2. Electrocardiography Dr. Shafali Singh

3. Objective: Electrocardiography To define ECG Genesis of ECG
Identify different waves of ECG with their causes

4. Introduction
Electrocardiography is the method of recording an Electrocardiogram (ECG) with the help of the machine Electrocardiograph.

5. Definition
Electrocardiogram (ECG) is a graphic recording of the electrical changes that occur within the heart during the cardiac cycle.

6. Characteristics of SA Nodal Cells
depolarization is achieved mainly by influx of Ca++ through L-type Ca++ channels instead of influx of Na+ through fast Na+ channels. Repolarization is accomplished in these fibers by inactivation of the Ca++ channels and by the increased K+ conductance through the iK1 and iK channels
SA Nodal (Pacemaker) Action Potential

8. the slow response, occurs in the sinoatrial (SA) node, which is the natural pacemaker region of the heart, and in the atrioventricular (AV) node, which is the specialized tissue that conducts the cardiac impulse from the atria to the ventricles

9. Principal ionic currents and channels that generate the various phases of the action potential in a cardiac cell. Phase 0: The chemical and electrostatic forces both favor the entry of Na+ into the cell through fast Na+ channels to generate the upstroke. Phase 1: The chemical and electrostatic forces both favor the efflux of K+ through ito channels to generate early, partial repolarization. Phase 2: During the plateau, the net influx of Ca++ through Ca++ channels is balanced by the efflux of K+ through iK, iK1, and ito channels. Phase 3: The chemical forces that favor the efflux of K+ through iK, iK1, and ito channels predominate over the electrostatic forces that favor the influx of K+ through these same channels. Phase 4: The chemical forces that favor the efflux of K+ through iK and iK1 channels very slightly exceed the electrostatic forces that favor the influx of K+ through these same channels.
The action potential amplitude (the potential change during phase 0) is dependent on [Na+]o. When [Na+]o is decreased, the amplitude of the action potential decreases, and when [Na+]o is reduced from its normal value of about 140 mEq/L to about 20 mEq/L, the cell is no longer excitable. voltage-activated Na+ channels can be blocked by the puffer fish toxin tetrodotoxin. In addition, many drugs used to treat certain cardiac rhythm disturbances (cardiac arrhythmias) act by blocking these fast Na+ channels.
The Na+ channels remain in the inactivated state until the membrane begins to repolarize. With repolarization, the channel transitions to the closed state, from which it can then be reopened by another depolarization of Vm to the threshold. These properties of the Na+ channel underlie the basis of the action potential refractory period. When the Na+ channels are in the inactivated state, they cannot be reopened, and another action potential cannot be generated. During this period the cell is said to be in the effective refractory period. This prevents a sustained, tetanic contraction of cardiac muscle, which would retard ventricular relaxation and therefore interfere with the normal intermittent pumping action of the heart. As the cell repolarizes (phase 3), the inactivated channels begin to transition to the closed state. During this period, called the relative refractory period, another action potential can be generated, but it requires a larger than normal depolarization

10. conduction of cardiac impulse

11. Conduction system of the heart: SA node is the pacemaker.

12. Conduction Pathways and Velocity of Conduction
SA node → atrial muscle → AV node (delay) → Purkinje fibers → ventricular muscle
Velocity
Fastest conducting fiber = Purkinje cell
Slowest conducting fiber = AV node

13. Which of the following has the fastest conducting fibers in the heart?
SA node
Atrial muscle
AV node
Purkinje fibers
Ventricular muscle

14. ORIGIN & SPREAD OF CARDIAC IMPULSE

21. Correlation of ECG with phases of action potential

22. Principle
Electrical activities generated with each heart beat are conducted from the heart to the body surface, which are picked up and recorded by the electrocardiograph.

26. The muscle mass of the atria is small compared with that of the ventricles, and the electrical change accompanying the contraction of the atria is therefore small. Contraction of atria is associated with the ECG wave called ‘P’.
The ventricular mass is large and so there is a large deflection of the ECG wave when ventricle is depolarised. This is called the ‘QRS’ complex.
The ‘T’ wave of ECG is associated with the return of ventricular mass to its resting electrical state – ‘repolarization’.

28. Normal EKG

29. P wave Cause: atrial depolarization Configuration: upright and round
Abnormalities:
Absent- SA nodal block, A.Fibrillation and
hyperkalemia
Wide and notched- Left atrial enlargement
Tall and peaked- Right atrial enlargement (inTricuspid Stenosis)
Fibrillation- p wave replaced by flutter wave

30. QRS complex Q – Septal depolarization
R – Depolarisation of ant part of ventricle
S – Depolarisation of base of ventricle
Abnormalities:
Deep Q wave – sign of Myocardial ischemia
Tall R wave – ventricular hypertrophy
Low voltage QRS complex – hypothyroidism, pericardial effusion
Wide bizarre QRS complex – ventricular fibrillation
QRS duration: less than 0.12 sec

31. Shape of QRS complexes
If the QRS complex is predominantly upward, or positive (i.e. the R wave greater that the S wave), the depolarization is moving towards that lead and if predominantly downward, moving away from that lead.
When the depolarization wave is moving at right angles to the lead, the R and S waves are of equal size.
If the QRS complex is predominantly upward, or positive (i.e. the R wave greater that the S wave), the depolarization is moving towards that lead
if predominantly downward, moving away from that lead.
When the depolarization wave is moving at right angles to the lead, the R and S waves are of equal size.

