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ECG MASTERCLASS P ART 1 ASKAR KEEN NICHOLAS SAWHOverview • Cardiac conduction system • ECG waves and what they represent • How an ECG looks at the heart • Principles of ECG interpretation • Conduction abnormalities Cardiac conduction system Each normal heartbeat begins with the discharge (‘depolarization’) of the SA node. The impulse then spreads from the SA node to depolarize the atria and travels until it reaches the AV node, located in the lower RA. Once the impulse has traversed the AV node, it enters the bundle of His which then divides into left and right bundle branches as it passes into the interventricular septum. The right bundle branch conducts the wave of depolarization to the right ventricle, whereas the left bundle branch divides into anterior and posterior fascicles that conduct the wave to the left ventricle. The conducting pathways end by dividing into Purkinje fibres that distribute the wave of depolarization rapidly throughout both ventricles. Normal depolarization of the ventricles is therefore usually very fast, occurring in less than 0.12 s. PQRST : Where the waves come from • Impulse from SA node spreads and depolarizes the atria (P wave) • Time taken for depolarization wave to pass from its origin in SAN through the AVN into the ventricular muscle: PR interval • Current flowing L R between the bundle branches responsible for first deflection of QRS complex (whether it is an upward/downward deflection depends on which side of the septum the lead is looking from).PQRST : Where the waves come from • Twhich no more electrical current can ben passed through the myocardium. It is measured from the end of the S wave to the beginning of the T wave. (The ST segment is of particular interest in the diagnosis of myocardial infarction and ischaemia) • The T wave represents repolarization (‘recharging’) of the ventricular myocardium to its resting electrical state. • The QT interval measures the total time for activation of the ventricles and recovery to the normal resting statePQRST: Where the waves come fromHow an ECG looks at the heart • Each lead is given a name and its position on a 12-lead ECG is usually standardized to make pattern recognition easier • Limb leads: (I, II, III, aVR, aVL, aVF) • Chest leads (V1, V2, V3, V4, V5, V6)How an ECG looks at the heart • Once you know the view each lead has of the heart, you can tell that lead or away from it.ulses in the heart are flowing towards • Electrical current flowing towards a lead produces an upward (positive) deflection on the ECG, whereas current flowing away causes a downward (negative) deflection. • From the viewpoint of most leads, the electricity appears to flow towards them, and so the P wave will be a positive (upward) deflection. The exception is lead aVR, where the electricity appears to flow away, and so the P wave is negative in that lead.How an ECG looks at the heart • Leads II, III, aVF = Inferior • ST segment elevation present (Acute inferior MI) • Reciprocal ST segment depression in leads I and AVLPrinciples of ECG interpretation • 1). Assess the rhythm • 2). Assess the QRS axis and QRS morphology • 3). Assess the P waves, PR interval • 4). Assess the QRS complex, ST segments, T waves Principles of ECG interpretation: Rhythm A patient’s heart rhythm can be regular or irregular. Irregular rhythms can be either: • Regularly irregular (i.e. a recurrent pattern of irregularity) • Irregularly irregular (i.e. completely disorganised) • Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are similar. Principles of ECG interpretation: Rhythm Determine if the patient is • Normal: 60-100 bpm • Tachycardic: > 100 bpm • Bradycardic: < 60 bpm If a patient has a regular heart rhythm their heart rate can be calculated using the following method: • Count the number of large squares present within one R-R interval. • Divide 300 by this number to calculate heart rate . E.g. • 4 large squares in an R-R interval • 300/4 = 75 beats per minutePrinciples of ECG interpretation: Rhythm If a patient’s heart rhythm is irregular the first method of heart rate calculation doesn’t work (as the R-R interval differs significantly throughout the ECG). As a result, you need to apply a different method: • Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long). • Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute). E.g. • 10 complexes on a rhythm strip • 10 x 6 = 60 beats per minutePrinciples of ECG interpretation: Axis and morphology • the general direction thatf the wave of depolarization takes as it flows through the the overall vector of words, ventricular depolarization. • The QRS axis is therefore conventionally referred to as the angle, measured in electrical current flowingof through the ventricles. • This is why we like to look at lead II when monitoring ECG’sPrinciples of ECG interpretation: Axis and morphology • to +90°cardiac axis is between -30° • LAD: -30° to -90° • RAD: +90° to -180° • Anything outside is termed ‘extreme axis deviationPrinciples of ECG interpretation: Axis and morphology How to determine? Key: As a general rule of thumb; If the QRS complexes are predominantly normal (–30° to +90°)nd II, the QRS axis is • First look at Lead I, is there a +ve/-ve QRS? (positive or negative deflection) • Then look at Lead II and do the samePrinciples of ECG interpretation: Axis and morphologyPrinciples of ECG interpretation: P waves questions:e P waves and answer the following • 1. Are P waves present? • 2. If so, is each P wave followed by a QRS complex? • duration, direction and shape? – check • activity?aves are absent, is there any atrial • Sawtooth baseline → flutter waves • Chaotic baseline → fibrillation waves • Flat line → no atrial activity at allPrinciples of ECG interpretation: PR interval The PR interval should be between 0.12-0.2s (3-5 small squares) Prolonged