Medicine

The structure and functionality of biological membranes Work of electrocardiograph

Physical base electrography.

 

Heart Electrical Phenomena

Описание: D:\..\..\..\..\..\KAFEDRA\VUKLADACHI\DIMA\Hiper_Phizics\hyperphysics\heartsketch4.gif

The rhythmic contractions of the heart which pump            the life-giving blood occur in response to periodic electrical control pulse sequences. The natural pacemaker is a specialized bundle of nerve fibers called the sinoatrial node (SA node). Nerve cells are capable of producing electrical impulses called action potentials. The bundle of active cells in the SA node trigger a sequence of electrical events in the heart which controls the orderly pattern of muscle contractions that pumps the blood out of the heart.

The electrical potentials (voltages) that are generated in the body have their origin in membrane potentials where differences in the concentrations of positive and negative ions give a localized separation of charges. This charge separation is called polarization. Changes in voltage occur when some event triggers a depolarization of a membrane, and also upon the repolarization of the membrane. The depolarization and repolarization of the SA node and the other elements of the heart's electical system produce a strong pattern of voltage change which can be measured with electrodes on the skin. Voltage measurements on the skin of the chest are called an electrocardiogram or ECG.

Описание: D:\..\..\..\..\..\KAFEDRA\VUKLADACHI\DIMA\Hiper_Phizics\hyperphysics\heartpump.gif

The heart's electrical control system must properly synchronize the pumping functions illustrated above.

Описание: A009_ECG_trace

Electrocardiogram (EKG, ECG)

The basis of a method of registration of biopotentials is the theory Einthoven. According to this theory heart is considered as current dipole, which is in homogeneous environment with which there are tissues, environmental heart.

Vector of the electrical moment current dipole, which is examined as well as a vector muscles of heart, traveling along a line, which refers to as an electrical axis of heart and rather close coincides with its anatomic axis. Dipole forms in an environment an electrical field, the lines of which intensity reach a surface of a body, on which, accordingly, there can be revealed points of different potential, and till them the constructed potential surfaces schematically shown in a fig. 2.

Einthoven has offered to register a difference of potentials between

fig. 1                              everyone by two of electrodes located on the right hand, left hand, and left leg, in tops & sides of a triangle АВС

As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body.  This electrical activity generated by the heart is generally measured by an array of electrodes placed on the body surface and the resulting tracing is called an electrocardiogram (ECG, or EKG).  A "typical" ECG tracing is shown below.  The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles. 

fig. 2

The P-wave represents the wave of depolarization that spreads from the SA node throughout the atria and is usually 0.08 to 0.1 seconds (80-100 ms) in duration.  The brief isoelectric (zero voltage) period after the P-wave represents the time in which the impulse is traveling within the AV node where the conduction velocity is greatly retarded. 

The period of time from the onset of the P-wave to the beginning of the QRS is termed the PR interval and normally ranges from 0.12 to 0.20 seconds.  This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization.  If the PR interval is >0.2 sec, a conduction defect (usually within the AV node) is present (first-degree heart block). 

The QRS complex represents ventricular depolarization.  The duration of the QRS complex is normally 0.06 to 0.1 seconds indicating that ventricular depolarization normally occurs very rapidly.  If the QRS complex is prolonged (> 0.1 sec), conduction is impaired within the ventricles.  This can occur with bundle branch blocks or whenever a ventricular foci becomes the pacemaker driving the ventricle. Such an ectopic foci nearly always results in impulses being conducted over slower pathways within the heart, thereby increasing the time for depolarization and the duration of the QRS complex. 

The isoelectric period (ST segment) following the QRS is the time at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential.  The ST segment is important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.

The T-wave represents ventricular repolarization and is longer in duration than depolarization (i.e., conduction of the repolarization wave is slower than the wave of depolarization). 

The QT interval represents the time for both ventricular depolarization and repolarization to occur, and therefore roughly estimates the duration of an average ventricular action potential.  This interval can range from 0.2 to 0.4 seconds depending upon heart rate.  At high heart rates, ventricular action potentials shorten in duration, which decreases the QT interval.  Because prolonged QT intervals can be diagnostic for susceptibility to certain types of arrhythmias, it is important to determine if a given QT interval is excessively long.  In practice, the QT interval is expressed as a "corrected QT (QTc)" by taking the QT interval and dividing it by the square root of the RR interval (interval between ventricular depolarizations).  This allows an assessment of the QT interval that is independent of heart rate.  Normal corrected QTc intervals are less than 0.44 seconds. 

