Therapeutic optimization of atrioventricular delay in cardiosurgical ICU patients by noninvasive cardiac output measurements versus pulse contour analysis.

BACKGROUND: Optimizing atrioventricular (AV) delay improves cardiac output and postoperative outcome. Impedance cardiography (ICG) is a non-invasive method for CO measurement. This study evaluates the ability of two ICG methods to determine the optimal AV delay (OAVD) and to compare ICG with invasive PICCO measurements. METHODS: In 14 cardiosurgical ICU patients (age 70.4 +/- 12.0 yrs) with temporary pacing wires, OAVD was determined by pulse contour analysis (PICCO) and ICG (conventional ICG [CI] and electrical velocimetry [EV] ICG monitors). Cardiac output (CO) and stroke volume (SV) were measured during DDD pacing with AVD varying from 70 to 270 ms in 20-ms increments. RESULTS: Measured OAV showed a linear correlation between PICCO and ICG: CI (r = 0.82, P < 0.0002) and EV (r = 0.84, P < 0.0002). The mean OAVD deviation between PICCO and ICG was 15.7 +/- 21.0 ms (CI) and 17.1 +/- 20.5 ms (EV). Hemodynamic parameters (SV increase OAVD against worst case) improved significantly (+ 11.7 +/- 7.2 %, P < 0.0001). CONCLUSION: Inappropriate selection of AVD can compromise the hemodynamic situation of cardiosurgical patients. As it is totally noninvasive, ICG is a reliable and effective tool for tailoring AVD. Both systems (CI and EV) offer valid OAV determination.

Tumour tissue monitoring during photodynamic and hyperthermic treatment using bioimpedance spectroscopy.

Electrical bioimpedance spectroscopy is a fast and relatively easily applicable method for tissue characterization. In the frequency range up to 10 MHz, current conduction through tissue is mainly determined by tissue structure, i.e. the extra- and intra-cellular compartments and the insulating cell membranes. Therefore, changes in the extra- and intra-cellular fluid volumes are reflected in the impedance spectra. Investigations of tumours (DS sarcoma, implanted on the hind foot dorsum of rats) during treatment with localized hyperthermia (HT), photodynamic therapy (PDT) and the combination of these two components were carried out using impedance spectroscopy in the frequency range of 37 Hz to 3.7 MHz. Data collected reveal totally different, but characteristic, behaviour patterns for the three treatments. HT caused a slow increase in conductance at high frequencies (G(HF)) and in the extracellular space index (ECSI), indicating an increase in extracellular fluid volume and in total fluid content. With PDT, G(HF) increased immediately upon commencement of irradiation and was accompanied by a distinct decrease in ECSI, indicating the development of a pronounced intracellular oedema.

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.

Impedance analyser module for EIT and spectroscopy using undersampling.

In this paper we present the concept, the design and the test procedure for a DSP-based high-precision and high-performance wide-band (up to 10 MHz) bioimpedance analyser module for application in EIT or bioimpedance spectroscopy. The module implements a digital concept with appropriate signal conditioning hardware for voltage and current measurement, early signal digitization and subsequent digital signal processing in order to calculate the components of impedance (or admittance). At low frequencies, the module utilizes the conventional direct conversion method, whereas at high frequencies the undersampling technique is used. The advantages of the described system are the following: (a) the frequency range is extended to higher frequencies, (b) the number of data sampled per time interval is significantly reduced, and (c) the current consumption and the costs of the ADCs can be significantly reduced. The validation procedure is performed by comparing the measured and theoretical values of the magnitude and the phase of the impedance for a commonly used tissue model. The module offers an accuracy of better than 0.012% for the magnitude of impedance and better than 0.02 degrees for the phase.

Evaluation of fast time-domain based impedance measurements on biological tissue.

Bio-impedance measurements are widely used for characterization of biological objects. Although the measured impedance of such objects is independent of the measurement method used, slight differences between measurements in the frequency and time domain are found. For many practical applications time domain based measurements are advantageous, but they are often rejected as not accurate. In order to show their suitability for bio-impedance measurements we used a special arrangement of time domain and frequency domain based measurements at the same biological specimen (canine liver) with the same electrodes. A reasonable coincidence in the measurement results could be shown. Moreover we used only a fraction of the time domain measurement data in order to demonstrate a significant reduction in measurement time while maintaining a reasonable accuracy. An algorithm for fast processing of the time domain data without transformation into the frequency domain is provided.

