Acute hyperkalemia detected by alert from implantable cardioverter-defibrillator

Two cases of implantable cardioverter-defibrillator lead impedance changes that corresponded with changes in serum potassium are described. In the first case, an audible lead alert was triggered due to a significant change in lead impedance caused by hyperkalemia. In the second case, a patient was admitted with hyperkalemia and renal failure. Device interrogation revealed a change in lead impedance corresponding to changes in serum potassium. As devices and leads become more sophisticated, their role for monitoring parameters and triggering alerts may expand to include electrolyte and other metabolic changes. (PACE 2012; 35:e276-e279).     

A comparison of neonatal and adult lung impedances derived from EIT images

An objective method of extracting respiratory data from lung images is presented, together with a technique for automatically generating regions of interest delineating the anterior and posterior regions of the lungs. The method is used to extract data on the change in lung impedance with frequency, and on calculated Cole parameters, from 19 normal neonates (gestational age 32 to 42 weeks) and 8 normal adults (age 21 to 82 years). A comparison of the impedance properties of neonatal and adult lungs was made. The variation of lung impedance with frequency in neonates, as derived from EIT images, is significantly different from that found for adults. The implications for a model of the electrical impedance of lung tissue are discussed.     

Steroid Eluting High Impedance Pacing Leads Decrease Short and Long-Term Current Drain: Results from a Multicenter Clinical Trial.

Pacemaker lead technology has changed considerably over the past decades. The widespread use of low polarization highly porous electrodes and steroid elution electrodes has resulted in low chronic pacing thresholds, as well as a decrease in the incidence of exit block. Efforts to develop pacing leads with high impedance might theoretically lead to lower lead current drain, which is a component of battery capacity. Pulse generator longevity can be increased without sacrificing pacemaker capabilities if pacing current drain can be decreased. Decreasing the size of the stimulation electrode results in increased pacing impedance, and if pacing thresholds are unchanged, a decreased current drain is predicted by Ohm's law (I = V/R). There is limited data available on the pacing characteristics of large numbers of patients with high impedance leads, despite their recent general availability and increasing widespread use. This multicenter, controlled trial examined the differences in performance between standard steroid-eluting pacing leads in the atrium (Medtronic model 5524) and ventricle (Medtronic model 5024), and new high impedance steroid-eluting pacing leads in the atrium (Medtronic model 5534) and ventricle (Medtronic model 5034). Measurements of bipolar pacing thresholds at 2.5 V, pacing impedance, and sensing thresholds were determined within 24 hours of pacemaker implantation, and at 0.5, 1, 3, 6 and 12 months after pacemaker implantation in 609 patients. Pacing and sensing thresholds were similar for the control and high impedance leads at all times except for a slightly larger R wave with the high impedance leads at implantation and 12 months. The mean impedance of the high impedance pacing leads in the atrium and ventricle at 12 months was 992 ± 175 and 1,080 ± 220 Ω, compared to 522 ± 69 and 600 ± 89 Ω for the standard pacing leads in the atrium and ventricle (P ≤ 0.001 for the high impedance leads compared to standard leads in each chamber). The mean atrial lead current (measured at 2.5 V) at 12 months was 2.6 ± 0.5 mA with the high impedance lead, and 4.9 ± 0.7 mA with the standard lead in the atrium (P ≤ 0.001). In the ventricle, the mean lead current at 12 months was 2.4 ± 0.4 mA with the high impedance pacing lead and 4.3 ± 0.6 mA with the standard lead (P ≤ 0.001). High impedance leads are associated with lower lead current drain than standard pacing leads in the atrium and ventricle for up to 1 year. No clinically important differences in sensing characteristics was noted with the high impedance leads in the atrium or ventricle compared to standard pacing leads. High impedance leads may result in increased pulse generator longevity.     

