How to read an Electrocardiogram (ECG), Lecture notes of Cardiology

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How to read an ECG

CONTENTS

 INTRODUCTION

 CONDUCTION SYSTEM

 RECORDING AN ECG

 12-LEAD ECG

 ECG SHAPE

 ECG WAVEFORMS

To enable an accurate and reproducible recording of an ECG between patients, ECG machines (and ECG paper) need to be standardised and calibrated appropriately. Normal ECG  Standard speed: the speed of tracing on ECG paper. Set at 25 mm/s. Every 5 large squares equals 1 second.  Standard paper: large square = 5 mm (0.2 seconds). Small square = 1 mm (0.04 seconds).  Calibration: 1 millivolt of electrical activity moves stylus 1 cm on ECG paper 12-lead ECG A standard 12-lead ECG utilises 10 leads to offer 12 different ‘views’ of the heart. A 12-lead ECG is recorded by attaching 10 leads (or electrodes) to different areas of skin on a patient. This allows us to visualise the heart from different angles. The word ‘lead’ is often used for both the wires that connect the ECG machine to the patients skin and the electrical picture of the heart. The image shows the standard ECG electrode locations. The ECG recorder compares electrical activity detected from each electrode. This allows formation of an ‘electrical picture’ that views the heart from different positions. Each electrical picture is termed a lead. With a 12-lead ECG we get 12 characteristic views of the heart, which we divide into limb leads and chest leads.

Limb leads There are six standard limbs leads, which are recorded from the electrodes that attached to the patients limbs. These leads look at the heart from the vertical plane.  II, III, aVF: inferior leads. Look at the inferior surface of the heart  I, aVL: lateral leads. Look at the left lateral surface of the heart  aVR: right arm lead. Looks at the right atrium of the heart. NOTE: due to its location, aVR should always be negative (predominantly negative deflection in QRS complex). If it is positive it may suggest wrong lead placement. Chest leads There are six standard chest leads, which are recorded from the electrodes that attach to the patient’s chest. These leads look at the heart from the horizontal plane.  V1, V2: septal leads. View the right ventricle of the heart and septum between ventricles.  V3, V4: anterior leads. View the anterior wall of the left ventricle  V5, V6: lateral leads. Look at the anterior and lateral wall of the left ventricle.

A normal ECG waveform is composed of:  P-waves: atrial depolarisation  QRS complexes (<120 ms): ventricular depolarisation. If first deflection is down it is a Q- wave, if the first deflection is up it is an R-wave.  T-waves: ventricular repolarisation.  U-waves: sometimes seen, origin disputed. May be pathological if follows abnormal T-wave The ECG may be deconstructed into additional key components:  PR-interval (120-200 ms): time taken for the electrical impulse to travel between the atria and ventricles.  ST-segment: should be isoelectric (i.e. on the baseline). Can be depressed or elevated (changes typical in ischaemia).  QT-interval:* varies with heart rate, long QT has many causes but may predispose to polymorphic ventricular tachycardia. *NOTE: normal QT interval is 350-440 ms in men and 350-460 ms in women QRS shape

The shape of the QRS complex is due to the imbalance between right and left ventricle and depolarisation of the septum first. Q waves develop because the septum between the ventricles undergoes depolarisation before the walls. The wave of depolarisation within the septum is from left to right. This means towards the septal leads (V1/V2) and away from the lateral leads (V5/V6). The more muscular left ventricle then exerts more influence on the ECG than the right ventricle leading to a dominant R wave in the lateral chest leads (V5/V6). In the septal leads (V1/V2) we see a small R wave and dominant S wave. After depolarisation of the whole myocardium, the ECG trace of the QRS complex moves back to the baseline, otherwise known as the isoelectric line.

Rate & rhythm Introduction

Rate and rhythm are the first things to assess when analysing an ECG. The rate refers to the frequency of electrical activity. It correlates with muscular contraction and therefore heart rate. Normal electrical activity in the absence of contraction is termed ‘pulseless electrical activity’. This rhythm is not compatible with life and can be seen in cardiac arrest. The rhythm refers to the area of the heart that is controlling electrical activity. In other words, the part of the heart that is initiating electrical activity, which then spreads throughout the heart. Due to spontaneous depolarisation, different parts of the heart can initiate electrical activity at different set rates. This acts as a ‘back-up’ if the normal pathway fails. Rate Rate is defined as the number of times the heart beats per minute. Rate on the ECG can be one of three broad categories:  Normal: 60-100 bpm  Slow (bradycardia): < 60 bpm  Fast (tachycardia): > 100 bpm When looking at the ECG, we can determine both the atrial rate (i.e. how frequently the atria are contracting) and the ventricular rate (i.e. how frequently the ventricles are contracting). Under normal circumstances, we simply determine the ventricular rate, which will reflect the atrial rate. There are two ways of calculating the rate: division method and multiplication method. Division method Look at two adjacent QRS complexes and count the number of large squares between the two R waves. Then divide 300 by this number. For example, if there are 4 large squares between two adjacent R waves, the corresponding rate is 75 bpm. This method is used when the rhythm is regular (i.e. the distance between two adjacent R waves is constant).

