28 August 2017

Understanding of heart's electrical system

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Heart is a two stage pump whose electrical activity can be measured by electrodes placed on the skin. Electrocardiogram (ECG or EKG) is nothing else than a recording of the heart’s electrical activity, which is routinely used to assess the electrical and muscular functions of the heart.

Heart’s main job is to pump blood and each heartbeat is a complex cycle in which your heart’s chambers relax and contract to pump blood. These activities are coordinated by a very important set of structures – the heart’s electrical system. It is made of three main parts:

  • The sinoatrial (SA) node located in the right atrium of your heart
  • The atrioventricular (AV) node located in the interatrial septum close to the tricuspid valve
  • The His-Purkinje system located along the walls of your heart’s ventricles
Heart’s conduction system
Heart’s conduction system (source: OpenStax College)

In a normal healthy heart, each heartbeat begins with an electrical signal from the SA node, which is usually called heart’s natural pacemaker. The number of signals that SA node produces per minute is known as a pulse. The signal is generated as the vena cavae fill heart’s right atrium with blood from other parts of your body. Signal spreads across the cells of right and left atria, causing the atria to contract (recorded as P wave on the ECG). This pushes blood through the open valves from the atria into both ventricles. When a signal arrives at the AV node, it slows down to allow the ventricles to fill with blood (recorded as a flat line between the end of P wave and the start of Q wave). Then signal moves along a pathway called the bundle of His, located in the walls of the ventricles. Passing through the Purkinje fibres, the signal divides into left and right bundle branches. These fibres connect the wall cells to left and right ventricles. As the signal spreads across the ventricles wall cells, both ventricles contract (a QRS complex on the ECG).

ECG tracing with heart contraction
ECG tracing with heart contraction (source: OpenStax College)

The left ventricle contracts an instant before the right ventricle. This pushes blood through the pulmonary valve (from the right ventricle) to your lungs, and through the aortic valve (from the left ventricle) to the rest of your body. As the signal passes, the walls of the ventricles relax and stop contracting which allows the heart to refill with blood (the T wave on the ECG). This entire process continues over and over with each new heartbeat. The abnormalities in the heart’s electrical system can range from minor premature beats (skipped beats) that do not require treatment, to slow or irregular beats that require an artificial pacemaker.

Sources:

https://www.nhlbi.nih.gov/health/health-topics/topics/hhw/electrical

https://www.nhlbi.nih.gov/health/health-topics/topics/hb/understanding

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