Physiological Basis of Electrocardiogram

describes everything that should be known about Electrocardiograph and the heart

1. PHYSIOLOGICAL BASIS OF ELECTROCARDIOGRAM (ECG) JAMES UDUAKABASI AKPANDEM

2. TABLE OF CONTENT 1 2 3 4 5 6 7 8 9 10 11 Defination and History. Understanding the ECG. Actions of the heart Cardiac action potentials Uses of ECG. Electrographic Grid . ECG Leads. Waves of ECG. Intervals and segments of ECG. ECG Interpretation. Problems associated with ECG.

3. Defination and History Electrocardiogram Electrocardiogram (ECG or EKG from electrokardiogram in Dutch) is the record or graphical registration of electrical activities of the heart, which occur prior to the onset of mechanical activities. Electrocardiograph Electrocardiograph is the instrument (machine) by which electrical activities of the heart are recorded. (K_Sembulingam, 2012)

4. Electrocardiography Electrocardiography is the technique by which electrical activities of the heart are studied. (K_Sembulingam, 2012) Dutch physiologist Willem Einthoven developed electrocardiogram in 1903, and for many years the tracing was called an EKG after the German Elektrokardiogramm. During the late 1960s, computerized electrocardiography came into use in many of the larger hospital. (Encyclopaedia Britannica, 2000)

5. Understanding ECG An ECG captures the electrical activity produced by the heart's contraction cycle. During each heart beat there is an action potential that is propagated from the SA node, through the internodal tract, the AV node, to the bundle of HIS, the left and right bundle branches and finally the Perkinje Fibers. The electrical impulses generated from the flow of charged particles along this pathway are detectable on the surface of the skin. The detection of this current forms the basis for the electrocardiogram. (PT Reviewer, 2019)

6. Cardiac cycle of the heart (K_Sembulingam, 2012)

7. Actions of the heart Actions of the heart are classified into four types: 1. Chronotropic action 2. Inotropic action 3. Dromotropic action 4. Bathmotropic action. CHRONOTROPIC ACTION Chronotropic action is the frequency of heartbeat or heart rate. It is of two types: i. Tachycardia or increase in heart rate ii. Bradycardia or decrease in heart rate. ( K_Sembulingam, 2012 )

8. INOTROPIC ACTION Force of contraction of heart is called inotropic action. It is of two types: i. Positive inotropic action or increase in the force of contraction ii. Negative inotropic action or decrease in the force of contraction. DROMOTROPIC ACTION Dromotropic action is the conduction of impulse through heart. It is of two types: i. Positive dromotropic action or increase in the velocity of conduction ii. Negative dromotropic action or decrease in the velocity of conduction. BATHMOTROPIC ACTION Bathmotropic action is the excitability of cardiac muscle. It is also of two types: i. Positive bathmotropic action or increase in the excitability of cardiac muscle ii. Negative bathmotropic action or decrease in the excitability of cardiac muscle. ( K_Sembulingam, 2012 )

9. Sinoatrial node and conductive system of the heart ( DROMOTROPIC ACTION) (K_Sembulingam, 2012)

10. Cardiac action potentials ( BATHMOTROPIC ACTION) The resting membrane potential is determined by the conductance to K+. ■ Inward current brings positive charge into the cell and depolarizes the membrane potential. ■ Outward current takes positive charge out of the cell and hyperpolarizes the membrane potential. ■ The role of Na+,K+-adenosine triphosphatase (ATPase) is to maintain ionic gradients across cell membranes. (Linda S. Costanzo, , 2011)

11. Sinoatrial (SA) node Is normally the pacemaker of the heart. Has an unstable resting potential. The AV node and the His-Purkinje systems are latent pacemakers and override the SA node if it is suppressed. The intrinsic rate of phase 4 depolarization (and heart rate) is fastest in the SA node and slowest in the His-Purkinje system: SA node > AV node > His-Purkinje AV node Upstroke of the action potential in the AV node is the result of an inward Ca+ current (as in the SA node). (Linda S. Costanzo, 2011)

