What do ECGs tell us?

Ever seen an electrocardiograph? You probably haven’t seen one of your own, but movies, cartoons and TV serials often use these jagged lines on a computer screen near a sick person in a hospital. Typically when the line goes flat, it indicates that the person has died.

That series of peaks and troughs is what is known as an electrocardiograph (ECG or EKG). What this pattern actually represents is the electrical activity of the heart. An ECG can show how fast your heart is beating, whether the heartbeat is steady or irregular, and the strength and the timing of the electric signals.

Electricity in the heart?

Our heart is basically just a pump ensuring that all organs of our body are supplied with blood. But just like any other machine, it needs to be powered by some form of energy. An electrical signal originating in the heart’s right atrium (the top right chamber of the heart) begins the process.

A group of cells in the right atrium called the sinoatrial node (SA node) starts the signal, which then travels all through the heart. This journey of the electrical signal is what is depicted in the ECG.

What do peaks in the ECG mean?

The signal from the SA node travels all through the right and left atria. This is represented by the first tiny bump on the ECG, the P wave. The electrical signal causes these two chambers of the heart to squeeze, thereby pumping blood into the lower chambers or the ventricles.

The electrical signal then passes into the ventricles via a group of cells at the atrioventricular node (AV node). At the AV node the signal slows down until the ventricles are filled with blood from the atria. This slowing down is what the flat line after the P wave represents.

The electrical signal now begins travelling through the left and right ventricles through a bundle of muscle fibres. This journey of the signal down the ventricles is represented by the small depression called the Q wave.

The signal is now rapidly spreading all through the ventricles, prompting the right and left ventricle to squeeze. This pumps blood out of the ventricles into the tubes that carry the blood to your lungs and other parts of the body. This highly active part is what the pointy QRS wave represents.

The ventricles then recover, and the heart starts filling with blood again, and the whole cycle begins again with each new heartbeat. In the time you’ve taken to read this your heart would have repeated this hundreds of times!

nandita.j@thehindu.co.in