ECG (Electrocardiogram)

Unlock the secrets of the heart's electrical language. Learn the fundamentals of ECG interpretation, from basic waveforms to common abnormalities.

See ECG Simulation

Live ECG Simulation

⚠️ DISCLAIMER: This is a visual animation ONLY for illustrative purposes. It shows the *appearance* of P-QRS-T waves scrolling. It does NOT represent real-time data, accurate intervals, specific rhythms, or medical advice, and MUST NOT be used for diagnosis.

BPM 72

Stylized Normal Sinus Rhythm Simulation

Decoding the Heartbeat: A Comprehensive Guide to ECG Interpretation

The electrocardiogram is a cornerstone of cardiovascular diagnostics. This guide breaks down its principles, interpretation, and clinical significance.

The ECG (Electrocardiogram), also sometimes called EKG (from the German *Elektrokardiogramm*), is one of the most fundamental and widely used diagnostic tools in medicine. It provides a real-time, non-invasive window into the electrical activity of the heart. By recording tiny electrical signals generated by the heart muscle as it depolarizes and repolarizes with each beat, the ECG translates this intricate electrical dance into a visual waveform. For the astute clinician, this waveform is a rich source of information, revealing details about the heart's rate, rhythm, conduction pathways, and even evidence of muscle damage or stress.

While interpreting complex ECGs requires significant expertise, mastering the fundamentals is an essential skill for all medical students and healthcare professionals. It allows for the rapid identification of potentially life-threatening conditions, guides further diagnostic workups, and helps monitor response to treatment. This comprehensive guide will walk you through the basic principles, the components of a normal ECG, a systematic approach to interpretation, and common abnormalities you might encounter.

What Does the ECG Actually Measure? The Electrical Symphony

It's crucial to understand that the ECG does not directly measure the mechanical contraction of the heart muscle or blood flow. It solely measures electrical activity. The heart beats because of a carefully orchestrated wave of electrical activation (depolarization) followed by electrical recovery (repolarization). As millions of cardiac muscle cells undergo these cycles, they create small electrical currents that spread through the conductive tissues of the body. Electrodes placed strategically on the skin surface act like tiny microphones, detecting these minute voltage changes over time.

The ECG machine amplifies these signals immensely and records them, plotting voltage changes on the vertical axis against time on the horizontal axis. The resulting trace reflects the *sum total* and *net direction* of electrical activity from the entire heart, viewed from the specific vantage point of each electrode placement (lead).

Performing the ECG: The Standard 12-Lead Setup

The standard clinical ECG uses 10 physical electrodes (4 limb, 6 chest) to generate 12 different electrical "views" or leads of the heart's activity. Each lead provides a unique perspective, like looking at an object from different angles.

Frontal Plane Leads (View from Front/Back):

Bipolar Limb Leads (I, II, III): Created by Einthoven, these record the voltage difference between two limbs.
Augmented Unipolar Limb Leads (aVR, aVL, aVF): These record the voltage at one limb relative to a composite reference point.

Horizontal (Transverse) Plane Leads (View from Cross-Section):

Precordial (Chest) Leads (V1-V6): These unipolar leads record voltage from specific points on the chest wall, providing detailed views of the septum (V1-V2), anterior wall (V3-V4), and lateral wall (V5-V6) of the ventricles.

Together, these 12 leads offer a comprehensive assessment of the heart's electrical function.

Understanding ECG Paper and Calibration: The Grid Rules

ECG paper is standardized graph paper:

Horizontal Axis (Time):** Standard speed is 25 mm/second. One small square = 0.04 seconds; one large square = 0.20 seconds.

Vertical Axis (Voltage):** Standard calibration is 10 mm = 1 mV. One small square = 0.1 mV; one large square = 0.5 mV.

Always verify calibration marks.

Components of the Normal ECG Waveform: The PQRST Complex

A typical ECG complex represents one cardiac cycle:

P Wave: Atrial depolarization.

PR Interval: Atrial depolarization + AV nodal delay (Normal: 0.12-0.20s).

QRS Complex: Ventricular depolarization (Normal: <0.12s).

ST Segment: Early ventricular repolarization (Normally isoelectric).

T Wave: Ventricular repolarization.

QT Interval: Total ventricular electrical activity (Corrected QTc varies, generally <440-460ms).

U Wave:** Sometimes present after T wave.

A Systematic Approach to ECG Interpretation

Develop a consistent method:

Basics & Calibration:** Verify patient info, speed, voltage.

Rate:** Calculate ventricular (and atrial, if different) rate.

Rhythm:** Regular or irregular? Sinus or arrhythmia? P:QRS relationship?

Axis:** Determine frontal plane axis (Normal, LAD, RAD, Extreme).

Intervals:** Measure PR, QRS, QT/QTc.

P Wave Morphology:** Assess for atrial enlargement.

QRS Morphology:** Assess for hypertrophy, pathological Q waves, bundle branch blocks.

ST-T-U Waves:** Look for signs of ischemia, injury, infarction, electrolyte changes, drug effects.

Overall Impression & Comparison:** Synthesize findings and compare to previous ECGs.

Common ECG Abnormalities (Brief Overview)

Arrhythmias:** AFib, Flutter, SVT, VT, Blocks (AV, BBB).

Ischemia/Infarction:** ST elevation/depression, T wave changes, Q waves.

Hypertrophy:** Voltage criteria for LVH/RVH.

Electrolytes:** Peaked T (Hyperkalemia), U waves (Hypokalemia).

Conclusion: An Essential Clinical Language

The ECG provides unique insights into the heart's electrical behavior. Mastering systematic interpretation is crucial for identifying urgent conditions and providing better care. It requires practice and clinical correlation.

ECG FAQs

Your common questions about electrocardiograms, answered.

Is getting an ECG painful?

No, it's completely painless. Electrodes only record signals, they don't send electricity.

What can an ECG diagnose?

It diagnoses electrical issues: arrhythmias, conduction blocks, ischemia/infarction (heart attack), chamber enlargement, electrolyte imbalances affecting the heart.

What *can't* an ECG reliably diagnose?

It doesn't show mechanical pumping (use echo) or blocked arteries (use angiogram). A normal ECG doesn't rule out heart disease. It's a snapshot in time.

What are the "normal" values for PR, QRS, and QTc?

Approximate normal ranges for adults:

PR Interval: 0.12 - 0.20s

QRS Duration: < 0.12s (often < 0.10s)

QTc Interval: < 440ms (males), < 460ms (females)

Interpret in clinical context.

Why is comparing to an old ECG so important?

It establishes the patient's baseline. What looks abnormal might be old/unchanged. *New* changes, even subtle ones, are often highly significant.