Lung volume calculated from electrical impedance tomography in ICU patients at different PEEP levels

Purpose  To study and compare the relationship between end-expiratory lung volume (EELV) and changes in end-expiratory lung impedance (EELI) measured with electrical impedance tomography (EIT) at the basal part of the lung at different PEEP levels in a mixed ICU population. Methods  End-expiratory lung volume, EELI and tidal impedance variation were determined at four PEEP levels (15–10–5–0 cm H₂O) in 25 ventilated ICU patients. The tidal impedance variation and tidal volume at 5 cm H₂O PEEP were used to calculate change in impedance per ml; this ratio was then used to calculate change in lung volume from change in EELI. To evaluate repeatability, EELV was measured in quadruplicate in five additional patients. Results  There was a significant but relatively low correlation (r = 0.79; R ² = 0.62) and moderate agreement (bias 194 ml, SD 323 ml) between ∆EELV and change in lung volume calculated from the ∆EELI. The ratio of tidal impedance variation and tidal volume differed between patients and also varied at different PEEP levels. Good agreement was found between repeated EELV measurements and washin/washout of a simulated nitrogen washout technique. Conclusion  During a PEEP trial, the assumption of a linear relationship between change in global tidal impedance and tidal volume cannot be used to calculate EELV when impedance is measured at only one thoracic level just above the diaphragm. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Evaluation of changes in cardiac output from the electrical impedance waveform in the forearm

We tested the validity of regional impedance cardiography (RIC) for measuring changes in both cardiac output and stroke volume by comparing the values with a 2D ultrasound technique in response to the breath-hold manipulation. Among 13 subjects, changes in the maximum amplitude of the regional impedance waveform from the forearm conformed to those in stroke volume (r = 0.86, p < 0.001) and cardiac output (r = 0.76, p < 0.003) measured with the ultrasound technique in baseline and immediately after a 30 s breath-hold maneuver. We also found that the per cent change in cardiac output (r = 0.73, p < 0.005) and the per cent change in stroke volume (r = 0.84, p < 0.0003) by the echocardiography were both positively correlated with the per cent change in the peak impedance amplitude. In addition, both the change and the per cent change in the mean area under the impedance curve were consistent with those in the stroke volume, respectively. Accordingly, the regional electrical impedance waveform from lower limbs may be helpful in providing a non-invasive and continuous assessment of left ventricular output, especially during cardiac procedures.     

A method for analyzing electrical impedance spectroscopy data from breast cancer patients

Research on freshly-excised malignant breast tissues and surrounding normal tissues in an in vitro impedance cell has shown that breast tumors have different conductivity and permittivity from normal or non-malignant tissues. This contrast may provide a basis for breast cancer detection using electrical impedance imaging. This paper describes a procedure for collecting electrical impedance spectroscopy data simultaneously and in register with tomosynthesis data from patients. We describe the methods used to analyze the data in order to determine if the electrodes are making contact with the breast of the patient. Canonical voltage patterns are applied and used to synthesize the data that would have resulted from constant voltage patterns applied to each of two parallel mammography plates. A type of Cole-Cole plot is generated and displayed from each of the currents measured on each of the electrodes for each of the frequencies (5, 10, 30, 100 and 300 kHz) of applied voltages. We illustrate the potential usefulness of these displays in distinguishing breast cancer from benign lesions with the Cole-Cole plots for two patients--one having cancer and one having a benign lesion--by comparing these graphs with electrical impedance spectra previously found by Jossinet and Schmitt in tissue samples taken from a variety of patients.     

Preliminary results from an EIT breast imaging simulation system

An electrical impedance tomography (EIT) simulation system for breast imaging has been developed using impedance data from a previous study and a finite-element model (FEM). This system has the functionality to construct various models of the breast, image the boundary voltages developed from any injection schema and provide parametric Cole-Cole modelling. Simulations indicate that breast carcinoma can be imaged and multi-frequency Cole-Cole dispersion data can be extracted. This is intended as the first stage towards providing an artificial intelligence based system capable of producing clinically relevant analytical data.     

Evaluation of algorithms for calculating bioimpedance phase angle values from measured whole-body impedance modulus

This paper addresses the problem of calculating the bioimpedance phase angle from measurements of impedance modulus. A complete impedance measurement was performed on altogether 20 healthy persons using a Solatron 1260/1294 system. The obtained impedance modulus (absolute impedance value) values were used to calculate the Cole parameters and from them the phase angles. In addition, the phase angles were also calculated using a Kramers-Kronig approach. A correlation analysis for all subjects at each frequency (5, 50, 100 and 200 kHz) for both methods gave R(2) values ranging from 0.7 to 0.96 for the Cole approach and from 0.83 to 0.96 for the Kramers-Kronig approach; thus, both methods gave good results compared with the complete measurement results. From further statistical significance testing of the absolute value of the difference between measured and calculated phase angles, it was found that the Cole equation method gave significantly better agreement for the 50 and 100 kHz frequencies. In addition, the Cole equation method gives the four Cole parameters (R(0), R(∞), τ(z) and α) using measurements at frequencies up to 200 kHz while the Kramers-Kronig method used frequencies up to 500 kHz to reduce the effect of truncation on the calculated results. Both methods gave results that can be used for further bioimpedance calculations, thus improving the application potential of bioimpedance measurement results obtained using relatively inexpensive and portable measurement equipment.     

