We studied the influence of three gravity levels (0, 1 and 1.8 g) on unilateral lung aeration in a left lateral position by the application of absolute electrical impedance tomography. The electrical resistivity of the lung tissue was considered to be a meaningful indicator for lung aeration since changes in resistivity have already been validated in other studies to be proportional to changes in lung volume. Twenty-two healthy volunteers were studied during parabolic flights with three phases of different gravity, each lasting similar to 20-22 s. Spontaneous breathing at normal tidal volume V-T and at increased V-T was performed. During transition to hyper-gravity mean expiratory resistivities (+/- SD in Omega m) increased at normal V-T in the upper (right) lung from 7.6 +/- 1.5 to 8.0 +/- 1.7 and decreased from 5.8 +/- 1.2 to 5.7 +/- 1.2 in the lower ( left) lung. Inspiratory resistivity values are 8.3 +/- 1.6 to 8.8 +/- 1.8 (right) and 6.3 +/- 1.3 to 6.0 +/- 1.3 (left). At increased V-T, the changes in resistivities at end-expiration were 7.7 +/- 1.5 to 8.0 +/- 1.7 (right) and 5.8 +/- 1.2 to 5.7 +/- 1.2 (left). Corresponding end-inspiratory values are 9.9 +/- 1.9 to 10.0 +/- 2.0 (right) and 8.6 +/- 2.1 to 7.9 +/- 2.0 (left). During weightlessness, the distortion in the lungs disappeared and both lungs showed a nearly identical aeration, which was between the levels displayed at normal gravity. The small increase in resistivity for the upper lung during transition to hyper-gravity from 1 to 1.8 g at increased V-T suggests that the degressive part of the pressure-volume curve has already been reached at end-inspiration. The results for a left lateral position are in agreement with West's lung model which has been introduced for cranio-caudal gravity dependence in the lungs.

We investigated five different methods which can be applied to quantitatively construct functional tomograms of the lungs. The focus was on the sensitivity of functional tomograms to errors in acquired data. To quantify this sensitivity, theoretical, error-free data sets of well-known properties were artificially generated based on a 'living thorax model'. Physiological time courses and a typical distribution of errors caused by a typical Goe-MF II EIT system were used for the calculations which encompassed a range up to 50 times greater than the initial error level (4 µVrms max–400 µVrms max). Additionally, low-pass filtering and principal component analysis (PCA) were used to quantify the effect of preprocessing the raw data. The results demonstrate that all methods based on fitting the local to the global time course were superior to the common functional tomograms utilizing standard deviation or maximum and minimum detection. Ventilation distribution was best quantified by the so-called VT methods. Filling capacity—a lung tissue property—was least dependent on increasing error levels. The errors introduced by filtering are significant with respect to a quantitative analysis of ventilation distribution. A preprocessing of raw data by applying a PCA performed well on the data sets which had been constructed but were, nonetheless, realistic. This approach appears to be highly promising for application on real data which is known to be erroneous.

A new transpulmonary echo contrast agent (SH U 508) was injected intracoronally to six anaesthetised sheep to examine its possible direct cardiac effects.SH U 508 was injected in randomised order in three different volumes (2, 4 and 8 ml; n = 12, 10, 9) with the same drug concentration of 200 mg/ml.The 2 ml and 4 ml injections had no relevant effect on the arterial, pulmonary-arterial and ventricular pressures, on the left ventricular contraction velocity and on the myocardial blood flow (less than +/- 10% of the control value). The left ventricular relaxation velocity decreased by 20%. The disturbance of the left ventricular relaxation at a volume of 8 ml was pronounced (about 40% decrease). Slight left ventricular dysfunction further manifested itself in a decreased systolic pressure (-15%) and increased left ventricular enddiastolic pressure (20%).In summary the overall effect of the intracoronary injections of SH U 508 exhibited only minor cardiac side effects. If the current results are extrapolated to peripheral-venous application, the clinically required central-venous 8 ml injection of a 400 mg/ml suspension is not expected to produce any coronary haemodynamic side effects, due to drug dilution in the pulmonary circulation and resulting low intra-coronary concentrations.

