Electrocardiogram

An electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. It is the prime tool in cardiac electrophysiology, and has a prime function in screening and diagnosis of cardiovascular diseases.

Lead placement

An ECG is constructed by measuring electrical potential between various points of the body using a galvanometer. Leads I, II and III are measured over the limbs: I is from the right to the left arm, II is from the right arm to the left leg and III is from the left arm to the left leg. From this, the imaginary point V is constructed, which is located centrally in the chest above the heart. The other nine leads are derived from potential between this point and the three limb leads (aVR, aVL and aVF) and the six precordial leads (V1-6).

Related Topics:
Electrical potential - Galvanometer

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Therefore, there are twelve leads in total. Each, by their nature, record information from particular parts of the heart:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior region (wall) of the heart. This is the apex of the left ventricle.
• The lateral leads (I, aVL, V₅ and V₆) look at the electrical activity from the vantage point of the lateral wall of the heart, which is the lateral wall of the left ventricle.
• The anterior leads, V₁ through V₆, and represent the anterior wall of the heart, or the frontal wall of the left ventricle.
• aVR is rarely used for diagnostic information, but indicates if the ECG leads were placed correctly on the patient.

Understanding the usual and abnormal directions, or vectors, of depolarization and repolarization yields important diagnostic information. The right ventricle has very little muscle mass. It leaves only a small imprint on the ECG, making it more difficult to diagnose than changes in the left ventricle.

Related Topics:
Ventricle

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The leads measure the average electrical activity generated by the summation of the action potentials of the heart at a particular moment in time. For instance, during normal atrial systole, the summation of the electrical activity produces an electrical vector that is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium (since the SA node resides in the right atrium). This turns into the P wave on the EKG, which is upright in II, III, and aVF (since the general electrical activity is going towards those leads), and inverted in aVR (since it is going away from that lead).

Related Topics:
Systole - Atrium

~ ~ ~ ~ ~ ~ ~ ~ ~ ~