Biomedical subjects

I Frerichs

Publications and source records attributed to I Frerichs.

At least 19 recordsLinked to original sources

PubMed2006

Regional filling characteristics of the lungs in mechanically ventilated patients with acute lung injury.

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.

Adult↗

PubMed2006

Imaging pathologic pulmonary air and fluid accumulation by functional and absolute EIT.

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.

Algorithms↗

PubMed2006

Regional pulmonary pressure volume curves in mechanically ventilated patients with acute respiratory failure measured by electrical impedance tomography.

BACKGROUND: We hypothesized, 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.

Acute Disease↗

PubMed2004

Transarterial aortic valve replacement with a self expanding stent in pigs.

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 microg/kg/min, 10 microg/kg/min, and 15 microg/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.

Animals↗

PubMed2001

Non-invasive radiation-free monitoring of regional lung ventilation in critically ill infants.

OBJECTIVE: Established techniques used to examine lung function in critically ill infants cannot continuously follow regional aspects of lung ventilation although this information would be beneficial for proper therapy planning. We have studied the applicability and clinical relevance of a relatively new non-invasive radiation-free imaging method, electrical impedance tomography (EIT), in monitoring regional lung function in paediatric intensive care patients. DESIGN: Prospective study. SETTING: Neonatal and paediatric intensive care unit (ICU) at a university hospital. PATIENTS: Eight infants (1 day-7 years old) suffering from miscellaneous diseases requiring intensive care therapy. INTERVENTIONS: Adjustment of ventilator settings, surfactant administration, and postural changes. MEASUREMENTS AND RESULTS: Repeated EIT measurements were performed with the intention to monitor regional lung ventilation in mechanically ventilated and spontaneously breathing infants. The follow-up time ranged between 1 and 11 days. During individual EIT measurements of 100-s duration electrical voltages resulting from repetitive injection of small electrical currents were continuously measured on the thoracic circumference using conventional surface electrodes. Acquired data were used to generate functional cross-sectional thoracic images of regional lung ventilation. A total of 638 EIT measurements were performed. The redistribution of lung ventilation and changes in regional ventilation magnitude resulting from adjusted positive end-expiratory pressure, peak inspiratory pressure, inspiration-expiration ratio, surfactant instillation, and prone or supine positioning were identified. CONCLUSIONS: Provided that EIT hardware and software are further developed to guarantee stable and undisturbed measurements in the ICU and that practical handling is improved, this non-invasive method may become a useful bedside monitoring tool of regional lung ventilation in critically ill infants.

Child↗

PubMed2001

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.

Adult↗

PubMed2001

Quantitative evaluation of the performance of different electrical tomography devices.

Two widely used electrical tomography systems, the Sheffield Mark I and the DAS-01P, were quantitatively evaluated and compared to the newly developed Goe-MF system. The performance was quantified using a hardware phantom which closely matches the real input and transfer impedances of the human thorax and allows measurements equivalent to different states of lung inflation. Our results demonstrate that adequate averaging is necessary for noise reduction for the Mark I and especially for the DAS-01P system to get meaningful results even in visualizing maximal respiratory manoeuvres. The Goe-MF system showed a notably improved signal-to-noise ratio which allows also dynamic measurements at low levels of lung volume changes, e.g., in intensive care lung injury patients.

Calibration↗

PubMed2000

A simple method to check the dynamic performance of electrical impedance tomography systems.

The test concept as well as the design of a simple resistor phantom suitable for the evaluation of the properties of electrical impedance tomographic (EIT) systems is presented. Input and transfer impedance of the phantom are matched with those of the human thorax. Amplitude of the local impedance variations similar to in vivo conditions (ventilation) can be intentionally set to perform measurements on different states. The theoretical potential differences between the electrodes are calculated. The evaluation procedure is performed in terms of the local amplitude of the relative impedance change as well as the local distribution of noise. The whole procedure can be applied either to compare quantitatively the performance of different EIT data acquisition systems or to determine the amount of measurement disturbance caused by the external electrical environment in clinical settings.

Cardiography, Impedance↗

PubMed2000

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.

Animals↗

PubMed1999

Monitoring regional lung ventilation by functional electrical impedance tomography during assisted ventilation.

A new approach in discriminating the regional air volume changes in the lungs associated with either spontaneous or mechanical ventilation during assisted ventilation is presented. Impedance data are obtained by conventional electrical impedance tomography (EIT). The data are filtered in the range of either the spontaneous or the ventilator rate and processed by the functional EIT (f-EIT) evaluation technique, whereby the variation of the respective EIT data with time is determined and imaged. EIT measurements performed in an infant during synchronized intermittent mandatory ventilation were evaluated with this method and indicated that the specific local lung volume swings related to spontaneous and mechanical inhalations can be separated and imaged as tomograms. This noninvasive approach may become useful in optimizing the ventilatory pattern during advanced forms of artificial ventilation and may help the clinician in the therapy management of individual patients.