We present a simple method to determine systematic errors that will occur in the measurements by EIT systems. The approach is based on very simple scalable resistive phantoms for EIT systems using a 16 electrode adjacent drive pattern. The output voltage of the phantoms is constant for all combinations of current injection and voltage measurements and the trans-impedance of each phantom is determined by only one component. It can be chosen independently from the input and output impedance, which can be set in order to simulate measurements on the human thorax. Additional serial adapters allow investigation of the influence of the contact impedance at the electrodes on resulting errors. Since real errors depend on the dynamic properties of an EIT system, the following parameters are accessible: crosstalk, the absolute error of each driving/sensing channel and the signal to noise ratio in each channel. Measurements were performed on a Goe-MF II EIT system under four different simulated operational conditions. We found that systematic measurement errors always exceeded the error level of stochastic noise since the Goe-MF II system had been optimized for a sufficient signal to noise ratio but not for accuracy. In time difference imaging and functional EIT (f-EIT) systematic errors are reduced to a minimum by dividing the raw data by reference data. This is not the case in absolute EIT (a-EIT) where the resistivity of the examined object is determined on an absolute scale. We conclude that a reduction of systematic errors has to be one major goal in future system design.

We present an improved approach to image ventilation in functional electrical impedance tomography (f-EIT). It combines the advantages of the two established procedures of calculating standard deviation as a functional parameter of ventilation (SD method) and the so-called filling capacity (FC method). The SD method quantifies the local impedance variation over a series of tomograms for each pixel; the FC method is based on the slope of a linear fit of regional versus the global impedance change. Tidal volume VT is displayed linearly by the SD method in f-EIT; it is, however, sensitive to noisy data. The FC method is much more robust with respect to noise but does not display the tidal volume VT. We combined the advantages of both techniques in a new VT method which is based on raw data. It saves computing time and is suitable for both f-EIT and absolute EIT (a-EIT). We separated the raw data into two representative sets: end expiratory and end inspiratory. This was accomplished by calculating the global time course of the relative impedance changes from the raw data. In this time course, we determined all frame numbers (indices) of end expiration and end inspiration. These frame numbers were used to calculate one mean expiratory and one mean inspiratory raw data frame. Reconstruction by difference imaging directly reflects the mean tidal volume VT during the acquired frame series. The effect of the improvement by the VT method was investigated at different noise levels by adding artificial noise from 0 to 100 µVrms to a real raw dataset. The robustness with regard to noise of the VT method was similar to that of the FC method. The practical value of suppression of non-ventilatory impedance changes, artefacts and noise was tested by studying ten healthy subjects (four females, six males) during normal breathing. We found a highly significant improvement in the image quality (p < 0.001) of ventilation for this group of volunteers.

An established procedure in functional EIT (f-EIT) to image ventilation is the calculation of the standard deviation of the local impedance variation over a series of tomograms for each pixel. We present an improved approach based on the raw data. It saves computing time and is suitable for f-EIT and absolute EIT (a-EIT) just as well. Furthermore, tidal volume, end-expiratory and end-inspiratory ventilatory level can be quantified separately. From the raw data we calculate the time course of the global relative impedance changes. It is used to determine separately the indices (frame number) of all frames that correspond to the maximum (inspiratory) and minimum (expiratory) level of ventilation. According to these indices two data sets of raw data are generated: one mean inspiratory and one mean expiratory frame. Based on these two frames functional images of ventilation or absolute images of lung tissue resistivity can be calculated representing the whole time series. These functional images of ventilation now correspond directly to the tidal volume of respiration which is more common in physiology than standard deviation.

For image reconstruction in absolute EIT (a-EIT) on the human thorax, often a simple body geometry as a circular or elliptical cylinder is assumed. However, deviations from this assumption lead to distortions or artefacts in the resulting tomograms. Furthermore, inexact electrode positions affect the results. In order to investigate the influence of these factors on image reconstruction, synthetic data sets were calculated using 3D-FE modelling (COMSOL Multiphysics, COMSOL AB) with varying body shape and electrode positions. Additionally, mea-surements on a healthy volunteer were performed accompanied by exact determination of individual body shape and slight variation of electrode positions. Two different reconstruction algorithms were used: one based on an unstructured triangular parametrisation with FE forward modelling in combination with a Gauss-Newton inversion and another one based on a modified SIRT-algorithm with FD forward modelling using cylindric segments. The results show, that the ignorance of individual body shape and electrode positioning errors lead to distortions or artefacts with both algorithms. If the geometrical peculiarities are known, they easily can be included in the inversion process by the use of the unstructured parametrisation and so their influences are mainly diminished.

