Electrocardiograph PDF

‘ECG’ stands for electrocardiogram, or electrocardiograph. In some countries, the abbreviation used is ‘EKG’. Clinical diagnosis depends mainly on a patient’s history, and to a lesser extent on the physical examina...

‘ECG’ stands for electrocardiogram, or electrocardiograph. In some countries, the abbreviation used is ‘EKG’. Clinical diagnosis depends mainly on a patient’s history, and to a lesser extent on the physical examination. The ECG can provide evidence to support a diagnosis, and in some cases it is crucial for patient management. The ECG is essential for the diagnosis, and therefore the management, of abnormal cardiac rhythms. It helps with the diagnosis of the cause of chest pain, and the proper use of early intervention in myocardial infarction depends upon it. THE DIFFERENT PARTS OF THE ECG The muscle mass of the atria is small compared with that of the ventricles, and so the electrical change accompanying the contraction of the atria is small. Contraction of the atria is associated 4 with the ECG wave called ‘P’. The ventricular mass is large, and so there is a large deflection of the ECG when the ventricles are depolarized: this is called the ‘QRS’ complex. The ‘T’ wave of the ECG is associated with the return of the ventricular mass to its resting electrical state (‘repolarization’). The letters P, Q, R, S and T were selected in the early days of ECG history, and were chosen arbitrarily. The P Q, R, S and T deflections are all called waves; the Q, R and S waves together make up a complex.  In some ECGs an extra wave can be seen on the end of the "T" wave, and this is called a "U" wave. Its origin is uncertain, though it may represent repolarization of the papillary muscles. If a "U" wave follows a normally shaped T wave. The different parts of the QRS complex are labelled. If the first deflection is downward, it is called a Q wave. An upward deflection is called an R wave, regardless of whether it is preceded by a Q wave or not. Any deflection below the baseline following an R wave is called an S wave, regardless of whether there is a preceding Q wave. TIMES AND SPEEDS ECG machines record changes in electrical activity by drawing a trace on a moving paper strip. ECG machines run at a standard rate of 25 mm/s and use paper with standard-sized squares. Each large square (5 mm) represents 0.2 second (s), i.e. 200 milliseconds (ms). Relationship between the squares on ECG paper and time. Here, there is one QRS complex per second, so the heart rate is 60 beats/min Therefore, there are five large squares per second, and 300 per minute. So an ECG event, such as a QRS complex, occurring once per large square is occurring at a rate of 300/min. The heart rate can be calculated rapidly by remembering the sequence in Table.  Table: Relationship between the number of large squares between successive R waves and the heart rate Just as the length of paper between R waves gives the heart rate, so the distance between the different parts of the P–QRS–T complex shows the time taken for conduction of the electrical discharge to spread through the different parts of the heart. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex, and it is the time taken for excitation to spread from the SA node, through the atrial muscle and the AV node, down the bundle of His and into the ventricular muscle. Logically, it should be called the PQ interval, but common usage is ‘PR interval’. The components of the ECG complex The normal PR interval is 120–220 ms, represented by 3–5 small squares. Most of this time is taken up by delay in the AV node.  Normal PR interval and QRS complex If the PR interval is very short, either the atria have been depolarized from close to the AV node, or there is abnormally fast conduction from the atria to the ventricles. The duration of the QRS complex shows how long excitation takes to spread through the ventricles. The QRS complex duration is normally 120 ms (represented by three small. squares) or less, but any abnormality of conduction takes longer, and causes widened QRS complexes.  Normal PR interval and prolonged QRS complex Remember that the QRS complex represents depolarization, not contraction, of the ventricles – contraction is proceeding during the ECG’s ST segment. THE ECG – ELECTRICAL PICTURES The word ‘lead’ sometimes causes confusion. Sometimes it is used to mean the pieces of wire that connect the patient to the ECG recorder. Properly, a lead is an electrical picture of the heart. The electrical signal from the heart is detected at the surface of the body through electrodes, which are joined to the ECG recorder by wires. One electrode is attached to each limb, and six to the front of the chest. The ECG recorder compares the electrical activity detected in the different electrodes, and the electrical picture so obtained is called a ‘lead’. The different comparisons ‘look at’ the heart from different directions. For