There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization.  Because the wave of atrial repolarization is relatively small in amplitude, it is masked by the much larger ventricular-generated QRS complex.

ECG tracings recorded simultaneous from different electrodes placed on the body will produce different characteristic waveforms.  To learn where ECG electrodes are place. 

Electrocardiogram Leads

As the heart undergoes depolarization and repolarization, the electrical currents spread throughout the body because the body acts as a volume conductor.  The electrical currents generated by the heart are commonly measured by an array of electrodes placed on the body surface and the resulting tracing is called an electrocardiogram (ECG, or EKG).  (Electrical measurements can also be made within the heart using special catheters; these are useful for specialized electrophysiological diagnostic procedures).  By convention, electrodes are placed on each arm and leg, and six electrodes are placed at defined locations on the chest.  These electrode leads are connected to a device that measures potential differences between selected electrodes in order to produce the characteristic electrocardiographic tracings.  

There are two basic types of electrocardiogram (ECG) leads: bipolar and unipolar.  Bipolar leads utilize a single positive and a single negative electrode between which electrical potentials are measured.  Unipolar leads (augmented leads and chest leads) have a single positive recording electrode and utilize a combination of the other electrodes to serve as a composite negative electrode.

Limb Leads (Bipolar)

Описание: A013_Einthovens_triangleBipolar recording is represented by standard limb lead configurations depicted at the right.  By convention, Lead I has the positive electrode on the left arm, and the negative electrode on the right arm, and therefore measures the potential difference between the two arms.  In this and the other two limb leads, an electrode on the right leg serves as a reference electrode for recording purposes.  In the Lead II configuration, the positive electrode is on the left leg and the negative electrode is on the right arm.  Lead III has the positive electrode on the left leg and the negative electrode on the left arm.  These three bipolar limb leads roughly form an equilateral triangle (with the heart at the center) that is called Einthoven's triangle in honor of Willem Einthoven who developed the electrocardiogram in 1901.  Whether the limb leads are attached to the end of the limb (wrists and ankles) or at the origin of the limb (shoulder or upper thigh) makes no difference in the recording because the limb can simply be viewed as a long wire conductor originating from a point on the trunk of the body.

Based upon universally accepted ECG rules, a wave a depolarization heading towards the left arm will give a positive deflection in Lead I because the positive electrode is on the left arm.  Maximal positive ECG deflection will occur in Lead I when a wave of depolarization travels parallel to the axis between the right and left arms.  If a wave of depolarization heads away from the left arm, the deflection will be negative.  Also by these rules, a wave of repolarization moving away from the left arm will be seen as a positive deflection.  Similar statements can be made for Leads II and III where the positive electrode is located on the left leg.  For example, a wave of depolarization traveling towards the left leg will give a positive deflection in both Leads II and III because the positive electrode for both leads is on the left leg.  A maximal positive deflection will be obtained in Lead II when the depolarization wave travels parallel to the axis between the right arm and left leg.  Similarly, a maximal positive deflection will be obtained in Lead II when the depolarization wave travels parallel to the axis between the left arm and left Описание: A013_axis_reference_1leg.

If the three limbs of Einthoven's triangle (assumed to be equilateral) are broken apart, collapsed, and superimposed over the heart, then the positive electrode for Lead I is said to be at zero degrees relative to the heart (along the horizontal axis) (see figure at right).  Similarly, the positive electrode for Lead II will be +60º relative to the heart, and the positive electrode for Lead III will be +120º relative to the heart as shown to the right.  This new construction of the electrical axis is called the axial reference system.  With this system, a wave of depolarization traveling at +60º will produce the greatest positive deflection in Lead II.  A wave of depolarization oriented +90º relative to the heart will produce equally positive deflections in both Lead II and III.  In this latter case, Lead I will show no net deflection because the wave of depolarization is heading perpendicular to the 0º, or Lead I, axis (see ECG rules).

