Electrocardiograph. How Stuff Works

Electrocardiograph is the Medical diagnostic instrument used to record the electrical activities of the heart. It gives a recorded data in the form of a graph called ElectroCardioGram or ECG. Human heart functions through the electrical impulses produced by the specialized groups of cells called Pace makers (SA node and AV node) located on the right surface of the heart. The electrical signals generated by the pace makers will be available on the surface of skin also. The Electrocardiograph detects these signals and output as a graph. By observing the waveform, it is easy to confirm the variations in the functioning of heart.

The ECG works mostly by detecting and amplifying the tiny electrical changes on the skin that are caused when the heart muscle “depolarizes” during each heart beat. At rest, each heart muscle cell has a charge across its outer wall, or cell membrane. Reducing this charge towards zero is called de-polarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat a healthy heart will have an orderly progression of a wave of depolarisation that is triggered by the cells in the Sinoatrial node (SA Node) , spreads out through the atrium, passes through “intrinsic conduction pathways” and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.

The Electrocardiograph has many electrodes to collect the electric signals available on the different parts of the body. For this, electrodes are attached to the left chest, sides of chest, hands and feet. To get firm electrical conductivity, petroleum jelly is applied to the site of fixing the electrodes. In conventional ECG, an ink pen moves over a moving graph paper and the waveform is marked on the paper as the paper moves.

It is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances. In a Myocardial Infarction (MI), the ECG can identify if the heart muscle has been damaged in specific areas, though not all areas of the heart are covered. The ECG cannot reliably measure the pumping ability of the heart, for which ultrasound-based (Echocardiography) or nuclear medicine tests are used.

Waves and intervals

A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a QRS complex, a T wave, and a U wave which is normally visible in 50 to 75% of ECGs

1.P Wave- During normal Atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG.

2. QRS Complex – The QRS complex reflects the rapid depolarization of the right and left ventricles. They have a large muscle mass compared to the atria and so the QRS complex usually has much larger amplitude than the P-wave.

3. T Wave – The T wave represents the Repolarization (or recovery) of the ventricles.

4. R Wave has highest Amplitude