32. T wave Cause- repolarization of ventricles
Configuration- upright and rounded
Abnormalities;
Inverted T wave- Myocardial ischemia
young children
Tall and peaked- Myocardial infarction,
hyperkalemia

33. U Wave Small rounded, upright wave following T wave;
Due to slow repolarisation of papillary muscles
Prominent in hypokalemia

34. Intervals
PR Interval - Distance between beginning of P wave and beginning of QRS complex;
Measures time during which a depolarization wave travels from the atria to the ventricles;
It measures the AV conduction time including AV conduction delay
Normal: 0.12–0.20 sec
Abnormalities;
Incr PR – AV conduction block, ischemic heart disease, rheumatic fever
Dec PR – AV nodal rhythm, WPW syndrome
PR segment – iso electric line
WPW syndrome- due to passage of impulse through an abnormal path bypassing the AV junction

35. Intervals
QT Interval - Measured from beginning of QRS to end of T wave;
Represents total ventricular activity.(entire duration of depolarization and repolarization of the ventricular muscles)
Normal: less than or equal to 0.4 sec

36. ST Segment – It is an isoelectric period between the end of QRS complex and beginning of T wave.
Measures time between complete ventricular depolarization and beginning of repolarization
Abnormalities –
Depressed in acute myocardial ischemia
Elevated in myocardial infarction

37. ST segment elevation and depression

38. WAVE/INTERVAL/SEGMENT
DURATION (sec)
EVENT
P WAVE
Atrial depolarisation
P-R INTERVAL (onset of P to onset of QRS)
Conduction of impulse from the atria to the ventricles
QRS COMPLEX
Ventricular depolarization
T WAVE
VARIABLE
Ventricular repolarization
Q-T INTERVAL (from onset of QRS to end of T wave) *
Ventricular depolarization and repolarization
S-T SEGMENT

39. Recording an ECG
The electrical signal from the heart is detected at the surface of the body through electrodes attached to the surface.
As a result of the sequence and the timing of the spread of depolarization and repolarization in the myocardium, potential differences are established between different portions of the heart, which can be detected by electrodes placed on the body surface
The ECG recorder compares the electrical activity detected in different electrodes and the electrical picture so obtained is called a ‘lead’.

40. Requirements ECG machine
Cardiac jelly – to establish proper contact of the electrode plates to the body
ECG paper
ECG leads

41. ECG PAPER Heat sensitive plastic coated paper
Has vertical (amplitude) & horizontal (time) lines 1mm apart
Heavy line every 5mm in both planes
Conventional speed – 25mm/sec (5 big squares)
Small square = 0.04sec
Big square = 0.20sec
Sensitivity – Voltage is measured along vertical axis
1mV = 10mm (2 big squares)

42. Standard ECG Paper Grid

43. ECG leads Direct leads Indirect leads
Limb leads – Unipolar & Bipolar (record electrical potentials transmitted onto the frontal plane)
Chest leads – Unipolar (record electrical potentials transmitted onto the transverse plane)
Esophageal leads

44. The 12 lead ECG Has 6 limb leads and 6 chest leads Limb Leads
Electrodes are placed on the right arm (RA), left arm (LA), right leg (RL), and left leg (LL). With only four electrodes, six leads are viewed.
■ Standard leads: I, II, III
■ Augmented leads: aVR, aVL, aVF

45. BIPOLAR LIMB LEADS
Einthoven’s leads to record electrical potential on the frontal plane
Two similar electrodes are placed on the body surface & the potential difference between them are recorded

46. BIPOLAR LIMB LEADS
Lead I – Between the right arm (-) & the left arm (+)
Lead II - Between the right arm (-) & the left leg (+)
Lead III - Between the left arm (-) & the left leg (+)
Right leg acts as ground wire to prevent external disturbance

47. AUGMENTED UNIPOLAR LIMB LEADS
One limb is the active electrode & the other 2 electrodes are connected to zero potential through 5000 ohms resistance (indifferent electrode)
aVR- RIGHT ARM ACTIVE
aVL- LEFT ARM ACTIVE
aVF- LEFT FOOT ACTIVE

48. UNIPOLAR CHEST LEADS Exploring electrode on the chest V1 – V6
Reference electrode is connected to the right arm, left arm & the left leg through a high resistance – Wilson’s terminal
Right leg – grounding electrode

49. UNIPOLAR CHEST LEADS

51. PROCEDURE Make the subject to lie down on a couch comfortably
Clean the skin over both wrists & both legs above the ankle joint & apply jelly
Connect the leads in these positions
Switch on the machine & place the stylus at the centre of the paper
Adjust the sensitivity to get a standard calibration
Adjust the lead selector in order – I, II, III, aVR, aVL, aVF & record
Place the chest electrodes V1 – V6 & record

52. ANATOMY OF THE WAVES
In the ECG, each wave has either a positive or a negative deflection
When the current flows towards the electrode, a positive deflection is recorded
When the current flows away from the electrode, a negative deflection is recorded

56. HEART RATE At a speed of 25mm/sec (1500mm/min),
Ventricular rate = 1500/ RR interval in mm
1500/no of small squares
(or)
300/no of big squares

57. Extent, location and progress of ischaemic damage to the myocardium
Uses
A very useful noninvasive, inexpensive and versatile diagnostic tool but must be interpreted with the clinical features of the patient and other investigations
Extent, location and progress of ischaemic damage to the myocardium
Relative sizes of the chambers
Disturbances of rhythm and conduction
Effects of altered electrolyte concentration
Anatomical orientation of the heart
Influence of drugs

58. Thank you