The heart is made up of four chambers: two upper atria and two lower ventricles. The right atrium receives deoxygenated blood from the systemic venous circulation. From the right atrium, blood flows into the right ventricle, which pumps it back to the lungs via the pulmonary artery. Carbon dioxide is released and oxygen is absorbed within the lungs. This blood, which is now oxygenated, returns to the heart via the pulmonary vein and empties into the left atrium. The blood then passes into the left ventricle, which pumps it into the aorta and the arterial systemic circulation (Waugh and Grant [281]).

The atrioventricular septum completely separates the right and left sides of the heart. From shortly after birth, the two sides of the heart never directly communicate, with blood flowing from the right side to the left side via the lungs only. However, the right and left atria and the right and left ventricles work together, contracting simultaneously (Marieb and Hoehn [147]). To prevent the backflow of blood, the heart has four valves. The valves open and close in response to pressure changes as the heart contracts and relaxes (Tortora and Derrickson [272]). The valve between the right atrium and ventricle is known as the tricuspid valve, while the valve between the left atrium and ventricle is called the mitral valve. The pulmonary valve prevents backflow of blood from the pulmonary artery into the right ventricle. Likewise, the aortic valve sits within the aorta and prevents regurgitation into the left ventricle.

The sinoatrial (SA) node is the natural pacemaker of the heart. It releases an electrical stimulus at a regular rate that will vary depending on whether the body is at rest or in action. As each stimulus passes through the myocardial cells, it creates a wave of contraction, which spreads rapidly through both atria and is known as ‘atrial depolarization’ (Tortora and Derrickson [272]).

The rapidity of atrial contraction is such that around 100 million myocardial cells contract in less than one‐third of a second. When the electrical stimulus from the SA node reaches the atrioventricular (AV) node, within the septum, it is delayed briefly so that the contracting atria have enough time to pump the blood into the ventricles. Once the atria are empty of blood, the valves between the atria and ventricles close. At this point, the atria begin to refill and the electrical stimulus passes through the AV node and the bundle of His, along the left and right bundle branches, and finally terminates in the Purkinje fibres within the ventricles. In this way, all the myocardial cells (around 400 million) in the ventricles receive an electrical stimulus, which causes them to contract (Marieb and Hoehn [147]). This process is known as ‘ventricular depolarization’ and also happens in less than one‐third of a second. As the ventricles contract, the right ventricle pumps blood to the lungs, where carbon dioxide is released and oxygen is absorbed, while the left ventricle pumps blood into the coronary and arterial circulation via the aorta. At this point the ventricles are empty, the atria are full and the valves between them are closed.

Prior to the process starting again, the SA and AV nodes must recharge. This process is known as ‘atrial and ventricular repolarization’. The SA and AV nodes recharge while the atria and ventricles are refilling with blood (Herring and Paterson [107]). This process takes less than one‐third of a second, resulting in a minimal pause in heart function. The times given for the three different stages are based on a heart rate of 60 beats per minute, or 1 beat per second.

The three stages of a single heart beat are therefore:

Cardiac arrest occurs when the cardiac output of the heart stops due to cessation of its mechanical activity. This is usually as a result of heart disease, such as coronary heart disease, cardiomyopathy, aortic valve stenosis, cardiac arrhythmias or congenital heart abnormalities.