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I now have enough rich, authoritative material from Goldman-Cecil Medicine to build a complete ECG teaching guide. Let me compile the full response.

ECG Interpretation: A Complete Guide for Medical Students

1. Why the ECG Works - The Electrical Foundation

Before reading a strip, you need to understand what is being recorded.

The heart has a dedicated electrical conduction system. Depolarization starts spontaneously at the SA node (high right atrium), spreads through both atria, is briefly delayed at the AV node, then races down the Bundle of His → right and left bundle branches → Purkinje fibers → ventricular muscle.

Cardiac conduction system showing SA node, AV node, Bundle of His, right/left bundle branches, and Purkinje fibers

Goldman-Cecil Medicine, cardiac conduction system

Each time a wave of depolarization moves toward a lead, the ECG draws an upward deflection. Movement away from a lead draws a downward deflection.

2. The ECG Paper

The paper runs at 25 mm/second.

Box	Width (time)	Height (voltage)
Small box	0.04 sec (40 ms)	0.1 mV
Large box (5 small)	0.20 sec (200 ms)	0.5 mV

3. The Leads - 12 Viewpoints of the Heart

A 12-lead ECG looks at the heart from 12 different angles simultaneously.

Limb leads (frontal plane):

I, II, III - standard bipolar
aVR, aVL, aVF - augmented unipolar

Chest (precordial) leads (horizontal plane):

V1 - right side of the septum
V2 - right ventricle
V3-V4 - anterior wall (transition zone)
V5-V6 - lateral left ventricle

Regional groupings (critical for MI localization):

Region	Leads
Inferior	II, III, aVF
Anterior	V2, V3, V4
Lateral	I, aVL, V5, V6
Septal	V3, V4
Right ventricular	V1

4. The Waveforms - What Each One Means

Normal 12-lead ECG showing all waveforms and leads

Normal ECG - Goldman-Cecil Medicine. HR ~78 bpm, PR 140 ms, QRS 90 ms, QTc 360 ms

The Waves Explained

Wave/Segment	What It Represents
P wave	Atrial depolarization (atrial contraction)
PR interval	AV node delay (conduction from atria to ventricles)
QRS complex	Ventricular depolarization (ventricular contraction)
ST segment	Early ventricular repolarization (should be on the baseline)
T wave	Ventricular repolarization
QT interval	Total duration of ventricular electrical activity
U wave	Rarely seen; may reflect Purkinje repolarization
J point	Junction between end of QRS and start of ST segment

QRS naming convention:

Capital letters (Q, R, S) = deflection ≥5 mm (0.5 mV)
Lowercase (q, r, s) = deflection <5 mm
First negative deflection = Q/q
First positive deflection = R/r
Negative deflection after R = S/s
All-negative complex = QS

5. Normal Values - Memorize These

From Goldman-Cecil Medicine (the most clinically used reference ranges):

Parameter	Normal Range
Heart rate	50-100 bpm
P wave duration	<120 ms (3 small boxes)
PR interval	90-200 ms (0.09-0.20 sec)
QRS duration	75-110 ms (<3 small boxes)
QTc (males)	390-450 ms
QTc (females)	390-460 ms
QRS axis	-30° to +90°

6. The Systematic Approach - Never Miss Anything

Always read ECGs in the same order, every time. This is the Goldman-Cecil systematic approach:

Step 1: Rate

Count large boxes between two consecutive R waves
- 1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50
For irregular rhythms: count beats in 10 seconds × 6

Step 2: Rhythm

Is it regular or irregular?
Is every P wave followed by a QRS?
Is every QRS preceded by a P wave?
Normal sinus rhythm: P wave positive in II, negative in aVR, followed by QRS at PR 120-200 ms

Step 3: Axis

Look at leads I and aVF
- Both positive → Normal axis (0° to +90°)
- I positive, aVF negative → Left axis deviation (LAD, <-30°)
- I negative, aVF positive → Right axis deviation (RAD, >+90°)
- Both negative → Extreme axis ("no man's land")

Step 4: P Wave

Shape: smooth, rounded, <120 ms, <2.5 mm tall
Positive in I and II, biphasic in V1 (normal)
Abnormal: broad/notched (left atrial enlargement), tall/peaked (right atrial enlargement)

