Electrocardiogram (ECG/EKG) — How It Works, Benefits & Recovery — Procedure Guide, Recovery & Risks | MyMedicPlus

An electrocardiogram (ECG or EKG, from the German Elektrokardiogramm) is a non-invasive test that graphically records the heart's electrical activity by detecting voltage changes on the body surface generated by the sequential depolarisation and repolarisation of cardiac muscle cells during each heartbeat. Standard electrodes are placed on the limbs and chest to capture twelve different views (leads) of the heart's electrical vectors simultaneously: six limb leads (I, II, III, aVR, aVL, aVF) and six precordial chest leads (V1–V6). The ECG tracing consists of the P wave (atrial depolarisation), PR interval (atrioventricular conduction), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation). Measurement of intervals, amplitudes, morphology, and axis allows diagnosis of a wide spectrum of cardiac conditions. The resting twelve-lead ECG records approximately ten seconds of cardiac electrical activity and is the most frequently performed cardiac investigation worldwide, providing immediate, low-cost, non-invasive information about cardiac rhythm, conduction, ischaemia, and structural changes. An abnormal ECG does not necessarily indicate heart disease, and a normal ECG does not exclude all cardiac pathology — clinical correlation is essential. ECG interpretation requires training and is performed by physicians, cardiologists, and increasingly by AI-assisted software.

An ECG is indicated for a broad range of cardiac and non-cardiac clinical presentations. Urgent indications include chest pain or pressure (to exclude STEMI requiring immediate reperfusion), palpitations with presyncope or syncope (to detect arrhythmia), sudden onset breathlessness (heart failure or PE), bradycardia below forty beats per minute causing haemodynamic compromise, and resuscitation or post-cardiac arrest care. Routine indications include pre-operative cardiac risk assessment before major surgery, known or suspected atrial fibrillation, hypertension assessment, QT interval monitoring for drugs known to prolong repolarisation (amiodarone, sotalol, antipsychotics, antihistamines), electrolyte abnormality monitoring (hyperkalaemia, hypokalaemia, hypomagnesaemia), and detection of left ventricular hypertrophy, bundle branch blocks, and Wolff-Parkinson-White syndrome. Occupational and insurance medical assessments include a resting ECG from age forty. Athletes require screening ECGs under Preparticipation Cardiovascular Evaluation programmes to detect inherited conditions including hypertrophic cardiomyopathy, Brugada syndrome, and long QT syndrome that predispose to sudden cardiac death. Baseline ECG documentation before drug initiation and annual cardiac surveillance in patients with established cardiac disease are standard clinical practice.

The patient lies supine on a couch, relaxed and still — movement artefact is the most common technical cause of ECG misinterpretation. The skin at electrode sites on the wrists, ankles, and chest is cleaned and, if necessary, shaved to improve electrical contact. Conductive gel or pre-gelled adhesive electrodes are applied to the left and right wrists, left and right ankles, and six chest positions (V1 at the fourth intercostal space right of sternum, V2 at the fourth ICS left of sternum, V4 at the fifth ICS midclavicular line, V3 between V2 and V4, V5 anterior axillary line at V4 level, V6 mid-axillary line at V4 level). Leads are connected from the ECG machine to the electrode sites. The operator instructs the patient to breathe normally and remain still, then activates the recorder. The twelve-lead tracing is recorded at twenty-five millimetres per second paper speed and ten millimetres per millivolt gain calibration as standard. The total recording time is approximately ten seconds. The tracing is printed or displayed on screen and automatically annotated by the machine's interpretation algorithm. Manual review by a physician is essential as automated ECG interpretation has error rates of five to twenty percent for less common diagnoses. The entire procedure including attachment and removal of electrodes takes five to ten minutes.

The twelve-lead ECG provides unique, rapid, and highly cost-effective diagnostic information unavailable from any other single test. For ST-elevation myocardial infarction (STEMI), ECG sensitivity exceeds ninety percent in the acute phase with ST elevation greater than one millimetre in two contiguous leads as the diagnostic criterion — enabling immediate activation of the cardiac catheterisation laboratory for primary percutaneous coronary intervention (PCI) within ninety minutes of first medical contact. Atrial fibrillation, the most common sustained cardiac arrhythmia, is detected with near one hundred percent sensitivity when present during the recording. Left ventricular hypertrophy (Sokolow-Lyon criteria, voltage criteria) is identified with seventy-five percent specificity in symptomatic patients. Complete left and right bundle branch blocks are diagnosed with high reliability and have important prognostic and management implications. The QTc interval is measurable to within a few milliseconds, critical for identifying congenital long QT syndromes and drug-induced QT prolongation that risk torsades de pointes arrhythmia. ECG is non-invasive, painless, radiation-free, inexpensive, and reproducible — making it suitable for serial monitoring of progression and treatment response. Wearable ECG technology (single-lead smartwatch ECG, skin patch monitors) extends rhythm monitoring capability into daily life.

The resting twelve-lead ECG carries no health risks. It delivers no electrical current to the patient — it only measures naturally occurring bioelectrical signals. The procedure is non-invasive, radiation-free, and painless. Skin preparation products may cause rare contact dermatitis in hypersensitive individuals. Adhesive electrode removal may cause minor skin irritation, particularly with repeated testing in hospitalised patients. The primary limitation of the standard resting ECG is its brief recording window of ten seconds: arrhythmias that are intermittent (paroxysmal atrial fibrillation, supraventricular tachycardia) may not occur during the recording window, yielding a normal ECG despite the arrhythmia. For this reason, ambulatory monitoring (Holter monitor, extended patch monitoring, or implantable loop recorder) is required for infrequent symptomatic arrhythmias. ECG must always be interpreted in clinical context: normal variants such as early repolarisation, left ventricular hypertrophy voltage criteria in athletes, and sinus bradycardia in trained athletes must be distinguished from pathological changes. The standard resting ECG cannot assess myocardial perfusion, valve function, or structural anatomy — echocardiography, cardiac MRI, and coronary CT angiography complement ECG in comprehensive cardiac assessment.

No recovery is required after an ECG. Electrode removal takes less than one minute and patients may resume all activities immediately. There are no dietary, activity, or medication restrictions related to the ECG itself. Results may be interpreted immediately at the point of care by a physician or transmitted electronically for remote reporting. In acute settings such as the emergency department, ECG interpretation occurs within minutes of acquisition to allow rapid decision-making for ACS, arrhythmia, and other time-sensitive conditions. In outpatient settings, ECG results are reviewed by the requesting physician and incorporated into the overall clinical assessment at the consultation. A normal ECG does not exclude serious cardiac disease — troponin, echocardiogram, or stress testing may be required to complete the cardiac evaluation depending on clinical presentation. Patients should be advised that a single normal ECG does not rule out intermittent arrhythmia if their symptoms suggest this, and further monitoring with a Holter or event recorder may be recommended.