The Electrocardiogram (ECG) is a widely used, non-invasive diagnostic tool that captures the electrical activity generated by the heart. This technology translates the heart’s electrical impulses into a visual graph, allowing for the assessment of cardiac function. The ECG provides a graphic representation of the cardiac cycle, charting the depolarization and repolarization of the heart muscle. By recording these voltage fluctuations, the ECG offers a time-sensitive window into the heart’s rhythm and electrical health. It remains the most common method for initial assessment and monitoring of heart abnormalities.
Generating the ECG Signal
The process of capturing the heart’s electrical events begins with placing electrodes on the patient’s skin. These electrodes, often made of silver and silver chloride (\(\text{Ag/AgCl}\)), use a conductive gel to ensure optimal contact and minimize impedance. The electrodes detect minute bioelectric signals generated by the migration of ions across heart muscle cells, creating potential differences on the body’s surface.
These recorded signals are extremely small, typically ranging from \(0.4\) to \(5\) millivolts (\(\text{mV}\)), requiring amplification. The ECG machine uses instrumentation amplifiers to boost this weak signal while simultaneously rejecting common-mode noise, such as interference from power lines. Filtering circuits, including high-pass and low-pass filters, further clean the signal by removing unwanted artifacts caused by muscle movement or baseline drift.
The final graph is constructed from multiple “leads,” which provide different electrical views of the heart. A standard \(12\)-lead ECG uses \(10\) electrodes strategically positioned on the limbs and chest to create these distinct perspectives. Each lead records the voltage difference between two points, allowing clinicians to observe the electrical wave as it travels through the heart from various angles. This process transforms the heart’s electrical energy into the characteristic waveform.
Decoding the Standard Waveform
The ECG tracing is centered on the PQRST complex, representing one complete cardiac cycle. The initial deflection, known as the P wave, signifies atrial depolarization—the electrical activation preceding the contraction of the upper chambers. This wave is usually small and rounded, indicating the smooth spread of the electrical impulse from the sinoatrial node.
The PR interval measures the time taken for the electrical signal to travel from the atria to the ventricles. This interval includes the brief delay as the signal passes through the atrioventricular (AV) node, allowing the ventricles time to fill before contraction. In a healthy adult, the PR interval typically falls within \(0.12\) to \(0.20\) seconds.
The QRS complex is the largest feature, representing the rapid depolarization of the ventricles. This complex is composed of the initial negative Q wave, the large positive R wave, and the final negative S wave. The entire QRS complex should be completed quickly, normally in \(0.06\) to \(0.10\) seconds, reflecting fast conduction through the Purkinje fibers.
The ST segment immediately follows the QRS complex and represents the period when the ventricular muscle mass is fully depolarized. This segment is normally isoelectric, resting on the baseline, as there is no net electrical movement. Finally, the T wave signifies ventricular repolarization, which is the electrical recovery of the ventricles for the next beat.
Clinical Significance of Signal Deviations
Variations from the standard PQRST morphology and timing provide clues about underlying cardiac pathology. Changes in the R-R interval, the distance between consecutive R waves, indicate problems with heart rate or rhythm. For example, rapid, chaotic electrical activity in the atria resulting in an “irregularly irregular” rhythm and absent P waves is the hallmark of atrial fibrillation.
Deviations in the PR interval point to issues with electrical conduction between the chambers. A prolonged PR interval, exceeding \(0.20\) seconds, suggests delayed signal transmission through the AV node, often seen in a first-degree atrioventricular block. A shortened PR interval may indicate an accessory pathway that bypasses the normal delay, suggesting a pre-excitation syndrome.
The QRS complex reveals structural or electrical abnormalities through changes in duration or shape. A prolonged QRS duration, exceeding \(0.10\) or \(0.12\) seconds, suggests impaired conduction within the ventricles, such as a bundle branch block. The presence of a deep and wide Q wave (pathological Q wave) can be a lasting sign of heart muscle damage from a previous myocardial infarction.
The ST segment is a primary indicator of myocardial ischemia or injury. Significant ST segment elevation, defined as \(1 \text{ mm}\) or more in limb leads, is a specific finding for an acute, ongoing heart attack. ST segment depression, where the segment drops below the baseline, is often a sign of myocardial ischemia, indicating reduced blood flow to the heart muscle.