Students try to read strips by recognizing pictures, and then panic when the picture is
slightly off. This page teaches the other way: a five-question procedure you run in the same order on every
strip, so the rhythm names itself at the end. This page is the method. Its sister page
NG-090 — the 9 ECG strips is the rhythm album; come here
to learn how to measure, go there to learn what each rhythm looks like and what to do about it.
📏 0.04 & 0.20Small box = 0.04 s. Large box = 0.20 s. Every measurement is box-counting.
🔢 ×10 or 300-150-1006-second strip ×10 works on any rhythm. The 300 sequence only on regular ones.
🖐️ 5 questionsRate → Regular? → P waves? → PR 0.12–0.20? → QRS <0.12? Then name it.
🚨 Patient firstAny rhythm change → check a pulse and check the patient. Leads lie. Patients don't.
📐
THE PAPER, THE WAVES, THE ORDER
STEP 0 · SET-UP
Before you can measure anything, you have to know what one box is worth and what each bump is.
🫀 Where each wave comes from
The ECG is not the heartbeat — it is the electricity that triggers the
beat. Every wave on the strip is one structure in the conduction system doing its job.
🧠 P = the little atria, QRS = the big ventricles, T = the recharge. The atria are small,
so their wave is small. The ventricles are huge, so their wave is huge.
📏 The paper is a ruler
ECG paper runs under the stylus at a standard 25 mm/second.
That is the whole trick: distance across the page IS time.
🧠 “Small four, big twenty.” Small box 0.04 s,
large box 0.20 s. Five large boxes = one second.
🧭 The loop — run it in this order, always
🧠 “Really Rushed People Pick Quick.”Rate · Rhythm · P waves ·
PR interval · QRS. Say it on the way into the room.
🧪 What every wave, segment and interval means
MARK
WHAT IT IS
NORMAL (adult)
P wave
Atria depolarizing
<0.12 s wide, <2.5 mm tall, upright in II
PR interval
Start of P → start of QRS: atria + the AV delay
0.12–0.20 s (3–5 small boxes)
QRS complex
Ventricles depolarizing
<0.12 s (under 3 small boxes)
ST segment
Ventricles fully depolarized, plateau
flat on the baseline
T wave
Ventricles repolarizing (recharging)
upright, rounded, same direction as QRS
QT interval
Total ventricular depolarize + recharge
≈0.36–0.44 s; rate-dependent
R-R interval
Beat to beat — this is what regularity means
equal, and equal to the P-P
Values are typical adult figures and vary a little between references and labs.
🧠 Wave = a bump. Segment = flat line between bumps. Interval = a wave PLUS a segment.
🚨 STEP 0 — look at the patient, not the monitor
Before you interpret anything, answer three things:
Is there a pulse? A perfect-looking rhythm with no pulse is PEA — a cardiac arrest.
Is the patient symptomatic? Chest pain, syncope, hypotension, altered mental status, shortness of breath. Symptoms drive urgency far more than the rhythm name.
Is it real, or is it artifact? Shivering, brushing teeth, a loose lead or a disconnected electrode all mimic dangerous rhythms.
Never treat a monitor tracing without laying hands on the patient.
🧠 “Feel before you fix.” Pulse first, print the strip second, call second.
✅ Artifact vs a real rhythm — how to tell fast
Artifact usually has real QRS complexes marching through it — look for the underlying beats inside the noise.
Artifact often stops when you touch the leads or ask the patient to hold still.
The patient looks fine: talking, pink, good color, normal blood pressure.
Real V-tach or V-fib has no pulse or a very sick patient to go with it.
🧠 “The toothbrush rhythm.” Chaotic strip + a chatty patient brushing their teeth = artifact.
📍 Why lead II is the monitoring lead
Lead II runs from the right arm to the left leg — almost exactly along the normal direction of atrial
depolarization. That makes the P wave biggest and clearly upright in lead II.
Since three of your five steps depend on seeing the P wave, this is the lead you monitor in.
If the P waves are hard to see, change the lead before you decide they are absent.
🧠 “Two to see the P.” Lead II is where P waves live.
⭐ The three questions this method answers
⏱️
How fast? Step 1 — the rate.
📍
Where did the beat start? Steps 3 and 5 — P waves say atria/SA, QRS width says above or below the ventricles.
🚦
Did it get through? Steps 2 and 4 — regularity and PR interval tell you whether every signal made it to the ventricle.
GO DEEPER Once you have those three answers, the rhythm name is
almost automatic — look it up on NG-090 · The 9 ECG strips.
1️⃣
STEP 1 — RATE
HOW FAST?
Normal 60–100 /min. Two counting methods; you must know which one is legal on which strip.