12. Phase 0 ■ is the upstroke of the action potential. ■ is caused by an increase in Ca2+ conductance. This increase causes an inward Ca2+ c urrent that drives the membrane potential toward the Ca2+ equilibrium potential. Phase 1 and 2 ■ are not present in the SA node action potential. Phase 3 ■ is repolarization. ■ is caused by an increase in K+ conductance. This increase results in an outward K+ current that causes repolarization of the membrane potential. 1 2 3 Sinoatrial (SA) node and AV node action potential 4 Phase 4 ■ is slow depolarization. ■ is caused by an increase in Na+ conductance, which results in an inward Na+ current . 5 AV node ■ Upstroke of the action potential in the AV node is the result of an inward Ca+ current (as in the SA node). (Linda S. Costanzo, 2011)

13. Sinoatrial (SA) node and AV node action potential (Linda S. Costanzo, 2011)

14. Ventricles, atria, and the Purkinje system Have stable resting membrane potentials of about –90mV. Action potentials are of long duration, especially in Purkinje fibers, where they last 300 milliseconds (msec). (Linda S. Costanzo, 2011)

15. Phase 0 ■ is the upstroke of the action potential. ■ is caused by a transient increase in Na+ conductance. This increase results in an inward Na+ current that depolarizes the membrane. Phase 1 ■ is a brief period of initial repolarization. ■ Initial repolarization is caused by an outward current, because of the movement of K+ io n out of the cell and because of a decrease in Na+ conductance. Phase 2 ■ is the plateau of the action potential. ■ is caused by a transient increase in Ca2+ conductance, which results in an inward Ca2+ current, and by an increase in K+ conductance. ■ During phase 2, outward and inward currents are approximately equal, so the membrane potential is stable at the plateau level . 1 2 3 Ventricles, atria and the Purkinje system action potential 4 Phase 3 ■ is repolarization. ■ Ca2+ conductance decreases, and K+ conductance increases . ■ The high K+ conductance results in a large outward K+ current (IK), which hyperpolarizes the membrane . 5 Phase 4 ■ is the resting membrane potential. ■ is a period during which inward and outward currents (IK1) are equal and the membrane potential approaches the K+ equilibrium potential. (Linda S. Costanzo, 2011)

16. Ventricles, atria, and the Purkinje system action potential (Linda S. Costanzo, 2011)

17. Uses of ECG The ECG is of greatest use in diagnosing the following: Heart rate Heart rhythm Abnormal electrical conduction Poor blood flow to heart muscle (ischemia) Heart attack Coronary artery disease Hypertrophy of heart chambers. Cardiac arrhythmia Pericardial disease (K_Sembulingam, 2012)

18. The presence of hypertension, thyroid disease, and certain types of malnutrition also may be revealed by ECG It can be used to determine whether a slow heart rate is physiological or is caused by heart block The Exercise ECG, or ECG stress test, is used to assess the ability of the coronary arteries to deliver oxygen while the heart is undergoing strain imposed by a standardized exercise protocol (Encyclopaedia, Britannica, 2000)

19. ELECTROCARDIOGRAPHIC GRID The paper that is used for recording ECG is called ECG paper. ECG machine amplifies the electrical signals produced from the heart and records these signals on a moving ECG paper. Electrocardiographic grid refers to the markings (lines) on ECG paper. ECG paper has horizontal and vertical lines at regular intervals of 1 mm. Every 5th line(5 mm) is thickened. Duration Time duration of different ECG waves is plotted horizontally on X-axis. On X-axis 1 mm = 0.04 second 5 mm = 0.20 second ( K_Sembulingam, 2012)

20. AMPLITUDE Amplitude of ECG waves is plotted vertically on Y-axis. On Y-axis 1 mm = 0.1 mV 5 mm = 0.5 mV SPEED OF PAPER Movement of paper through the machine can be adjusted by two speeds, 25 mm/second and 50 mm/second. Usually, speed of the paper during recording is fixed at 25 mm/second. If heart rate is very high, speed of the paper is changed to 50 mm/second. (K_Sembulingam, 2012)

21. Electrographic Grid (K_Sembulingam, 2012)

22. ECG Leads ECG is recorded by placing series of electrodes on the surface of the body. These electrodes are called ECG leads and are connected to the ECG machine. Einthoven Triangle Einthoven triangle is defined as an equilateral triangle that is used as a model of standard limb leads used to record electrocardiogram. The heart is said to lie in the center of Einthoven Triangle. (K_Sembulingam, 2012)