PR interval (>0.2s) suggests the presence of atrioventricular delay (AV block) Shortened PR interval can mean one of two things: • S(the SA node is not in a fixed place and some people’s atria are smaller than others).n takes less time • The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall. This is an accessory pathway and can be associated with a delta wave (see below which demonstrates an ECG of a patient with Wolff Parkinson White syndrome.Principles of ECG interpretation: QRS complex In a similar method to PR interval, measure the small boxes in-between QRS complex. Normal value: 0.06-0.12s. (one small box=0.04s) Is the QRS complex: • Narrow: <0.12s • Broad: >0.12s • Small: defined as < 5mm in the limb leads or < 10 mm in the chest leads. • Tbe due to body habituse.g. tall slim people).an Anything >0.12 may suggest a dysrhythmia such as a PVC (Premature ventricular contractions):extra heartbeats that begin in one of the heart's two lower pumping chambers (ventricles).Principles of ECG interpretation: ST segment • In a healthy individual it should be an isoelectric line. • Abnormalities should be investigated to rule out pathology. • ST elevation most commonly caused by acute full-thickness myocardial infarction. • ST depression typically indicates myocardial ischemia. Principles of ECG interpretation: T Waves T waves represent repolarisation of the ventricles. T waves are considered tall if they are: • > 5mm in the limb leads AND • > 10mm in the chest leads Tall T waves can be associated with: • Hyperkalaemia (“tall tented T waves”) • Hyper-acute STEMI Inverted T waves in other leads are a nonspecific sign of a wide variety of conditions: • Ischaemia • Pulmonary embolism • LV hypertrophy (in lateral leads) • Bundle branch blocks ((V4-6 in LBBB and V1-V3 in RBBB) • General illness Conduction Abnormalities SA Node Bundle of His AV Node Left Bundle Branch Right Bundle BranchConduction Abnormalities Atrioventricular Blocks Blocked or delayed conduction between atria and ventricles ➢ 1 Degree ➢ 2 Degree ➢ 3 Degree Bundle Branch Blocks Blocked conduction through left or right bundle branches ➢ Left Bundle Branch Block ➢ Right Bundle Branch BlockAtrioventricular Blocks ➢ Blocked conduction between atria and ventricles ➢ 1 Degree – Delayed impulse = prolonged PR ➢ 2 Degree – Impulses intermittently become completely blocked from reaching ventricles – ‘skipped beats’ ➢ 3 Degree – Complete AV dissociation All about the relationship between P wave and QRS! st 1 Degree AV Block • Delayed conduction through AV node - impulse is late to arrive at ventricles st 1 Degree AV Block PR interval >1 big box • Delayed conduction through AV node - impulse is late to arrive at ventricles • Prolonged PR interval - (>200ms / 1 big box) • Regular rhythm – no missed beats st 1 Degree AV Block PR interval >1 big box • 1º AV block is nonspecific: ➢ Patients on beta blockers ➢ Normal variant in athletes / young ppl ➢ Coronary artery disease, rheumatic heart disease and other causes of fibrosis1 Degree AV Block 2 nd Degree AV Block • Heart ‘skips a beat’ • Occasional impulse completely fails to pass through the AV node and never reaches the ventricles • Results in a dropped QRS • 2 Subtypes based on PR interval: ➢ Mobitz Type I ➢ Mobitz Type II nd 2 Degree AV Block (Mobitz I) 2 nd Degree AV Block (Mobitz I) In Mobitz Type I: • The PR interval gets progressively longer and longer until a QRS is dropped • This cycle then repeats • aka ‘Wenckebach’ rhythm nd 2 Degree AV Block (Mobitz II) 2 nd Degree AV Block (Mobitz II) In Mobitz Type II: • The PR intervals stay the same, QRS complexes drop suddenly • This cycle then repeats • May result in severe bradycardia Mobitz I or Mobitz II? ➢ Mobitz Type II ➢ (3:2 conduction ratio) 3 rd Degree AV Block • Complete AV dissociation, atrial impulses do not reach ventricles • P waves and QRS complexes have no relationship • Ventricles generate their own impulses (escape beat) – bradycardia ++ rd 3 Degree AV BlockBundle Branch Blocks • Blockage in left or right bundle branch Bundle Branch Blocks • Conduction blocked in left or right bundle branch • Unblocked pathway depolarises first, followed by slower depolarisation of affected ventricle • Results in widened QRS (>3 small squares) and strange QRS shapes • Seen in V1 and V6 V1 V6 Right Bundle Branch Block V6 V1 R’ r S • V1: Notched, M-shape • V6: Slurred S wave pattern (rSR’) • Terminal R’ wave in V1 Right Bundle Branch Block • Wide QRS (>3 small squares) • V1: Notched pattern (RSR’) → M-shape • V6: Slurred S wave in V6 Causes of RBBB include: ➢ RVH ➢ PE ➢ Pulmonary Hypertension ➢ Ischaemic Heart Disease ➢ May be a normal variant in elderlyRight Bundle Branch BlockRight Bundle Branch BlockRight Bundle Branch Block Left Bundle Branch Block V6 V1 • V1: Wide, negative QRS • V6: Wide, upright QRS • May be notched • Dominant S wave Left Bundle Branch Block • Wide QRS • V1: Deep S wave (downward deflection) • V6: Broad upright R wave, notched at top New LBBB is always pathological: ➢ Acute MI ▪ –*chest pain + new LBBB? = Think STEMI!* ➢ Aortic Stenosis ➢ HTN ➢ Previous ischaemiaLeft Bundle Branch Block Left Bundle Branch Block V6 = Wide QRS with upwards V1 = Wide QRS with dominant deflection and T wave downwards deflection inversions (dominant S wave) LBBB vs RBBB - A Simple Approach Spotting a bundle branch block ➢ Is the QRS widened in V1? (> 3 small squares) Look at V1 and V6 ➢ QRS in V1 ends in a large upwards deflection? (terminal R’) = RBBB ➢ QRS in V1 ends after large downwards deflection? (dominant S wave ) = LBBBLBBB or RBBB?LBBB or RBBB?LBBB or RBBB?Clinical Case Clinical Case A 49-year-old man is admitted to the coronary care unit following an inferior myocardial infarction. His ECG is as follows: FEEDBACK Please scan the QR code and fill out our feedback form to receive your certificate and a copy of the presentations!