Monitoring temperature-induced changes in tissue during hyperthermia by impedance methods.

The electrical conduction in living tissue depends on temperature in two ways: (1) the temperature coefficients of conductivity of the intra- and extracellular electrolytes and (2) temperature-induced fluid volume shifts in the tissue. Measurements in rat skeletal muscle and tumors (DS sarcoma) during hyperthermic treatment reveal that the contribution of fluid volume shifts to changes in conductance is of the same order of magnitude as the change in fluid conductivity. In skeletal muscles, blood volume changes are caused by the temperature-dependent regulation of the vessel diameter (vasodilatation). In tumors, fluid content changes irregularly. These effects render temperature measurements by impedance methods, for example, electrical impedance tomography (EIT), questionable. However, monitoring fluid volume changes in tissue and the state of cell membranes is an interesting application of impedance (or admittance) spectroscopy and tomography as well.

Quantitative analysis of impedance spectra of organs during ischemia.

We have developed a rapid, quantitative procedure to fit the spectra of the real and imaginary part of tissue impedance, providing characteristic parameters: time constants, their distribution, and the amplitudes of associated dispersions. Based on the time course of tissue impedance during ischemia, we have derived the evolution of characteristic parameters for both myocardial and liver tissue. The similar evolution of the distribution of time constants for myocardial and liver tissue is emphasized and discussed.

Problems involved in temperature measurements using EIT.

In cancer therapy, hyperthermic treatment by microwaves requires a non-invasive and reliable method for measuring the temperature distribution inside the body. EIT seems to be able to evaluate the temperature-dependent tissue impedance for delivering the temperature profile in a cross-section of the body. Assuming a temperature coefficient of the resistivity of an electrolyte of about -2% degrees C-1 and temperature measurement to an accuracy of 0.5 degree C, the error in impedance measurement must be lower than 1%. Irrespective of the accuracy of the tomographic measuring system itself, a problem arises from the fact that the fluid content in the tissue as well as the fluid distribution between the extracellular and the intracellular compartment change with temperature. Measurements of the impedance spectra of skeletal muscle and tumours of rats during hyperthermic treatment deliver very different temperature coefficients of the resistivity from -1.3% degree C-1 to -3% degree C-1, thus questioning the feasibility of the EIT as a temperature measuring method. However, changes in the tissue caused by hyperthermia (e.g., fluid shifts, development of oedema and membrane disintegration) can be detected.

Tissue impedance spectra and the appropriate frequencies for EIT.

The complex impedance of each kind of tissue depends on the frequency in a characteristic manner. Using appropriate measuring frequencies, EIT can provide a differentiating insight into the interior of a body. Therefore, a knowledge of the tissue impedance spectra of various organs is essential for choosing the appropriate frequencies. The impedance data of various tissues in different states (normal, altered by ischaemia or cancerous) show that the characterizing differences occur at frequencies below 500 kHz and down to a few kilohertz. Moreover, the spectra show that the imaginary component of impedance essentially contributes to the characterization of the kind and state of a tissue, even though the dissipative and reactive components are connected by the Kramers-Kronig relations. The course of a dispersion and the position in the frequency range, determined by the distribution of the time constants in the tissue, are clearly presented by the imaginary component. Tomographic imaging combined with spectroscopy for tissue characterization requires a frequency range of at least 10-800 kHz. The upper frequency limit depends on the fluid content of the tissue under investigation.

EIT using magnitude and phase in an extended frequency range.

The electrical impedance is a characteristic tissue property that can be used for imaging cross sections of the body. The full information contained in the complex tissue impedance can be utilized if not only the real part Re(Z) or the magnitude of the impedance but also the imaginary part Im(Z) or the phase is considered. Impedance measurements provide information about tissue structure, particularly extracellular space and cell membranes. Therefore, an electrical impedance tomograph was constructed which uses alternatively the real component, the imaginary component, the magnitude or the phase in an extended frequency range. The components are evaluated by digital correlation. The device allows state-different or frequency-different (almost static) imaging. 16 electrodes are used. Image reconstruction is arrived at by a back-projection algorithm. For frequency-different imaging the measured imaginary part values can be used after normalization (division by the measuring frequency); instead of the phase values, the quotients Im(Z)/Re(Z) are taken and divided by the actual frequency, representing time constants of the tissue. Frequency-different measurements on a tank filled with saline containing a metallic rod and an insulator show in a very illustrative manner the impedance of the metal/electrolyte boundary layer (phenomenon of electrode polarization). The first in vivo measurements are very promising, state-different as well as frequency-different images of the human thorax represent, for example, the lungs with higher contrast using the phase than using the magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelations between electrical and biochemical processes in ischemic porcine livers at low temperature.

