ECG - 12-lead electrocardiogram

Recording of the electrical activity of the heart

Electrical activity of the heart

ECG (electrocardiogram) is a tracing of the heart's electrical activity. Several sticky patches called electrodes are attached to the skin of the chest, arms, and legs. Leads (wires) then connect the electrodes to the ECG machine. Every cardiac cycle (heartbeat) produces tiny electrical signals. The ECG machine amplifies and records these signals on paper for a period of time, allowing detailed analysis of abnormalities. The procedure takes about 5-10 minutes and is noninvasive and painless. The ECG machine does not generate any external electrical current and only passively records electrical potentials produced by cardiac activity.

Standard electrocardiogram uses 12 leads:

6 limb leads: 3 unipolar, aVR connected to the right arm, aVL to the left arm, aVF to left foot;
and 3 bipolar, I from left arm to the right arm, II from left leg to the right arm, III from left leg to left arm

6 unipolar precordial chest leads: V1 - 4th intercostal space right, V2 - 4th intercostal left, V3 – between V2 and V4, V4 – midclavicular (mid collar bone), V5 – 5th intercostal space in anterior axillary line, and V6 – 5th intercostal in the midaxillary line

A typical electrocardiogram of one cardiac cycle consists of a P wave, a QRS complex, a T wave, and sometimes a U wave. The baseline voltage of the ECG is known as the isoelectric line.

An ECG will be normally recorded during the first appointment with Dr Ruzicka. It is usually unnecessary during follow-up appointments, except for patients with ongoing heart rhythm problems or baseline abnormalities. It is a completely safe test with no known risks.


An ECG shows heart rate, indicating whether the heart rhythm is steady or irregular, and it can locate the part of the heart muscle responsible for the problems. An ECG is a basic part of a cardiology consultation for assessing patients with chest pain, palpitations, murmurs, dizziness and blackouts, monitoring during anaesthesia, and critically ill patients.

ECG can detect:
  • enlargement of heart chambers (atrial and ventricular hypertrophy) caused, e.g. by high blood pressure, valve diseases, such as aortic stenosis, or chronic lung disease
  • extra beats (premature contractions, atrial/supraventricular and ventricular ectopics) sometimes corresponding to palpitations
  • sustained arrhythmias, such as atrial fibrillation or atrial flutter
  • conduction defects (problems with conduction of electric signals within the heart), i.e. left anterior and posterior hemiblock (LAH and LPH), right and left bundle branch block (RBBB, LBBB), 1st, 2nd (Wenckebach, Mobitz) and 3rd degree AV (atrioventricular) block
  • presence and effect of pacemaker (atrial, ventricular and atrioventricular pacing)
  • lack of oxygen supply to the heart muscle (myocardial ischaemia) corresponding to anginal chest pain
  • presence, location and type of the heart attack (myocardial infarction)
  • effect of drugs or electrolyte disturbances in the blood (e.g. effect of digoxin or hypo- and hyperkalaemia – lack and excess of potassium)
  • susceptibility to arrhythmias (e.g. LQTS – long QT interval, delta wave indicating the presence of accessory pathway in WPW syndrome, Brugada syndrome etc.)

Before the procedure

The test will be explained, and you will be asked to remove any clothing above the waist. Since access to your ankles is needed, it is helpful to avoid wearing tights. Ten electrode stickers will be attached to your chest, shoulders and ankles and connected to an ECG machine.

During the procedure

You will be lying on a couch with attached leads and will be asked to relax completely and be ‘floppy’ for 10-20 seconds to minimize any electrical activity coming from outside your heart. Artefacts can be caused by excessive muscle tension, shaking (tremor), talking or movements. You will not feel anything during the test.

After the procedure

The stickers will be removed, and you will be able to get dressed. The ECG tracing will be reported by Dr Ruzicka and discussed with you during the consultation.

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