Empedans pletismografisi yöntemiyle kan akış hacminin ve kalbin fizyolojik parametrelerinin bilgisayar destekli ölçümü

Abstract

Bu çalışmada Empedans Pletismografisi yöntemini kullanarak kol ve bacak uzuvlarındaki kan akış hacminin ve kalbin fizyolojik parametrelerinin bilgisayar yardımıyla ölçülmesi için gerçekleştirilen bir insan- enstrümantasyon sistemi uygulaması tanıtılmaktadır. Birinci bölümde, kalp debisi ve vuru hacminin tanımları yapılmakta ve bu parametrelerin ölçümünün klinik uygulamadaki önemi belirtilmektedir. ikinci bölümde, kan akış hacminin, kalp debisinin ve vuru hacminin ölçülmesi için günümüzde yaygın olarak kullanılan yöntemlerden bahsedilmektedir. Üçüncü bölümde, Empedans Pletismografisi yöntemi kullanılarak kan akış hacminin ölçülmesi amacıyla gerçekleştirilen bilgisayar destekli bir insan- enstrümantasyon sistemi uygulaması tanıtılmakta ve bu amaçla gerçekleştirilen veri işleme programı algoritması anlatılmaktadır. Sonuç olarak, sistemin uygulama alanlarından bahsedilmekte ve önemi belirtilmektedir. Dördüncü bölümde, Empedans Kardiyografisi yöntemi ile kalbin fizyolojik parametrelerinin ölçülmesi için geliştirilen bilgisayar destekli bir sistem ve bu sistemi kontrol eden veri işleme programı algoritmaları tanıtılmaktadır. Ayrıca yöntemin diğer yöntemlerle bir karşılaştırması yapılmakta ve klinik uygulamadaki yeri belirtilmektedir.