Recently it has been shown that during liver ischemia at 25 degrees C the presence of glycogen, by supporting glycolytic supply, not only retards ATP decay but also leads to a corresponding delay of the rise of the electrical impedance of the ischemic organ. A sudden rise of impedance during ischemia is supposed to indicate the closure of gap junctions. Although similar effects on energy state do exist at low temperature, the impact of glycogen on the electrical impedance under storage conditions has still to be evaluated. Therefore, in a model with protected porcine livers, we examined the intraischemic effects of a preischemic glucose and potassium feeding on impedance changes, lactate production and ATP-content at a storing temperature of 5 degrees C. Impedance was measured both in the low frequency alpha- and the higher frequency beta-dispersion range. In addition, the same parameters were determined in a group of unprotected livers. In this group all animals had received glucose and potassium orally prior to ischemia. Whereas in case of preischemic glucose feeding the rise of impedance in the range of the beta-dispersion (e.g. 5kHz) roughly coincided with the exhaustion of ATP, the corresponding impedance changes in the protected group without a glucose premedication only occurred when glycolysis had already stopped and ATP had reached basal values for some hours. In contrast, in the alpha-dispersion range the impedance changes in the latter group just began at the time when ATP became exhausted and lactate production ceased.(ABSTRACT TRUNCATED AT 250 WORDS)

[Measuring impedance for evaluating ischemia damage to the human liver in preparation for transplantation]. / Die Impedanzmessung zur Beurteilung von Ischämieschäden der humanen Leber in der Vorbereitung zur Transplantation.

In 22 human donor livers the measurement of the non-invasive bioelectrical impedance was performed prospectively to evaluate the degree of tissue damage sustained during cold ischemia. The results of the measurement were correlated with liver function, the method of organ preservation and the period of ischemia. The impedance was measured in vivo as 620 ohm (at 192 Hz), the phase angle as -7.4 degrees (at 5 kHz). The results were compared with the data obtained from 72 patients who underwent elective laparotomies. The 22 donor livers were studied further during ischemia. The method was found to be a reliable way of detecting severe damage to the hepatocytes during the cold ischemia.

[Measuring electric impedance of organs--methodologic principles]. / Messung der elektrischen Impedanz von Organen--Methodische Grundlagen.

Ischemia causes changes in organ tissue (e.g. during operation or transplantation) which may finally lead to irreversible injury, so that the organ can no longer be resuscitated. To the extent that these changes affect the electrical properties of the tissue they are manifested in the impedance spectrum. As an example, the course of impedance of a HTK-protected porcine liver is presented in the frequency range of 0.1 Hz to 10 MHz, which includes two dispersion--alpha- and beta-dispersion. Using a suitable electrical equivalent circuit analogue to the structure of the liver, the behavior of the alpha- and beta-dispersion is explained on the basis of gap junction closure and narrowing of the extracellular space due to cell swelling.

Influence of tissue acidification and halothane anesthesia on hepatic electrical and biochemical properties during ischemia.

In order to further corroborate the recent findings on liver ischemia after perfusion with solutions containing an additive of heptanol, that the intraischemic loss of cell-to-cell communication in protected livers can be detected by electrical impedance measurement (1), we tried to induce uncoupling in porcine liver by tissue acidification applying acidified protective solutions. Moreover, the effects of preischemic inhaling of high concentrations of the decoupling anesthetic halothane were examined in unprotected ischemic livers (2-5). Electrical impedance, biochemical analyses, and pH measurements were applied in parallel. In addition, typical time courses of impedance parameters of unprotected liver and skeletal muscle were compared, because the latter is devoid of gap junctions. In spite of overlapping side-effects of the respective uncoupling measure, the results suggest that the loss of cell-to-cell communication in the liver is associated with measureable effects on the electrical impedance.