Objectives: The objective of the study was to determine regional pulmonary filling characteristics in 20 mechanically ventilated patients with acute lung injury. Methods: Regional filling characteristics were calculated from tracings of regional tidal volumes vs. global tidal volumes measured by electrical impedance tomography (EIT). These plots were fitted to a polynomial function of the second degree. Regional polynomial coefficients of the second degree characterized the curve linearity of the plots. Near-zero values of the polynomial coefficient indicated a homogeneous increase in regional tidal volumes during the whole inspiration. Positive values hinted at initial low regional tidal volume change suggesting lung volume recruitment. Negative values indicated late low regional tidal volume change implying hyperinflation of this lung region. Results: We found a broad heterogeneity of regional lung filling characteristics. The minimal regional polynomial coefficients varied from -2.80 to -0.56 (median -1.16), while the maximal regional polynomial coefficients varied from 0.58 to 3.65 (median 1.41). Conclusions: Measurements of regional filling characteristics by EIT may be a helpful tool to adjust the respiratory settings during mechanical ventilation to optimize lung recruitment and to avoid overdistension. It applies a non-pressure-related assessment to the mechanics of lung inflation and gives a view of the real problems underlying ventilatory strategies dependent on global characteristics.

Background: We hypothized, that in mechanically ventilated patients with acute respiratory failure, regional pressure volume curves differ markedly from conventional global pressure volume curves of the whole lung.Methods: In nine mechanically ventilated patients with acute respiratory failure during an inspiratory low‐flow manoeuvre, conventional global pressure volume curves were registered by spirometry and regional pressure volume curves in up to 912 regions were assessed simultaneously using electrical impedance tomography. We compared the lower (LIP) and upper (UIP) inflection points obtained from the conventional global pressure volume curve and regional pressure volume curves.Results: We identified from the conventional global pressure volume curves LIP [3–11 (8) cmH2O] in eight patients and UIP [31–39 (33) cmH2O] in three patients. Using electrical impedance tomography (EIT), LIP [3–18 (8) cmH2O] in 54–264 (180) regions and UIP [23–42 (36) cmH2O] in 149–324 (193) regions (range and median) were identified. Lung mechanics measured by conventional global pressure volume curves are similar to the median of regional pressure volume curves obtained by EIT within the tomographic plane. However, single regional pressure volume curves differ markedly with a broad heterogeneity of lower and upper inflection points.Conclusion: Lower and upper inflection points obtained from conventional global pressure volume curves are not representative of all regions of the lungs.

The increasing use of EIT in clinical research on severely ill lung patients requires a clarification of the influence of pathologic impedance distributions on the validity of the resulting tomograms. Significant accumulation of low-conducting air (e.g. pneumothorax or emphysema) or well-conducting liquid (e.g. haematothorax or atelectases) may conflict with treating the imaging problem as purely linear. First, we investigated the influence of stepwise inflation and deflation by up to 300 ml of air and 300 ml of Ringer solution into the pleural space of five pigs on the resulting tomograms during ventilation at constant tidal volume. Series of EIT images representing relative impedance changes were generated on the basis of a modified Sheffield back projection algorithm and ventilation distribution was displayed as functional (f-EIT) tomograms. In addition, a modified simultaneous iterative reconstruction technique (SIRT) was applied to quantify the resistivity distribution on an absolute level scaled in Omega m (a-EIT). Second, we applied these two EIT techniques on four intensive care patients with inhomogeneous air and fluid distribution and compared the EIT results to computed tomography (CT) and to a reference set of intrathoracic resistivity data of 20 healthy volunteers calculated by SIRT. The results of the animal model show that f-EIT based on back projection is not disturbed by the artificial pneumo- or haematothorax. Application of SIRT allows reliable discrimination and detection of the location and amplitude of pneumo- or haematothorax. These results were supported by the good agreement between the electrical impedance tomograms and CT scans on patients and by the significant differences of regional resistivity data between patients and healthy volunteers.

