Biyotelemetri sistemi

Abstract

Bu çalışma Biyotelemetri sisteminin EKG kısmını oluşturmaktadır. Sistem pratik olarak gerçeklenmektedir. Sistem genel olarak beş kısımdan oluşmaktadır. İlk kısımda; yüzey elektrodları ile EKG işaretleri algılanmaktadır. Bu işaretler sağ kol (RA),sol kol (LA), sol bacak (LF) derivasyonlandır; bunlar Einthoven üçgeninde L II, HI nolu bipolar derivasyonları ve frontal düzlemdeki aVR, aVL, aVF unipolar derivasyonlarmı (Goldberger) oluşturmak için kullanılmaktadır. İkinci kısımda; sağ bacak sürücüsü kullanılmakta ve bu RA, LA, LF'den alınan işaretler kalbin vektör açısı bozulmayacak şekilde 3 eşit direnç ile sürücü üzerinden sağ bacağa verilir; sürücünün görevi vücut üzerine şebeke veya diğer elektromanyetik etkilerden dolayı ortak modda oluşan gerilimlerin meydana getirdiği kaçak akımların sağ bacak sürücüsündeki direnç sayesinde düşük akımlara dönüştürülerek vücudun oluşacak akımlardan etkilenmesi önlenmiş olmaktadır. Üçüncü kısımda; filtreler, kuvvetlendiriciler ve offset ayar devresi bulunmaktadır. İlk kat Kİ =40 kazançlı olup, filtreler ise önce alçak geçiren filtre olup üst kesim frekansı 1 10 Hz ve bu katm kazancı K2 = 2 seçilmiştir. Üçüncü katta çentik (Notch) filtre konulup frekansı 50 Hz'e ayarlanmıştır, kazancı ise K3 = l'dir. Dördüncü kat K4 = 6.23 kazançlı olarak düzenlenmiş; beşinci kata yüksek geçiren filtre konulup bunun kesim frekansı 0.34 Hz olarak düzeıüenmistir; burada önceki katlardan gelen DC offset kaymalar da önlenmektedir. Altıncı katta ise K6 = 2 kazançlı ve tüm kanallar için +2.5 Voltluk pozitif yönde offset kayma sağlayacak offset devresi düzenlenmiştir. Bu sayede her kanalda toplam K = 1993.6lık kazanç sağlanmıştır. Tepeden tepeye 2 mV olan EKG işaretleri bu kuvvetlenme ile yaklaşık 4V seviyesine yükselmiştir. H8/532'nin ADC girişleri 0-5 V arası çalıştığından 2.5 V'luk pozitif yöndeki offset öteleme,işaretin 0.5 V ile 4.5 V arasında H8'in ADC girişlerinde gözükmesine neden olmaktadır. Dördüncü kısmı; H8/532 16 bit mikrodenetleyicisi ve bunun çevre birimlerinden oluşmaktadır. Burada H8'in görevi 3 kanaldan gelen bilgileri dijitale çevirmek ve bunları seri çıkışından vermektir. Yani burada her kanal 8 bit' e çevirilip Ff/8'in seri çıkışından her kanala bir start bir de stop biti eklenerek gönderilmektedir. Senkronizasyonu sağlamak için 1, 2 ve 3 nolu kanalların önüne sıfir bigjsi içeren sıfir byte'ı eklenerek bilgisayarın RS-232'sine 9600 baud'luk bir hızla gönderilmektedir. Beşinci kısım; PC bilgisayar tarafında H8'den gelen bilgilerin porttan okunması, kodlanması, I, İL Di, aVR, aVL, aVF derivasyonlannm elde edilmesi ve bu derivasyonlarin real time süpürme (sweep) grafiği ile monitörda gösterilmesinden oluşmaktadır. Hasta isimleri girilerek ekrandaki gerekli istenen grafikler hard diskte IV depolanmakta ve bunlar istenildiğinde tekrar hard diskten çağrılarak ekranda gösterilmektedir.

