Normal Heart Rhythm

The normal human heart beats about 100,000 times per day and about 35 million times in a year. During an average lifetime, the human heart will beat more than 2.5 billion times. The heart does so with incredible accuracy and coordination most of the time ensuring optimal heart function.
The normal resting heart rate is not fixed and varies significantly during the day. The rate is constantly modified depending on changing body requirements. For example, the heart rate falls during sleep (sometimes as low as 30 beats per minute in young people) and increases with exercise, anxiety and emotion. The resting heart rate for most people runs normally in the range between 60-80 beats per minute but rates can easily vary between 50 and 100 beats per minute in normal individuals.
The heart is a complex organ that functions predominantly as a mechanical pump to circulate the blood across the body. For this pump to function properly, all its components need to function well (arteries, heart muscle, electrical system). Failure of any of these components will lead to heart disease For example, a blockage in the arteries that supply the heart muscle itself can cause a heart attack, diseases of the heart muscle itself will weaken the heart leading to heart failure, and disturbance of the heart electrical activity will cause arrhythmias.
The heart is composed of 4 muscular pumping chambers. The two upper, relatively small chambers are called the atria and they act mainly as reservoirs for the blood returning from the body. The two lower chambers are larger and are called the ventricles and they act as the main pumping chambers. Most heart cells have the ability to contract independently, however it is vital for heart cells to contract in a co-ordinated fashion to pump the blood properly. The heart uses electrical signals to co-ordinate the contraction of all heart cells in a synchronized way. This allows both atria to contract together, followed a fraction of a second later by both ventricles
Fig 1 the cardiac conduction system Converted
The normal heart rhythm (called sinus rhythm) arises from the sinus node which is a small structure in the upper part of the right atrium. The sinus node acts as the main timing clock in the heart and works like a maestro to orchestrate the contraction of heart cells and set the heart rate. The heart beat starts with an electrical impulse from the sinus node which travels rapidly through the right and left atria causing their simultaneous contraction. The electrical signal then enters the only electrical connection (or little nerve) that connects the atria to the ventricles called the AV node (atrioventricular node). The electrical signal then travels through a specialised electrical conduction system that distributes the impulse rapidly to all areas in both ventricles. This leads to their simultaneous contraction shortly after atrial contraction has filled them up with blood.
This highly organised electrical process allows the heart to beat in a very efficient way. When this process is disturbed, arrhythmias occur. This can be due to slowing or delay of the electrical impulse, an extra connection between the atria and ventricles, or due to a localized area in the heart muscle producing abnormal rapid electrical impulses.