Neuromagnetic field strength outside the human head due to impedance changes from neuronal depolarization

The holy grail of neuroimaging would be to have an imaging system, which could image neuronal electrical activity over milliseconds. One way to do this would be by imaging the impedance changes associated with ion channels opening in neuronal membranes in the brain during activity. In principle, we could measure this change by using electrical impedance tomography (EIT) but it is close to its threshold of detectability. With the inherent limitation in the use of electrodes, we propose a new scheme based on recording the magnetic field resulting from an injected current with superconducting quantum interference devices (SQUIDs), used in magnetoencephalography (MEG). We have performed a feasibility study using computer simulation. The head was modelled as concentric spheres to mimic the scalp, skull, cerebrospinal fluid and brain using the finite element method. The magnetic field 1 cm away from the scalp was estimated. An impedance change of 1% in a 2 cm radius volume in the brain was modelled as the region of depolarization. A constant current of 100 microA was injected into the head from diametrically opposite electrodes. The model predicts that the standing magnetic field is about 10 pT and changed by about 3 fT (0.03%) on depolarization. The spectral noise density in a typical MEG system in the frequency band 1-100 Hz is about 7 fT, so this places the change at the limit of detectability. This is similar to electrical recording, as in conventional EIT systems, but there may be advantages to MEG in that the magnetic field directly traverses the skull and instrumentation errors from the electrode-skin interface will be obviated.     

High-Frequency Acoustic Impedance Imaging of Cancer Cells

Variations in the acoustic impedance throughout cells and tissue can be used to gain insight into cellular microstructures and the physiologic state of the cell. Ultrasound imaging can be used to create a map of the acoustic impedance, on which fluctuations can be used to help identify the dominant ultrasound scattering source in cells, providing information for ultrasound tissue characterization. The physiologic state of a cell can be inferred from the average acoustic impedance values, as many cellular physiologic changes are linked to an alteration in their mechanical properties. A recently proposed method, acoustic impedance imaging, has been used to measure the acoustic impedance maps of biological tissues, but the method has not been used to characterize individual cells. Using this method to image cells can result in more precise acoustic impedance maps of cells than obtained previously using time-resolved acoustic microscopy. We employed an acoustic microscope using a transducer with a center frequency of 375 MHz to calculate the acoustic impedance of normal (MCF-10 A) and cancerous (MCF-7) breast cells. The generated acoustic impedance maps and simulations suggest that the position of the nucleus with respect to the polystyrene substrate may have an effect on the measured acoustic impedance value of the cell. Fluorescence microscopy and confocal microscopy were used to correlate acoustic impedance images with the position of the nucleus within the cell. The average acoustic impedance statistically differed between normal and cancerous breast cells (1.636 ± 0.010 MRayl vs. 1.612 ± 0.006 MRayl), indicating that acoustic impedance could be used to differentiate between normal and cancerous cells.     

Inappropriate mode switching in a dual chamber pacemaker due to oversensing of a high frequency signal from a conductor/ring discontinuity (loose set screw)

Two unusual cases are reported of pacemaker oversensing of the impedance pulse by the atrial channel attributed to a loose set screw.     

