Reading a digital Electrocardiogram (ECG) involves a systematic approach to interpret the electrical activity of the heart, providing crucial insights into its health and function. Understanding the waveforms, intervals, and segments is key to identifying normal rhythms and potential abnormalities.
Understanding the Basics of an ECG
A digital ECG displays the heart's electrical signals as waves on a grid. This grid typically shows time on the horizontal axis (small square = 0.04 seconds, large square = 0.20 seconds) and voltage on the vertical axis (small square = 0.1 millivolt, large square = 0.5 millivolts).
Key Components of an ECG Waveform
Before diving into interpretation, it's essential to recognize the fundamental components of an ECG tracing:
- P Wave: Represents atrial depolarization (contraction of the atria).
- PR Interval: Measures the time from the beginning of atrial depolarization to the beginning of ventricular depolarization. It reflects the time taken for the electrical impulse to travel from the atria through the AV node to the ventricles.
- QRS Complex: Represents ventricular depolarization (contraction of the ventricles). It's typically the largest and most prominent wave.
- ST Segment: The flat line following the QRS complex and preceding the T wave. It represents the period when the ventricles are fully depolarized.
- T Wave: Represents ventricular repolarization (relaxation of the ventricles).
- QT Interval: Measures the time from the beginning of ventricular depolarization to the end of ventricular repolarization.
Step-by-Step Guide to Interpreting a Digital ECG
A structured approach ensures that no critical details are missed. Here’s a common sequence for interpreting a digital ECG:
1. Check Calibration and Standardisation
Always begin by verifying the ECG's calibration. Standard paper speed is 25 mm/s, and amplitude is 10 mm = 1 mV. This is usually indicated by a calibration mark (a square wave) at the beginning of the tracing. Incorrect calibration can lead to misinterpretation.
2. Determine the Heart Rate
Calculating the heart rate is a fundamental first step.
- For Regular Rhythms:
- Method 1: Locate an R wave that falls on a thick line. Then, count down using the sequence 300, 150, 100, 75, 60, 50 for subsequent thick lines until the next R wave. The number associated with the thick line closest to the next R wave is the approximate heart rate.
- Method 2: A more precise method for regular rhythms is to divide 300 by the number of large squares between two consecutive QRS complexes (R-R interval).
- For Irregular Rhythms:
- Method: Count the number of QRS complexes within 30 large squares (which represents 6 seconds at a standard paper speed of 25 mm/s). Then, multiply this number by 10 to estimate the heart rate per minute.
3. Assess the Rhythm
Is the rhythm regular or irregular?
- Regular: R-R intervals are consistent.
- Irregular: R-R intervals vary. If irregular, is it regularly irregular (e.g., a repeating pattern) or irregularly irregular (no discernable pattern)?
- Determine Sinus Rhythm: Is there a P wave before every QRS complex? Is the PR interval constant? Is the QRS complex narrow? If yes, it's likely a sinus rhythm originating from the SA node.
4. Evaluate the P Waves
Examine the P waves for:
- Presence: Are P waves present before every QRS?
- Morphology: Are they upright and smooth in most leads (especially Lead II)?
- Consistency: Do they all look similar?
- Relation to QRS: Is there one P wave for every QRS, and is it consistently associated?
5. Measure the PR Interval
Measure the PR interval from the beginning of the P wave to the beginning of the QRS complex.
- Normal Range: Typically 0.12 to 0.20 seconds (3 to 5 small squares).
- Prolonged PR: Suggests a conduction delay (e.g., first-degree AV block).
- Short PR: May indicate an accessory pathway (e.g., Wolff-Parkinson-White syndrome).
6. Examine the QRS Complex
Focus on the duration, morphology, and axis of the QRS complex.
- Duration:
- Normal: 0.08 to 0.12 seconds (2 to 3 small squares).
- Wide QRS (> 0.12s): Suggests a bundle branch block, ventricular rhythm, or other intraventricular conduction delay.
- Morphology: Look for specific patterns like R, S, or Q waves, and their amplitudes.
- Axis: Determine the electrical axis of the heart using leads I and aVF. Normal axis is generally between -30° and +90°.
7. Analyze the ST Segment
The ST segment should be isoelectric (flat and at the baseline).
- ST Elevation: Can indicate myocardial infarction (heart attack) or pericarditis.
- ST Depression: Can suggest myocardial ischemia (reduced blood flow), subendocardial infarction, or digitalis effect.
8. Look at the T Waves
Assess the T wave's direction, amplitude, and symmetry.
- Normal: Upright and rounded in most leads, typically in the same direction as the QRS complex.
- Inverted T Waves: Can suggest ischemia.
- Peaked T Waves: Often seen in hyperkalemia.
- Flattened T Waves: Can be seen in hypokalemia.
9. Measure the QT Interval
Measure the QT interval from the beginning of the QRS complex to the end of the T wave. This interval is rate-dependent and is often corrected (QTc) for heart rate using formulas like Bazett's.
- Normal QTc: Typically < 0.44 seconds in adults.
- Prolonged QTc: Can increase the risk of dangerous arrhythmias (e.g., Torsades de Pointes).
10. Compare Across Leads
Always examine the ECG across all 12 leads. Changes may be localized to certain leads, indicating the area of the heart affected. For example, inferior wall abnormalities appear in leads II, III, and aVF.
Practical Tips for Digital ECG Interpretation
- Systematic Approach: Always follow a consistent step-by-step method to avoid missing details.
- Clinical Correlation: Interpret ECG findings in the context of the patient's symptoms, medical history, and other clinical data.
- Practice: Regular practice with various ECGs is crucial for developing proficiency.
- Technology: Digital ECG systems often provide automated measurements and preliminary interpretations, but these should always be reviewed and confirmed by a human expert.
- Baseline Comparison: If available, compare the current ECG with previous ones to identify new changes.
Normal ECG Intervals (Approximate Values)
| Component | Normal Duration/Range |
|---|---|
| P Wave | 0.08 - 0.12 seconds (2-3 small squares) |
| PR Interval | 0.12 - 0.20 seconds (3-5 small squares) |
| QRS Complex | 0.08 - 0.12 seconds (2-3 small squares) |
| ST Segment | Isoelectric (at baseline) |
| T Wave | Upright in most leads, typically < 10 mm in limb leads |
| QTc Interval | < 0.44 seconds (varies with age/gender) |
| Heart Rate | 60 - 100 beats per minute (bpm) |
For further reading and in-depth understanding, resources like the American Heart Association and Mayo Clinic provide extensive information on heart health and ECG interpretation.