Augmented Limb Leads (Unipolar)

Описание: A013_6_lead_axial_refIn addition to the three bipolar limb leads described above, there are three augmented unipolar limb leads.  These are termed unipolar leads because there is a single positive electrode that is referenced against a combination of the other limb electrodes.  The positive electrodes for these augmented leads are located on the left arm (aVL), the right arm (aVR), and the left leg (aVF).  In practice, these are the same electrodes used for Leads I, II and III.  (The ECG machine does the actual switching and rearranging of the electrode designations).  The three augmented leads are depicted using the axial reference system as shown to the right.  The aVL lead is at -30º relative to the Lead I axis; aVR is at -150º and aVF is at +90º.  It is very important to learn which lead is associated with each axis.

The three augmented unipolar leads, coupled with the three bipolar leads, constitute the six limb leads of the ECG.  These leads record electrical activity along a single plane, termed the frontal plane relative to the heart.  Using the axial reference system and these six leads, it is rather simple to define the direction of an electrical vector at any given instant in time.  If a wave of depolarization is spreading from right-to-left along the 0º axis, then Lead I will show the greatest positive amplitude.  Likewise, if the direction of the electrical vector for depolarization is directed downwards (+90º), then aVF will show the greatest positive deflection.  If a wave of depolarization is moving from left-to-right at +150º, then aVL will show the greatest negative deflection according to the rules for ECG interpretation.

Chest Leads (Unipolar)

Описание: A013_chest_leadsThe last ECG leads to consider are the precordial, unipolar chest leads.  These are six positive electrodes placed on the surface of the chest over the heart in order to record electrical activity in a plane perpendicular to the frontal plane (see figure at right).  These six leads are named V1 - V6.  The rules of interpretation are the same as for the limb leads.  For example, a wave of depolarization traveling towards a particular electrode on the chest surface will elicit a positive deflection.  

In summary, the twelve ECG leads provide different views of the same electrical activity within the heart.  Therefore, the waveform recorded will be different for each lead.  To understand how cardiac electrical currents actually generate and ECG tracing and why the different leads display that electrical activity differently, it is necessary to understand volume conductor principles and vectors

The difference of potentials, which are registered at Electrocardiography, turns out at excitation nerves - muscles of device of heart. Nervous or muscles the fibers in a condition of rest is polarized so, that the external surface of its environment has a positive charge, and internal negative. At excitation this difference of potentials sharply decreases, and then changes a mark to opposite. In process of passage of a wave of excitation along a fibers the difference of potentials on its sites comes back to initial state.

The device is included between an external surface of an environment and internal environment of a fiber, will register change of potentials shown on a

The part of a curve (a) answers a phase "depolarization", part (b) - "repolarization" of an environment and part (c) - "remain" to potential. The phenomenon as a whole name as formation" of potential of action ".

Biopotentials, sum on all elements nervously - muscles of the device, form a common difference of potentials, which refers to as electromotive force of heart.

          The size of the loops is determined in mm from a zero line to upwards for positive P, R, and T, and downwards - for negative Q, S and is compared with calibrated by a signal, which the voltage U = 1mV determined. Size greatest loops R: UR=2,5mV. The duration loops and intervals of absence of a signal is determined on a special grid located on electrocardiograms. All intimate cycle lasts approximately 1c, and most short-term loops - 100-th shares of second. Thus, electrocardiograph should register a difference of potentials with frequency from 0,3 up to 120-150 Hz and amplitude about 1mV. It requires amplification biopotentials in tens thousand times.

          There are many different marks electrocardiograph we shall work with Cardio complex. A principle of action electrocardiograph based on direct amplification and registration as a curve (electrocardiograms) of a voltage of signals from electrodes of the body, imposed on the appropriate point, of the patient. The electrodes join to electrocardiographs through a cable of loops, which consists of conductors, which correspond  to number of electrodes, and come to an end by probes with multi-colored cables. The display of the information can be on the monitor of computers or can be printed out on a paper.

Considered us loops are basic. In the further number loops was increased at the expense of electrodes, which are imposed on a surface thorax of a crate in the field of an arrangement of heart. These loops correspond to a projection of a vector electric of force of heart a horizontal plane.