Background and objective: For the treatment of patients with adult respiratory distress syndrome and acute lung injury bedside measurements of regional lung ventilation should be considered for optimizing ventilatory settings. The aim was to investigate the effect of positive end‐expiratory pressure (PEEP) on regional ventilation in mechanically ventilated patients at the bedside by electrical impedance tomography. Methods: Eight mechanically ventilated patients were included in the study. PEEP levels were increased from 0 to 5, 10, 15 mbar and back to 0 mbar. Regional ventilation in 912 regions of the thorax was investigated at each PEEP by electrical impedance tomography. The obtained regions were divided in four groups: none (none and poorly ventilated regions including chest wall and mediastinum), bad, moderate and well‐ventilated regions. Results: Increasing the PEEP stepwise from 0 to 15 mbar decreased the non‐ventilated regions (none: 540 regions at PEEP 0 and 406 regions at PEEP 15). In contrast, the other regions increased (bad: 316 regions at PEEP 0 and 380 regions at PEEP 15; moderate: 40 regions at PEEP 0 and 100 regions at PEEP 15; well: 0 region at PEEP 0 and 34 regions at PEEP 15 (median values)) indicating an improvement of regional ventilation. Conclusions: Increasing PEEP in mechanically ventilated patients reduces none ventilated regions (atelectasis). Furthermore, it leads to a shift from none and bad ventilated regions to moderately and well‐ventilated regions. Electrical impedance tomography is a bedside technique and might be an alternative to computed tomography scan to assess aerated lung regions.

Background and objective: It has been shown that racemic ketamine increases coronary blood flow and that this effect is at least in part due to a direct vasorelaxing effect of this substance. This study was designed to determine whether ketamine might stereoselectively relax isolated porcine coronary arteries.Methods: Using the model of isolated vessels we studied the effects of S(+) ketamine, R(-) ketamine, and racemic ketamine (5-500 mu g mL(-1)) on artery strips pre-contracted by either potassium chloride (KCl) or prostaglandin F-2 alpha (PGF(2 alpha)). To elucidate possible mechanisms of action these experiments were repeated in the presence of one of the following compounds: N-omega-nitro-L-arginine (L-NNA), indomethacin, glibenclamide, and tetraethylammonium (TEA) chloride, an inhibitor of the BKCa K+ channel.Results: Both isoforms and racemic ketamine relaxed isolated coronary arteries in a concentration-dependent manner in concentrations beyond those used in clinical practice. S(+) ketamine exerted the strongest vasorelaxing effect, followed by racemic ketamine and R(-) ketamine. Pretreatment with L-NNA, indomethacin, or glibenclamide did not alter the vasodilating properties of ketamine, whereas TEA chloride significantly attenuated the vasorelaxing. effects of all the three forms of ketamine..Conclusions: Ketamine dilates coronary arteries in vitro when administered in high concentrations. There is a stereoselective difference with a stronger vasorelaxing effect of S(+) ketamine compared to racemic and R(-) ketamine. The impact of TEA chloride suggests that the activation of the BKCa channel may contribute to the vasodilating effect of ketamine.

Elektrische Impedanztomographie (EIT) ist ein relativ neues bildgebendes Verfahren, das nicht-invasiv und ohne Strahlenbelastung Querschnittsbilder des menschlichen Körpers erstellen kann. Die EIT-Bilder können mit einer hohen zeitlichen Auflösung über lange Zeiträume erfasst werden. Das Messprinzip von EIT beruht auf der Bestimmung elektrischer Eigenschaften biologischer Gewebe. Bei einer Anwendung am Brustkorb können mit EIT unterschiedliche Aspekte der Lungenfunktion regional im Thoraxquerschnitt ermittelt werden. Unter der Nutzung neuartiger Auswertungsalgorithmen werden funktionelle EIT-Bilder generiert, die die Verteilung der regionalen Lungenventilation und die Änderungen der lokalen Lungenvolumina erfassen können. Die funktionelle EIT hat die Perspektive in der Zukunft als ein kontinuierliches bettseitiges Monitoringverfahren zur Überwachung von beatmeten Intensivpatienten eingesetzt zu werden. Das klinische Potential von EIT in dieser Anwendung wird in dieser Arbeit durch Ergebnisse aus experimentellen und klinischen Untersuchungen dokumentiert.

