Virtual Medical Centre

ECG / EKG (Electrocardiogram)

• What is an Electrocardiogram (ECG/ EKG)? 
• When is an ECG Used?
• How to Prepare for an ECG
• What Can You Expect During an ECG?
• What Does the ECG Record?
• What Can You Expect After an ECG?
   

 

When is an ECG Used?

The ECG is used to investigate suspected problems with the electrical conduction system of the heart as well as some other abnormalities such as metabolic disturbances (e.g. excess potassium). The ECG is an essential tool for health professionals in making a diagnosis of abnormal heart rhythms when one is suspected. It is a routine investigation in people who complain of symptoms such as chest pain as well as breathlessness. The findings of the ECG can also have an effect on treatment of certain conditions, for example it is helpful in deciding some aspects of treatment in people who have suffered from a heart attack.

How to Prepare for an ECG

• First reassure yourself that there is no danger or pain involved in performing the procedure and try to relax breathing steadily.
• Rest your arms by your side with your legs flat, making sure that your legs aren't touching one another.
• Make sure that your chest is exposed as well as your arms and legs.
• The nurse should then clean your skin with a sterile wipe prior to placing the electrodes in place

What Can You Expect During an ECG?

After the preparations that take place before an ECG is performed, various electrodes with a plastic head and an adhesive gel are placed at strategically located points on your body. These points are shown in the diagram to the right, and include:

• 6 points on your chest starting just to the right of your breast bone extending sideways towards the left hand border of your rib cage called V1 - V6.
• 4 points, one on each limb, the ones on the arms are placed below the levels of the shoulders and on the legs they are placed below the level of your groin (note that the closer the electrodes are placed to your heart the more intense the reading is, and the easier it is to interpret).

What Does the ECG Record?

During the process of the recording the signal is calculated and amplified from the 10 electrodes placed on your body and subsequently recorded on a piece of graph paper with specific dimensions. Depending what 'lead' (the vector formed between the different electrodes) you are looking at the ECG appears different. There are 12 such leads in a regular 12 lead ECG, each of which analyse a different plane of the heart and are therefore useful in diagnosing different conditions and localising disease. The 'leads' or vectors on the chest wall are named V1, V2, V3, V4, V5 & V6 going from right to left on the patient, and therefore analyse the heart in order from the right to the left i.e. from V1 to V6. The leads on the chest are called unipolar leads because their vector is only pointing in one direction i.e. in a direction perpendicular to the chest wall. There are 4 electrodes on the patients limbs, but only 3 of which are used to form leads and one which is used as an earth, just like the one found on a plug for an electrical device. The 3 limb electrodes used are the right arm, the left arm and the left leg. From these 3 electrodes 6 limb leads are formed, these are named I, II, III, aVR, aVL, and AVF. The leads are formed by using various combinations of the 3 leads, with leads I-III using 2 electrodes to form a vector (bipolar leads) and leads aVR, aVL and aVF using 3 electrodes (augmented bipolar leads).

• Bipolar leads
  • I = Right arm to left arm
  • II= Right arm to left leg
  • III = Left arm to left leg
• Augmented bipolar leads
  • aVR = Right arm to left arm and left leg
  • aVL = Left arm to left leg and right arm
  • aVF = Left leg to left arm and right arm

Usually the ECG is recorded on special graph paper that is divided into 1-mm² boxes. Each box represents a specific time interval since the ECG always records at a particular velocity. By counting these boxes horizontally, the doctor is able to discern the heart rate as well as the timings of the different parts of the ECG. The heart rate, the timings of the intervals (as seen in the diagram below), the height of the recording as well as the leads in which the abnormalities are present all help to make a diagnosis. An ECG readout will look like a series of 'waveforms', a single one of which is shown below. The image also highlights the electrical activity in the heart at the time, that causes the distinctive wave. Depolarisation indicates that a wave of electricity has just passed through an area of the heart such as the ventricles of atria, causing them to contract.