Electrocardiogram (ECG) Courtesy of Graham and Emma

These signals occur due to a p.d. between polarised and depolarised heart cells. An ECG is used to measure the rate and regularity of heartbeats as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart (such as a pacemaker). Alternative names: Electrocardiogram; EKG What is an ECG… An electrocardiogram is a display through an oscilloscope of the electrical signals passing through the heart.

How to prepare for a test… Adults: Before the ECG, tell your health care provider if you are taking any medications. There are no restrictions for food or fluids. However, ingestion of cold water immediately before an ECG may produce changes in one of the waveforms recorded (the T wave). Exercise (such as climbing stairs) immediately before an ECG may significantly increase your heart rate. You may be asked to remove all jewellery and to wear a hospital gown.

Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust.

How the test is performed… You are asked to lie down, and electrodes are affixed to each arm and leg and to your chest. This requires cleaning the site and, if necessary, shaving or clipping hair. The standard number of leads attached is 12 to 15 for a diagnostic ECG but may be as few as 3 to 5 for a monitoring procedure. You are usually required to remain still, and you may be asked to hold your breath for short periods during the procedure. Sometimes this test is performed while you are exercising or under minimal stress to monitor changes in the heart. This type of ECG is often called a stress test. The results are recorded on graph paper.

How the test will feel… An ECG is painless. When first applied, the disks may be cold and in rare circumstances, you may develop a localized rash or irritation where the patches are placed.

What the risks are… There are generally no risks. Because this procedure merely monitors the electrical impulses and does not emit electricity, there is no risk of shock. During an exercise electrocardiogram, some patients experience arrhythmias or heart distress. Equipment for dealing with these occurrences is located in the testing area.

Why the test is performed… An ECG is very useful in determining whether a person has heart disease. If a person has chest pain or palpitations, an ECG is helpful in determining if the heart is beating normally. If a person is on medications that may affect the heart or if the patient is on a pacemaker, an ECG can readily determine the immediate effects of changes in activity or medication levels. An ECG may be included as part of a routine examination in patients over 40 years old.

About the heart… To understand an electrocardiogram, you must first understand the conduction system of the heart…

Conduction system of the heart… The heartbeat can be described as myogenic (meaning it is controlled/maintained within the muscle). This means it is stimulated within the heart and does not need external control to keep the beat functioning. This control comes from the SA node, or the Sinoatrial node. This group of specialised cells are located high up in the right atrium and produce impulses averaging 70 times a minute (when the body is at rest). The SA node is found at 1 on the diagram.

Conduction system of the heart… The SA node signal fires once the Right atrium is filled with blood from the body and the left atrium is filled with oxygenated blood from the lungs. When the SA node fires the signal is passed through the right then the left atria causing it to contract and push the blood in to the ventricles. The signal is passed down to the AV node (atrioventricular node, 2 on diagram) where it is delayed for approximately 0.1 seconds while the ventricles fill up then it too fires, pushing the signal across the ventricles rapidly down the main fibres, the bundle of His (3). The ventricles do not yet contract.

Conduction system of the heart… Once the signal reaches the apex of the heart, the signal is sent up both sides of the ventricles via the Purkynjie fibres. It is then that the ventricles contract from the bottom upwards, causing the blood to be pushed up through the semi lunar valves in the arteries. The process then begins again. The valves of the heart (bicuspid, tricupsid, semilunar) ensure that these contractions force blood only one way.

The electrocardiogram… In a single heart beat, the electrocardiogram would show the following phases…