23. Einthoven Law Einthoven's law states that if the electrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third one can be determined mathematically by simply summing the first two (but note that the positive and negative signs of the different leads must be observed when making this summation). (Guyton and Hall, 2006)

24. Einthoven Triangle (ptreviewer.com, 2018)

25. Electrical potential generated from the heart appears simultaneously on the roots of the three limbs, namely the left arm, right arm and the left leg. ECG is recorded in 12 leads, which are generally classified into two categories. I. Bipolar leads II. Unipolar leads. (K_Sembulingam, 2012)

26. The term bipolar means that the electrocardiogram is recorded from two electrodes located on different sides of the heart, in this case, on the limbs. Thus, a lead is not a single wire connecting from the body but a combination of two wires and their electrodes to make a complete circuit between the body and the electrocardiograph. From the position of a limb towards the heart makes it positive, while from the position of the heart to anothe limb makes it positive (Guyton and Hall, 2006) Standard limb leads are of three types: a. Limb lead I b. Limb lead II c. Limb lead III. (K_Sembulingam, 2012) Bipolar leads

27. Lead I Lead I is obtained by connecting right arm and left arm. Right arm is connected to the negative terminal of the instrument and the left arm is connected to the positive terminal. Lead II Lead II is obtained by connecting right arm and left leg. Right arm is connected to the negative terminal of the instrument and the left leg is connected to the positiveterminal. Lead III Lead III is obtained by connecting left arm and left leg. Left arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal. (K_Sembulingam, 2012) 1 2 3

28. Bipolar leads (K_Sembulingam, 2012)

29. Here, one electrode is active electrode and the other one is an indifferent electrode. Active electrode is positive and the indifferent electrode is serving as a composite negative electrode. Unipolar leads are of two types: 1. Augmented unipolar limb leads 2. Unipolar chest leads. Augmented Unipolar Limb Leads Augmented unipolar limb leads are also called augmented limb leads or augmented voltage leads. Active electrode is connected to one of the limbs. They are of three types: i. aVR lead ii. aVL lead iii. aVF lead. (K_Sembulingam, 2012) Unipolar leads

30. aVR lead Active electrode is from right arm. Indifferent electrode is obtained by connecting left arm and left leg. aVL lead Active electrode is from left arm. Indifferent electrode is obtained by connecting right arm and left leg. aVF lead Active electrode is from left leg (foot). Indifferent electrode is obtained by connecting the two upper limbs. (K_Sembulingam, 2012) 1 2 3

31. Unipolar Limb leads

32. Often electrocardiograms are recorded with one electrode placed on the anterior surface of the chest directly over the heart. The negative electrode, called the indifferent electrode, is connected through equal electrical resistances to the right arm, left arm, and left leg all at the same time, Active electrode is placed on six points over the chest. This electrode is known as the chest electrode and the six points overnthe chest are called V1, V2, V3, V4, V5 and V6. V indicates vector In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because, the chest electrode in these leads is nearer to the base of the heart than to the apex. which is the direction of electronegativity during most of the ventricular depolarization process. Conversely, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is nearer the heart apex, which is the direction of electropositivity during most of depolarization. (K_Sembulingam, 2012) Unipolar Chest leads

33. V1 : Over 4th intercostal space near right sternal margin V2 : Over 4th intercostal space near left sternal margin V3 : In between V2 and V4 V4 : Over left 5th intercostal space on the mid clavicular line V5 : Over left 5th intercostal space on the anterior axillary line V6 : Over left 5th intercostal space on the mid axillary line. (K_Sembulingam, 2012) Position Unipolar Chest leads

34. Unipolar Chest leads (K_Sembulingam, 2012)

35. Waves of ECG Normal electrocardiogram has the following waves, namely P, Q, R, S and T. Major Complexes and Waves in ECG 1. ‘P’wave, the atrial depolarization complex 2. ‘QRS’complex, the initial ventricular depolarization complex 3. ‘T’wave, the final ventricular repolarization complex 4. ‘QRST’, the ventricular complex. (K_Sembulingam, 2012)

36. ‘ P’ wave is a positive wave and the first wave in ECG. It is also called atrial complex. Cause ‘P’ wave is produced due to the depolarization of atrial musculature. Depolarization spreads from SA node to all parts of atrial musculature. Duration Normal duration of ‘P’ wave is 0.1 second. Amplitude Normal amplitude of ‘P’ wave is 0.1 to 0.12 mV. (K_Sembulingam, 2012) P waves