🔢 Method A — the 6-second strip × 10
Most printed strips are exactly 6 seconds long and have 3-second markers
along the top edge. Count the QRS complexes between the two outer markers and add a zero.
🧠 “Count the beats, add a zero.” 8 beats → 80. 5 beats → 50. 13 beats → 130.
📐 Method B — the 300 sequence
Land on an R wave that sits on a heavy line, then count the heavy lines to the next
R wave saying 300 · 150 · 100 · 75 · 60 · 50.
Where does the sequence come from? There are 300 large boxes in a minute, so
300 ÷ (large boxes between R waves) = the rate. Two boxes → 150. Three → 100. It is just division you memorized.
🧠 “Three Hundred Silly Hens Sat Silently.” 300-150-100-75-60-50. Sing it once and it sticks.
⭐ Which method, when — the rule that gets tested
METHOD
USE IT WHEN
NEVER USE IT WHEN
6-second × 10
Any strip. Regular or irregular.
— (always allowed)
300-150-100
Rhythm is regular and you want a fast answer
Rhythm is irregular — the answer changes with every pair you pick
1500 ÷ small boxes
Regular, and you need a precise number
Irregular rhythms
If the rhythm is irregular, the 6-second method is the only correct one.
🧠 Irregular = only the 6-second ruler. Uneven gaps make box-counting meaningless.
🧮 Method C — the 1500 rule (the precise one)
1500 ÷ number of SMALL boxes between two R waves = rate
Why 1500? There are 1500 small boxes in one minute
(25 mm/s × 60 s = 1500 mm).
20 small boxes → 1500 ÷ 20 = 75
15 small boxes → 1500 ÷ 15 = 100
25 small boxes → 1500 ÷ 25 = 60
🧠 1500 small · 300 large · 30 large in a 6-second strip. Three numbers, one ruler.
📈 Count the ATRIAL rate too when they disagree
The rate you report is normally the ventricular rate — count QRS complexes, because that is what
produces a pulse and a blood pressure.
But if there are more P waves than QRS complexes, count both: count P waves for the atrial rate and
QRS complexes for the ventricular rate.
Calipers do the same job faster; the paper trick works when you have no calipers,
which is most of the time on a med-surg floor.
🧠 Two ticks and march. Same technique for P-P as for R-R.
📊 The four regularity patterns
PATTERN
WHAT YOU SEE
CLASSIC CAUSE
Regular
All R-R gaps identical
Sinus rhythms, SVT, V-tach, flutter with fixed conduction
Regularly irregular
An irregularity that repeats in a pattern
Wenckebach; a bigeminal pattern of extra beats
Irregularly irregular
No pattern at all, ever
Atrial fibrillation — the classic answer
Regular with an interruption
Regular, then one odd beat or one pause
A single premature beat (PAC/PVC) or a dropped beat
🧠 “Irregularly irregular” is nearly always A-fib on an exam. Say the phrase, pick the answer.
⚠️ Measure P-P and R-R separately
Two rulers, two answers:
P-P regular but R-R irregular → the atria are steady; some signals are not getting through. Think AV block.
Both regular and equal → normal conduction, one signal one beat.
P-P not measurable at all → there are no organized P waves. Think A-fib or flutter.
🧠 Atria have their own ruler. When R-R is a mess, always go back and check P-P.
🚨 Why irregular matters clinically
An irregular rhythm is not just a naming problem:
Pulse deficit — some beats are too weak to reach the wrist. Apical and radial pulses differ. Count apical for a full minute.
Loss of atrial kick — in A-fib the atria quiver instead of contracting, costing roughly a fifth to a third of cardiac output.
Clot risk — blood pooling in a quivering atrium forms clots, which is why A-fib is an anticoagulation conversation.
🧠 Quiver → pool → clot → stroke. That chain is the whole reason A-fib is scary.
🧾 Wording your answer for Step 2
Write it the way the chart wants it: “R-R regular”,
“R-R irregularly irregular”, or
“regular with one premature beat”.
Avoid “kind of regular”. If the gaps are not equal, it is irregular, and that changes which rate
method you are allowed to use in Step 1.
🧠 Step 2 feeds back into Step 1. If you find irregular, go back and re-count with the 6-second method.
3️⃣
STEP 3 — P WAVES
WHO STARTED IT?
Three questions: is a P there, do they all look alike, and does each one own a QRS?
🖼️ The four P-wave pictures
🧠 “Every P needs a partner.” A lonely P (no QRS after it) means the signal was blocked.
A lonely QRS (no P before it) means the beat did not start in the SA node.
✅ The three P questions, in order
1
PRESENT? Is there a P wave at all? No P → the SA node is not driving.
2
UNIFORM? Do all the Ps look the same? Different shapes = beats starting in different places.
