ELECTROCARDIOGRAM (ECG) & ASSOCIATED INVESTIGATIONS:

Electrocardiogram is abbreviated as ECG (though sometimes is seen as EKG taken from the Greek word 'Kardia') and is the recording of electrical activity form the heart. It is a simple, painless examination that takes around 10 minutes to perform.

Electrode Placement

It consists of applying 10 leads to the body connected usually to small adhesive pads (electrodes) that will provide 12 different recordings of the electrical activity that is given off by the heart. The electrodes are attached on each limb (called the Limb Leads) and across the left side of the chest (called the V Leads) in specific positions:

• V1 - In the 4th intercostal space on the right side of the sternal border
• V2 - In the 4th intercostal space on the left side of the sternal border
• V3 - Midway between V2 and V4, no anatomical positioning required
• V4 - In the 5th intercostal space in line with the middle of the clavicle
• V5 - In line with the anterior axilla (front of the armpit) in line with V4
• V6 - In line with the mid-axilla (middle of armpit) in line with V4 and V5

There are times when the leads will be placed in different positions or there may be less or more leads attached:

If information about the back of the heart (posterior) is required then the Chest (V leads) may be extended to:

• V7 - Left posterior axilla in (back of armpit) in line with V4/V5/V6
• V8 - At the tip of the scapula (shoulder blade) in line with V7
• V9 - Left para-spinal region (next to the spine) in line with V8
  

In Intensive Care or Cardiac Wards three leads may be attached to provide basic monitoring information.

In amputees the limb electrode would be placed as near to the limb position as possible and this may be on the shoulder or hips.

In neonate babies the chest wall is so small V3 is omitted and the chest progression leads begins with V4 on the right side, denoted V4R, then V1, V2, V4, V5 & V6.

In a very rare condition called dextrocardia, the Chest leads (V Leads) may be reversed to cover the right side of the chest because the heart is transposed, that is a mirror image of the correct positioning of the heart. So V1 would be in the 4th intercostal space on the left sternal border and called V1R.

If Atrial activity is of particular interest a lead placement may be performed to increase the visibility of the activity. This is called the Lewis lead or S5, and is created by placing the right arm lead in the 2nd intercostal space on the right sternal border and the left arm electrode is placed in the 4th intercostal space on the right sternal border. The Lewis lead would then be viewed on Lead 1 of the ECG enhancing the atrial activity, such as atrial flutter.

Uses of ECG

The ECG may only be the starting point for screening and detecting cardiac problems. There are various other methods or means of recording the ECG such as:

If ECG information is required over an extended period of time then the ECG may be repeated every so often every hour. This would enable clinicians to monitor any rapidly changing ECG such as a heart attack 'evolving' and 'resolving'.

If ECG information is required over a longer period then an Ambulatory ECG may be performed, this is where the patient is connected to a small recording device that they take away and wear for 24 - 72 hours.The ECG is recorded constantly and rate and rhythm trends are seen and any symptomatic episodes or abnormalities can be looked at in detail more carefully. However a person's problems may be more infrequent than every 1-3 days. 

If a patients symptoms are more infrequent then they may be given a 'Cardio-Memo' device. This device has no leads but is the size of a mobile phone and is placed on the chest wall when the person is symptomatic.This reduces the inconvenience of wearing a device constantly but may miss irregularities that are not symptomatic or when the subject is asleep. Cardio-Memo's are given on loan for 3 days or longer or until the patient captures a symptomatic episode.

An ECG with Exercise (sometimes called a Stress test or exercise stress test) may be indicated when the symptoms include chest pain, breathlessness or palpitations. The ECG is recorded whilst the person performs exercise that gradually gets more difficult until they fatigue, become symptomatic or a certain heart rate is obtained. The exercise may be performed on a stationary bike or a treadmill, it is also possible to emulate exercise by using drugs/medication.

History of the ECG

The first accurate recording of an ECG was in 1895 and the waveforms were eventually  designated P, Q, R, S and T. Each waveform is representative of a certain portion of the cardiac cycle or heart beat. More detailed information can be found in the ECG Screening Course PDF but simply; the P wave is atrial contraction (Depolarisation), the QRS is the various parts of the ventricle contracting (Depolarisation) and the T wave is the 'recovery' (Repolarisation) of the ventricles. Occasionally there is a further, small wave called the U wave and this is thought to be the late repolarisation of papillary muscles or Purkinje fibres.

The information that can be gained from an ECG is taken by analysing the rate and overall rhythm of the ECG, the morphology (shape) and measurements of the ECG and by evaluating the timing of the components of the ECG waveforms.

Artefacts

Care should be taken when performing an ECG as the recorded waveform may be contaminated with artefacts. Artefacts can be considered to be any electrical potentials that do not arise from the heart or something that affects the portrayal of the true ECG.

The main sources of artefact come from:

• The recording equipment/leads or electrodes (ECG machine)
• Electrical interference external to the recorder
• The patient
• The Electrocardiographer or person performing the ECG

ECG Screening Course