The influence of high versus normal impedance ventricular leads on pacemaker generator longevity

As pacemaker generator longevity is dependent on current consumption and resistance of the pacing lead, the use of a high impedance pacing lead theoretically results in an extension of battery longevity. Therefore, the effect of high versus standard impedance ventricular leads on generator longevity was studied. In 40 patients (21 women, age 73 +/- 13 years) with a standard dual chamber pacemaker indication, a bipolar standard impedance ventricular lead was implanted in 20 patients, the remaining patients received a bipolar high impedance lead in a randomized fashion. All patients received identical pacemaker generators and atrial leads. The estimated longevity of the generator was calculated automatically by a programmed pacemaker algorithm. After a mean follow-up of 39 +/- 4.8 months, no significant differences were observed with respect to mean pacing and sensing thresholds of the atrial and ventricular leads in both groups. However, the high impedance leads displayed a significantly higher impedance and a significantly lower current drain as compared to standard impedance leads (1,044 +/- 139 vs 585 +/- 90 Omega, and 2.2 +/- 0.4 vs 4.3 +/- 1.1 mA). The extrapolated generator longevity was significantly longer in the high impedance lead group, as compared to the standard impedance lead group (107.3 +/- 8.5 vs 97.6 +/- 9.0 months; P = 0.02). In conclusion, implantation of a high impedance lead for ventricular pacing results in a clinically relevant extension of generator longevity.     

Diuretic induced change in lung water assessed by electrical impedance tomography

Monitoring patients with left ventricular failure can be difficult. Electrical impedance tomography (EIT) produces cross-sectional images of changes in the impedance of the thorax. We measured changes in the electrical impedance of the lung in nine volunteers following a diuretic challenge. The hypothesis was that lung impedance would increase with diuretic induced fluid loss. Heart rate, blood pressure and urine output were also recorded. After diuretic the mean urine output was 1220 ml compared with 187 ml after placebo. Following diuretic administration, mean thoracic impedance increased by 13.6% (p < 0.01) and lung impedance increased by 7.8% (p < 0.05). Taken as a group there was a correlation between overall impedance change and total urine output. However, for each individual, the time course of change in impedance and urine output did not correlate significantly. Our findings show that EIT may offer a better guide to the response of the lung to diuretic treatment than simply measuring urine output. The urine output is neither specific nor sensitive in the assessment of lung water. Mean lung impedance, however, is largely determined by lung water. The study showed that lung impedance can be recorded at supra-normal values. EIT may help in the management of patients with excess lung water.     

Three-dimensional electrical impedance tomography of human brain activity

Regional cerebral blood flow and blood volume changes that occur during human brain activity will change the local impedance of that cortical area, as blood has a lower impedance than that of brain. Theoretically, such impedance changes could be measured from scalp electrodes and reconstructed into images of the internal impedance of the head. Electrical Impedance Tomography (EIT) is a newly developed technique by which impedance measurements from the surface of an object are reconstructed into impedance images. It is fast, portable, inexpensive, and noninvasive, but has a relatively low spatial resolution. EIT images were recorded with scalp electrodes and an EIT system, specially optimized for recording brain function, in 39 adult human subjects during visual, somatosensory, or motor activity. Reproducible impedance changes of about 0.5% occurred in 51/52 recordings, which lasted from 6 s after the stimulus onset to 41 s after stimulus cessation. When these changes were reconstructed into impedance images, using a novel 3-D reconstruction algorithm, 19 data sets demonstrated significant impedance changes in the appropriate cortical region. This demonstrates, for the first time, that significant impedance changes, which could form the basis for a novel neuroimaging technology, may be recorded in human subjects with scalp electrodes. The final images contained spatial noise and strategies to reduce this in future work are presented.     

Impedance to Defibrillation Countershock: Does an Optimal Impedance Exist?

Defibrillation is thought to occur because of changes in the transmembrane potential that are caused by current flow through the heart tissue. Impedance to electric countershock is an important parameter because it is determined by the magnitude and distribution of the current that flows for a specific shock voltage. The impedance is comprised of resistive contributions from: (Ij extra-tissue sources, which include the defibrillator, leads, and electrodes; (2) tissue sources, which include intracardiac and extra-cardiac tissue; and (3) the interface between electrode and tissue. Tissue sources dominate the impedance and probably contribute to the wide range of impedance values presented to the defibrillation pulse. Hucause impedance is not constant within or between subjects, defibrillators must be designed to accommodate these differences without compromising patient safety or therapeutic efficacy. Experimental investigations in animals and humans suggest that impedance changes at several different time scales ranging from milliseconds to years. These alterations are believed to be a result of both electrochemical and physiological mechanisms. It is commonly thought that impedance is optimized when it has been decreased to a minimum, since this allows the most current flow for a given voltage shock. However, if the impedance is lowered by changing the location or size of the electrodes in such a way that current flow is decreased in part of the heart even though current flow is increased elsewhere, then the total voltage, current, and energy needed for defibrillation may increase, not decrease, even though impedance is decreased. A simple boundary element computer model suggests that the most even distribution of current flow through the heart is achieved for those electrode locations in which the impedance across the heart is at or near the maximum cardiac impedance for any location of these particular electrodes. Thus, the optimum shock impedance is achieved when impedance is minimized for extra-tissue and extra-cardiac tissue sources and is at or near a maximum for intracardiac tissue sources.     