3
1 : 1? One P before every QRS, one QRS after every P? Extra Ps = something is being blocked.
🧠 “P·U·1” — Present, Uniform, One-to-one. Three ticks and Step 3 is done.
🧪 What a normal P wave looks like
Upright in lead II (the impulse is traveling toward the positive electrode).
Rounded and smooth — not notched, peaked or biphasic.
Under 0.12 s wide (under 3 small boxes) and under 2.5 mm tall.
All identical to each other across the strip.
One per QRS, at the same distance in front of each QRS.
A P wave that satisfies all five means the beat is sinus in origin — this is what earns the word
“sinus” in a rhythm name.
🧠 Upright + uniform + 1:1 = the SA node is in charge.
⚠️ Inverted, buried or missing P waves
If the beat starts in the AV junction instead of the SA node, the atria depolarize backwards.
That gives you one of three pictures:
Inverted P before the QRS — the atria fired just before the ventricles.
No visible P — the P is hidden inside the QRS because both fired together.
Inverted P after the QRS — the atria fired late.
All three = a junctional beat or rhythm. The QRS stays narrow, because the signal still uses
the normal ventricular highway below the AV node.
🧠 Junctional = backwards P, normal QRS. The pacemaker moved down one floor, not into the ventricle.
❌ The trap: calling a P wave absent when it is only hidden
Before you write “no P waves”, do two things:
Change the lead. P waves can be tiny in one lead and obvious in another. Lead II is your friend.
Look inside the T wave. At fast rates the P wave sits on top of the previous T, giving it a
strange notch or extra peak. That notch is the P wave.
Never diagnose A-fib on a single noisy lead without checking regularity as well.
🧠 “Camel-hump T” = a P wave hiding. Fast and regular with a lumpy T is SVT, not A-fib.
4️⃣
STEP 4 — PR INTERVAL
DID IT GET THROUGH?
Start of P to start of QRS. Normal 0.12–0.20 s — three to five small boxes.
📐 Measuring it — and what the number means
Measure from the very first upstroke of the P wave to the
very first deflection of the QRS — not from the top of the P and not to the R peak.
Some quick-reference sheets round the range to 0.10–0.20 s; the standard adult
teaching range is 0.12–0.20 s, so use 3–5 small boxes.
🧠 “Three to five, alive.” Under 3 boxes = the signal skipped the delay.
Over 5 boxes = the AV node is dragging its feet.
🪜 The block ladder — what a bad PR looks like
🧠 “If the R is far from P, then you have a first-degree.
Longer, longer, longer, drop — then you have a Wenckebach.
If some Ps just don't get through, then you have a Mobitz II.
If Ps and Qs don't agree, then you have a third-degree.”
✅ Step 4 in three moves
1
Count the small boxes from the start of P to the start of QRS. 3–5 is normal.
2
Is it the SAME on every beat? Constant vs lengthening is the whole difference between type I and type II.
3
Is every P followed by a QRS? A P with nothing after it is a dropped beat.
🧠 Length · consistency · completeness. Three checks, one interval.
📊 Sorting the AV blocks side by side
BLOCK
PR
DROPPED QRS?
WORRY LEVEL
1st degree
>0.20 s, constant
No
Usually benign; watch and document
2nd degree type I (Wenckebach / Mobitz I)
Progressively longer
Yes, cyclically
Often stable; watch, treat if symptomatic
2nd degree type II (Mobitz II)
Constant
Yes, suddenly
Unstable — can progress to complete block
3rd degree (complete)
No relationship at all
P and QRS independent
Emergency — needs pacing
Type I lengthens then drops. Type II drops without warning.
🧠 “Type I is polite — it warns you. Type II just leaves.”
🚨 The blocks that need a pacemaker conversation
Mobitz II and complete (3rd degree) heart block are the two that classically progress and
classically need pacing — transcutaneous pacing at the bedside, then a transvenous or permanent pacemaker.
Have the transcutaneous pacer pads and the crash cart available; keep the patient on continuous
monitoring and on bed rest until a decision is made.
Atropine is the standard first drug for symptomatic bradycardia by protocol, but it works at the AV node
and is often ineffective in high-grade/infranodal block — do not let it delay pacing.
Never leave a patient in new complete heart block unmonitored or send them off the floor alone.
Follow your facility's ACLS protocol for all drug choices and doses.
🧠 “Two and three, pace with me.” Mobitz II and third degree → pacer pads on.
⚠️ A short PR is a finding too
A PR under 0.12 s means the signal reached the ventricle faster than the
AV node should allow — it took a shortcut, or the beat did not start in the SA node.
Junctional beats — starts below the atria, so there is less distance to travel.
Pre-excitation (an accessory pathway) — the signal bypasses the AV delay entirely.
