Electrocardiography

Electrocardiography is the medical process of recording the electrical activity of the heart across repeated cardiac cycles. The resulting electrocardiogram is a graphical representation of voltage variations over time, obtained using electrodes placed on the surface of the skin. These electrical signals reflect the depolarisation and repolarisation of cardiac muscle cells and provide essential clinical insight into cardiac structure, rhythm and function. Standard electrocardiograms typically involve twelve leads, offering multiple perspectives on the heart’s electrical activity, although numerous specialised and portable alternatives are also widely used.

Terminology and Physiological Basis

The term electrocardiogram derives from a combination of international scientific vocabulary, incorporating elements denoting electrical activity and the Greek root for heart. Although the abbreviation ECG is standard in British and international practice, the variant EKG—based on early German nomenclature—is retained in certain contexts to avoid confusion with EEG when similar pronunciation may arise.
Each heartbeat begins with the initiation of an electrical impulse by pacemaker cells located in the sinoatrial node. The impulse spreads across the atria, passes through the atrioventricular node and travels via the bundle of His and Purkinje fibres throughout the ventricles. This orderly process produces the characteristic ECG waveform, which includes:

• P wave – atrial depolarisation.
• QRS complex – rapid ventricular depolarisation.
• T wave – ventricular repolarisation.

The ECG thus serves as a non-invasive, real-time indicator of the heart’s conduction system and electrical mechanics.

Recording Techniques and Lead Systems

A conventional twelve-lead ECG employs ten electrodes placed on the patient’s limbs and chest. These electrodes measure the heart’s electrical field from twelve distinct angles, capturing the magnitude and direction of electrical depolarisation over approximately ten seconds.
Alternative recording systems include:

• Holter monitors, which provide continuous ambulatory monitoring over 24 hours or longer.
• Single-lead devices incorporated into smartwatches and fitness trackers.
• Implantable devices, such as pacemakers and implantable loop recorders, which record electrograms from within the thoracic cavity.
• Portable multi-lead recorders, capable of producing diagnostic-quality tracings outside clinical settings.

Modern ECG machines use analogue-to-digital conversion to process the minute voltages detected across the skin. Many offer automated interpretation algorithms that calculate intervals, axes and rhythm characteristics, though these outputs require clinical verification.

Interpretation and Clinical Insight

The ECG provides vital diagnostic information. Trained clinicians can evaluate:

• Heart rate and rhythm, including atrial fibrillation, atrial flutter and various tachycardias.
• Chamber enlargement, such as atrial enlargement or ventricular hypertrophy.
• Conduction abnormalities, including bundle branch blocks and atrioventricular block.
• Myocardial ischaemia and infarction, identified by changes such as ST-segment elevation or depression and pathological Q waves.
• Electrolyte imbalances, including features associated with hypokalaemia or hyperkalaemia.
• Drug effects, such as QT prolongation or digoxin-related changes.
• Pacemaker function, through analysis of pacing spikes and capture responses.

Continuous ECG monitoring is indispensable in intensive care units, operating theatres and emergency departments, where real-time detection of arrhythmias and haemodynamic instability is essential.

Indications and Medical Applications

Electrocardiography is employed in a range of clinical scenarios, including:

• Investigation of chest pain or suspected acute myocardial infarction.
• Evaluation of palpitations, syncope, seizures or unexplained episodes of collapse.
• Monitoring during general anaesthesia and postoperative care.
• Assessment of drug toxicity, including tricyclic antidepressant overdose.
• Timing imaging studies such as CT angiography or magnetic resonance angiography using ECG gating to minimise motion artefact.
• Intracardiac mapping during electrophysiological studies via catheter-based electrode arrays.

Continuous monitoring is advised for critically ill patients, individuals undergoing major procedures and those with intermittent arrhythmias unlikely to be captured during a brief tracing.
In asymptomatic adults at low risk of cardiovascular disease, routine ECG screening is not recommended due to limited benefit and potential for false-positive findings. However, screening may be required in certain occupations, such as aviation, and in adolescents at risk of inherited cardiomyopathy.

Electrocardiograph Machines and Safety Considerations

Modern electrocardiograph units incorporate compact digital systems with instrumentation amplifiers, filters and shielding to manage the low-amplitude cardiac signals and to reduce electrical noise from mains power. Safety features include:

• Earthing and isolation circuitry to prevent electric shock.
• High-voltage protection, enabling the machine to withstand defibrillation discharges.
• Driven right-leg circuits, reducing common-mode interference.
• Simultaneous multi-lead recording, now standard in clinical devices.

Portable and wearable ECG systems have expanded significantly, with innovations ranging from wireless patch-based monitors to integrated smartwatch sensors. These devices enable long-term rhythm tracking and facilitate early detection of arrhythmias in outpatient and community settings.

Cardiac Monitoring Technologies

Beyond traditional electrocardiographs, a wide array of devices can measure cardiac electrical activity:

• Holter monitors, introduced in the 1960s, providing continuous ambulatory ECG recordings.
• Patch-based monitors, offering single-use, cable-free continuous monitoring over days or weeks.
• Implantable cardioverter-defibrillators and pacemakers, capable of recording internal electrograms between implanted leads and device enclosures.
• Implantable loop recorders, offering prolonged monitoring for unexplained syncope or suspected arrhythmias.
• Consumer-grade sensors, integrated into wearables or electronic kits for educational or low-level monitoring purposes.