Published August 22, 2026 | Clinical Skills

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How to Read an EKG: A Systematic Method for Interns

Every July I watch a new intern pick up an EKG, stare at the middle of it, and announce a diagnosis. Sometimes they are right. That is the problem. Being right by luck teaches you nothing, and the one time you eyeball a strip and miss the third degree block, someone gets hurt. You do not read an EKG by looking at it. You read it by interrogating it, in the same order, every single time, until the order is so automatic you could do it half asleep. Which is convenient, because you will be doing it half asleep.

Here is the method the way I would teach it to you at 2 AM, standing at the nurses station with a strip in my hand. Rate, rhythm, axis, intervals, hypertrophy, ischemia. Six steps. Say them out loud until they annoy you.

Step 1: Rate

Find an R wave that lands on a heavy grid line. Count heavy lines to the next R wave: 300, 150, 100, 75, 60, 50. That is your rate. If the rhythm is irregular, count the QRS complexes in the ten second strip along the bottom and multiply by six.

Then commit to a category. Under 60 is bradycardia. Over 100 is tachycardia. Do not write "borderline." A rate of 48 in a sleeping 25 year old and a rate of 48 in an 80 year old on diltiazem who just fainted are the same number and completely different problems. The number means nothing without the patient.

Step 2: Rhythm

This is where interns skip ahead and get burned. Ask three questions, in order.

• Is it regular? Take calipers, or the edge of a paper, and march out the R waves. Regularly irregular and irregularly irregular are different answers.
• Is there a P wave before every QRS, and a QRS after every P? Look in lead II and V1, where P waves live. If you cannot find P waves, say so. "I do not see P waves" is a legitimate and important finding.
• Is the QRS narrow or wide? Narrow means the ventricles are being driven from above. Wide means ventricular origin or aberrant conduction, and wide plus fast means you stop reading and go see the patient.

Those three answers sort almost everything. Irregularly irregular with no P waves is atrial fibrillation until proven otherwise. Regular, narrow, and 150 should make you think atrial flutter with 2 to 1 block, because flutter loves to hide at exactly 150. Sawtooth waves in the inferior leads will give it away if you look.

Rhythm recognition is a pattern skill, and pattern skills are built by reps, not by reading paragraphs like this one. The interactive rhythm strips on the practice page exist for exactly this. Ten strips a day for a month and you will stop sweating telemetry calls. There is no shortcut. Do the reps.

Step 3: Axis

Axis intimidates interns because it gets taught with vectors and hexaxial diagrams. Forget all that at the bedside. Look at leads I and aVF.

• Both upright: normal axis. Move on.
• Lead I up, aVF down: left axis deviation. Think left anterior fascicular block, inferior MI, or LVH.
• Lead I down, aVF up: right axis deviation. Think right heart strain, and in the right story, pulmonary embolism.
• Both down: extreme axis. Check your lead placement, then think ventricular rhythm.

That is 95 percent of what you need on call. The fine points can wait for board study.

Step 4: Intervals

Three numbers. Learn them cold.

• PR: 120 to 200 ms. Long PR is first degree AV block, which alone is usually benign. The real question is whether it is stable or progressing. Compare with the old EKG. Always compare with the old EKG.
• QRS: under 120 ms. Wide QRS means bundle branch block, ventricular origin, hyperkalemia, or drug toxicity. A new wide QRS is never something to shrug at.
• QT: eyeball rule first. If the QT is more than half the R to R interval, it is probably long. Then use the machine's QTc as a starting point and verify it yourself, because the computer measures QT badly in bundle branch blocks and atrial fibrillation. A QTc creeping past 500 in a patient on haloperidol, methadone, or a fluoroquinolone is a phone call, not a footnote.

Step 5: Hypertrophy

Quick screen, not a dissertation. For LVH, add the deepest S in V1 to the tallest R in V5 or V6. Over 35 mm suggests LVH, and the strain pattern, asymmetric ST depression with T inversion in the lateral leads, supports it. For RVH, a dominant R wave in V1 is your flag, in a patient without a posterior MI or right bundle to explain it. Left atrial enlargement shows as a deep terminal negative P in V1. Note it, correlate it, and keep moving.

Step 6: Ischemia and infarction

You saved this for last on purpose. If you look for ST elevation first, you will find it or exclude it and then stop reading, and you will miss the long QT or the new left bundle sitting right next to it.