Blood Gas Analysis↗

PubMed1999

Errors of the backextrapolation method in determination of the blood volume.

Backextrapolation is an empirical method to calculate the central volume of distribution (for example the blood volume). It is based on the compartment model, which says that after an injection the substance is distributed instantaneously in the central volume with no time delay. The occurrence of recirculation is not taken into account. The change of concentration with time of indocyanine green (ICG) was observed in an in vitro model, in which the volume was recirculating in 60 s and the clearance of the ICG could be varied. It was found that the higher the elimination of ICG, the higher was the error of the backextrapolation method. The theoretical consideration of Schröder et al (Biomed. Tech. 42 (1997) 7-11) was proved. If the injected substance is eliminated somewhere in the body (i.e. not by radioactive decay), the backextrapolation method produces large errors.

Artifacts↗

PubMed1999

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

The aim of the study was to analyze thoracic electrical impedance tomographic (EIT) measurements accomplished under conditions comparable with clinical situations during artificial ventilation. Multiple EIT measurements were performed in pigs in three transverse thoracic planes during the volume controlled mode of mechanical ventilation at various tidal volumes (V(T)) and positive end-expiratory pressures (PEEP). The protocol comprised following ventilatory patterns: 1) V(T)(400, 500, 600, 700 ml) was varied in a random order at various constant PEEP levels and 2) PEEP (2, 5, 8, 11, 14 cm H2O) was randomly modified during ventilation with a constant V(T). The EIT technique was used to generate cross-sectional images of 1) regional lung ventilation and 2) regional shifts in lung volume with PEEP. The quantitative analysis was performed in terms of the tidal amplitude of the impedance change, reflecting the volume of delivered gas at various preset V(T) and the end-expiratory impedance change, revealing the variation of the lung volume at various PEEP levels. The results showed: 1) an increase in the tidal amplitude of the impedance change, proportional to the delivered V(T) at all constant PEEP levels, 2) a rising end-expiratory impedance change, with PEEP reflecting an increase in gas volume, and 3) a PEEP-dependent redistribution of the ventilated gas between the planes. The generated images and the quantitative results indicate the ability of EIT to identify regional changes in V(T) and lung volume during mechanical ventilation.

Animals↗

PubMed1999

Optimizing deconvolution techniques by the application of the Münchhausen meta algorithm.

A deconvolution applied to disturbed data often gives poor results, due to fundamental difficulties associated with ill-posed problems. Many numerical and theoretical methods have been invented to circumvent this phenomenon. Their performance varies, depending on the given problem and data. The main aim of this paper is to provide a decision rule for choosing a method for deconvolution and application of this method to the same data. We have called this meta-algorithm Münchhausen. In this paper we introduce and describe for the first time the basic principle of artificial disturbance of the data in the set-up of deconvolution. We demonstrate some interesting features of the random procedure Münchhausen, such as the non parametric set-up, robustness to disturbance of the data and last but not least good performance.

Algorithms↗

PubMed1998

Electrical impedance tomography in monitoring experimental lung injury.

OBJECTIVE: To apply electrical impedance tomography (EIT) and the new evaluation approach (the functional EIT) in monitoring the development of artificial lung injury. DESIGN: Acute experimental trial. SETTING: Operating room for animal experimental studies at a university hospital. SUBJECTS: Five pigs (41.3 +/- 4.1 kg, mean body weight +/- SD). INTERVENTIONS: The animals were anaesthetised and mechanically ventilated. Sixteen electrodes were attached on the thoracic circumference and used for electrical current injection and surface voltage measurement. Oleic acid was applied sequentially (total dose 0.05 ml/kg body weight) into the left pulmonary artery to produce selective unilateral lung injury. MEASUREMENTS AND RESULTS: The presence of lung injury was documented by significant changes of PaCO2 (40.1 mmHg vs control 37.1 mmHg), PaO2 (112.3 mmHg vs 187.5 mmHg), pH (7.35 vs 7.42), mean pulmonary arterial pressure (29.2 mmHg vs 20.8 mmHg) and chest radiography. EIT detected 1) a regional decrease in mean impedance variation over the affected left lung (-41.4% vs control) and an increase over the intact right lung (+ 20.4% vs control) indicating reduced ventilation of the affected, and a compensatory augmented ventilation of the unaffected lung and 2) a pronounced fall in local baseline electrical impedance over the injured lung (-20.6% vs control) with a moderate fall over the intact lung (-10.0% vs control) indicating the development of lung oedema in the injured lung with a probable atelectasis formation in the contralateral one. CONCLUSION: The development of the local impairment of pulmonary ventilation and the formation of lung oedema could be followed by EIT in an experimental model of lung injury. This technique may become a useful tool for monitoring local pulmonary ventilation in intensive care patients suffering from pulmonary disorders associated with regionally reduced ventilation, fluid accumulation and/or cell membrane changes.