example, when the recorder is set to ‘lead I’ it is comparing the electrical events detected by the electrodes attached to the right and left arms. Each lead gives a different view of the electrical activity of the heart, and so a different ECG pattern. Strictly, each ECG pattern should be called ‘lead...’, but often the word ‘lead’ is omitted. The ECG is made up of 12 characteristic views of the heart, six obtained from the ‘limb’ leads (I, II, III, VR, VL, VF) and six from the ‘chest’ leads (V1–V6). It is not necessary to remember how the leads (or views of the heart) are derived by the recorder, but for those who like to know how it works. The electrode attached to the right leg is used as an earth, and does not contribute to any lead. THE 12-LEAD ECG ECG interpretation is easy if you remember the directions from which the various leads look at the heart. The six ‘standard’ leads, which are recorded from the electrodes attached to the limbs, can be thought of as looking at the heart in a vertical plane. The ECG patterns recorded by the six ‘standard’ leads Leads I, II and VL look at the left lateral surface of the heart, leads III and VF at the inferior surface, and lead VR looks at the right atrium.  The six V leads (V1–V6) look at the heart in a horizontal plane, from the front and the left side. Thus, leads V1 and V2 look at the right ventricle, V3 and V4 look at the septum between the ventricles and the anterior wall of the left ventricle, and V5 and V6 look at the anterior and lateral walls of the left ventricle. The relationship between the six chest leads and the heart As with the limb leads, the chest leads each show a different ECG pattern. In each lead the pattern is characteristic, being similar in individuals who have normal hearts.  The cardiac rhythm is identified from whichever lead shows the P wave most clearly – usually lead II. When a single lead is recorded simply to show the rhythm, it is called a ‘rhythm strip’, but it is important not to make any diagnosis from a single lead, other than identifying the cardiac rhythm. The ECG patterns recorded by the chest leads THE SHAPE OF THE QRS COMPLEX We now need to consider why the ECG has a characteristic appearance in each lead. THE QRS COMPLEX IN THE LIMB LEADS The ECG machine is arranged so that when a depolarization wave spreads towards a lead the stylus moves upwards, and when it spreads away from the lead the stylus moves downwards. Depolarization spreads through the heart in many directions at once, but the shape of the QRS complex shows the average direction in which the wave of depolarization is spreading through the ventricles. If the QRS complex is predominantly upward, or positive (i.e. the R wave is greater than the S wave), the depolarization is moving towards that lead.  If predominantly downward, or negative (the S wave is greater than the R wave), the depolarization is 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. Q waves, when present, have a special significance. Depolarization and the shape of the QRS complex THE CARDIAC AXIS Leads VR and II look at the heart from opposite directions. When seen from the front, the depolarization wave normally spreads through the ventricles from 11 o’clock to 5 o’clock, so the deflections in lead VR are normally mainly downward (negative) and in lead II mainly. upward (positive).  The cardiac axis The average direction of spread of the depolarization wave through the ventricles as seen from the front is called the ‘cardiac axis’. It is useful to decide whether this axis is in a normal direction or not. The direction of the axis can be derived most easily from the QRS complex in leads I, II and III. A normal 11 o’clock–5 o’clock axis means that the depolarizing wave is spreading towards leads I, II and III, and is therefore associated with a predominantly upward deflection in all these leads; the deflection will be greater in lead II than in I or III. The normal axis If the right ventricle becomes hypertrophied, it has more effect on the QRS complex than the left ventricle, and the average depolarization wave – the axis – will swing towards the right. The deflection in lead I becomes negative (predominantly downward) because depolarization is spreading away from it, and the deflection in lead III becomes more positive (predominantly upward) because depolarization is spreading towards it. It is associated mainly with pulmonary conditions that put a strain on the right side of the heart, and with congenital heart disorders. Right axis deviation When the left ventricle becomes hypertrophied, it exerts more influence on the QRS complex than the right ventricle. Hence, the axis may swing to the left, and the QRS complex becomes predominantly negative in lead III.  ‘Left axis deviation’ is not significant until the QRS complex deflection is also predominantly negative in lead II. Although left axis deviation can be due to excess influence of an enlarged left ventricle, in fact this axis change is usually due to a conduction defect rather than to increased bulk of the left ventricular muscle. Left axis deviation The cardiac axis is sometimes measured in degrees, though this is not clinically particularly useful. Lead I is taken as looking at the heart from 0°; lead II from +60°; lead VF from +90°; and lead III from +120°. Leads VL and VR look from – 30° and –150°, respectively. The cardiac axis and lead angles The normal cardiac axis is in the range –30° to +90°. If in lead II the S wave is greater than the R wave, the axis must be more than 90° away from lead II. In other words, it must be at a greater angle than –30°, and closer to the vertical, and left axis deviation is present. Similarly, if the size of the R wave equals that of the S wave in lead I, the axis is at right angles to lead I or at +90°. This is the limit of normality towards the ‘right’. If the S wave is greater than the R wave in lead I, the axis is at an angle of greater than +90°, and right axis deviation is present. WHY WORRY ABOUT THE CARDIAC AXIS? Right and left axis deviation in themselves are seldom significant – minor degrees occur in tall, thin individuals and in short, fat individuals, respectively. However, the presence of axis deviation should alert you to look for other signs of right and left ventricular hypertrophy…. THE QRS COMPLEX IN THE V LEADS The shape of the QRS complex in the chest (V) leads is determined by two things:  The septum between the ventricles is depolarized before the walls of the ventricles, and the depolarization wave spreads across the septum from left to right.  In the normal heart there is more muscle in the wall of the left ventricle than in that of the right ventricle, and so the left ventricle exerts more influence on the ECG pattern than does the right ventricle. Leads V1 and V2 look at the right ventricle; leads V3 and V4 look at the septum; and leads V5 and V6 at the left ventricle. In a right ventricular lead the deflection is first upwards (R wave) as the septum is depolarized. In a left ventricular lead the opposite pattern is seen: there is a small downward deflection (‘septal’ Q wave). In a right ventricular lead there is then a downward deflection (S wave) as the main muscle mass is depolarized – the electrical effects in the bigger left ventricle (in which depolarization is spreading away from a right ventricular lead) outweighing those in the smaller right ventricle. In a left ventricular lead there is an upward deflection (R wave) as the ventricular muscle is depolarized. When the whole of the myocardium is depolarized, the ECG trace returns to the baseline When the whole of the myocardium is depolarized, the ECG trace returns to the baseline. The QRS complex in the chest leads shows a progression from lead Vl, where it is predominantly downward, to lead V6, where it is predominantly upward. The ‘transition point’, where the R and S waves are equal, indicates the position of the interventricular septum.  Conduction and its problems CONDUCTION PROBLEMS IN THE AV NODE AND HIS BUNDLE The time taken for the spread of depolarization from the SA node to the ventricular muscle is shown by the PR interval, and is not normally greater than 220 ms (six small squares). Interference with the conduction process causes the phenomenon called ‘heart block’. FIRST DEGREE HEART BLOCK If each wave of depolarization that originates in the SA node is conducted to the ventricles, but there is delay somewhere along the conduction pathway, then the PR interval is prolonged. SECOND DEGREE HEART BLOCK Sometimes excitation completely fails to pass through the AV node or the bundle of His. When this occurs intermittently, ‘second degree heart block’ is said to exist. THIRD DEGREE HEART BLOCK Complete heart block (third degree block) is said to occur when atrial contraction is normal but no beats are conducted to the ventricles. When this occurs the ventricles are excited by a slow ‘escape mechanism’, from a depolarizing focus within the ventricular muscle.  Abnormalities of P waves, QRS complexes and T waves ABNORMALITIES OF THE P WAVE 1. Anything that causes the right atrium to become hypertrophied (such as tricuspid valve stenosis or pulmonary hypertension) causes the P wave to become peaked. 2. Left atrial hypertrophy (usually due to mitral stenosis) causes a broad and bifid P wave.  ABNORMALITIES OF THE QRS COMPLEX The normal QRS complex has four characteristics: 1. Its duration is no greater than 120 ms (three small squares). 2. In a right ventricular lead (V1), the S wave is greater than the R wave. 3. In a left ventricular lead (V5 or V6), the height of the R wave is less than 25 mm. 4. Left ventricular leads may show Q waves due to septal depolarization, but these are less than 1 mm across and less than 2 mm deep.  ABNORMALITIES OF THE ST SEGMENT The ST segment lies between the QRS complex and the T wave