The electrocardiogram (ECG or EKG) The electrocardiogram (ECG) is a record of the electrical activity of the heart from the surface of the body. The body tissues function as electrical conductors because they contain electrolytes.

* If a recording electrode is applied on any point on the surface of the trunk, it will detect electrical waves reflecting the electrical activity in the heart. * A positive wave is recorded when depolarization is propagating towards the electrode or when repolarization is propagating away from the electrode. * A negative wave is recorded when depolarization is propagating away from the electrode or when repolarization is propagating towards the electrode. * When there are no propagating potentials, no waves are recorded and the recording needle will be on the line of zero potential, which is called the isoelectric line.

The active and reference electrodes For recording an ECG, two electrodes are required. One of the them is the active electrode (also called searching electrode or exploring electrode) which is applied to a recoding point on the surface of the body. The other is the reference electrode which serves a reference to the active electrode.

Recording points on the body surface By convention, there are nine standard points on the surface of the body from which an ECG should be recorded. Six points are on the chest wall and the other three points are on the limbs.

Chest points V1: at the right fourth intercostals space near the sternum. V2: at the left fourth intercostals space near the sternum. V3: midway between V2 and V4. V4: at the left fifth intercostals space at the midcalvicular line. V5: at the left fifth intercostals space at the anterior axillary line. V6: at the left fifth intercostals space at the midaxillary line.

Chest points

Chest points

Limb points VL: at the junction of the left arm with the trunk. Any point on the left upper limb has the same potential.   VR: at the junction of the right arm with the trunk. Any point on the right upper limb has the same potential. VF: at the junction of the left lower limb with the trunk. Any point on the left or right lower limbs has the same potential.

Limb points

The ECG leads A lead is the ECG record obtained when the recording electrodes are placed at specific points on the body.

Unipolar leads These are the ECG records obtained when the reference electrode is at zero potential. The active electrode is applied to the recording points on the body surface. So, the records are actually the vectors of these points. There are six standard unipolar chest leads recorded from the six standard chest points and designated as V1, V2, V3, V4, V5 and V6. There are another three standard unipolar limb leads recorded from the standard limb points and designated as aVL, aVR, aVF.

Unipolar Limb leads

Unipolar Chest leads

Bipolar leads These are the ECG records obtained when the active electrode is applied to a recording point and the reference electrode is applied to another recording point. The ECG will be a record of the changes in electrical potential at the active electrode relative to the reference electrode.

Lead I: records the potential between left arm and right arm Lead I: records the potential between left arm and right arm. The active electrode is at VL and the reference electrode is at VR.   Lead II: records the potential between left leg and right arm. The active electrode is at VF and the reference electrode is at VR. Lead III: records the potential between left leg and left arm. The active electrode is at VF and the reference electrode is at VL.

Bipolar Limb Leads - - + +

Normal ECG The normal ECG has five waves, designated as P, Q, R, S, and T waves. The shape and amplitude of the waves differ in different leads.

Normal ECG P wave: It is caused by atrial depolarization. Depolarization wave starts at the S-A node then moves to the atria. It is a positive wave.   QRS complex: It is caused by ventricular depolarization. Q wave: Depolarization of the ventricular septum (direction of depolarization: left to right → -ve wave). R wave: Depolarization of the ventricular wall. Depolarization wave of the left ventricle predominates over that of the right ventricle (direction of depolarization: right to left → +ve wave). T wave: It is caused by ventricular repolarization. (direction of repolarization: left to right → + ve wave).

Electrical Activity in the Heart

The Normal EKG recorded on the Bipolar Limb Leads

Normal Unipolar and Chest EKG

Arrhythmia means abnormal rate or rhythm of the heat beats. Cardiac arrhythmias Arrhythmia means abnormal rate or rhythm of the heat beats. It may be caused by abnormal excitability of some cardiac fibers or abnormal conduction of impulses in the heart.

Arrhythmias due to abnormal excitability Extrasystoles (premature beats = premature complexes = ectopic beats) Paroxysmal atrial tachycardia (PAT) Paroxysmal ventricular tachycardia (VT) Atrial fibrillation (AF) Ventricular fibrillation (VF)

Arrhythmias due to abnormal conductivity (heart block) Heart block means failure of conduction of impulses from the S-A node down to the ventricles. A-V block:   In this condition, the A-V node and bundle conduct impulses at an abnormally slow rate or fail to conduct some or all of the impulses from the atria to the ventricles.