 

 

 

Medical equipment for mesument electrical signals of heart. Medical Equipment for functional diagnostics

Functional diagnosis (FD) - a diagnostic partition, based on the use of instrumental and laboratory methods for the study of patients with an objective assessment of the functional condition different systems, organs and tissues of the body at rest and during exercise, as well as for monitoring the dynamics of the functional changes occurring under the influence of treatment.

Currently, the most extensive group of instruments and apparatus, whereby the perception of information (identification, measurement, registration, remember) and processing of bioelectric signals.

Classification methods of functional diagnostics depending on the area of study:

1.     Methods and devices for diagnostic tests of cardiovascular system.

 

Electrocardiography - a method of registration the electrical activity of the myocardium, delivered in the heart muscle during the cardiac cycle. Graphic representation of the electrical activity of the myocardium is called the electrocardiogram (ECG). As it is defined by the frequency and timing of cardiac activity. Chance diagnosis of arrhythmia, angina, coronary heart disease, myocardial infarction and other diseases of the cardiovascular system.

For ECG is used electrocardiograph. As the number of leads from the electrodes superimposed on the wrists, left leg and chest, they are divided into one-, two-, three-, four-and six-channel. Multichannel devices quickly registering bioelectric potentials of the heart, as occurs simultaneously record several leads.

The main characteristics of the ECG is a form and height of the teeth and the length of intervals. Table 1 shows the values ​​of the characteristics of ECG is normal:

Teeth ECG. The amplitude A mV. Duration D, s

Р

Q

R

S

А

Д

А

Д

А

Д

А

Д

0,05-0,25

0-0,1

0-0,2

max 0,03

0,3-1,6

max 0,03

0-0,03

max 0,03

 

 

 

Teeth ECG. The amplitude A mV. Duration D, s

Intervals, s

А

Д

РQ

QRS

QRSТ

RR

0,25-0,6

max 0,25

0,12-0,2

0,06-0,09

0,30-0,49

0-0,15

0,7-1

(depending on heart rate)

 

In  pathological changes in the heart there is a change of characteristics that can be used electrocardiograms to diagnose heart disease.

Knowing the height of the teeth ECG may identify angles formed by the dipole moment vector heart with lines leads. Determine the angle α, formed by dipole with lines and drainage. It is believed that the line AB (Fig. 3) corresponds to the assignment and, UАВ = UI, UАС = UІІ, UВС = UІІІ and αАВ = α. According to this we get

where UI, UII, UIII - ECG R wave height respectively, leads I, II, II.

Electrocardiographs are produced portable and stationary.

Depending on the type of element, writing, and media type information electrocardiographs are distinguished: pen (with ink writing on a chart or heat sensitive paper) and jet (a record at a regular or photo paper).

Currently available in specialized ECG - systems for traditional and long-term (24 h) cardiograms, including automatic data processing.

 

 

 

 

The device, which is the recording of ECG, electrocardiograph called.

 

 

 

 


AC

 

 

Fig. 12 Block diagram of the electrocardiograph

Bioelectric signals through leads cable and the switch leads (SL) served on the amplifier input voltage (AV). To input voltage amplifier connected as a source of tension gauge (STG). The signal amplified by the amplifier voltage is fed to the input of the power amplifier (PA), after which the signal is sent to an electromechanical transducer (ET), which provides an electrical signal converting heat into motion the pen. Heat sensitive paper moving  uniformly relative to the pen with tape extended mechanism (TEM). To power biopotential amplifier, electric belt-extended mechanism of thermal pen device used power adapter AC.

The main components of electrocardiograph:

• Remote control

• power supply

• power gain,

• galvanometer

• tape drive mechanisms,

• cable leads.

 

On the control panel are:

• switch the power supply,

• Button sedation

• gain regulator signal

• button calibration signal

• Rotary feathers

• handle switch assignments,

• Switch the speed of the belt,

• Record button,

• connector for connecting cable leads,

• AC power cord and grounding,

• socket for connecting consoles and sensors.

The principle of the electrocardiograph.

 

 

 

 

 

 

 

The principle of the electrocardiograph (fig. 12) is that electrical signals to the electrodes were passing through the cable leads through the switch on the power amplifier, amplified hundreds of thousands times and passed on the galvanometer. Electrical oscillations in the galvanometer converted to mechanical, thus moves anchor of an electromagnet and galvanometer driven device is recording.

