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Essay on Electrocardiogram
The electrocardiogram (ECG) is a graphic record of electrical events of the cardiac cycle i.e. heartbeat. The cardiac impulse is spontaneously generated in the sinus node and this spontaneous rhythmic contractility is a unique property of cardiac muscle.
This electrical impulse is formed in the sinus node at regular intervals at the rate of 60 to 90 beats per minute in a healthy adult and the cardiac muscle contracts at the rate generated by the sinus node.
The cardiac impulse traverses the auricle, the bundle of His and ultimately the ventricles and then contraction of heart takes place i.e. systole. This is followed by diastole when heart muscle remains relaxed as well as electrically refractory at the onset of diastole. As the heart is situated within the body in fluid medium containing electrolytes, the path of cardiac impulse can be recorded by applying electrodes in specific positions of body surface.
The impulse formation and its transmission in the myocardium, produces weak electrical currents, which are magnified by the ECG machine and recording can be made in moving graph paper. The recording paper has graphic inscription of large and small squares and it moves at 25 mm/sec. or 50 mm/sec. for recording of ECG. Each small square is 0.04 sec (1/25 sec) and each large square is 0.2 (1/5 sec).
E.C.G. has been standardised in relation to deflection caused by one milli volt. One mi 111 volt causes deflection of the stylus by 10 milli meter. Only properly standardised E.C.G. should be read.
Voltage Measurements:
The voltage measurements are made with reference to base line. The deflection above base line is considered positive, while below the base line is negative.
How to Read ECG:
In any ECG the following points should be assessed
1. Rhythm and rate
2. Standardisation
3. Voltage
4. P – wave
5. P – R interval
6. QRS interval
7. QRS complex
8. ST segment
9. T and U wave
10. QT interval
For routine purposes a 12 lead ECG is taken but more elaborate studies can be made in special circumstances and special leads e.g. esophageal lead or His-bundle ECG can be taken.
In order to properly read an ECG at least 4 to 5 complexes should be recorded for each lead and one long lead of 8 to 10 complexes should be recorded specially to study the cardiac rhythm, for routine work. For special purposes continuous monitoring may be necessary by using cardiac monitoring device.
The ECG is specially helpful in the diagnosis of the following conditions:
1. Arrhythmias
2. Atrial and ventricular hypertrophy
3. Bundle branch block and conduction defects
4. Ischaemic heart disease — Routine ECG as well as exercise testing and Holter monitoring methods may be required.
5. Myocardial diseases, cardiomyopathy and pericarditis
6. Drug induced ECG changes especially due to digitalis, quinidine, electrolyte abnormalities especially hypo or hyperkalaemia.
Rhythm:
Proceed as mentioned below:
1. Regular or irregular.
2. If normal P waves precede every QRS complex, rhythm is sinus.
3. If deformed P waves precede QRS complex, rhythm is junctional or nodal.
4. If there are no P waves preceding QRS complex, rhythm is junctional if QRS interval is normal. If QRS interval has increased duration, the rhythm is ventricular.
5. Mention if QRS complexes are grouped or not, and also if they are dropped or not.
Rate:
The distance between two vertical lines (small square) on ECG paper = 0.04 seconds. Count the number of squares between two consecutive QRS complexes. Divide 1500 by this number. If this number is 15, the heart-rate is 1500 divided by 15 = 100 per minute. If distance between two consecutive QRS complexes is not uniform, count at three places, take the average of it and use this average number for dividing 1500. ECG-calipers are also available for direct measurement of rates.
Normally the auricular and ventricular rates do not differ, but they should be counted individually as these may vary in some disorders of rhythm e.g. complete heart block (i.e. third degree heart block), AV block or some cases of second degree AV block. In auricular fibrillation, only wavy lines are seen, hence auricular rates cannot be properly counted.
Intervals:
PR intervals (A-V-con duct ion time)- normal value 0.12 to 0.2 sec. It is measured from the beginning of P-wave to the onset of QRS complex. Strictly speaking, therefore, it is P – Q and not PR which has become accepted by usage. The PR interval should not exceed 0.2 second but possibly 0.21 sec. may also be considered normal. The age, body build and heart rate are actually important.
Nomogram produced by Ashman R & Hull, E- Essentials of Electrocardiography, Macmillan, 1937 shows that at heart rate of 70 p.m. for large adults PR interval may be 0.21 which for small adults is 0.20. At heart rate of iii – 130 p.m., PR interval for large adults is 0.18 which for small adults it is 0.17. Similarly for children the PR interval is shorter than in adults.
PP Interval:
If the rhythm is regular, PP interval, RR interval should be equal. If the ventricular rhythm is irregular or if auricular and ventricular rates are different but regular, PP interval and RR intervals will differ. The auricular contraction rate should be measured from counting between two successive P waves.
