Development of a miniature and ASIC based impedance cardiograph

Abstract

In view of cardiac output monitoring application of impedance cardiography a miniature impedance cardiograph has been developed, which is based on an application specific integrated circuit (analog front end chip AFE4300). Carrier current capability of this chip has been enhanced four times (up to 3.2 milliamperes) for cardiology applications with the help of an external resistance, not provisioned in the original design. Also the sensed signal is externally amplified before feeding to AFE chip to increase resolution in the impedance range of 10–50 Ohms.

The AFE chip is interfaced to a Personal Computer with the help of a microcontroller board through a USB cable. Application software programs the AFE chip for sending a carrier current of 3.2 milliamperes (peak to peak) to the subject’s body segment through current output ports and sensing the chest impedance through voltage input ports. The chest impedance is read in detector mode and communicated to Laptop through serial peripheral interface of the chip. Chest impedance, change in impedance as a function of time and rate of change of impedance are displayed on the graphic user interface. Fifteen minute data from volunteers have shown consistent recordings, as described in this paper.     

New ambulatory impedance cardiograph validated against the Minnesota Impedance Cardiograph

The validity and reliability of a new ambulatory impedance cardiograph (AZCG) was tested against the Minnesota Impedance Cardiograph (ZCG) during rest, orthostasis, and mental stress. Impedance cardiography allows noninvasive assessment of stroke volume, cardiac output, and systolic time intervals. A reliable ambulatory device would allow studies outside the lab. The devices were compared at two sites in healthy subjects. In both studies, the AZCG tracked changes across conditions closely with the ZCG (all Period x Device interactions were nonsignificant). Pearson rs, were .65 to .93, random intraclass correlation coefficients ranged from .80 to .98, indicating high degrees of shared measurement variance, and Cronbach's alpha indicated very good internal reliabilities (.91 to .99). Relative to the ZCG, the new AZCG appears to provide valid and reliable estimates of cardiac function at rest and during behavioral challenges in the lab.     

Non-invasive cardiac output evaluation during a maximal progressive exercise test, using a new impedance cardiograph device

One of the greatest challenges in exercise physiology is to develop a valid, reliable, non-invasive and affordable measurement of cardiac output (CO). The purpose of this study was to evaluate the reproducibility and accuracy of a new impedance cardiograph device, the Physio Flow, during a 1-min step incremental exercise test from rest to maximal peak effort. A group of 12 subjects was evaluated to determine the reproducibility of the method as follows: (1) each subject performed two comparable tests while their CO was measured by impedance cardiography using the new device (CO_Imp1, CO_Imp2), and (2) in a subgroup of 7 subjects CO was also determined by the direct Fick method (CO_Fick) during the second test. The mean difference between the values obtained by impedance (i.e. CO_Imp1–CO_Imp2) was –0.009 l·min^–1 (95% confidence interval: –4.2 l·min^–1, 4.2 l·min^–1), and CO ranged from 3.55 l·min^–1 to 26.75 l·min^–1 (n=146). When expressed as a percentage, the difference (CO_Imp1–CO_Imp2) did not vary with increasing CO. The correlation coefficient between the values of CO_Imp and CO_Fick obtained during the second exercise test was r=0.94 (P<0.01, n=50). The mean difference expressed as percentage was –2.78% (95% confidence interval: –27.44%, 21.78%). We conclude that CO_Imp provides a clinically acceptable evaluation of CO in healthy subjects during an incremental exercise. Electronic Publication     

ECGLAB-H-A型动态心电图分析系统临床应用

ECGLAB-H-A型动态心电图分析系统采用符合国际标准十二导联体系，与常规十二导联心电图标准完全一致，24h不间断记录心电图。本文就我院应用ECGLAB-H-A型动态心电图分析系统检查40例病人，与检测前的常规心电图进行对比分析，其结果动态心电图分析系统对严重心律失常检出率比常规心电图检出率增加30％，根据动态心电图中ST-T改变不同来鉴别心肌缺血还是冠状动脉供血不足。     

High-sensitivity C-reactive protein, adiposity, and blood pressure in the Yakut of Siberia.

