You're handed a 12-lead EKG on the ward. Twelve squiggly lines, a rhythm strip, and someone waiting for your answer. Most students freeze not because they don't know the patterns, but because they look at the EKG in a random order and miss things.
The fix is a system: the same eight steps, every time, in the same order. Do that and you won't miss the heart block hiding behind a normal rate or the ST elevation in two small leads. Below is that system, followed by the five patterns you must recognise on sight.
Before you start: the EKG paper
| Element | Value (standard 25 mm/s, 10 mm/mV) |
|---|---|
| 1 small square | 0.04 s (40 ms) |
| 1 large square | 0.20 s (200 ms) |
| 5 large squares | 1 second |
| Calibration | 10 mm = 1 mV |
Always check the patient's name, date and time, and the calibration first. Then compare with any previous EKG; change matters more than any single finding.
The 8-step EKG interpretation system
Step 1: Rate
- Regular rhythm: 300 ÷ number of large squares between two R waves.
- Irregular rhythm: count the QRS complexes on a 10-second strip and multiply by 6.
- Normal: 60–100 bpm. Below 60 = bradycardia; above 100 = tachycardia.
Step 2: Rhythm
Is the R–R interval regular, regularly irregular, or irregularly irregular? Mark R waves on a strip of paper and slide it along to check.
Step 3: P waves and their relationship to QRS
Is there a P wave before every QRS, and a QRS after every P wave? Upright P waves in lead II followed by a QRS = sinus rhythm.
Step 4: PR interval
Normal 0.12–0.20 s (3–5 small squares). Longer than 0.20 s = first-degree heart block. Short PR raises the possibility of pre-excitation (e.g. Wolff–Parkinson–White).
Step 5: QRS complex
Normal width < 0.12 s (3 small squares). A broad QRS suggests bundle branch block, a ventricular rhythm, hyperkalaemia or drug effect. Look for pathological Q waves.
Step 6: Axis
Quick method using leads I and aVF:
| Lead I | Lead aVF | Axis |
|---|---|---|
| Positive | Positive | Normal |
| Positive | Negative | Left axis deviation (check lead II: if also negative, it's true LAD) |
| Negative | Positive | Right axis deviation |
| Negative | Negative | Extreme axis deviation |
Step 7: ST segments and T waves
Look for ST elevation or depression in contiguous leads (leads looking at the same area of the heart), T-wave inversion, and tall or peaked T waves.
Step 8: QT interval
Measure from the start of QRS to the end of the T wave and correct for heart rate (QTc). As a rough guide, a QTc above about 450 ms in men or 460–470 ms in women is prolonged, and above 500 ms is high risk for torsades de pointes.
Tip: say it out loud on ward rounds: "Rate, rhythm, P waves, PR, QRS, axis, ST/T, QT." Saying the order is how it becomes automatic.
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See the Complete Edition – $595 EKG patterns you must recognise on sight
1. Atrial fibrillation (AF)
- Look for: an irregularly irregular rhythm with no clear P waves.
- Clue: an irregular, narrow-complex rhythm with a chaotic (fibrillating) baseline.
- Why it matters: stroke risk, and fast AF can cause haemodynamic compromise.
2. Atrial flutter
- Look for: sawtooth flutter waves, best seen in II, III, aVF and V1.
- Clue: a regular ventricular rate of around 150 bpm often means flutter with 2:1 block. Look hard for a hidden second flutter wave.
3. Heart blocks: first-degree, Mobitz I, Mobitz II and complete
| Block | EKG finding | Risk |
|---|---|---|
| First-degree | PR > 0.20 s, every P conducted | Usually benign |
| Mobitz I (Wenckebach) | PR gets progressively longer until a beat is dropped | Usually benign |
| Mobitz II | Constant PR, then a sudden dropped beat | Can progress to complete block |
| Complete (third-degree) | P waves and QRS complexes march independently (AV dissociation) | Emergency: often needs pacing |
4. STEMI by territory
| Leads with ST elevation | Territory | Usual artery |
|---|---|---|
| II, III, aVF | Inferior | Right coronary artery (sometimes circumflex) |
| V1–V4 | Anterior / anteroseptal | Left anterior descending |
| I, aVL, V5–V6 | Lateral | Circumflex or diagonal branches |
Reciprocal ST depression in the opposite leads supports a true infarction. In an inferior STEMI, also record right-sided leads (V4R) to look for right ventricular involvement.
5. Hyperkalaemia
- Sequence: peaked T waves → PR prolongation and flattened P waves → QRS widening → sine-wave pattern.
- Why it matters: it can progress to VF or asystole. Hyperkalaemia with EKG changes is an emergency. Full management is in our hyperkalaemia guide.
Reading EKGs is a skill. Skills need reps.
The Complete Edition's 15 practice exams and doctor-written guides give you structured practice, not random strips.
Get the Complete EditionWorked example: put the system together
A 72-year-old has palpitations. The EKG shows: rate about 130 bpm (22 QRS complexes in 10 seconds × 6 = 132), irregularly irregular rhythm, no P waves, narrow QRS, normal axis, no ST elevation, normal QTc.
Answer: atrial fibrillation with a fast ventricular response. Notice how steps 1–3 gave the diagnosis, and steps 4–8 ruled out anything dangerous hiding alongside it.
Quick EKG recap table
| Step | Normal |
|---|---|
| Rate | 60–100 bpm |
| Rhythm | Regular, sinus |
| PR | 0.12–0.20 s |
| QRS | < 0.12 s |
| Axis | Leads I and aVF both positive |
| ST/T | No elevation/depression; T waves upright in most leads (aVR inverted) |
| QTc | Below about 450 ms (men) / 460–470 ms (women) |
Frequently asked questions
What is the easiest way to read an EKG?
Use the same order every time: rate, rhythm, P waves, PR interval, QRS, axis, ST segments and T waves, and QT interval. A fixed system stops you missing findings.
How do you calculate heart rate on an EKG?
For a regular rhythm, divide 300 by the number of large squares between two R waves. For an irregular rhythm, count the QRS complexes on a 10-second strip and multiply by 6.
How do you tell atrial fibrillation from atrial flutter?
AF is irregularly irregular with no clear P waves. Flutter shows regular sawtooth flutter waves and often a regular ventricular rate around 150 bpm with 2:1 block.
What is the difference between Mobitz I and Mobitz II?
In Mobitz I the PR interval lengthens progressively until a beat is dropped. In Mobitz II the PR stays constant and beats drop suddenly. Mobitz II is more dangerous because it can progress to complete heart block.
What are the EKG changes of hyperkalaemia?
Peaked T waves first, then PR prolongation and flattened P waves, then QRS widening and finally a sine-wave pattern that can lead to VF or asystole.
Keep going
- How to interpret an ABG in 6 steps
- Beta-blocker side effects: the high-yield cheat sheet
- Hyperkalaemia: EKG changes, causes and emergency management
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21 doctor-written guides, 15 practice exams, the ABG handbook and a 12-week study plan. The Med Notes Guide Complete Edition, $59.
Get the Complete Edition – $59Educational content only. Not medical advice. Always check your local guidelines and protocols.