Introduction
The contraction of any muscle is associated with electrical changes called ‘depolarisation’, and these changes can be detected by electrodes attached to the surface of the body. Since all muscular contractions will be detected, the electrical changes associated with the heart muscle will only be clear if the patient is fully relaxed and no skeletal muscles are contracting.
The ECG can be used to help determine;
- the anatomical orientation of the heart and the relative sizes of its chambers eg- left ventricular hypertrophy
- disturbances of rhythm and conduction eg- arrhythmias
- the extent and location of ischemic damage to the heart muscle
- the effects of abnormal concentrations of various plasma electrolytes
- digital imbalance of medications eg- Digoxin
Path of electrical impulse through the heart
the electrical discharge fro each cardiac cycle normally starts at the sinoatrial node in the right atrium. Depolarization then spreads through the atrial muscle fibres. There is a delay whilst the depolarization spreads through the atrioventricular node, after which it travels through the bundle of his, through the right and left bundle branches and through specialized fibres known as purkinje fibres.
diagram 1 + 2
How to interpret and ECG
The muscle mass of the atria, when in comparison with that of the ventricles is small, and thus the electrical change accompanying the contraction of the atria is small.
Contractions of the atria are associated with the ‘p’ wave.
The ventricular mass is large and the large deflection of the ECG when the ventricles are depolarized is seen by the ‘QRS’ complex.
The ‘T’ wave of the ECG is associated with the return of the ventricular mass to its resting state- repolarisation.
*in some ECGs and extra wave ‘U’ can be seen, the origin of which is uncertain. However if a ‘U’ wave follows a normally shaped ‘T’ wave, it can be assumed normal.
PR interval- the period between the beginning of the p wave and the QRS complex. I.e.- time taken for excitation to spread from sa node, through the atrial muscles to the AV node. Normally takes ~ 0.12-0.20 sec.
QT interval- beginning of Q wave till the end of T wave. varies with heart rate. Its prolonged in patients with some electrolyte abnormalities, and more importantly it is prolonged by some drugs. [ a prolonged QT interval ,greater then 450ms may lead to ventricular tachycardia.]
ST segment- reflects a time when the entire ventricular myocardium is depolarized, and should therefore have the same potential ad that present when the muscle is relaxed. –isoelectric line.
Few Problems that you might encounter-
-abnormalities of the p wave ~ enlargement of one or other atria
-prolonged pr interval ~ disorder of the AV node
-abnormalities of the QRS complex ~ disorders of cardiac conduction, ischeamic damage to heart muscle
-ST segment deviate from resting potential ~ hypoxia [may indicate myocardial ischaemia]
-abnormalities of T wave~ may indicate myocardial damage, electrolyte disturbances or cardiac hypertrophy.
- a higher than normal T wave~ excess ca++
- missed p wave~ sa node malfunction [av node taken over]
How to attach the leads
diagram 3
arms- attach onto medial anterior side towards the ulna, at the feet- on the medial anterior aspect over the tibia.
The unipolar electrodes on the chest;
V1- 4th right intercostal space to the right of the sternum
V2- 4th left intercostal space adjacent to the sternum
V3- between V2 and V4
V4- 5th intercostal space in the mid clavicular line
V5- in the same horizontal plane as V4 in the anterior axillary line
V6- in the same horizontal plane as V5 in the mid axillary line
~ they all look at the heart in a horizontal plane
V1 and v2- right ventricle
V3 and v4- septum between the ventricles and the anterior wall of the left ventricle
V5 and v6- anterior and lateral walls of the left ventricle
Measurements
A typical electrocardiograph runs at a paper speed of 25 mm/s, although faster paper speeds are occasionally used. Each small block of ECG paper is 1 mm². At a paper speed of 25 mm/s, one small block of ECG paper translates into 0.04 s (or 40 ms). Five small blocks make up 1 large block, which translates into 0.20 s (or 200 ms). Hence, there are 5 large blocks per second. A diagnostic quality 12 lead ECG is calibrated at 10 mm/mV, so 1 mm translates into 0.1 mV. A calibration signal should be included with every record. A standard signal of 1 mV must move the stylus vertically 1 cm, that is two large squares on ECG paper.
To calculate heart rate- 300 divided by the number of large squares between each QRS complex.
References-
1)Tutorial notes
2)The ECG made easy by John R. Hampton - 7th ed
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