Consider Fig.13 , which shows a diagram of electrocardiograph . The figure shows the controls electrocardiograph , dashed lines indicate a unit managed by using this switch. The connecting wire connecting the electrodes contained on patients with electrocardiograph . Button "Standard 1 mV " on the front panel to set the calibration voltage 1 mV calibration electrocardiograph . Although modern electrocardiographs stable and their sensitivity does not change with time , the introduction of calibration pulse before or after each entry when removing the 12-lead ECG is still practiced. From the switch leads ECG signal goes to the original signal amplifier (DCA ). This device is a differential amplifier with a high degree of suppression ( resection ) general ( common mode ) signal. MAP also includes a switch for sensitivity adjustment or enhancement. For most patients, this switch is in position "1". If the ECG curve has a very small scale , the sensitivity can double by sliding the switch to " 2". For patients with high signal intensity of ECG signals can be reduced by half by setting the switch to " 1/ 2". In the ECG , which was used previously used continuous sensitivity adjustment , so-called " Set Calibration ". With this setting you can select as electrocardiograph sensitivity to 1 mV calibration pulse triggered stylus deflection of 10 mm ( position "1" gain switch ). In the current gain amplifiers usually remains stable if its once adjusted the so continuous gain control can now be found only occasionally and is in the form of screw adjustment (which can be adjusted with a screwdriver ) located on the side or back of the electrocardiograph

Fig.13 Basic blocks and controls of modern electrocardiograph

Next element after initial signal amplifier is amplifier of constant voltage, called the amplifier recorder (AR). AR provides the necessary power to move the recording pen, which achieved record ECG. The input of this amplifier can usually present the signal from an external source that is done with a special connector. Thus, the ECG can be used to record the output signals to other devices.

Usually all modern electrocardiograph uses heat sensitive paper and thermal pen, the pen is a needle with electric heating, the temperature of which can be controlled by the handle " heating pen" that allows you to get the best recording signal. In addition to recording pen in ECG using a " time marker ", which is activated by a button. This allows the operator to apply coded label at the beginning of recorded ECG leads. Usually ECG recorded at a speed of movement of paper 25 mm / s, but in the device and provides higher speed of 50 mm / s, which allows you to get more information about QRS complex at very high heart rate or rhythm where we need to investigate some specific details of the recorded curve .

The power switch has three positions. To the "On" power supplied to the amplifier, but the paper does not move. To enable broaches of paper should switch set to "broach". On older electrocardiograph with button or special metal contact the operator can check the correct polarity of device connect to power line. Because wrong connections can create a danger of electric shock to the patient, so this test is a mandatory before connect the electrodes to the patient.

Modern devices are complicated and contain additional elements. In the figure, for example, shows that of RL lead connected to the ground. In modern devices often used the so-called Slave (excitatory) lead RL, which reduces device sensitivity to interference caused by AC voltage. Instead of connecting resistors central point directly to the electrodes used in modern electrocardiographs isolating amplifiers are installed in all active connections to the patient. Thus, the input impedance of the amplifier increases and preparing places for the imposition of the electrodes is not as important as in this case, you can prevent and higher impedance of electrodes.

Initial signal amplifier in modern electrocardiographs are often isolated from the ground, while power line and the line for transmitting signals joined by optical devices or transformers. Isolated lead from the patient reduce the risk of electric shock, which can occur in some cases.

Modification of electrocardiography is a vector cardiography as a method of recording the electrical activity of the heart, including the magnitude and direction of the electric field of the heart during the cardiac cycle. The clinic is a method used to identify focal lesions infarction, ventricular hypertrophy, especially early on.

Phonocardiography - a method of recording sounds (tone, noise) arising from activities of  heart. It is used to determine violations of heart valve including defects. Phonocardiogram obtained using instruments Phonocardiograph.

Physical base electrography

 

Electrocardiogram (EKG, ECG)

As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body.  This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG, or EKG).  A "typical" ECG tracing is shown to the right.  The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles. The ECG is recorded at a speed of 25 mm/sec, and the voltages are calibrated so that 1 mV = 10 mm in the vertical direction. Therefore, each small 1-mm square represents 0.04 sec (40 msec) in time and 0.1 mV in voltage. Because the recording speed is standardized, one can calculate the heart rate from the intervals between different waves.