37. QRS’ complex is also called the initial ventricular complex.‘Q’ wave is a small negative wave. It is continued as the tall ‘R’ wave, which is a positive wave. ‘R’wave is followed by a small negative wave, the‘S’ wave. Cause ‘QRS’ complex is due to depolarization of ventricular musculature. ‘Q’ wave is due to the depolarization of basal portion of interventricular septum. ‘R’ wave is due to the depolarization of apical portion of interventricular septum and apical portion of ventricular muscle. ‘S’wave is due to the depolarization of basal portion of ventricular muscle near the atrioventricular ring. Duration Normal duration of ‘QRS’ complex is between 0.08 and 0.10 second. Amplitude Q’ wave = 0.1 to 0.2 mV. R’ wave = 1 mV. S’ wave = 0.4 mV. (K_Sembulingam, 2012) QRS Complex

38. ‘T’ wave is the final ventricular complex and is a positive wave. Cause ‘T’ wave is due to the repolarization of ventricular musculature. Duration Normal duration of ‘T’ wave is 0.2 second. Amplitude Normal amplitude of ‘T’ wave is 0.3 mV. (K_Sembulingam, 2012) T waves

39. Waves of Normal ECG (K_Sembulingam, 2012)

40. 12 Leads ECG (K_Sembulingam, 2012)

41. Intervals and Segments of ECG ‘P-R’ Interval ‘P-R’ interval is the interval between the onset of ‘P’wave and onset of ‘Q’ wave.‘P-R’ interval signifies the atrial depolarization and conduction of impulses through AV node. It shows the duration of conduction of the impulses from the SA node to ventricles through atrial muscle and AV node. Duration Normal duration of ‘P-R interval’ is 0.18 second and varies between 0.12 and 0.2 second. If it is more than 0.2 second, it signifies the delay in the conduction of impulse from SA node to the ventricles. Usually, the delay occurs in the AV node. So it is called the AV nodal delay. (K_Sembulingam, 2012)

42. Clinical Significance Variation in the duration of ‘P-R’ intervals helps in the diagnosis of several cardiac problems such as: It is prolonged in bradycardia and first degree heart block It is shortened in tachycardia, Wolf-Parkinson-White syndrome, Lown-Ganong-Levine syndrome, Duchenne muscular dystrophy and type II glycogen storage disease. (K_Sembulingam, 2012)

43. Q-T Interval ‘Q-T’ interval is the interval between the onset of ‘Q’wave and the end of ‘T’ wave.‘Q-T’ interval indicates the ventricular depolarization and ventricular repolarization, i.e. it signifies the electrical activity in ventricles. Duration Normal duration of Q-T interval is between 0.4 and 0.42 second. Clinical Significance ‘Q-T’ interval is prolonged in long ‘Q-T’ syndrome, myocardial infarction, myocarditis, hypocalcemia and hypothyroidism ‘Q-T’ interval is shortened in short ‘Q-T’ syndrome and hypercalcemia. (K_Sembulingam, 2012)

44. S-T Interval ‘ S-T’ segment is the time interval between the end of‘S’ wave and the onset of ‘T’ wave. It is an isoelectric period. Duration of ‘S-T’ Segment Normal duration of ‘S-T’ segment is 0.08 second. Clinical Significance Elevation of ‘S-T’ segment occurs in anterior or inferior myocardial infarction, left bundle branch block and acute pericarditis. Depression of ‘S-T’ segment occurs in acute myocardial ischemia, posterior myocardial infarction, ventricular hypertrophy and hypokalemia. S-T’ segment is prolonged in hypocalcemia S-T’ segment is shortened in hypercalcemia (K_Sembulingam, 2012)

45. R-R Interval ‘R-R’ interval is the time interval between two consecutive ‘R’ waves . Significance ‘R-R’ interval signifies the duration of one cardiac cycle. Duration Normal duration of ‘R-R’ interval is 0.8 second. Significance of Measuring ‘R-R’ Interval Measurement of ‘R-R’ interval helps to calculate: Heart rate Heart rate variability. (K_Sembulingam, 2012)