Impedance plethysmography is defined as the measurement of physiologic volume changes through the measurement of electrical impedance. öbjective evaluation of volume changes is important in the management of many medical problems. Patient safety and comfort are increased when these measurements are made by a noninvasive technique, and the cost of testing is reduced. Plethysmographs based on air ör fluid displacement are difficult to use, limiting their application f ör routine clinical testing. impedance plethysmography, on the other hand, is simple and not difficult to use. Much of the controversy concerning the relationship between electrical impedance and blood volume was the result of studies in which a single pair of spot elecrodes was used. it is now recognized that a four electrode (tetra-polar) technique, which is used in this study, is necessary to accurately guantify electric impedance changes from skin electrodes. in this method, a constant magnitude high freguency eletric current is passed between two electrodes and the resultant voltage is sampled at between two electrodes. This arrangement effectively eliminates skin impedance and reduces the sensitivity to events close to the electrodes. in impedance plethysmography, the electrodes are applied to the skin över the body region of interest. in the extremities like legs ör arms, electrodes are usually applied as circumferential bands. A high freguency current is passed through the outer (current) electrode pair. Freguencies between 10 and 100 kHz with a vi magnitude of a few milliamperes ör less are generally employed. Thus the current magnitude is too low to be perceived by the patient and the frequency is too high to stimulate the muscles ör heart. The phase angle of the eiectrical impedance, which is the arctangent of the ratio of the reactive component to the resistive component, is less then 12° f ör the frequency range in which most instruments operate. For this reason, nonresistive effects are small, and, f rom an eiectrical point of view, the body can be modeled as a set of purely resistive elements. The validity of impedance plethysmography is dependent on demonstration of a predictable relationship between a volume change (/W) and the corresponding eiectrical impedance change (AZ). Assuming that the segmental blood volume change (AV) could be modeled as the impedance change (AZ) electrally parallel with the basal tissue impedance (Zo), is led to the well-known " Nyboer Formula ", Av = -(g*L2*Az)/Zo2 in this expression, g is the resistivity of the blood (in ohm.cm) for the packed celi volume that exists, and Zo is the basal tissue impedance between potential measuring electrodes, which are L cm apart. Two computer aided applications of impedance plethysmography are discussed in this study. The objective of these studies was to further explore the validity and applicability of signal averaging of impedance waveforms and specifically to compare volume changes calculated by the ensemble averaging and the standard hand-digitizing method on the same impedance records from normal subjects at rest. Volume changes in the extremities are calculated in the first application of this study. The purpose of this study was to develop a pre-diagnostic system that uses the signal averaging of the waveforms and to compare volume changes calculated by the ensemble averaging and the standard hand-digitizing method on the same impedance records from normal subjects at rest. vii The two sub.jects were healthy females, aged 26-29 and weights 50 kg. Tests were run in istanbul in May wîth typical temperatures of 22°C and relative humidity between 80-85 percent. in the two subjects, impedance plethysmographic measurements were made during seated rest and impedance electrodes were attached on arms. Ali hemodynamic data were acquired by a human instrumentation system developed in the Medical Electronics Laboratory of the istanbul Technical University. The system consists of an impedance plethysmograph, a monitör oscilloscope, which are the units of an eight channel poligraph, a heat writing recorder, a data acquisition system and a personal computer. Analog/Digital converter unit of the data acguisition system was used f ör the experiments. This system is used with a personal computer which contains a software package för data acguisition. The system is run by an application software including signal processing algorithms, developed under the aid of the package för the tests. Test procedures started with the calibration of the impedance plthysmograph. This unit was calibrated för the values of 50 kHz and 350 uA. Af ter the subject was attached to the system by means of the band electrodes, the data acguisition system was configured by the operatör, using the program developed. The constant parameters g, Zo and L, defined in the Nyboer Formula, were entered by the operatör before the tests. The impedance plethysmograph was then run. The subject was then asked to hold his breath at the end of normal expiration f ör 10 to 15 seconds. After the impedance record had become relatively stabilized (in 5-7 s), the acguisition was started by the operatör. Sampling was done at 500 Hz. 2500 points of data were sampled and digital values continually were stored in a binary array opened in the buffer of the computer. After the scaling of the binary data to voltage, the real values were represented on the monitör of the personal computer. Af ter the acceptance of values by the operatör, the signal processing algorithms were run and, the volume change was calculated for each period by using the Nyboer Formula and an ensemble average was obtained automatically. Calculations were also made manually. viii The signals on the heat writing recorder were also examined for the calculation of cardiac output by using hand digitizing method, and the results were compared. As a result, the data denote good agreement between the two methods. Direct or invasive methods of cardiovascular monitoring by catheterization provide valuable information in general, but the obvious risks associated with invasive techniques limits its use to hospitalized patients and has been a key factor in the search for a simple, inexpensive, and atraumatic technique for the study of heart and blood vessels. This situation is especially important for use with neonates, infants, pregnant women, outpatients, and volunteers. The electrical impedance technique, used in this study, is very simple to use and highly reproducible. As a transducer it offers a convinient, continious, and noninvasive means of acquiring information concerning cardiac and other variables of the cardiovascular system, especially when a knowledge of the trends is important rather then the absolute values. Unfortunately, at present, impedance cardiography is not commonly used in the hospitals. The claims and controversies about stroke volume and cardiac output measurements among the early investigators were detrimental to the growth of this simple technique. Accepting that the method suffers from limitations that are common to other noninvasive techniques of monitoring, its ease of use is attractive in a number of clinical situations. Started from a simple model ( "Nyboer Formula" ) and emprical relationships (Kubicek expression), impedance changes are now being explained by detailed investigation into the origin of the impedance cardiogram. On the basis of this new insight, the importance of the differentiated impedance cardiogram is emphasized in clinical practice. With recent improvements in instrumentation and understanding of the advantages and limitations of impedance cardiography, it is hoped that this simple technique can be used as an adjunct to ECG to provide vital cardiac information in the intensive care unit and other clinical areas. XI During an operation, the anesthesiologist monitors the health of the patient. In order to measure cardiac output (flow), a catheter tube is threaded through a vein in the arm into the heart. Ten millimeters of cold saline is injected at the entrance of the heart. But to measure cardiac output, it is desirable not to invade the body. 3y the impedance cardiography method, we can measure voltage between electrodes on the neck and chest. From the resulting electrical impedance we can calculate stroke volume, and the cardiac output. In the second application of this study, the physiological parameters of the heart were determined by a human- instrumentation system, which is mentioned before, with additional devices. The objective of this study was to explore the validity and applicibility of signal differentiating and averaging of impedance waveforms and to compare SV calculated by ensemble averaging and the standard hand digitizing methods on the same impedance records from normal subjects at rest. In four healthy volunteer male subjects, 23-26 yr of age, impedance cardiographic measurements were made during seated rest. Tests were run in istanbul in April with typical temperatures of 18 °C and relative humidity between 80-85 percent. In addition to impedance plethysmgraph unit in the measurement system mentioned before, a differentiator, an ECG amplifier and a PCG amplifier units were used for simultaneously recording. All hemodynamic data were simultaneously recorded on a four channel heat writing recorder and only impedance change signals were acquired in the way which has been described for impedance plethysmography before. After the data of volume change for 5 s were acquired by the data acquisition system, an algorithm for differentiation of the digital data was run and the algorithms for signal processing were applied for automatically calculation. Kubicek's expression was used in determining of the stroke volume and cardiac output. In this expression, for each period, dz/dW&x was recorded as the deviation (Q/s) registered between isoelectric base line and the minimum peak of dz/dt wave form, T was measured as the interval between the indicies of the %15 of the minimum peak and the maximum peak. After the parameters were obtained for each period, an average was determined. These calculations were also made manually for comparison. The signals on the heat writing recorder were also examined for the calculation of cardiac output by using hand digitizing method, and the results were compared. As a result, the data denote good agreement between the two methods. Direct or invasive methods of cardiovascular monitoring by catheterization provide valuable information in general, but the obvious risks associated with invasive techniques limits its use to hospitalized patients and has been a key factor in the search for a simple, inexpensive, and atraumatic technique for the study of heart and blood vessels. This situation is especially important for use with neonates, infants, pregnant women, outpatients, and volunteers. The electrical impedance technique, used in this study, is very simple to use and highly reproducible. As a transducer it offers a convinient, continious, and noninvasive means of acquiring information concerning cardiac and other variables of the cardiovascular system, especially when a knowledge of the trends is important rather then the absolute values. Unfortunately, at present, impedance cardiography is not commonly used in the hospitals. The claims and controversies about stroke volume and cardiac output measurements among the early investigators were detrimental to the growth of this simple technique. Accepting that the method suffers from limitations that are common to other noninvasive techniques of monitoring, its ease of use is attractive in a number of clinical situations. Started from a simple model ( "Nyboer Formula" ) and emprical relationships (Kubicek expression), impedance changes are now being explained by detailed investigation into the origin of the impedance cardiogram. On the basis of this new insight, the importance of the differentiated impedance cardiogram is emphasized in clinical practice. With recent improvements in instrumentation and understanding of the advantages and limitations of impedance cardiography, it is hoped that this simple technique can be used as an adjunct to ECG to provide vital cardiac information in the intensive care unit and other clinical areas.