P wave

The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.1 seconds (80-100 ms) in duration.  The brief isoelectric (zero voltage) period after the P wave represents the time in which the impulse is traveling within the AV node (where the conduction velocity is greatly retarded) and the bundle of His. Atrial rate can be calculated by determining the time interval between P waves. Click here to see how atrial rate is calculated.

The period of time from the onset of the P wave to the beginning of the QRS complex is termed the P-R interval, which normally ranges from 0.12 to 0.20 seconds in duration.  This interval represents the time between the onset of atrial depolarization and the onset of ventricular depolarization.  If the P-R interval is >0.2 sec, there is an AV conduction block, which is also termed a first-degree heart block if the impulse is still able to be conducted into the ventricles.

QRS complex

The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes. Click here to see how ventricular rate is calculated.

The duration of the QRS complex is normally 0.06 to 0.1 seconds.  This relatively short duration indicates that ventricular depolarization normally occurs very rapidly.  If the QRS complex is prolonged (> 0.1 sec), conduction is impaired within the ventricles.  This can occur with bundle branch blocks or whenever a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. Such anectopic foci nearly always results in impulses being conducted over slower pathways within the heart, thereby increasing the time for depolarization and the duration of the QRS complex.

The shape of the QRS complex in the above figure is idealized. In fact, the shape changes depending on which recording electrodes are being used. The shape will also change when there is abnormal conduction of electrical impulses within the ventricles. The figure to the right summarizes the nomenclature used to define the different components of the QRS complex.

ST segment

The isoelectric period (ST segment) following the QRS is the time at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential.  The ST segment is important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.

T wave

The T wave represents ventricular repolarization and is longer in duration than depolarization (i.e., conduction of the repolarization wave is slower than the wave of depolarization). Sometimes a small positive U wave may be seen following the T wave (not shown in figure at top of page). This wave represents the last remnants of ventricular repolarization. Inverted or prominent U waves indicates underlying pathology or conditions affecting repolarization.

Q-T interval

The Q-T interval represents the time for both ventricular depolarization and repolarization to occur, and therefore roughly estimates the duration of an average ventricular action potential.  This interval can range from 0.2 to 0.4 seconds depending upon heart rate.  At high heart rates, ventricular action potentials shorten in duration, which decreases the Q-T interval. Because prolonged Q-T intervals can be diagnostic for susceptibility to certain types of tachyarrhythmias, it is important to determine if a given Q-T interval is excessively long.  In practice, the Q-T interval is expressed as a "corrected Q-T (QTc)" by taking the Q-T interval and dividing it by the square root of the R-R interval (interval between ventricular depolarizations).  This allows an assessment of the Q-T interval that is independent of heart rate.  Normal corrected Q-Tc intervals are less than 0.44 seconds.

There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization.  Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger ventricular-generated QRS complex.

 

 

 

 

 

 

 

ECG tracings recorded simultaneous from different electrodes placed on the body produce different characteristic waveforms.  To learn where ECG electrodes are placed, CLICK HERE.

 

 

Chest Leads (Unipolar)

The last ECG leads to consider are the precordial, unipolar chest leads.  These are six positive electrodes placed on the surface of the chest over the heart in order to record electrical activity in a plane perpendicular to the frontal plane (see figure at right).  These six leads are named V1 - V6.  The rules of interpretation are the same as for the limb leads.  For example, a wave of depolarization traveling towards a particular electrode on the chest surface will elicit a positive deflection.  

In summary, the twelve ECG leads provide different views of the same electrical activity within the heart.  Therefore, the waveform recorded will be different for each lead.  To understand how cardiac electrical currents actually generate and ECG tracing and why the different leads display that electrical activity differently, it is necessary to understand volume conductor principles and vectors

The difference of potentials, which are registered at Electrocardiography, turns out at excitation nerves - muscles of device of heart. Nervous or muscles the fibers in a condition of rest is polarized so, that the external surface of its environment has a positive charge, and internal negative. At excitation this difference of potentials sharply decreases, and then changes a mark to opposite. In process of passage of a wave of excitation along a fibers the difference of potentials on its sites comes back to initial state.