Extrasystoles (premature beats = premature complexes = ectopic beats) An extrasystole is an abnormal systole that occurs early during diastole. Extrasystoles are caused by impulses generated in a hyperexcitable focus, other than the S-A node. If the focus is in the ventricle, the extrasystole is called ventricular extrasystole. If the focus is in the atrial wall, the extrasystole is called atrial extrasystole.

An atrial extrasystole is followed by the normal period of diastole. This is because when the abnormal impulse reaches the S-A node, it depolarizes the node. The node then repolarizes and starts the depolarization again for the next normal impulse.

A ventricular extrasystole is usually followed by a longer diastole called the compensatory pause. The long compensatory pause occurs because the conducting system does not allow the abnormal impulse from the ventricular ectopic focus to pass up to the atria. The next impulse from the S-A node reaches the ventricles in its normal timing. It finds the ventricles still in the absolute refractory period of the extrasystole. So, it will be ineffective. The ventricles then, wait in diastole for a longer period (compensatory pause) until the next impulse from the S-A node arrives to excite them and start the next systole.

Paroxysmal atrial tachycardia (PAT) This is tachycardia that occurs in attacks that last from few seconds up to few days. Usually the atria beat at a regular rate of about 200 beats/minute. PAT is caused by one of three mechanisms:

* An ectopic hyperexcitable focus that discharges impulses at a high rate. * Lewis’ circus movement (reentry mechanism): Normally, there are circles of cardiac muscle fibers in which impulses travel. When an impulse enters the circle, it splits into two, each one travels in the opposite direction to the other. Each impulse is followed by an absolute refractory period. When they normally meet, they both die off and vanish. If a transient block occurs at one side, one impulse is blocked and dies off, the other one goes on around the ring. By the time it reaches the block area, the block would have finished and the impulse passes this area and continues to circulate. * Wolff-Parkinson-White (WPW) syndrome.

Re-entry by Partial conduction Block The long cardiac refractory period normally prevents re-excitement and extinguishes the AP wave. Partial conduction block can allow re-entry of the AP into previously stimulated regions. This allows the AP to continue cycling through the heart.

Wolff-Parkinson-White (WPW) syndrome: Normally the only conducting pathway between the atria and the ventricles is the A-V bundle. Persons with WPW have another abnormal conducting bundle; the bundle of Kent. This bundle conducts faster than the A-V node but has a longer absolute refractory period. It can conduct impulses in both directions. Under normal conditions, impulses from the S-A node pass through the bundle of Kent and the A-V bundle. Both impulses meet then die off in the right ventricle. If an early impulse is fired in the atrium, it passes through the A-V node, but finds the bundle of Kent still in the absolute refractory period. The impulse passes through the right ventricle then passes through the bundle of Kent up to the right atrium. The impulse reenters the A-V node, to the ventricles and reentry circuit is established.

Paroxysmal ventricular tachycardia This is tachycardia that occurs in attacks that last from few seconds up to few days. The heart rate is about 200 beats/minute. The mechanism of VT is a high rate of discharge from an ectopic focus or circus movement in the ventricles.

An attack of SVT can be terminated by reflex stimulation of the vagus nerve by massaging the carotid sinus area. This depresses the excitability of the myocardial cells. However, carotid sinus massage can not stop an attack of VT because the vagus nerves do not supply the ventricles.

Atrial fibrillation (AF) This is a pathological condition in which atria beat at an extremely high rate (350-500 beats/min). AF is caused by multiple circus movements in the atria operating at the same time leading to desynchronization of the atrial muscle fibers.

Ventricular fibrillation (VF) This is a pathological condition in which the ventricles beat at an extremely high rate (350-500 beats/min). VF is caused by multiple circus movements in the ventricles operating at the same time leading to desynchronization of the ventricular muscle fibers

Arrhythmias due to abnormal conductivity (heart block) Heart block means failure of conduction of impulses from the S-A node down to the ventricles.   A-V block: In this condition, the A-V node and bundle conduct impulses at an abnormally slow rate or fail to conduct some or all of the impulses from the atria to the ventricles.

ECG AV NODE AND AV BLOCKS FOCUS ON N REGION NORMAL 1ST DEGREE PROLONGED AV CONDUCTION TIME 2ND DEGREE 1/2 ATRIAL IMPULSES CONDUCTED TO VENTRICLES 3RD DEGREE VAGAL MEDIATION IN N REGION/COMPLETE BLOCK