Acute Disease↗

PubMed1998

Monitoring perioperative changes in distribution of pulmonary ventilation by functional electrical impedance tomography.

BACKGROUND: Electrical impedance tomography (EIT) is a noninvasive technique providing cross-sectional images of the thorax. We have tested an extended evaluation procedure, the functional EIT (f-EIT), to identify the local shifts of ventilation known to occur during the transition between spontaneous, controlled and assisted ventilation modes. METHODS: Ten patients scheduled for elective laparotomy were studied in the surgical ward, operating theatre and ICU during spontaneous and different modes of mechanical ventilation. Sixteen ECG electrodes were placed on the circumference of the thorax and connected with an EIT device (APT System Mark I, IBEES, Sheffield, UK). Measurements lasting 180 s were performed and f-EIT images of regional ventilation computed. The geometrical centre of ventilation was determined to quantify the regional distribution of lung ventilation during individual modes of ventilation. RESULTS: F-EIT confirmed the differences in the distribution of ventilation associated with various modes of artificial ventilation. Accentuated ventilation of the dependent lung regions was observed during spontaneous breathing, whereas a shift of the centre of ventilation to the nondependent regions was found during controlled ventilation. In the course of assisted ventilation a continuous displacement of the centre of ventilation back towards the dependent lung regions, consistent with an increased proportion of spontaneous breathing, was detected. Unassisted spontaneous breathing after weaning from mechanical ventilation resulted in a similar ventilation distribution as during tidal breathing prior to surgery. CONCLUSION: F-EIT determined the redistribution of lung ventilation during different modes of mechanical ventilation. We expect that f-EIT will become a useful noninvasive bedside monitoring technique for imaging regional ventilation in pulmonary diseased patients during mechanical ventilation.

Abdomen↗

PubMed1998

Simulation of the initial concentration-time course after intravenous application of the drug.

In this paper we present a widely applicable computational method for the description of the initial concentration-time-course after intravenous injection of a substance. The intravascular concentration-time course, r, is described as r = c0 + g x r, where the asterisk denotes the convolution operation, c0 is the concentration-time course during the first passage of the substance and g is the transport function of the body. If the body transport function is known, then the concentration-time course of a substance can be predicted. The site of interest can be chosen arbitrarily, i.e. the concentration-time course in the arterial circulation supplying any organ can be described. This might be of special interest for the optimal design of intravenous injections of contrast media, where initial concentrations at the region of interest determine the success of the diagnostic procedure.

Animals↗

PubMed1998

Development and initial in vivo testing of a new hydraulic drive system (Paedipump) for circulatory support in infants.

The main limitation in the use of circulatory support in children is the lack of an adequate system with regard to size and pumping capacity. Recently, two pneumatically driven ventricular support systems with low volume chambers for use in a pediatric population became available. We have developed a hydraulic drive system with an advantageous exact control of the stroke volume. The system enables two different modes of operation: the full-empty and the filled-empty modes. In both cases the ventricle is empty at the end of systole. This new system was tested in experimental animals (6 pigs, body weight 9.5-14.0 kg) with normal and reduced left ventricular function (MAP<45 mmHg). A 25 ml ventricle (HIA-Medos) was implanted. The full-empty and the filled-empty mode used led to a significant load reduction, both in animals with normal and impaired cardiac function. Plasma lactate levels, pH-values and total body O2-consumption were in the normal range during circulatory support indicating adequate organ perfusion. Results showed that sufficient ventricular support was achieved during all pumping modes due to the possibility of controlling and modifying the stroke volume of the hydraulically driven support system employed according to necessity. This is a promising feature for its future application in infants with congenital or acquired heart diseases.

Animals↗

PubMed1997

[LOGNORMAL-NLSQ-technique. Evaluation of a new mathematical method for determining blood volume].

This paper describes the investigation of a new mathematical method of calculating blood volume. The new method determines the blood volume by calculating the product of the mean circulation transit time. The mean transit time is calculated from the body transport function. To examine the accuracy of the LOGNORMAL-NLSQ technique, 45 concentration time curves were measured in an in vitro recirculation model with variable clearance. The calculated volume was 4% smaller than the actual volume. This may be attributed to the functional dead space within the model, and is tolerable for clinical situations. The LOGNORMAL-NLSQ technique might acquire considerable importance in future, especially since it provides accurate results very quickly.

Blood Flow Velocity↗