[Measuring electrical impedance of organs--instrumental equipment for research and clinical use]. / Messung der elektrischen Impedanz von Organen--Apparative Ausrüstung für Forschung und klinische Anwendung.

An apparatus for measuring the impedance of intact biological organs or parts of organs in the frequency range of 10 Hz to 10 MHz is described. In this range impedance exhibits a large dispersion, which is dependent on tissue structures. The time course of alterations of electrical impedance such as occur during ischemia can be recorded with this equipment. Five specimens in five measuring chambers can be examined simultaneously at different temperatures. In the second part of the article, a portable impedance meter for measuring the modulus of impedance near 200 Hz, the phase of impedance at 5 kHz and the local temperature at the measuring point, is described. These parameters permit an intra-operative evaluation of the changing state of ischemic organs. Sterilizable probes with four surface electrodes and an integrated temperature sensor permit atraumatic measurements at the organ surface. The measurement itself is harmless to the tissue.

Heptanol effects on protected livers.

Heptanol, an agent known for inducing closure of gap junctions in a variety of organs, was used to evaluate the influence of uncoupling on the electrical impedance of livers during ischemia. Heptanol was added to a modified HTK solution or to Belzer's UW-CSS solution. Livers of swine were then perfused for 8 min with either one of the solutions containing heptanol or a solution devoid of this additive. During the following ischemia the phase angle of impedance at 5 kHz, pH and different biochemical parameters were determined. Heptanol fundamentally changed the time course of impedance and made the otherwise characteristic fast increase of the phase angle of impedance disappear. Already early during ischemia the phase angle was raised in a dose-dependent manner up to even highest values at the beginning of the whole observation period in case of a fully developed effect. Heptanol also stimulated anaerobic energy turnover. The results suggest that, besides unspecific effects, heptanol induces uncoupling which is detectable by electrical impedance measurement.

Glycogen effects on energy state and passive electric properties of liver during protection.

In order to evaluate the importance of glycogen for the hepatic tolerance to ischemia, livers of swine fed a glucose-potassium solution for premedication were perfused with either Bretschneider's HTK-solution (histidine-tryptophan-ketoglutarate) or with Euro-Collins-solution (EC) prior to subsequent ischemia at 25 and 5 degrees C. During ischemia, in regular intervals or continuously, energy rich phosphates, lactate, intrahepatic pH and the electrical impedance of liver tissue were determined. The results were compared with corresponding data from swine which had starved for 48 h. Corresponding to the higher glycogen content, energy supply during ischemia was markedly improved by the premedication. Despite high amounts of glucose in the EC-solution, energy supply after glucose-potassium premedication was no better with EC-solution than with HTK-solution. Moreover, glucose uptake led to concomitant cellular water uptake. Electrical impedance measurements during ischemia mirrored improved energetical protection by the glucose-potassium premedication.

Impedance spectroscopy: a method for surveillance of ischemia tolerance of the heart.

UNLABELLED: During myocardial ischemia the phase angle phi of the complex electric impedance of myocardial tissue at 5 kHz AC exhibits a characteristic behaviour, the progress of which depends on the cardioplegic method applied. By extending the frequency range to 200 Hz and 10 MHz and by analyzing in addition to phase and magnitude also real and imaginary part of the impedance it was possible to elucidate which ischemic changes in the myocardium are responsible for the course of phi (5 kHz). This method we call impedance spectroscopy. Canine hearts were cardioplegically perfused with either the standard solution HTK[4] or the solution HTK[4] + 50 mumol/l Ca++. During the following ischemia at 25 degrees C energy-rich phosphate level, the ultrastructure, the real part, imaginary part and phase angle of the impedance between 200 Hz and 10 MHz were analyzed. RESULTS: phi (5 kHz) displays very similar characteristics during the ischemic period to those of the real part of the impedance at 200 Hz, Re (200 Hz). Re (200 Hz) increases, when--according to electron microscopic findings--an intracellular myocardial edema begins to develop. The changes of Re(200 Hz) are always smaller, however, than those of phi (5 kHz). This indicates that phi (5 kHz) increases in the course of ischemia not only as a consequence of confinement of the extracellular space by myocardial cellular edema but also because of changes of passive electrical characteristics of the myocardial cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)