Now be systematic here too. Walk the lead groups: inferior is II, III, aVF. Lateral is I, aVL, V5, V6. Anterior is V3 and V4. Septal is V1 and V2. ST elevation in a regional distribution with reciprocal depression in the opposite wall is a STEMI until proven otherwise, and reciprocal changes are your best friend, because they are what separates real infarction from pericarditis and early repolarization.

Learn the patterns that kill quietly: ST depression in V1 to V3 with tall R waves may be a posterior MI showing you its mirror image. Deep symmetric T inversions in V2 and V3 in a patient with recent chest pain is a proximal LAD lesion signature that deserves respect. And hyperacute T waves, broad and tall and out of proportion to the QRS, come before the ST elevation does. If the story fits, get serial EKGs. One normal tracing rules out nothing.

The classic rookie mistakes

• Trusting the computer read. The machine is decent at rate and intervals and unreliable at rhythm and ischemia. Read it yourself, then look at what the computer said, in that order.
• Not pulling the old EKG. Half of EKG interpretation is the word "new." An old left bundle is background noise. A new left bundle in a diaphoretic patient is a cath lab conversation.
• Reading the strip instead of the patient. A rate of 130 on telemetry means nothing until you know whether the patient is febrile, bleeding, in pain, or in pulmonary edema. The EKG is one vital sign, not a verdict.
• Missing lead misplacement. Bizarre axis, inverted P in lead I, or a tracing that looks nothing like yesterday should prompt one question first: were the leads on right?
• Anchoring on the obvious finding. You spotted the atrial fibrillation. Good. Did you check the QT before you wrote for the antiarrhythmic? Finish the six steps every time, especially when the diagnosis seems easy.

When to wake someone up

Interns lose more sleep over whether to call than over the call itself. Here is the rule: you never get in trouble for waking a senior about a real change. You get in trouble for sitting on one. Call immediately for any of these.

• New ST elevation, or ST depression that is deep, diffuse, or accompanied by symptoms.
• Any sustained wide complex tachycardia. Treat it as VT until someone senior says otherwise, and know your ACLS algorithms before you need them, not during.
• New high grade AV block: Mobitz II or third degree. First degree block can wait until morning. These cannot.
• Bradycardia or tachycardia with hypotension, chest pain, altered mental status, or pulmonary edema. Unstable plus arrhythmia equals now.
• A QTc over 500 in a patient on QT prolonging drugs, or any new finding you cannot explain.

Everything else, look at the patient, compare with the old tracing, write down what you see, and present it in the morning. The middle of the night pages that generate these strips have their own playbook, and the common night calls guide covers the ones that will actually hit your pager.

How you actually get good at this

Nobody becomes competent at EKGs by reading about EKGs. You get good the same way you got good at anything in medicine: volume plus feedback. Read every EKG on every patient you admit, commit to an interpretation before you see the official read, and score yourself. Then add deliberate practice on top. Work through the rhythm strip trainer until the common arrhythmias are boring, and when you are ready to go deeper than survival level, the cardiology board review material covers the electrophysiology behind what you are pattern matching. Understanding why the flutter waves are negative in the inferior leads makes the pattern stick permanently.

Six steps. Same order. Every strip. Do that for six months and you will be the resident the interns come find.

If this is the kind of teaching that works for you, it is what the whole site is built on. The board review section takes the same blunt approach to the exam material, and membership gets you the full question banks and practice tools. Start with the free EKG trainer, and when it stops being hard, come find the rest.

Where HistoryandPhysical.net fits in

A systematic method is worth having only if it survives being tired, which means repetitions.

• Sixty questions with real strips. The ECG practice bank draws rhythm strips on a true ECG grid and pairs them with reasoning questions, so you practice the read and the decision that follows.
• Confusion pairs named explicitly. Every explanation says which rhythm this one is most often mistaken for, because that pair is where the error actually happens.
• Practise only what you missed. Membership saves your progress and gives you a one click run through just the strips you got wrong.
• The guides stay free. This method, the night calls guide and every calculator cost nothing, with no ads anywhere on the site.

Ten questions are free without an account. Membership is $10 a month or $59 a year at the founding rate, cancellable in two clicks.

This is not medical advice. It is a teaching outline for clinicians and clinicians in training. Thresholds and examples above are illustrative. Follow your own institution's protocols, verify every finding and threshold against a current reference, and use your own clinical judgment.

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