Biomedical Telemetry is a special area of biomedical instrumentation that pennits transmission of physiologic information from an often inaccessible location to a remote site. The goals of biotelemetry include the capability for monitoring humans and animals with minimum restraint and to provide faithful reproduction of the transmitted data. Although some telemetry of physiologic is done via telephone lines, the majority is carried via radio link. The encoding of physiologic data into some unique format is common to ali biotelemetry systems. Formats differ according to system performance requirements, the number of required data channels, size, and cost. The transmitting unit can be carried outside the monitored subject as a backpack unit ör can be implanted \vithin the subject's body after appropriate miniaturization and sealing against body fluid. Thanks to the innovativenes and resourcefulness of investigators, who since the 1950s have developed biomedical telemetry to the useful and flexible instrumentation tool it is today, biotelemetry method are found in many areas of medical research and clinical monitoring. Biomedical telemetry offers complete electrical patient isolation and ambulatory freedom m the clinical monitoring setting. With telemetry portable emergency çare units, physiologic data such as ECGs can be communicated to hospital base stations from remote emergency sites. The freedom from attached wires and physical restraint is often desirable in human and animal research. Biomedical telemetry has provided this freedom in a very broad and diverse spectrum of experiments and animal species. Human stress and exercise physiology, pH, CSF pressure, fetal and meternal physiology, and many others have benefitted from the technique. Animals reported to have been monitored with biotelemetry include cockroaches, Hzards, fishsnakes, seals, birds, elk, girafFes,dolphins horses, and türdeş in the wild and dogs, cats, rats, rabbits, monkeys, and baboons in. the laboratory. Morover, virtually physiologic parameter has been monitored vvith biomedical telemetry. Electrocardiogram (ECG), electromiyogram (EMG), hidrogen ion consantration (pH), temperature, pressure, muscle contractile forces, gait, and blood flow are the phiysiologic data that are most frequently transmitted vvith the benefits of biomedical telemetry. Electrical Activity of excitable cells Bioelectric potentials are produced as result of electrochemical activity of a certain class of cells, knovvn as excitable cells, that are components of nervous, muscular, ör glandular tissue. Electrically they exhibit a resting potential, and, vvhen appropriately stimulated, an action potential, as we shall explained in the following paragraphs. vi The resting state The individual excitable celi maintains a steady electrical potential difference between its intemal and external environments. This resting potential of the intenıal medium lies in the range - 50 to -100 mV, relative to the external medium. The celi membrane is a very thin (7-15 nm) lipoprotein complex that is essentially impermeable to intracelluar protein and other organic anions (A"). The membrane in the resting state is moderately permeable to Na+ and rather freely permeable to K+ and Cl". The permeablity of the resting membrane to potassium ion (?K) is approximately 50-100 times larger than its permeabüty to sodium ion (PNa). The reason for these differences in permeabüty of the membrane to various ionic species is as yet unknovvn, but it may be based on the size of pores in the membrane. For frog skeletal muscle, the K* concentration of the internal media is 140 mmol / Hter, while that of the external media is 2.5 mmol / üter. Thus there is a diffiısion gradient that is directed outward across to membrane duen to the concentration imbalance. The movement of the K+ along this diffusion gradient (vvhile the nondifîusible anion components stays within the celi) is in such a direction as to make the interior of the celi more negative rektive to the outside of the celi (i. e, positive charge is removed from the interior). A transmembrane potential difference is thus established. The membrane may than be described electrically as a leaky capacitor. That is, it acts as a charge separator, yet it has a dielectric material (the lipoprotein complex of the membrane itself) that allows a leakage flow of ions across the membrane via pores. The electric field supported by the membrane capacitor is directed inward from positive to negative across the membrane, and tends to inhibit the outvvard flow of positively charged ions (such as Cl"). Thus the diöusional and electrical forces acting across the membrane are opposed to öne another, and a steady stead is ultimately achieved. The membrane potential at which this steady state exists (considering K+ to be the main ionic species involved in the resting state, that is, PK » ?Na) is called the equüibrium potential for potassium EK. it is measured in volts, and is calculated from the Nerst equation : EK = *L. h ö = 0.0615. logı. ö (V) (i) nF [K], & [K], at 37° C (body temperature). Here n is the valance of K; [K]; and [K]0 are the intra- and extracellular concentrations of K+ in moles per liter; R is the universal gas constant; T is absolute temperature in K ; and F is the Faraday constant Equation (1) gives a reasonably good approximation to the potential of the resting membrane, which indicates that the resting membrane is effectively a potassium. membrane. A more accurate expression for membrane equiKbrium potential E that accounts for the influence of other ionic species in the internal and external media was first developed by Goidman (1943) and kter modifîed by Hodgkin and Katz (1949), who assumed a constant electric field across the membrane vii _ RT.,P*[K]0 + PJNa]0 + Pa[Cll E = ln(-^ P-^ FT-) (Y) (ü) F V P*[K], + P»[Na], + Pa[Ciy Here E is the equilibrium transmembrane resting potential when net current trough the membrane is zero and P is the permeability coefficient of the membrane. Mamtaining steady-state ionic imbalance betvveen the internal and external medk of the celi requires the continual active transport of ionic species diflûsing gradients. The active transport mecanism is located \vithin the membrane, and is sometimes referred to as the sodhım-potassium pump. it activly transport Na"1" out of the celi and K+ into the celi Enegy for the pump is provided by a common source of celluar energy, adenosine triphosphate (ATP). The Active State Another property of an excitable celi is its ability to conduct an action Potential ( Figüre 2.2 ) \vhen adequately stimulated. An adequate stimulus is öne that brings about a depolarization in a membrane that is sufficent to exeed the threshold potential of the membrane and therby elicit an all-or-none action potantial that travels in an unattenuated fashion at a constant conduction velocity along the membrane of the excitable celL Because of the steady restinig potential, the membrane is said to be polarized. A lessening of the magnitude of this polarization is called depolarization, while an increase in magnitute is referred to as hyperpolarization. The all-or-none property of the action potential means that the membrane potential goes through a very characteristic cycle; a change in potential from the resting level of a certain amount for a rbced duration of time; for a nerve fiber V = 120 mV and the duration is approximateby l ms. Further increases in intensity ör duration of stimulus beyond that required for exeeding the threshold level only produce the same result. The origm of the action potential lies in the voftage in the voltage and time -dependent nature of the membrane permiabilities to specific ions, notably sodium and potassium. As the membrane is depolarized, the permeability of the membrane to sodium PNa (ör, equivalently, the conductance the conductance of the membrane to sodium gNa) is significantly increased. As aresult, Na+ rushes into the internal medium of the celi, bringmg about further depolarization, whşch in turn brings about a further increase in gNa (that is, gNa is depent on the voltage across the membrane). If the membrane threshold is exeeded, this proses is şelf- regenerative and leads to runaway depolarization. Under these conditions, the membrane potential tends to approach the Nerst potential of sodium, ENa, which has a value of about +60 mV. The membrane potential never achives this level, hovvever because of two factors: (1) gNa is not only voltage-depent, but also time-dependent and(2) Ther is a dekyed increase in gK that acts as a hyperpolarizing influence, tending to return the membrane to resting levels (Figüre 2. l ). As the membrane potential ultimately return to the resting level, gK is stili elevated \vith respect to its resting value and return slowly along an exponential time course. Since potassium ions continue to viii leave the cell during this time, the membrane hyperpolarizes and an undershoot is produced in the transmembrane potential waveform (Vm). Anatomy and function of the heart The heart serves as a four-chambered pump for the circulatory system (Figure 3.1). The main pumping function is supplied by the ventricels, and the atria are merely antechambers to store blood during the time the ventriculs are pumping. The resting or filling phase of the heart cycle is referred to as diastole. The contractile or pumping phase is called systole. The smoot, rhthmic contraction of the atria and ventricels has an underlying electrical precursor in form of a well- coordinated series of electrical events that take place within the heart. That this set of electrical events is intrinsic to the heart itself is well demonstrated when the heart (particularly that of cold-blooded vertebrates such as the frog or turtle) is removed from the body and placed in a nutrient medium (e.g., glucose-Ringer solution). The heart continues to beat rhythmically for many hours. The coordinated contraction of the atria and ventricels is set up by a specific pattern of electrical activation in the musculature of these structures. Moreover, the electrical activation patterns in the walls of the atria and ventricles are initiated by a coordinated series of events in the "specialized conduction system" of the heart(Figure 3.1 ). In relation to the heart as a whole, the specialized conduction system is very small. It constitutes only a minute portion of the total mass of the heart. The wall of the left ventricele (Figure 3. 