心阻抗图检测舒张期主动脉向外周供血量的初步探讨

目的探讨心舒张期主动脉向外周的供血量。方法基于心阻抗图非线性理论，导出舒张期流向外周的供血量（Ｖｄ）、流量（ＦＱ）、流量指数（ＦＱＩ）的计算公式，分别测量心衰组（ｎ＝５２）和正常对照组（ｎ＝５０）的Ｖｄ、ＦＱ、ＦＱＩ。结果心衰组的Ｖｄ、ＦＱ、ＦＱＩ非常显著地低于对照组（Ｐ＜０．０１），但收缩期向外周的供血量下降不多。结论患者心输出量的减少，对收缩期向外周的供血量影响不大，而主要表现为舒张期向外周供血的减少。     

Descending Aortic Flow Contribution to Intrathoracic Impedance—Development and Preliminary Testing of a Dual Impedance Model

Objective Impedance measurement of cardiac output has struggled to become established partly because there have been only a few attempts to establish a sound theoretical basis for this measurement. Our objective is to demonstrate that there is valuable aortic flow information available from an intrathoracic impedance signal which may eventually be useful in the measurement of cardiac output by impedance technology. Methods A model, using dual impedance measurement electrodes and the change in impedance when blood flows, has been developed based on an intrathoracic impedance model of the descending aorta and esophagus. Using this model as the basis for measurement by an esophageal probe, we provide solutions to the velocity of blood flow in the descending aorta. Results Five patients were studied. Only three patients had suitable signals for analysis but the aortic flow profiles from these three patients were consistent and realistic. Conclusion Aortic blood flow information may be obtained from the intrathoracic impedance signal using this dual impedance method. Baker AB, McLeod CN, Roxburgh AJ, Bannister P. Descending aortic flow contribution to intrathoracic impedance—development and preliminary testing of a dual impedance model.     

Effect of cooling and re-warming on cerebral and whole body electrical impedance

Cerebral electrical impedance is useful for the detection of cerebral edema following hypoxia in newborn infants. Thus it may be useful for determining neurological outcome or monitoring treatment. Hypothermia is a promising new therapy currently undergoing trials, but will alter impedance measurements. This study aimed to define the relationship between temperature and both cerebral and whole body electrical impedance, and to derive correction factors for adjustment of impedance measurements during hypothermia. In eight anaesthetized 1-2 day old piglets rectal, tympanic and scalp temperatures were monitored continuously. Following baseline readings at a rectal temperature of 39 degrees C, piglets were cooled to 32 degrees C. Four piglets were re-warmed. Cerebral and whole body impedance were measured at each 0.5 degrees C as rectal temperature decreased. There was a strong linear relationship between both cerebral and whole body impedance and each of the temperatures measured. There was no difference in the relationship between impedance and rectal, tympanic or scalp temperatures. The relationship for impedance and rectal temperature was the same during cooling and re-warming. Using the correction factors derived it will be possible to accurately monitor cerebral and whole body fluid distribution during hypothermic treatment.     

120例心脏起搏系统阻抗的术中检测及其与起搏阈值的关系

起搏器安装或更换术中测试了120例起搏系统阻抗并将其与同时测试的起搏阈值进行了对照分析。结果显示,急性期阻抗值为274Ω至1014Ω,平均621·02Ω,随阻抗值增大而有起搏阈值的减小。慢性期阻抗值362Ω至672Ω,平均484·75Ω,较急性期者显著降低,提示慢性期随起搏阈值增高,阻抗自行调整而降低以维持起搏器电压相对衡定输出。本文结论是,起搏系统阻抗的显著降低应作为更换起搏器的参考指标之一。