Also known as an electrocardiogram or an EKG, an ECG is a test that detects and records the strength and timing of the electrical activity in your heart. This information is recorded on a graph that shows each phase of the electrical signal as it travels through your heart.
The electrical signal begins in the sinoatrial node (1) which is located in the right atrium and travels to the right and left atria, causing them to contract and pump blood into the ventricles. This electrical signal is recorded as the P wave on the ECG. The PR Interval is the time, in seconds, from the beginning of the P wave to the beginning of the QRS complex.
The electrical signal passes from the atria to the ventricles through the atrioventricular (AV) node (2). The signal slows down as it passes through this node, allowing the ventricles to fill with blood. This slowing signal appears as a flat line on the ECG between the end of the P wave and the beginning of the Q wave. The PR segment represents the electrical conduction through the atria and the delay of the electrical impulse in the atrioventricular node.
After the signal leaves the AV node it travels along a pathway called the bundle of His (3) and into the right and left bundle branches (4, 5). The signal travels across the heart’s ventricles causing them to contract, pumping blood to the lungs and the body. This signal is recorded as the QRS waves on the ECG. Because these waves occur in rapid succession they are usually considered together as the QRS complex.
The ventricles then recover to their normal electrical state, shown as the T wave. The muscles relax and stop contracting, allowing the atria to fill with blood and the entire process repeats with each heartbeat. The ST segment connects the QRS complex and the T wave and represents the beginning of the electrical recovery of the ventricles.
The QT interval represents the time during which the ventricles are stimulated and recover after the stimulation. This interval shortens at a faster heart rate and lengthens at a slower heart rate.