The device is included between an external surface of an environment and internal environment of a fiber, will register change of potentials shown on a

The part of a curve (a) answers a phase "depolarization", part (b) - "repolarization" of an environment and part (c) - "remain" to potential. The phenomenon as a whole name as formation" of potential of action ".

Biopotentials, sum on all elements nervously - muscles of the device, form a common difference of potentials, which refers to as electromotive force of heart.

           The size of the loops is determined in mm from a zero line to upwards for positive P, R, and T, and downwards - for negative Q, S and is compared with calibrated by a signal, which the voltage U = 1mV determined. Size greatest loops R: UR=2,5mV. The duration loops and intervals of absence of a signal is determined on a special grid located on electrocardiograms. All intimate cycle lasts approximately 1c, and most short-term loops - 100-th shares of second. Thus, electrocardiograph should register a difference of potentials with frequency from 0,3 up to 120-150 Hz and amplitude about 1mV. It requires amplification biopotentials in tens thousand times.

           There are many different marks electrocardiograph we shall work with Cardio complex. A principle of action electrocardiograph based on direct amplification and registration as a curve (electrocardiograms) of a voltage of signals from electrodes of the body, imposed on the appropriate point, of the patient. The electrodes join to electrocardiographs through a cable of loops, which consists of conductors, which correspond  to number of electrodes, and come to an end by probes with multi-colored cables. The display of the information can be on the monitor of computers or can be printed out on a paper.

Considered us loops are basic. In the further number loops was increased at the expense of electrodes, which are imposed on a surface thorax of a crate in the field of an arrangement of heart. These loops correspond to a projection of a vector electric of force of heart a horizontal plane.

 

High resolution 16-channel ECG system

A CE-certified Reference-Electro-Cardiogram-Device (ECG-device) providing a number of special technical features has been developed and manufactured by the Physikalisch-Technische Bundesanstalt (PTB). The system records and stores bio-electrical potential differences at the body surface caused by the excitation of the heart.
In contrast to common ECG-systems the reference ECG-device stores the potential differences measured with respect to a reference electrode. There is no hardware lead network. The conventional ECG is calculated exactly by software.
This approach enables the simulation of human electrical heart activity. The signal can directly be fed into the patient cables of a commercial ECG-device for testing , -also for measuring and analysing ECG-devices- The system offers the possibility to record simultaneously up to 16 channels. This allows to record the data for the standard ECG-leads, to calculate the Frank-leads as well as to acquire reference signals regarding breathing and the frequency of the main power voltage.

·         Measurement device providing a total 16 channels, 14 channels for ECG's and one channel each for breathing and power voltage

·         Input voltage of ±16 mV with an offset of up to ±300 mV that can be compensated

·         Input impedance: 100 MOhm (DC)

·         Resolution: 16 bit with 0,5 µV/LSB

·         Signal band width: 0...1 kHz (synchronous sampling of all channels)

·         Noise: max. 10 µV (pp) or 3 µV (rms) for short circuit at input

·         Online-measurement of skin impedance before and after data acquisition

·         Noise-measurement during data acquisition

Описание: D:\..\..\..\..\..\Vaculenko\Metod\technology in the medical\med equipment\PTB - 8_21 - Bioelectricity and Biomagnetism.files\WEB_EKG.jpg                Описание: D:\..\..\..\..\..\Vaculenko\Metod\technology in the medical\med equipment\PTB - 8_21 - Bioelectricity and Biomagnetism.files\WEB_EKG_Verstaerker.jpg

 

Описание: Electrocardiogram taken on a patientFig 1. High resolution 16-channel ECG system              Fig 2. Amplifier module of the ECG systems

 

Technique of performance of laboratory work.

1.  On the basis of offered electrocardiogram execute the following tasks:                

2.  Results of amplitudes and intervals of loops electrocardiograms note in the table.

1.     On calibrated of a signal by a voltage 1 mV determine scale of a voltage   where n - height calibrated, signal in mm;

2.     Determine electric of force

3.     Determine a time scale

4.     Determine time intervals loops on a time scale and distance between loops;

8.     Find a rhythm of work of heart - time interval " between loops" R-R ";

9.     Calculate height of intimate reductions Fhb(Frequencyof  heartbeet) under the formula   where t - meanings of a time interval in c.