Lecture #5 PHYSICAL PRINCIPLES OF ELECTROGRAPHY

1. Lecture #5PHYSICAL PRINCIPLES OF ELECTROGRAPHY

2. Plan of the lecture • Some fundamental notions of electrodynamics • Principle of superposition of fields • Electrical dipole. • Current dipole. • Einthoven’s electrocardiac field theory • Electrocardiogram. Standard, augmented and chest leads • Vector-cardiography

3. The work of all human organs is connected with the electrical phenomena arising from generation of action potentials at membranes of cells. Registration of biopotential differences between various points of an organism underlies electrographic diagnostic methods.

4. The functioning of an alive cell is always associated with the flow of different ions (like K+, Na+, Ca2+, Cl- etc.) in it. • Ionic fluxes lead to the generation of bioelectric potentials (biopotentials). • Consequently, any alive tissue or organ (heart, brain, muscle, etc.) is a source of biopotentials. • By recording the biopotentials of the organ, we can evaluate its functional state.

5. Electrical activity of the organ is electrical phenomena in organ, connected with generation of biopotentials in the cell membranes.

6. The functional state of the cells, tissues or organs is determined by the degree of their electrical activity. • To determine the functional state different electrographic diagnostic methods (electrography) are used.

7. Electrography is a method of graphic recording of bioelectrical activity of an organ or tissue for diagnostic purposes. • During the electrographic a change of thepotential difference between two specific points of the body over time is recorded.

8. Electrographic diagnostic methods: • Electrocardiography (ECG) is a method of graphic recording of biopotential difference arising from the electrical work of the heart • Electroencephalography (EEG) is a method of registration of biopotential difference in the brain. • Electromyography (EMG) is a method of registration of biopotential difference at the muscle contraction. • Electroretinography (ERG) is a method of registration of biopotential difference in the retina.

9. Some Fundamental Notions of ELECTRODYNAMICS • Electric fieldis a type of matter that is generated by electrically charged bodies, or an alternating magnetic field and is manifested by acting on a charged body.

10. Characteristics of the electric field: 1)The intensity of electric field ( ) - is a force characteristic of the electric field. Unit of E is V/m. If F is the force effecting from the electric field on a charge (q)

11. 2) Potential of electric field() – is a power characteristic of the electric field. Unit of  is Volt (V). In some point the potential of the electric field is equal to the ratio of the potential energy (W) of a charge placed in this point to the value of this charge, i. e. W is a potential energy of electrical field, (J) q is charge placed in some poin of electrical field(C)

12. The intensity (E) is connected with the potential (φ) by the ratio where is a gradient of potential φ. The direction of the vector of the potential gradient coincides with the direction of the fastest increase of the potential.

13. If the field is created by several charges its characteristics are calculated fromthe principle of superposition of fields: the intensity of the electric field, created in some point by several charges, is equal to the vector sum of intensities of the fields created in this point by each of charges, and the potential is equal to the scalar sum of potentials created by each of charges, i. e. where and  are the resulting intensity and potential of the field created by charges; are the intensities of fields created by each of the charges, are the potentials of fields created by each of the charges.

14. Based on the principle of superposition of fields it is possible to calculate characteristics of the field, created by various systems of charges. • Electrical systems: • Electric dipole • Current dipole

15. 1. Electrical dipoleis the system of two point charges identical in absolute value and opposite in a sign (+q and –q), taking place on a distance from each other. • The distance is called thearm of a dipole. q q

16. The dipole moment(like and arm of dipole)isdirected from a negative charge of a dipole to a positive one: The unit of dipole moment is coulomb multiplied by meter (C·m).

17. Specific conductivity for various biological tissues: Values of the currents passing in mediums depend on conducting properties of these mediums and from the intensity of the electric field in them. Ohm’s law in the differential form establishes the connection between these quantities

18. 2. Current dipoleis a system of two point sources of current – a source and a drain. If the current is flowing from a point source of current that source is called a source, if it is flowing into a point current source – it is a drain. Current strength of a source is positive +I, current strength of a drain is negative − I. I −I (a drain) +I (a source)

19. Characteristics of the current dipole : • the current strenth I of point sources, • the arm of a dipole , • the dipole moment of a current dipole ( ) – the vector directed from the drain to the source.