46. Heart Rate Heart rate is calculated by measuring the number of ‘R’ waves per unit time. Calculation of heart rates Time is plotted horizontally (X-axis). On X-axis, interval between two thick lines is 0.2 sec. Time duration for 30 thick lines is 6 seconds. Number of ‘R’waves (QRS complexes) in 6 seconds (30 thick lines) is counted and multiplied by 10 to obtain heart rate. For the sake of convenience, the ECG paper has special time marking at every 3 seconds. So it is easy to find the time duration of 6 seconds. Or you can divide 300 by the number of big squares per R–R interval (assumes the UK standard ECG speed of 25mm/s, elsewhere 50mm/s may be used) (K_Sembulingam, 2012)

47. ECG Interpretation Interpretation of the ECG is fundamentally about understanding the electrical conduction system of the heart. Normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be pathological. Theory Interpretation of the ECG is ultimately that of pattern recognition. In order to understand the patterns found, it is helpful to understand the theory of what ECGs represent. The theory is rooted in electromagnetics and boils down to the four following points: depolarization of the heart toward the positive electrode produces a positive deflection depolarization of the heart away from the positive electrode produces a negative deflection repolarization of the heart toward the positive electrode produces a negative deflection repolarization of the heart away from the positive electrode produces a positive deflection (en.wikipedia.org, 2019)

48. Electrocardiogram grid ECGs are normally printed on a grid. The horizontal axis represents time and the vertical axis represents voltage. The standard values on this grid are shown in the adjacent image: A small box is 1 mm × 1 mm and represents 0.1 mV × 0.04 seconds. A large box is 5 mm × 5 mm and represents 0.5 mV × 0.20 seconds. (en.wikipedia.org, 2019)

49. Rate and rhythm In a normal heart, the heart rate is the rate in which the sinoatrial node depolarizes as it is the source of depolarization of the heart. A heart rate less than normal is called bradycardia (<60 in adults) and higher than normal is tachycardia (>100 in adults). Normal sinus rhythm produces the prototypical pattern of P wave, QRS complex, and T wave. Generally, deviation from normal sinus rhythm is considered a cardiac arrhythmia. Thus, the first question in interpreting an ECG is whether or not there is a sinus rhythm. A criterion for sinus rhythm is that P waves and QRS complexes appear 1-to-1, thus implying that the P wave causes the QRS complex. Once sinus rhythm is established, or not, the second question is the rate. For a sinus rhythm this is either the rate of P waves or QRS complexes since they are 1-to-1. If the rate is too fast then it is sinus tachycardia and if it is too slow then it is sinus bradycardia.

50. If it is not a sinus rhythm, then determining the rhythm is necessary before proceeding with further interpretation. Some arrhythmias with characteristic findings: Absent P waves with "irregularly irregular" QRS complexes is the hallmark of atrial fibrillation A "saw tooth" pattern with QRS complexes is the hallmark of atrial flutter Sine wave pattern is the hallmark of ventricular flutter Absent P waves with wide QRS complexes and a fast heart rate is ventricular tachycardia Determination of rate and rhythm is necessary in order to make sense of further interpretation. (en.wikipedia.org, 2019)

51. Axis The heart has several axes, but the most common by far is the axis of the QRS complex (references to "the axis" imply the QRS axis). Each axis can be computationally determined to result in a number representing degrees of deviation from zero, or it can be categorized into a few types. The normal QRS axis is generally down and to the left, following the anatomical orientation of the heart within the chest. An abnormal axis suggests a change in the physical shape and orientation of the heart or a defect in its conduction system that causes the ventricles to depolarize in an abnormal way. (en.wikipedia.org, 2019)

52. Amplitudes and intervals All of the waves on an ECG tracing and the intervals between them have a predictable time duration, a range of acceptable amplitudes (voltages), and a typical morphology. Any deviation from the normal tracing is potentially pathological and therefore of clinical significance. For ease of measuring the amplitudes and intervals, an ECG is printed on graph paper at a standard scale: each 1 mm (one small box on the standard ECG paper) represents 40 milliseconds of time on the x-axis, and 0.1 millivolts on the y-axis. (en.wikipedia.org, 2019)

53. Noise pollution: It can affect the amplitude Faulty leads or parts Improper positioning of leads/bulbs Presence of metallic objects Movement of the patient Electrical interferal Excess Fatty tissue Excess chest hair Problems associated ECG

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