The electrocardiogram (EKG or ECG) is one of the most widely used diagnostic tools in medicine. It provides a non-invasive, real-time assessment of the heart’s electrical activity and rhythm.
Interpreting an EKG may seem daunting at first, but breaking it down into six systematic steps makes it simple, reliable, and clinically valuable. This guide explains each step in detail, covering normal values, abnormalities, and practical methods for heart rate calculation.
Step 1: Examine the P Wave
The P wave represents atrial depolarization (atrial contraction).
What to Look For:
- Each P wave should be present and upright in lead II.
- There should be one P wave before every QRS complex.
- P waves should look similar throughout the strip (notchaed, peaked, or absent P waves may indicate pathology).
Abnormal P Wave Findings:
- Absent P waves: Atrial fibrillation.
- Multiple P waves: Atrial flutter (saw-tooth waves).
- Peaked P waves: Right atrial enlargement (P pulmonale).
- Wide or notched P waves: Left atrial enlargement (P mitrale).
Step 2: Measure the PR Interval
The PR interval reflects the time it takes for the electrical impulse to travel from the atria to the ventricles.
Normal Range:
0.12 – 0.20 seconds (3–5 small boxes).Abnormal PR Intervals:
- Prolonged PR (>0.20s): First-degree AV block.
- Short PR (<0.12s): Pre-excitation syndromes (Wolff-Parkinson-White).
Step 3: Evaluate the QRS Complex
The QRS complex represents ventricular depolarization (contraction).
Normal Range:
0.06 – 0.12 seconds (1.5 – 3 small boxes).Abnormal QRS Complex:
Widened QRS (>0.12s):
- Bundle branch blocks.
- Ventricular rhythms.
- Hyperkalemia or drug toxicity (e.g., digoxin).
Step 4: Check the R-R Interval
The R-R interval helps determine the rhythm’s regularity.
What to Assess:
- Are the R waves spaced evenly? → Regular rhythm.
- Are they irregular? → Could be atrial fibrillation, sinus arrhythmia, or ectopic beats.
Step 5: Determine the Heart Rate
There are two common methods:
1. 6-Second Method
- Count the number of R waves within a 6-second strip and multiply by 10.
- Example: 6 R’s × 10 = 60 beats per minute.
- Best for irregular rhythms.
2. Big Box Method
- Divide 300 by the number of big boxes between two R waves.
- Example: 300 ÷ 5 big boxes = 60 bpm.
- Best for regular rhythms.
Normal Heart Rates:
- Normal sinus rhythm: 60–100 bpm.
- Sinus tachycardia: >100 bpm.
- Sinus bradycardia: <60 bpm.
Step 6: Identify the EKG Findings
After checking waves, intervals, and heart rate, the final step is interpreting the rhythm and identifying abnormalities.
Examples of Common Findings:
- Normal sinus rhythm: Upright P before every QRS, PR and QRS intervals normal, rate 60–100 bpm.
- Atrial fibrillation: Irregularly irregular rhythm, absent P waves, irregular R-R intervals.
- Ventricular tachycardia: Wide QRS complexes, fast rate, no visible P waves.
- Myocardial infarction: ST elevation or depression, abnormal Q waves.
Basic Rhythm Categories
- Normal Sinus Rhythm: 60–100 bpm, regular R-R, normal PQRST.
- Sinus Tachycardia: >100 bpm, often seen with fever, anxiety, dehydration.
- Sinus Bradycardia: <60 bpm, common in athletes and vagal stimulation, but also in heart block.
EKG Grid Measurements
On ECG paper:
- 1 small box = 0.04 sec.
- 1 large box = 0.20 sec.
- 5 large boxes = 1 sec.
- 300 large boxes = 1 minute.
This scale is essential for calculating rates and intervals accurately.
Practical Table: 6 Steps of EKG Interpretation
| Step | What to Check | Normal Value | Abnormal Indications |
|---|---|---|---|
| 1. P Wave | Present, upright, one before every QRS | Upright in lead II | Absent = AFib, Peaked = RAE, Notched = LAE |
| 2. PR Interval | Time atria → ventricles | 0.12–0.20s | Long = AV block, Short = WPW |
| 3. QRS Complex | Ventricular depolarization | 0.06–0.12s | Wide = BBB, Tall = LVH, Toxins |
| 4. R-R Interval | Regularity of rhythm | Regular | Irregular = AFib, sinus arrhythmia |
| 5. Heart Rate | 6-sec or 300 method | 60–100 bpm | <60 = brady, >100 = tachy |
| 6. Identify Findings | Full interpretation | NSR | MI, arrhythmias, blocks |
Frequently Asked Questions (FAQ)
Q1. What is the easiest method to calculate heart rate on an EKG?
The 6-second method is easiest—count R waves in 6 seconds and multiply by 10.
Q2. What does a prolonged QRS mean?
It suggests ventricular conduction delay, such as bundle branch block or ventricular rhythm.
Q3. How do you know if a rhythm is regular or irregular?
Measure the R-R intervals—if consistent, it’s regular; if variable, it’s irregular.
Q4. What is the difference between sinus tachycardia and supraventricular tachycardia (SVT)?
- Sinus tachycardia has visible P waves and gradual onset.
- SVT has hidden or abnormal P waves and sudden onset.
Q5. What are the normal ranges of PR and QRS intervals?
- PR interval: 0.12–0.20 sec.
- QRS complex: 0.06–0.12 sec.