QRS Interval:
It measures ventricular depolarization time. It is measured from the beginning of 0 wave (or R wave where Q is absent) up to the end of S wave. The QRS interval is normally within 0.1 sec. but in V2 and V3 up to 0.11 sec may be considered normal.
RR Interval:
Will vary according to heart rate. It is the interval between two successive R waves. In regular rhythm RR interval in seconds (peak to peak of two successive R waves divided by 60, will indicate the heart rate. However, this can also be found out by a nomogram. If the ventricular rhythm is irregular, count the number of R waves in 10 seconds interval and calculate the rate per minute. If fifteen R waves are present in 10 seconds interval, the ventricular rate is 15 × 6 = 90 per minute.
QT Interval:
QT should be less than 35 mm. — It is the electrical systole measured as distance between beginning of Q wave and end of T wave. It varies with heart rate and therefore it is corrected for purpose of standardisation, which is known as corrected QT interval (Q-Tc). It is computed from a nomogram which gives Q-Tc. At the heart rate of 60 p.m. Q-Tc adult males should not exceed 0.42 sec and for females 0.43 second.
VAT (Ventricular Activation Time):
VAT is the time interval taken for the electrical impulse to travel from endocardium to pericardium i.e. it is the distance between onset of Q to the peak of R wave. Measured from a surface ECG, it cannot be very accurate as thickness of chest wall differs causing errors. It can be accurately measured by direct electrodes placed over the surface of heart and endocardium. Normally VAT should be within 0.03 sec in V1 – V3 and 0.05 sec. in V5 – V6.
P – Wave:
P – Wave is the electrical activation of atrium. Normally upright in leads I, II, aVF and inverted in aVR, it has not a constant configuration in other leads. Normally its duration is less than 0.1 sec and amplitude less than 2.5 mm. If P is taller in lead I than in lead III and is wide and notched, it is called P-mitrale which is characteristic of mitral stenosis. If P is taller in lead III than in lead I and is pointed, it is called P-pulmonale, which is characteristic of corpulmonale.
PR Segment and S – T Junction (J Junction):
PR segment is the portion of complex which begins from the end of P wave to the onset of QRS complex, which is normally isoelectric.
S – T Segment:
This RS – T segment which starts from where QRS ends and ST begins or the J – junction. The ST segment is usually isoelectric. It may be elevated or depressed above or below the base line (i.e. the time from end of T – wave to onset of next P – wave (T-P segment).
T – Wave:
ST segment is followed by T – wave. Normally T-waves are upright in leads I, III, AVL, AVF, V3 to V6 and inverted in aVR. In all the other leads it may vary. Height of T wave should not be more than 5 mm in limb leads and more than 10 mm in chest leads. If it is more than this its cause needs evaluation.
However, it must be remembered that T wave is the most vulnerable portion of the ECG and it may be affected by many factors. T – Wave may be tall due to hyperkalemia or even exposure to hot environment, while T – waves may be flattened or inverted due to hypokalemia, myxoedema, extreme cold exposure toxic myocarditis or cardiomyopathy. Tail T may also occur in I.H.D. (especially inferior wall infarction).
T – Waves are variable in shape and direction on III, aVL, aVF, Vl and V2. Interpretation of T-wave should be very cautiously done to avoid cardiac neurosis.
The most common causes of ST elevation are acute myocardial injury (infarction) or pericarditis.
Depression of ST segment (horizontal or significant down-sloping) may be due to ventricular strain, ischaemia or digitalis effect.
U-Wave:
T-wave may or may not be followed by a small wave called U-wave. Usually it is upright. When inverted, it indicates either myocardial ischaemia, ventricular stress or electrolyte imbalance.
Q – U Interval:
It is the distance between onset of Q wave to the end of U wave (when present). It measures the time of total ventricular repolarization including purkinje fibres in the myocardium.
Points to Remember:
1. Abnormal looking ECG may sometimes be seen with normal hearts.
2. ECG may be normal even though heart might be abnormal.
3. ECG should be assessed along with history and clinical background.
4. To be more conclusive, study of serial ECG’s is essential.
5. Do not read too much from an ECG.
A technician recording an ECG should not make any comments to the patient. If there are some abnormalities requiring immediate attention, the technician should urgently inform the physician concerned. A case report published in the Guy’s Hospital Gazette, London is a classic example of wrong interpretation about ECG relating to heart.
A patient’s ECG showed some minor abnormalities and his doctor diagnosed him as a case of I.H.D. The man lost his job and finally committed suicide. On post-mortem his coronary arteries were normal.
A good history, clinical findings, follow-up of the case, proper medication and exercise tests including echocardiography, coronary angiography, Holter monitoring and modern radioisotopic investigation wherever possible may be undertaken in controversial cases.