Electrocardiography Dr. Shafali Singh

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

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

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

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

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

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

conduction of cardiac impulse

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

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

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

ORIGIN & SPREAD OF CARDIAC IMPULSE

Correlation of ECG with phases of action potential

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.

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’.

Normal EKG

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

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

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.

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

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

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

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

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

ST segment elevation and depression

WAVE/INTERVAL/SEGMENT DURATION (sec) EVENT P WAVE 0.08 - 0.10 Atrial depolarisation P-R INTERVAL (onset of P to onset of QRS) 0.12 - 0.20 Conduction of impulse from the atria to the ventricles QRS COMPLEX 0.06 - 0.10 Ventricular depolarization T WAVE VARIABLE Ventricular repolarization Q-T INTERVAL (from onset of QRS to end of T wave) 0.30 - 0.40* Ventricular depolarization and repolarization S-T SEGMENT

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’.

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

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)

Standard ECG Paper Grid

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

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

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

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

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

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

UNIPOLAR CHEST LEADS

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

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

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

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

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