lateral, prone, supine.

MEASUREMENTS AND RESULTS

EIT measurements were performed using the Göttingen GoeMF I system. EIT scans of regional lung ventilation showing the distribution of respired air in the chest cross-section were generated during phases of rapid tidal breathing and deep breaths. During tidal breathing, 54.5+/-8.3%, 55.2+/-10.5%, 59.9+/-8.4% and 54.2+/-8.5% of inspired air (mean values +/- SD) were directed into the right lung in the supine, right lateral, prone and repeated supine postures respectively. During deep inspirations, the right lung ventilation accounted for 52.6+/-7.9%, 68.5+/-8.5%, 55.4+/-8.2% and 50.5+/-6.6% of total ventilation respectively.

CONCLUSION

The study identified the significant effect of breathing pattern and posture on the spatial distribution of lung ventilation in spontaneously breathing neonates. The results demonstrate that changes in regional ventilation can easily be determined by EIT and bode well for the future use of this method in paediatric intensive care.

Evaluation of an EIT reconstruction algorithm using finite difference human thorax models as phantoms

A finite difference model of the human thorax with 113,400 control volumes (nodes) based on ECG gated MRI images was used to evaluate the Sheffield DAS-01P EIT system. Sixteen simulated electrode positions equally spaced around the thorax model at approximately the fourth intercostals space level were selected. Pairs of adjacent positions were excited sequentially by injecting current in a manner similar to that used by the Sheffield DAS-01P EIT system. The resulting voltages on the non-excited electrode positions were calculated and used to reconstruct the image using the Sheffield filtered back projection algorithm. By changing the resistivities of the lungs, the ventricles and the atria over a range of 1% to 40%, the resulting changes in the images were quantified by measuring the average resistivity change over a region defined automatically by two thresholds, 40% or 80% of the average of the first four pixels with the largest change. The results show that the changes observed in the images are consistently less than the changes in the model, but changed in a nearly linear manner as a function of resistivity in the model. For 40% resistivity changes in the model for right lung, right ventricle and right atrium, the observed resistivity changes in the region of interest (ROI, defined by the 80% threshold) of the images are 32% for the right lung, 11% for the right ventricle and 5.5% for the right atrium, which suggests strong volume dependence of EIT imaging. The effect of structural (size) change between end diastole and end systole was also studied, which showed large resistivity changes caused in the heart region of the constructed image. The study demonstrates that the Sheffield DAS-01P EIT reconstruction algorithm tracks the change occurring in the lungs most closely and with proper scaling may be used to observe physiological changes.

Detection of local lung air content by electrical impedance tomography compared with electron beam CT

The aim of the study was to validate the ability of electrical impedance tomography (EIT) to detect local changes in air content, resulting from modified ventilator settings, by comparing EIT findings with electron beam computed tomography (EBCT) scans obtained under identical steady-state conditions. The experiments were carried out on six anesthetized supine pigs ventilated with five tidal volumes (VT) at three positive end-expiratory pressure (PEEP) levels. The lung air content changes were determined both by EIT (Goe-MF1 system) and EBCT (Imatron C-150XP scanner) in six regions of interest, located in the ventral, middle, and dorsal areas of each lung, with respect to the reference air content at the lowest VT and PEEP, as a change in either local electrical impedance or lung tissue density. An increase in local air content with VT and PEEP was identified by both methods at all regions studied. A good correlation between the changes in lung air content determined by EIT and EBCT was revealed. Mean correlation coefficients in the ventral, middle, and dorsal regions were 0.81, 0.87, and 0.93, respectively. The study confirms that EIT is a suitable, noninvasive method for detecting regional changes in air content and monitoring local effects of artificial ventilation.

Regional lung perfusion as determined by electrical impedance tomography in comparison with electron beam CT imaging

The aim of the experiments was to check the feasibility of pulmonary perfusion imaging by functional electrical impedance tomography (EIT) and to compare the EIT findings with electron beam computed tomography (EBCT) scans. In three pigs, a Swan-Ganz catheter was positioned in a pulmonary artery branch and hypertonic saline solution or a radiographic contrast agent were administered as boli through the distal or proximal openings of the catheter. During the administration through the proximal opening, the balloon at the tip of the catheter was either deflated or inflated. The latter case represented a perfusion defect. The series of EIT scans of the momentary distribution of electrical impedance within the chest were obtained during each saline bolus administration at a rate of 13/s. EBCT scans were acquired at a rate of 3.3/s during bolus administrations of the radiopaque contrast material under the same steady-state conditions. The EIT data were used to generate local time-impedance curves and functional EIT images showing the perfusion of a small lung region, both lungs with a perfusion defect and complete both lungs during bolus administration through the distal and proximal catheter opening with an inflated or deflated balloon, respectively. The results indicate that EIT imaging of lung perfusion is feasible when an electrical impedance contrast agent is used.

Gravity effects on regional lung ventilation determined by functional EIT during parabolic flights

Gravity-dependent changes of regional lung function were studied during normogravity, hypergravity, and microgravity induced by parabolic flights. Seven healthy subjects were followed in the right lateral and supine postures during tidal breathing, forced vital capacity, and slow expiratory vital capacity maneuvers. Regional 1) lung ventilation, 2) lung volumes, and 3) lung emptying behavior were studied in a transverse thoracic plane by functional electrical impedance tomography (EIT). The results showed gravity-dependent changes of regional lung ventilation parameters. A significant effect of gravity on regional functional residual capacity with a rapid lung volume redistribution during the gravity transition phases was established. The most homogeneous functional residual capacity distribution was found at microgravity. During vital capacity and forced vital capacity in the right lateral posture, the decrease in lung volume on expiration was larger in the right lung region at all gravity phases. During tidal breathing, the differences in ventilation magnitudes between the right and left lung regions were not significant in either posture or gravity phase. A significant nonlinearity of lung emptying was determined at normogravity and hypergravity. The pattern of lung emptying was homogeneous during microgravity.

Electrical impedance tomography (EIT) in applications related to lung and ventilation: a review of experimental and clinical activities

This review article is a summary of the publications dealing with the pulmonary applications of electrical impedance tomography (EIT). Original papers on EIT lung imaging published over 15 years are analysed and several aspects of the performed EIT measurements summarized. Information on the type of the EIT device and electrodes used, the studied transverse thoracic planes, the data acquisition rate, the number of studied animals, normal subjects or patients, the kind of lung pathology, the performed ventilatory manoeuvres and other interventions, as well as the applied reference techniques, is given. The type of the generated pulmonary EIT images and the quantitative analysis of the EIT data are described. Finally, the major results achieved are presented, followed by an analysis of the perspectives of EIT in clinical applications. A comparative analysis of the EIT hardware and the quality of the evaluation tools was not performed.