🧠 Short PR = the signal cheated. Long PR = the signal queued.
5️⃣
STEP 5 — QRS WIDTH
FROM ABOVE OR BELOW?
Under 0.12 s (under 3 small boxes) is narrow. Narrow and wide are two different worlds.
↔️ Narrow vs wide — and why the width tells you the origin
A narrow QRS means the signal used the His–Purkinje express lanes, so both
ventricles fired almost simultaneously — fast, so the complex is thin. A wide QRS means the signal spread
muscle cell to muscle cell, which is slow — so the complex smears out.
🧠 “Narrow = normal road. Wide = off-road.” Skinny QRS = the beat came from above the ventricles.
⭐ The single most useful line on the page
NARROW <0.12 sBeat started above the ventricles — sinus, atrial or junctional. “Supraventricular.”
WIDE ≥0.12 sBeat started in the ventricle, or a bundle branch is blocked, or a pacemaker is firing.
Combine it with rate and you have a working diagnosis before you name anything:
wide + fast + regular is treated as ventricular tachycardia until proven otherwise.
🧠 “Wide and fast — assume the worst.” Treat wide-complex tachycardia as V-tach.
📊 Four reasons a QRS is wide
CAUSE
CLUE ON THE STRIP
Ventricular origin (PVC, V-tach, idioventricular)
No related P wave; T wave points the opposite way to the QRS
Bundle branch block
Normal P with a normal PR, but the QRS is wide — often notched, “rabbit ears”
Paced beat
A sharp vertical pacer spike right before the wide QRS
Severe hyperkalemia / some drug toxicity
Progressive widening, peaked T waves, flattening P waves
Measure from the start of the QRS to the end of the T wave. Typical adult
≈0.36–0.44 s, but it shortens as the rate rises, so it is always read against the rate.
Quick bedside sanity check: at a normal rate, the QT should be less than half the R-R interval.
Long QT → risk of torsades de pointes. Causes include low potassium, low magnesium, low calcium and QT-prolonging drugs.
Check the QT whenever a patient is on a rhythm drug, an antipsychotic or certain antibiotics.
🧠 “Low K, low Mag, long QT, torsades.” Magnesium (1.3–2.1 mEq/L) is the one people forget.
🚨 The two strips that end the assessment early
Wide, fast, no pulse → pulseless V-tach. Shock. Start CPR.
Chaotic with no identifiable complexes → V-fib. Shock. Start CPR.
Flat line → asystole. Do NOT shock. CPR and epinephrine, and check leads in more than one lead first.
Never work through five interpretation steps on a pulseless patient — start compressions.
🧠 Shock the V's — V-tach (pulseless) and V-fib. Asystole and PEA are never shocked.
Rate 130, irregularly irregular, no Ps, QRS 0.08 → atrial fibrillation with rapid ventricular response.
Rate 180, regular, no Ps, QRS 0.16 → ventricular tachycardia.
🧠 Measure first, name last. The name is the output, never the starting point.
🧪
WORKED STRIPS — RUN ALL FIVE STEPS
PRACTICE
Cover the answer boxes, work the strip yourself, then check. Do this three times and the order sticks.
🟩 PRACTICE STRIP 1 — run the five steps before you read the boxes
🟦 PRACTICE STRIP 2 — the rate changed, nothing else did
🟧 PRACTICE STRIP 3 — two boxes fail at once
🟪 PRACTICE STRIP 4 — everything looks fine until Step 4
🟥 PRACTICE STRIP 5 — stop at Step 0 and check a pulse
🎓 Quick-fire self test — cover the answers
Q. A strip is irregularly irregular. Which rate method are you allowed to use?
▸ Only the 6-second strip × 10. Box-counting methods need equal R-R intervals.
Q. How many small boxes is a normal PR interval?
▸ 3 to 5 small boxes = 0.12–0.20 s.
Q. The PR gets longer, longer, longest, then a QRS is missing. Name it.
▸ Second-degree AV block type I (Wenckebach / Mobitz I).
Q. QRS is 0.14 s. What does that tell you about where the beat started?
▸ It is wide: either a ventricular focus, a bundle branch block, or a paced beat. It did not travel the normal express route.
Q. More P waves than QRS complexes, and the PR is different on every beat. Name it.
▸ Third-degree (complete) heart block — atria and ventricles are dissociated. Prepare for pacing.
Q. Rate 38, regular, P before every QRS, PR 0.18, QRS 0.08 — patient is dizzy with a BP of 78/40. First action?
▸ This is symptomatic sinus bradycardia. Assess and support the patient (airway, oxygen, IV access, continuous monitoring) and follow the bradycardia protocol — the symptoms, not the number, drive the response.