Background and objective: Hypotension, especially in elderly and hypovolaemic patients, is frequently associated with intravenous midazolam administration. The mechanisms are not completely understood. This study was designed to investigate the mechanisms involved in the relaxing effect of midazolam on coronary arteries.Methods: The substance was studied in isolated porcine coronary artery rings precontracted by either potassium chloride or prostaglandin F-2 alpha,Results: Midazolam caused vasodilatation in a concentration-dependent manner. Relaxation was more pronounced in prostaglandin F-2 alpha precontracted segments than in those treated with potassium chloride (P < 0.001). Vasodilatation was unaffected by N.-nitro-L-arginine, indomethacin and glibenclamide. Tetraethylammonium chloride, an inhibitor of the BKCa K+ channel (a high conductance Ca2+-sensitive K+ channel), dose dependently attenuated the vasodilating effect of midazolam (P < 0.01).Conclusions: Hyperpolarization of the smooth muscle cell in the vessel wall, elicited by the activation the BKCa K+ channel, may contribute to the vasorelaxing effect of midazolam.

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 (∼0g) and hypergravity (∼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° 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.

To determine the effect of age and posture on regional lung ventilation, eight young (26 ± 1 years, mean ± S.D.) and eight old (73 ± 5 years) healthy men were studied by electrical impedance tomography in four body positions (sitting, supine, right and left lateral). The distribution of gas into the right and left lung regions was determined in the chest cross-section during tidal breathing at the resting lung volume, near residual volume and total lung capacity, as well as forced and slow vital capacity maneuvers. In the young, significant posture-dependent changes in gas distribution occurred during resting tidal breathing whereas they were absent in the elderly. In the older subjects, the contribution of the right lung to global ventilation fell with the transition from sitting to supine posture during both full expiration maneuvers. During forced vital capacity, the high flow rate and early airway closure in the dependent lung, occurring at higher volumes in the elderly, minimized the posture-dependency in gas distribution which was present during the slow maneuver. Our study revealed the significant effect of age on posture-dependent changes in ventilation distribution.

Objectives: To evaluate the feasibility of percutaneous aortic valve replacement without cardiac arrest in animal experiments. Methods: A self expanding nitinol stent, containing pulmonary valves from pigs in its proximal part, was implanted in six pigs (94–118 kg) by means of a 25 French catheter through the left subclavian artery under guidance of fluoroscopy and transoesophageal echocardiography. During stent deployment the original aortic valve was pushed against the aortic wall by the self expanding force of the stent while the new valve was expanded. Results: It was possible to replace the aortic valve in the beating heart in four pigs (67%) with no complication or relevant drop in blood pressure. The procedure failed in two pigs (33%) due to dysfunction of the catheter device in one case and to problems with correct positioning in the left ventricular outflow tract in the other. After successful stent valve implantation, dopamine was infused in doses of 5 μg/kg/min, 10 μg/kg/min, and 15 μg/kg/min. Cardiac output increased from 4.4 to 8.8 l/min and the mean arterial pressure rose from 79 to 105 mm Hg. The maximum peak to peak pressure gradient across the valve carrying stent reached a maximum of 8 mm Hg under dopamine infusion. All pigs were killed six hours after transvascular aortic valve replacement. The chest was opened, and the left ventricle and the ascending aorta were carefully inspected. There were no signs of malfunction of the implant, of damage of the aortic vessel wall, or of obstruction of the coronary ostia. Conclusions: Percutaneous aortic valve replacement with a self expanding nitinol stent in the beating heart is possible. The device was safe under pharmacological stress test. After successful chronic animal experiments, this concept may become a feasible option for treating patients with relevant aortic valve disease but where open heart surgery would be risky.

BACKGROUND AND OBJECTIVE:Propofol may cause undesirable hypotension due to vasodilation. The underlying mechanisms are not completely understood. We investigated the mechanisms by which propofol relaxes vascular segments. METHODS:We studied the effect of propofol on isolated porcine coronary artery rings precontracted with potassium chloride or prostaglandin F2alpha. RESULTS:Propofol, in a concentration-dependent manner, relaxed all segments at concentrations of 5 microg mL(-1) and above. This relaxation was unaltered in the presence of N(omega)-nitro-L-arginine, indomethacin, diltiazem and glibenclamide. Tetraethylammonium chloride, an inhibitor of the BK(Ca) K+ channel (a high conductance Ca2+-sensitive K+ channel), dose-dependently attenuated the vasodilating effect of propofol (P < 0.001). CONCLUSIONS:Our results suggests that the activation of the BK(Ca) channel may contribute to the vasodilating effect of propofol, hereby causing hyperpolarization of the smooth muscle membrane and reduction of smooth muscle tone.