ECG simplified – Part 1

ECG simplified – Part 1

ECG stands for electrocardiogram, the recording of the electrical activity of the heart from the body surface. Direct recording of the electrical activity of the heart from within its various chambers are called electrograms. ECG is inexpensive widely available, and equipment needed for recording is quite portable. ECG is a very important tool to the clinician, though it was invented more than a century back.

Certain important disorders like a heart attack are still quickly diagnosed with an ECG than any other modalities. Heart rhythm disorders are diagnosed mainly by ECG, though sometimes electrograms recorded from within the heart are needed for finer details.

This is how ECG machine looked like when Willem Einthoven invented it way back in 1903! Subject had to immerse three limbs in jars filled with salt water in order to get the ECG recorded. With miniaturization and microminiaturization in electronics, now even small wearable devices can record ECG. ECG monitoring has become commonplace in all emergency departments, intensive care units and operating rooms.

A basic knowledge of the electrical system of the heart is essential to understand ECG tracings. Here is a rough sketch of the electrical system of the heart. Electrical activity starts in the pacemaker cells of the sinus node, known in short as SA node. SA node is the natural pacemaker of the heart which gives regular electrical signals to the rest of the heart. It is situated in the upper part of right atrium the right upper chamber of the heart.

Signals from the sinus node travel through muscle cells of the right atrium to the next relay centre in the lower part of right atrium known as atrioventricular node or AV node in short. There is some evidence of specialized conduction pathways within the right atrium known as internodal pathways as well. A branch of one of these pathways take signals to the left atrium, the left upper chamber.

Conduction through the AV node is quite slow compared to other regions of the heart. This delay is needed in order to complete contraction of the upper chamber before the lower chambers start contracting. Contraction of upper chambers give a booster filling to the lower chambers in addition to the passive filling of blood which they receive while relaxing after a contraction. This atrial booster function is lost in conditions like atrial fibrillation, a common rhythm disorder of the upper chambers.

From the AV node, signals pass through the specialized conduction bundle known as bundle of His into the lower chambers known as ventricles. It may be noted that the upper and lower chambers are electrically isolated and conduction between them occurs only through the specialized conduction pathways normally.

Two bundle branches, known as right bundle branch and left bundle branch, take the signals to the respective ventricles. The conduction in the bundles are quite fast, unlike in the AV node. Finer branches arise from the bundle branches, known as Purkinje fibres, which take the signals to individual heart muscle cells. Surface ECG is a sum total of the electrical activity of the heart, recorded at a distance.

This animation video shows the electrical signals travelling from the sinus node to the AV node, bundle of His and through the bundle branches. Normal activation of the heart is known as sinus rhythm and it has a rate between 60-100/minute in an adult. If the heart rate is fast, it is known as sinus tachycardia and if it is slow it is known as sinus bradycardia.