20. (-I) (+I) a drain Θ a source of of a currenta current r medium conductivityσ А (φ)

21. PHYSICAL PRINCIPLES of ElectroCardioGraphy • The cavity of the heart is divided longitudinally into two parts (left and right) by a thick septum. • Each side contains twochambers: a posterior chamber is calledthe atrium, where the blood is received from the veins and collected, and a thickly muscled anterior chamber calledthe ventricle, which pumps the blood out again into the arteries. Human heart contains 4 chambers.

22. A small collection of the specialized cardiac muscle fibers, known as the sinoatrial node, is found in the wall of the right atrium, near the entrance of the venue cava. • The sinoatrial node acts as a pacemaker, initiating the phase of contraction and controlling its regularity. • Another collection of specialized heart muscle, often referred to as the bundle of His. • Thus the rhythm of ventricular contraction is made to follow the rhythm of atrial contraction.

23. Willem Einthoven(1860—1927), Dutch physiologist is a pioneer of electrocardiography; he was awarded the Nobel prize for physiology and medicine in 1924. • According to the Einthoven theory • 1) the heart can be considered as a current dipoletaking place in infinite isotropic conducting medium ; • 2) he offered to measure the potential difference of the electrical field generated by heart in the following three points: right hand (R), left hand (L), left foot (F).

24. The first electrocardiograph (1903):

25. Triangle of standard (extremity) leads is a correct triangle which tops are points R, L, and F. Current dipole of heart is located in the centre of the triangle.

26. Standard leads: • first lead – is the potential differences between points R and L, • second lead – is the potential differences between points R and F, • third lead – is the potential differences between points L and F.

27. In Einthoven’s model the potential differences are proportional to the values of projections of the vector of the heart dipole moment ( ) on the corresponding sides of the triangle of leads: III lead I lead II lead

28. It is considered that the origin of vector is located in the sinus (sinoatrial, SA) node, • the endof vector D during the one cardiac cycle describes in space three loops with changing both in the magnitude and in the direction.

29. Electrocardiogram is the time dependence of a potential difference (∆φ = U) which is registered in each of three standard leads. Normally the signal registeredin the second standard lead is the strongest.

30. It is possible to allocate 5 typical waves on the cardiogram, which are accepted for denoting by letters P, Q, R, S, T. Out of three loops, described by the end of vector of heart current diapole (D) during a cardiac cycle, • the 1st loop is connected with the Р-wave, • the 2nd is connected with the complex of QRS-waves, • the 3rd is connected with the Т-wave.

31. The P-wave represents the wave of membrane depolarization that spreads from the SAnode 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.

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

33. 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.

34. As the heart undergoes depolarization and repolarization, the electrical currents spread throughout the body because the body acts as a volume conductor. • By convention, electrodes are placed on each arm and leg, and six electrodes are placed at defined locations on the chest. • There are two basic types of electrocardiogram (ECG) leads: bipolar and unipolar. Bipolar leads utilize a single positive and a single negative electrode between winch electrical potentials are measured. Unipolar leads (augmented leads and chest leads) have a single positive recording electrode and utilize a combination of the electrodes to serve as a composite negative electrode. • The positive electrodes for these augmented leads are located on the left arm (aV1), the right arm (aVR), and the left leg (aVF).

35. 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. Physiological segment q Pathological segment Q

36. 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.

37. Each cardiogram is compared with the known standard curves which conformity to these or those deviations from norm is fixed in practice. Many forms of irregularities of excitability of heart and arrhythmia are easily identified on ECG. It is possible to establish presence of the myocardial infarction and even its localization by the form of ECG. The important information can be received at other pathologies of heart.

38. With registration of the potentials caused by the heart functioning such diagnostic technique as a vector–cardiographyis also connected. At the vector-cardiography four electrodes are placed near to the heart area in two perpendicular planes. Chest Amplifier

39. At the present time other diagnostic techniques based on registration of changes of biopotentials at functioning those or other organs are also applied. So the potentials generated by the cerebrum are registered at the electroencephalography (EEG). • Biopotentials registered at the electroencephalography are connected basically with the functioning of pyramidal neurons of the cortex of cerebrum. Electroencephalograms have the complex form. The maximum value of the biopotentials registered at EEG is about several microvolts.

40. The potentials generated by various muscles of a body are registered at • the electromyography (EMG). • The potentials generated by the eye retina are registered at the electroretinography (ERG). The electromyogramma