C-reactive protein (CRP), an acute-phase reactant and marker of inflammatory response, is known to be an important predictor of future cardiovascular mortality, independent of other risk factors. The purpose of this research was to investigate the association between CRP, adiposity, and blood pressure in the Yakut, an indigenous Siberian population undergoing rapid cultural change. We conducted a cross-sectional study of 265 healthy Yakut adults in six villages in rural northeastern Siberia. Plasma CRP was measured by high-sensitivity immunoturbidimetric assay. The median CRP value was 0.85 mg/l, with values for the 25th, 50th, and 75th percentiles of 0.30, 0.85, and 2.28 mg/l, respectively. CRP was positively associated with age (r = 0.19; P = 0.002), but not plasma lipids or smoking status. CRP was associated with measures of central adiposity and characteristics of the metabolic syndrome, particularly in women. We found significantly higher CRP across quintiles (Q) of waist circumference for women (difference = 0.7 mg/l; P = 0.035), but not men (difference = 0.36 mg/l; P = 0.515). CRP was significantly associated with systolic blood pressure in men (difference, Q1 vs. Q5 = 1.1 mg/l; P = 0.044) but not women (difference, Q1 vs. Q5 = 0.03 mg/l; P = 0.713) after adjusting for age, waist circumference, and smoking status. CRP in the Yakut was considerably lower than was reported for other populations. The low CRP levels may be explained in part by a low prevalence of abdominal obesity. Among the Yakut, the high physical-activity demands of a traditional herding lifeway likely play a role through high energy expenditure and maintenance of negative energy balance. Our findings underscore the need for further research on the metabolic activity of adipose tissue, blood pressure, and inflammatory activation in non-Western populations.     

Passive,wireless transduction of electrochemical impedance across thin-film microfabricated coils using reflected impedance

A new method of wirelessly transducing electrochemical impedance without integrated circuits or discrete electrical components was developed and characterized. The resonant frequency and impedance magnitude at resonance of a planar inductive coil is affected by the load on a secondary coil terminating in sensing electrodes exposed to solution (reflected impedance), allowing the transduction of the high-frequency electrochemical impedance between the two electrodes. Biocompatible, flexible secondary coils with sensing electrodes made from gold and Parylene C were microfabricated and the reflected impedance in response to phosphate-buffered saline solutions of varying concentrations was characterized. Both the resonant frequency and impedance at resonance were highly sensitive to changes in solution conductivity at the secondary electrodes, and the effects of vertical separation, lateral misalignment, and temperature changes were also characterized. Two applications of reflected impedance in biomedical sensors for hydrocephalus shunts and glucose sensing are discussed.     

血浆同型半胱氨酸、超敏C反应蛋白与脑部大动脉粥样硬化性狭窄的相关性

目的探讨血浆总同型半胱氨酸（homocysteine，Hcy）、超敏C反应蛋白（high-sensitivity C-reactive protein，hs-CRP）水平与脑大动脉粥样硬化性（atherosclerosis，AS）狭窄的关系。方法对行数字减影全脑血管造影（digital subtraction angiography，DSA）的94例缺血性脑血管病患者进行分组，其中具有颅内外大动脉粥样硬化性狭窄（至少1支血管狭窄率〉50％）的68例患者为狭窄组，无狭窄或轻度狭窄（狭窄率≤50％）患者26例为对照组。对两组患者血浆总Hcy水平、hs-CRP及脑卒中的常规危险因素如年龄、性别、高血压、糖尿病、长期吸烟、甘油三酯（TG）、高密度脂蛋白胆固醇（HDL-C）、低密度脂蛋白胆固醇（LDL-C）水平等进行了分析。结果狭窄组血浆Hcy及hsCRP水平及升高的比例明显大于对照组；多元回归分析显示Hcy、hsCRP、高血压、糖尿病、吸烟与脑大动脉狭窄密切相关；血浆Hcy及hsCRP随大动脉硬化性狭窄程度加重而增高。结论血浆Hcy及hs-CRP水平升高与脑部动脉粥样硬化性狭窄密切相关，是脑大动脉粥样硬化性狭窄病变形成的危险因子。     

细菌性肺炎患者血清降钙素原和超敏C反应蛋白水平分析

目的:分析血清降钙素原(PCT)和超敏C反应蛋白(hs-CRP)水平与细菌性肺炎的关系。方法:按严重程度综合评分将127例下呼吸道细菌性肺炎患者(细菌性肺炎组)分为Ⅰ级40例,Ⅱ-Ⅲ级49例,Ⅳ-Ⅴ级38例;采用荧光免疫分析技术测定血清PCT水平,采用全自动免疫比浊法测定血清hs-CRP浓度,统计学分析其与35例下呼吸道非细菌性肺炎患者(非细菌性肺炎组)及20例健康体检者(对照组)的差异及细菌性肺炎严重程度与PCT和hs-CRP的相关性。结果:与对照组和非细菌性肺炎组相比,细菌性肺炎组血清PCT、hs-CRP水平均显著增高(P0.01);且细菌性肺炎严重程度与患者血清PCT、hs-CRP水平呈显著正相关(r分别为0.993、0.976,P均0.05)。结论:血清PCT和hs-CRP水平升高有助于鉴别下呼吸道细菌性和非细菌性感染,还能反映细菌性肺炎的严重程度。     

