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It releases electrical stimuli at a regular rate, which will vary depending on whether the body is at rest or in action. As each stimulus passes through the myocardial cells of the atria, it creates a wave of contraction which spreads rapidly through both atria.

The rapidity of atrial contraction is such that around 100 million myocardial cells contract in less than one-third of a second; this is so fast it appears instantaneous.

When the electrical stimulus from the SA node reaches the AV node, it is delayed briefly so that the contracting atria have enough time to pump all 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, through the bundle of His, along the left and right bundle branches and the Purkinje fibres. In this way all the cells in the ventricles receive an electrical stimulus causing them to contract (Becker 2007).

Around 400 million myocardial cells that make up the ventricles contract 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, whilst the left ventricle pumps blood into the aorta from where it passes into the coronary and arterial circulation.

At this point the ventricles are empty, the atria are full and the valves between them are closed. The SA node is about to release another electrical stimulus and the process is about to repeat itself. However, there is a third section to this process. The SA node and AV node contain only one stimulus. Therefore every time the nodes release a stimulus, they must recharge before they can do it again.

In the heart, the SA node recharges whilst the atria are refilling, and the AV node recharges when the ventricles are refilling. This means there is no need for a pause in heart function. Again, this process takes less than one-third of a second. (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 term used for the release of an electrical stimulus is ‘depolarization’, and the term for recharging is ‘repolarization’.

So, the three stages of a single heart beat are:

atrial depolarization

ventricular depolarization

atrial and ventricular repolarization (Moran 2010, Tortora and Derrickson 2009).

Related theory

Potentially reversible causes of a cardiopulmonary arrest

During cardiac arrest, potential causes or aggravating factors for which specific treatment exists should be considered. For ease of memory, there are eight common causes of arrest, four of which begin with the letter H and four with the letter T.

Hypoxia.

Hypovolaemia.

Hypothermia.

Hypo/hyperkalaemia.

Thromboembolism.

Tension pneumothorax.

Tamponade.

Toxicity (metabolic or drug induced) (Resuscitation Council 2010b).

Hypoxia

There are many reasons why a patient may become severely hypoxic (see Chapters X and X), the most common being the following.

Acute respiratory failure.

Airway difficulties.

Acute lung injury.

Severe anaemia.

Neuromuscular disorders.

For healthy cell metabolism, the body requires a constant supply of oxygen. When this is interrupted for more than 3 minutes in most situations (except when there is severe hypothermia), cell death occurs, followed by lactic acidosis and very rapidly a cardiorespiratory arrest. The risk of hypoxia is minimized by ensuring that the patient’s lungs are ventilated adequately with 100% oxygen (Resuscitation Council 2010b).

Hypovolaemia

Hypovolaemia in adults that results in PEA is usually due to severe blood loss. While it is not the nurse’s role to make a medical diagnosis, they may be aware of significant factors in the history of a patient that may have led to PEA.

The most common causes of severe blood loss are:

trauma

surgical procedure

gastrointestinal mucosa erosion

oesophageal varices

peripheral vessel erosion (by tumour usually)

clotting abnormality.

Note: blood loss,