the term SR, we should comment on the presence of any abnormalities (e.g. heart block, extra beats, abnormal ventricular activity). Regular or irregular As part of rhythm assessment, it is important to determine whether the ECG trace is regular or irregular. Assessment of whether a rhythm is regular or irregular refers to the distance between adjacent QRS complexes. If the R-R interval is constant, we can assume the rhythm is regular. If the R-R interval differs across beats we can say the rhythm is irregular. Causes of regular rhythm  NSR  Sinus tachycardia  Sinus bradycardia  Supraventricular tachycardia  Atrial flutter Causes of irregular rhythm  Sinus arrhythmia  Ectopic beats  Pauses & blocks  Atrial fibrillation  Atrial flutter with variable block Sinus arrhythmia

The SAN can be influenced by several external factors including the parasympathetic nervous system via the vagus nerve, and by respiration through reflexes originating in the lungs. Inspiration can inhibit vagal tone increasing the heart rate, whereas expiration can increase vagal tone reducing heart rate. This difference may lead to a small beat by beat variation, which is felt clinically as an irregular pulse. On the ECG, it is observed as SR with an alteration in the R-R interval >0.12 seconds between different R waves. We term this sinus arrhythmia. It is generally considered a normal variant in healthy young adults. Slow or fast We can broadly divide abnormal heart rhythms into slow (bradycardias) or fast (tachycardias). Under normal circumstances at rest, our heart should be in SR with an accompanying rate of 60-100 bpm. If the defining features of NSR are met but the heart rate is fast (> 100 bpm), we call it sinus tachycardia. If the defining features of NSR are met but the heart rate is slow (< 60 bpm), we call it sinus bradycardia. We term abnormal heart rhythms arrhythmias and they can be broadly divided into bradycardias and tachycardias. Causes of bradyarrhythmias  Sinus bradycardia  Sinus pauses and blocks  Escape rhythms: junctional, ventricular  Heart blocks: Wenckebach, type II Mobitz, complete heart block Causes of tachyarrhythmias  Sinus tachycardia  Supraventricular tachycardia  Ventricular tachycardia  Atrial fibrillation or flutter with fast ventricular response

Alternatively, an abnormal supraventricular rhythm may arise because another area has initiated, and potentially continued to initiate, electrical activity. Common supraventricular rhythms include atrial ectopics (discussed below), atrial tachycardia, atrial fibrillation, atrial flutter and supraventricular tachycardia (nodal and non-nodal). In each of these rhythms, initiation of ectopic electrical activity has a unique aetiology. Ventricular When a rhythm originates from within the ventricles, it is termed a ventricular rhythm. A ventricular rhythm may originate from anywhere within the ventricles. As the ectopic initiation of electrical activity does not pass through the normal conduction system, it can only slowly depolarise the myocardium leading to a broad QRS (> 120 ms). This helps us differentiate it from a supraventricular rhythm (unless there is a co-existent bundle branch block).

Origin of ventricular rhythm An abnormal ventricular rhythm may arise as an escape rhythm due to failure of propagation of electrical activity from the atria to the ventricles. This is seen in compete heart block due to disease of the AVN and presents with a very slow ventricular rate due to its much lower rate of spontaneous depolarisation. NOTE: Due to the fibrous rings of the heart, electrical activity may only pass through the AVN to the ventricles. Alternatively, a focus of electrical activity may occur within the ventricles leading to rapid depolarisation. This type of rhythm is referred to as ventricular tachycardia, which is a life- threatening arrhythmia that can lead to loss of cardiac output and subsequent cardiac arrest.

Ventricular ectopic  Absent P wave  Broad, abnormal QRS  No compensatory pause (does not reset P wave cycle)

INTRODUCTION

 NORMAL AXIS

 LEFT AXIS DEVIATION

 RIGHT AXIS DEVIATION

 WORKING OUT AXIS

 TRANSITION POINT