A sensitive,high impedance cardiac rhythm follower

This apparatus, which is designed for use during animal operations, allows the cardiac rhythm to be followed with the aid of sound pulses. These pulses are synchronized with the R wave of the Q.R.S. complex of the electrocardiogram. The employment of microelectrodes for the detection of the signal has required a high input impedance of the oder of 10⁹ Ω. On the other hand, because of the amplifier pass-band (0·2 Hz–100 kHz), an output has been obtained which has allowed the electrocardiogram to be recorded.     

Factors affecting electrode-gel-skin interface impedance in electrical impedance tomography

The magnitude, mismatch and temporal variations of the electrode-gel-skin interface impedance can cause problems in electrical impedance tomography (EIT) measurement. It is shown that at the high frequencies generally encountered in EIT the capacitive properties of the electrode interface, and especially those of the skin, are of primary importance. A wide range of techniques are reviewed that could possibly be used to minimise these problems. These techniques include the use of skin preparation, penetration enhancers, temperature and electrical impulses. Although several of these techniques appear very attractive, they are not without serious potential drawbacks. A combination of some of these techniques may well hold the key to success.     

Electrical impedance endotomography

In electrical impedance endotomography (EIE), the impedance measuring electrodes are placed at the centre of the region of interest instead of encircling it, as in usual electrical impedance tomography. EIE has been developed for prostate imaging. The developed mathematical model enables the derivation of analytical equations for electric potential, electric field and sensitivity. This paper focuses on the selection of an optimal current injection method capable of sensing distant points. Experimental measurements were carried out in vitro using an enlarged size, 50 mm diameter, 16-electrode mock-up probe placed in tap water. The collected data enabled the production of images of a circular insulating target, 10 mm in diameter located 100 mm from the axis of the probe. Future work includes extension of sensitivity range, improvement of image reconstruction, design of a multiple frequency hardware system and construction of a real-size, biocompatible impedance probe.     

Real-time extraction of tissue impedance model parameters for electrical impedance spectrometer

This paper presents a new algorithm for real-time extraction of tissue electrical impedance model parameters from in vivo electrical impedance spectroscopic measurements. This algorithm was developed as a part of a system for muscle tissue ischemia measurements using electrical impedance spectroscopy. An iterative least square fitting method, biased with a priori knowledge of the impedance model was developed. It simultaneously uses both the real and imaginary impedance spectra to calculate tissue parameters R₀, R_∞, α and τ. The algorithm was tested with simulated data, and during real-time in vivo ischemia experiments. Experimental results were achieved with standard deviations of , σ_α=0.72%, and σ_τ=1.26%. On a Pentium II based PC, the algorithm converges to within 0.1% of the results in 17 ms. The results show that the algorithm possesses excellent parameter extraction capabilities, repeatability, speed and noise rejection.     

Real-time extraction of tissue impedance model parameters for electrical impedance spectrometer

This paper presents a new algorithm for real-time extraction of tissue electrical impedance model parameters from in vivo electrical impedance spectroscopic measurements. This algorithm was developed as a part of a system for muscle tissue ischemia measurements using electrical impedance spectroscopy. An iterative least square fitting method, biased with a priori knowledge of the impedance model was developed. It simultaneously uses both the real and imaginary impedance spectra to calculate tissue parameters R0, R infinity, alpha and tau. The algorithm was tested with simulated data, and during real-time in vivo ischemia experiments. Experimental results were achieved with standard deviations of sigma R0 = 0.80%, sigma R infinity = 0.84%, sigma alpha = 0.72%, and sigma tau = 1.26%. On a Pentium II based PC, the algorithm converges to within 0.1% of the results in 17 ms. The results show that the algorithm possesses excellent parameter extraction capabilities, repeatability, speed and noise rejection.     

Theoretical pulmonary microvascular impedance

The basic equations governing transient oscillations in capillary blood vessels of a pulmonary alveolar sheet are presented. It follows from the nonlinearity of the differential equations that the concept of impedance is useful only in the case of small perturbations. Analytical and numerical results are given for input impedances, impedance matrix, conductance matrix, and transfer matrix for small pressure and flow oscillations. It is shown that the determinant of the transfer matrix is in general unequal to 1 for the pulmonary sheet; thus differing from electric circuits with lumped parameters.