1 ) is 2.5-3.0 times as thick as the wall of the right ventricle, while the intraventricular septum is nearly as thick as the left ventricular wall. The major portion of the muscle mass of the ventricle consists of the free walls of right and left ventricels and septum. Considering the heard as a bioelectric source, strenght of this source can be expected to be directly related to the mass of the active muscle (ie., the number of active myocardial cells). Therefore the atria and the free walls and septum of the ventricels can be considered the major contributors to external potential fields from the heart. In clinical electrocardiography,more than one lead must be recorded to fully describe the heart's electrical activity. In practice, several leads are taken in the frontal and transverse planes. Three basic leads make up the frontal -plane ECG. These are derived from the various permutations of pairs of electrodes when one electrode is located on right arm (RA in Figure 3.6 ), the left arm (LA), and the left leg (LL). Very often an electrode is also placed on the right leg (RL) and grounded or connected to special circuits. The resulting three leads are lead LLA to RA; lead II, LL to RA; and lead HI, LL to LA. The lead vectors formed can be approximated as an equilateral triangle, known as Einthoven's triangle, in the frontal plane of the body. The components of a particular cardiac vector can be easily determined by placing the vector within the triangle and determining its components along each side. The proses can also be reversed, allowing us to determine the cirdiac vector when we know the components along the three lead vectors, or at least two of them. It is this latter problem that usually concerns the electrocardiographer. ix Three additional leads in the frontal plane -as well as a group of leads in the transverse plane - are routinly used in taking clinical ECGs. These leads are based on signals obtained from more than öne pair of electrodes. They are often referred to as unipolar leads because they consist of the potential appearing on öne electrode taken with respect to an equivalent referance electrode, which is the average of the signals seen at two ör more electrodes. Öne such equivalent reference electrode is Wilson central terminal, shown in Figüre 3.9. Here the three-limb electrodes just described are connected through equal-valued resistors to a common node. The voltage at this node,which is the Wilson central terminal, is the average of the voltages at each electrode. The signal between LA and the central point is knovvn as VL; that at RA, as VR; and that at the left foot, VF. Note that for each of these leads, öne of the resistances R shunts the circuit between the central terminal and the limb electrode. This tends to reduce the the amlitude of the signal observed, and we can modify these leads to augmented leads by removing the connection between the limb being measured and the central terminal. This does not affect the direction of the lead vector, but results in a 50% increase in amplutude of the signal. The augmented leads - knovvn as aVL, aVR, and aVF - are illustrated in Figüre 3.9,which also illustrates their lead vectors, along with those of leads I, ü, and HL Note that when the negative direction for aVR is considered with the other fîve, ali six vectors are equally spaced, by 30°. it is thus possible for the cardiologist loking at an ECG consisting of meşe six leads to estimate the position of the cardiac cycle by seeing which of the six leads has the greatest signal amplirude at that point in the cycle. This project consists of ECG part of Biotelemetry System. The system is realized pratically. This project generally contains five different parts. s m the first part ECG signals are sensed by means of surface elektrodes. This signals are derivations of right arm (RA), left arm (LA), left foot (LF); vvhich are used for Ln,ni numbered bipolar derivations at Einthoven triangle and to constitute aVR, aVL, aW unipolar derivations at frontial plane. Ih the second part; the signals sensed from RA, LA and LF are connected through the driver by three equals resistors vvithout afTecting the vector angle of the heard; the function of the driver is to prevent the body from the leakage currents, constituted by the common mode voltage because of mains and electromagnetic wave effects on the body; are reduced from high to the low curren levels due to the effective output resistor of the right foot driver. in the third part; there are filters, amplifiers, and oflset adjusting circut. The first block has a gain of Kı=40, and the second block is a low pass filter having a cut offfrequency of 110 Hz and a gain of K2=2. Ih the third block; a Notch filter that frequency is of 50 Hz; is designed and its gain is of Ks=l. Ih the fourth block; a circuit having a gain of K4=6.23 is designed; after that in the fifth block; a high pass X filler is designed and its cut off freqency is of 0.34 Hz, in this block the DC offsets coming from the early blocks are also prevented. in the skth block; a circuit is arranged for a gam of K$=2 and its output DC level is shifted by +2.5 Volls. As a result in the every three channels a gain of KT= 1993.6 is totally ensured. ECG signals having a peak to peak voltage level of 2 mV on the body are approximately amplified to the level of 4 Volts peak to peak voltage levels ör -2 V to +2 V. Due to running of H8/532 microcontroller its ADC port accepts a range of 0-5 Volts input signaL The shifting of +2.5 Volts in the positive direction by oflset circuit causes the input signal to become in the level of+0.5 V to +4.5 V. The fourth part has H8/532 microcontroller of 16 bits and its peripheral devices. in this place, the first öne of the functions of H8 is to convert the signals coming from 3 different channels to the digital signals, 8 bits, having 224 Samples/second samphng rate and 9600 baud rate and is to send them to the serial output of the microcontroller. The second öne is to constitute a senkronization byte, used before first, second and third channels' sampled bytes. The fifth part includes receiving data; sent by H8; at RS-232 port side of the PC, computing 1,11,111, aVR, aVL,aW derivations and these derivations are monitored by real time sweep graph on the PC screen. Ih addition, entering the patient names information -graphs- on the screen can be saved into the hard disk and whenever they are necessary for the observer ( Medical Doctor ete.), they can be monitored on the PC screen.