Step 5: PR Interval

Normal: 120-200 ms
Long PR (>200 ms) = 1st degree AV block
Short PR (<120 ms) = WPW or junctional rhythm
Variable PR = Wenckebach (Mobitz I) or complete heart block

Step 6: QRS Complex

Duration: <110 ms
Wide QRS (>120 ms) = bundle branch block, ventricular rhythm, or hyperkalemia
Tall R in V1 = RBBB, RVH, posterior MI, WPW
R wave should grow V1→V5 ("R wave progression"). Poor progression suggests anterior MI or LBBB.
Deep Q waves (>40 ms wide or >25% of R wave height) = pathological; suggest prior MI

Step 7: ST Segment

Should be isoelectric (flat at baseline)
ST elevation: STEMI, pericarditis (diffuse saddle-shaped), Brugada, early repolarization
ST depression: NSTEMI/ACS, LVH strain pattern, digoxin effect

Step 8: T Wave

Normally concordant with QRS (same direction as terminal QRS)
Inverted T waves: ischemia, LVH, RBBB (V1-V3), PE (V1-V4)
Tall peaked T waves: hyperkalemia, hyperacute STEMI (very early finding)

Step 9: QT Interval

Correct for rate using Bazett formula: QTc = QT / √RR
Long QTc (>450 ms men, >460 ms women): drugs (antiarrhythmics, antipsychotics, antibiotics), hypokalemia, hypomagnesemia → risk of torsades de pointes

7. Key Patterns Every Medical Student Must Know

Sinus Rhythms

Rhythm	Rate	Features
Normal sinus	60-100	P before every QRS, regular
Sinus tachycardia	>100	Same morphology, fast
Sinus bradycardia	<60	Same morphology, slow
Sinus arrhythmia	Varies	Rate varies with breathing - normal!

Bundle Branch Blocks

RBBB: Wide QRS + RSR' ("rabbit ears") in V1 + wide S in I and V6
LBBB: Wide QRS + broad notched R in I, aVL, V5-V6 + QS in V1
- Memory trick: WiLLiaM MaRRoW (W pattern in V1 = LBBB; M in V1 = RBBB)

Hypertrophy

LVH: S in V1 + R in V5 or V6 > 35 mm (Sokolow-Lyon criteria)
RVH: Dominant R in V1 + right axis deviation

MI Localization

When you see ST changes, match the leads to the region and the coronary artery:

Territory	Leads	Artery
Inferior MI	II, III, aVF	RCA
Anterior MI	V2-V4	LAD
Lateral MI	I, aVL, V5-V6	LCx or LAD diagonal
Posterior MI	Tall R + ST depression in V1-V2	RCA or LCx

8. The Sequence of Change in STEMI

The ECG evolves over time in acute MI:

Hyperacute T waves (first minutes) - tall, broad-based, asymmetric T waves
ST elevation (minutes to hours) - J point elevation with upward sloping ST
Q wave formation (hours to days) - pathological Q = dead tissue
T wave inversion (hours to days)
ST normalization (days to weeks)
Persistent Q waves (permanent scar)

As Rosen's Emergency Medicine notes, the ST elevation criteria for STEMI are:

≥1 mm in ≥2 contiguous leads (most leads)
≥1.5 mm in females in V2-V3
≥2.5 mm in males <40 years in V2-V3
≥2.0 mm in males ≥40 years in V2-V3

9. Practical Tips for Exam Reading

Always state the rate and rhythm first - it orients everything else
Never call LBBB "normal" - an isolated new LBBB needs cardiac workup
Check the calibration box (should be 1 mV = 10 mm tall) - if missing or wrong, voltages are unreliable
Reciprocal changes confirm MI - ST elevation in inferior leads + ST depression in I/aVL = true inferior STEMI
Compare to old ECGs when available - "new" changes matter far more than chronic findings
Axis in lead I and aVF first, then refine - this two-lead trick gets you close enough in seconds

