Physiology of Electrocardiogram

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Electrocardiogram (ECG or EKG) is a graphic record of electrical activity of the heart, obtained by the help of electrodes placed on body surface. Electrocardiograph is the instrument used to record the ECG.

Cardiac impulse arises in the sinoatrial node and spreads to both atria and the atrioventricular node. There is a delay of 0.13 seconds in the atrioventricular node. The impulse then goes to the right and left bundle branches and through the purkinje fibers is distributed to the whole cardiac muscles.

Principles of ECG

  • Electrical activity is recorded when part of muscle fiber is depolarized or repolarized.
  • No electrical activity is recorded when the muscle fiber is fully depolarized or repolarized.

ECG Lead

Pair of two electrodes connected to electrocardiograph with wires constitute a lead. A lead measures voltage difference between two electrodes.

Lead Axis

Lead axis is an imaginary straight line connecting the two electrodes. By convention, axis of a lead directs towards the positive electrode. A lead records potential difference along its axis (between two electrodes).

Positive Electrode

If the electrode is present in an area of positivity, an upward deflection will be recorded.

Negative Electrode

If the electrode is present in the area of negativity, a downward deflection will be recorded.

Isoelectric line or segment

A flat line on ECG tracing without any wave

Interval

A flat line on ECG tracing having at least one ECG wave

Calculation of Heart Rate

Heart rate may be calculated by several methods:

60 divided by the time in seconds between two consecutive R waves
300 divided by the number of large squares between two consecutive R waves
1500 divided by the number of small squares between two consecutive R waves
6 second strip

12 Lead Electrocardiogram
ECG
There are 6 limb leads and 6 chest leads. Out of the limb leads, I, II, and III are bipolar leads while aVR, aVL and aVF are unipolar leads. Chest leads are all unipolar, V1, V2, V3, V4, V5 and V6.

Bipolar Limb Leads

In bipolar limb leads positive electrode is placed on one limb and negative on the other. These measure the electrical potential difference between the positive and negative electrodes.

Lead I -positive electrode is placed on left arm and negative an right arm

Lead II -positive electrode is placed on left foot and negative on right arm

Lead III -positive electrode is placed on left foot and negative on left arm
Standard-placement-of-limb-leads
Einthoven’s Law

Electrical potentials of bipolar limb leads I and III equal that of lead II.

Unipolar Limb Leads

Unipolar limb leads are also called the augmented limb leads. The positive electrode is placed on one limb and the other two limbs jointly act as negative electrode. The positive electrode is called the ‘exploring electrode’ while the negative is called the ‘indifferent electrode’.

Voltage of indifferent electrode is kept at almost zero. Actual voltage in the area of positive electrode is measured. Augmentation enhances the voltage by about 50%.

Lead aVF -positive electrode is  placed on left foot and both upper arms act as indifferent electrode

Lead aVL -positive electrode is placed on left arm and right arm and left foot act as indifferent elecctrodes

Lead aVR -positive electrode is placed on right arm and left arm and left foot act as indifferent electrodes

Chest (Precardial) Leads

Chest leads are all unipolar leads. The positive electrode is placed on the chest and all three limbs jointly act as negative (indifferent) electrode.
Precardial-leads-in-ECG
The limb leads record electrical activity in frontal plane (superior/inferior and right/left) while precardial leads record electrical activity in horizontal plane (anterior/posterior and right/left).

Cardiac Vector

Any entity having direction and magnitude can be represented by a vector. Electrical currents generated in the heart have both direction and magnitude, thus can be represented by vectors.

The moving wave of depolarization or repolarization can be represented by a vector. A vector is represented by an arrow. By convention the arrow head points towards the positive area. Length of the arrow represents the voltage.

Instantaneous Mean Vector

Many vectors summate/cancel each other and the net resultant vector is called the instantaneous mean vector. Instantaneous mean vector is recorded on the ECG according to its relation with a particular lead axis.

Current of Injury

Flow of current from damaged area to normal area of heart between heart beats is called current of injury. Damaged area or semi-viable area remains depolarized. Current of injury may be caused by:

Ischemia (most common cause)
Mechanical trauma
Infections

Whether the injury current is positive or negative depends on the relation between the current of injury vector and the positive electrode of the respective leads.

Clinical Uses of ECG

Finding heart rate
Determining heart rhythm
Infarction
Hypertrophy
Axis deviation
Electrolyte imbalance
Drug toxicity

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The writer enjoys medical education and has special interest in community medicine.