Electrocardiogram is a recording of the electrical potential (depolarization and repolarization) of the cardiac muscle using electrodes placed on the skin of the opposite site of the heart

Is a useful clinical diagnostic tool to measure electric activities of the heart in normal and abnormal conditions especially in:

*Rhythm disturbances (cardiac arrhythmias)

*Changes in electrical conduction (heart block) *Disruption of blood supply (cardiac ischemia and infarction)

*P wave - atrial depolarization - .08 to .1 seconds

*QRS complex - ventricular depolarization - .06 to .1

*T wave - ventricular repolarization - N/A

*P-R interval - atrial depolarization + AV nodal delay - .12 to .2

*ST segment - isoelectric period of depolarized ventricles - N/A

*Q-T interval - length of depolarization plus repolarization - corresponds to AP duration - .2 to .4

Electrodes are placed directly on cardiac muscle fibers to measure depolarization and repolarization

As positive charge propagates down the fiber a depolarization wave is measured

When negative charges propagate down the fiber during repolarization that wave is then measured

*Voltage in MAP is much higher (~110mV) compare to ECG (0.1 -4 mV) - MAP measures from the cardiac muscle fibers while ECG from the outside of the body

*No potential is recorded in ECG when the ventricular muscle is completely polarized or depolarized

There are two types of ECG leads:

1- Limbs (standard) leads - 6

2- Chest leads (precordial leads) - 6

 Limb leads:

a) Three bipolar standard limb leads

b) Three augmented unipolar limb leads

*Lead I - the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left arm

*Lead II - the negative terminal of the electrocardiograph is connected to the right arm and the positive terminal to the left leg

*Lead III - the negative terminal of the electrocardiograph is connected to left arm and the positive terminal to the left leg

Einthoven’s law: If the electrical potentials of any two of the three bipolar limb ECG leads are known at any given instant, the third one can be calculated by summing the first two (consider + and - signs)

Leads I, II, and III form Einthoven's triangle in the chest

In unipolar limb leads two of the limbs are connected to the the negative terminal of the electrocardiograph and the third limb is connected to the positive terminal

*aVR - augmented voltage right arm - positive terminal on the right arm

*aVL - positive terminal on the left arm

*aVF - positive terminal on the left leg

*A wave of depolarization traveling toward a positive electrode results in a positive deflection in the ECG trace.

*A wave of depolarization traveling away from a positive electrode results in a negative deflection.

*A wave of repolarization traveling toward a positive electrode results in a negative deflection.

*A wave of repolarization traveling away from a positive electrode results in a positive deflection.

*A wave of depolarization or repolarization traveling perpendicular to an electrode axis results in a biphasic deflection of equal positive and negative voltages (i.e., no net deflection).

Positive electrode connects to chest wall

Negative electrode connects to left arm, right arm, left leg

*V1 & V2 close to the base of the heart

*V5 & V6 close to the apex of the heart

Mean Vector: mean direction of the electrical potential of the ventricle during depolarization process

Arrow directed from the base to the apex of the heart, passing through the center of the ventricles

Mean QRS vector = + 59 degree

Vector: is an arrow that points in the direction of the electrical potential of the heart generated by current flow (arrowhead points to the positive direction). The length of the arrow is proportional to the voltage of the potential

*Axis of lead I = 0

*Axis of lead II = 60

*Axis of lead III = 120

Mean QRS vector shifts to the right of the normal 59 degree (remember when looking right for the heart is left on the ECG graphs)

Common causes are

*right ventricular hypertrophy

*left posterior fasicular block

*acute right heart strain, ex - massive pulmonary embolism 

The mean QRS vector shifts to the left from its normal 59 degrees axis (left on the heart, right on the graph)

Common causes:

*left ventricular hypertrophy

*left anterior fasicular block

*inferior wall myocardial infarction

Current of injury: Cardiac abnormalities that cause part of the heart to remain partially or totally depolarized all the time

Causes:

1- Mechanical trauma

2- Infectious process

3- Ischemia due to local coronary occlusion (most common) disturb normal membrane polarization due to lack of nutrients

The "J point", the exact point at which the wave of depolarization just completes its passage through the heart, is used for analysis

At this point all parts of the ventricles have become depolarized even the damaged parts and the current of injury disappears

A horizontal line is drawn for each lead at the level of the J point. This line is the zero potential level from which all potentials caused by the currents of injury must be measured

Sudden increase in heart beats (150-250 beats/min) originate either from atrial or AV ectopic foci

Occurs in young healthy individuals and they grow out of it after adolescence

Sudden fast heart beats (150-250/min) originate from ectopic ventricular foci

It is a serious condition most likely result from considerable ventricular ischemia

Can lead to the deadly condition ventricular fibrillation

A delay of the AV bundle conduction from the atria to the ventricle but not actual blockage of the conduction

Prolonged P-R interval > 0.2 sec

*AV bundle conduction slow down that results in missed beat to the ventricle

*Prolonged PR interval 0.25 to 0.45 + "dropped beats" atrial P wave but NO QRS-T waves

*Rythms of 2:1 or 3:2 or 3:1 can be seen

*Complete block of the impulse from atria to the ventricle (complete AV block)

*Ventricle establish its own signal (from AV node or bundle)

*P waves become dissociated from QRS-T waves

What is premature atrial systole?

Sudden atrial beat from ectopic origin results in an earlier abnormal P wave

*Abnormal P wave characterized by:

1) P wave occurs too early in the heart cycle,

2) PR interval is shorten (ectopic origin of the beat is near AV node)

3)The interval between premature beat and the next one is slightly prolonged

*Common causes: in healthy individual, mild toxic conditions such as lack of sleep, too much coffee or alcohol, smoking

Sudden ventricular beat from ectopic origin results in a big abnormal QRS complex

*QRS usually prolong in time and has high voltage, follows by inverted T wave

*More serious than premature atrial systole

*Multiple atrial beats originate from ectopic foci discharging rapidly (350-450 discharges/min)

*Either small very fast P waves or straight line due to opposite direction of the waves that electrically neutralize each other. Normal QRS complexes but irregular in timing

*Causes: atrial enlargement due to valves diseases

*Most serious cardiac arrhythmias, if not stopped within 1-3 min = Death

*Cardiac impulses go "berserk" within the ventricle -> no coordination of muscle contraction -> no cardiac output

*Causes: a) Electrical cardiac shock b) Ischemia of the heart muscle or conductive system