Summary: The One-Page Cheat Sheet

RATE   → Large box method (300/150/100/75/60/50)
RHYTHM → P before QRS? Regular? QRS rate = P rate?
AXIS   → I + aVF: both up = normal; I up/aVF down = LAD; I down/aVF up = RAD
P      → Smooth, <120 ms, positive in II
PR     → 120-200 ms. Long = AV block. Short = WPW/junctional
QRS    → <110 ms. Wide = BBB. Q waves = infarct
ST     → Flat at baseline. Up = STEMI/pericarditis. Down = ischemia/LVH
T      → Concordant with QRS. Inverted = ischemia/RVH. Peaked = hyperK
QT     → QTc < 440-460 ms. Long = drug effect / electrolytes

Sources: Goldman-Cecil Medicine, 26th Ed. | Rosen's Emergency Medicine, 9th Ed. | Braunwald's Heart Disease, 12th Ed.

Want me to go deeper on any section - arrhythmias, bundle branch blocks, ischemia patterns, or axis calculation? I can also walk you through reading actual ECG strips.

Walk me through reading an ECG strip step-by-step

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I have everything needed for a thorough, image-rich step-by-step walkthrough. Here it is:

Reading an ECG Strip: Step-by-Step

Think of this as a ritual - same order, every single time. Harrison's describes 14 parameters to check. I've grouped them into 9 logical steps you can memorize and apply in 90 seconds at the bedside.

Before You Start: Check the Calibration

Look at the top-left corner of any ECG. There should be a calibration box - a square pulse that is exactly 1 mV = 10 mm tall and 0.2 sec (1 large box) wide.

If the calibration is set to half (5 mm = 1 mV), all voltages look smaller than they are. If it's double, everything looks larger. Always check this first or you'll misread hypertrophy and low-voltage patterns.

Also verify: are leads labeled? Any artifact or lead reversal obvious?

Step 1: Heart Rate

The large-box method (fast, works for regular rhythms):

Count the number of large boxes between two consecutive R waves (the tallest peaks), then divide:

Large boxes	Rate (bpm)
1	300
2	150
3	100
4	75
5	60
6	50

Mnemonic: "300, 150, 100, 75, 60, 50" - learn this sequence cold.

For irregular rhythms (e.g., atrial fibrillation): count all QRS complexes in a 10-second strip and multiply by 6.

Normal rate: 60-100 bpm. <60 = bradycardia. >100 = tachycardia.

Step 2: Rhythm - Is It Regular?

Mark two consecutive R-R intervals with calipers (or a piece of paper). Walk that distance across the whole strip. If every R-R interval is the same → regular. If it varies → irregular.

Now ask three questions:

Question	What you're testing
Is there a P wave before every QRS?	Is atrial activity driving ventricular activity?
Is there a QRS after every P wave?	Is every atrial impulse conducted?
Do all P waves look the same?	Are they all from the SA node?

Normal sinus rhythm (NSR) criteria:

Rate 60-100 bpm
P wave positive in lead II, negative in aVR (this confirms SA node origin)
Regular P-P and R-R intervals
PR interval 120-200 ms, constant

Step 3: Axis - Which Way Is the Heart Pointing?

The QRS axis tells you the mean electrical direction of ventricular depolarization. Look at just two leads: I and aVF.

Lead I	Lead aVF	Axis	Meaning
Positive (R > S)	Positive (R > S)	Normal (-30° to +90°)	Normal
Positive	Negative	Left axis deviation (LAD)	LAFB, LVH, inferior MI
Negative	Positive	Right axis deviation (RAD)	RVH, LPFB, lateral MI, PE
Negative	Negative	Extreme axis ("northwest")	Ventricular rhythm, severe disease

A quick memory trick: leads I and aVF are like compass needles - both pointing "in" = normal axis.

Step 4: P Wave

Use lead II (best P wave view) and V1 (shows atrial enlargement patterns).

Ask:

Shape: smooth and rounded? (normal) or notched/peaked?
Duration: <120 ms (3 small boxes)?
Amplitude: <2.5 mm (2.5 small boxes) tall?
Axis: positive in II, I, aVF? Negative in aVR?

P wave abnormality	Appearance	Meaning
Normal	Smooth, <120 ms, positive in II	Sinus rhythm
P mitrale	Broad, notched in II; biphasic in V1	Left atrial enlargement
P pulmonale	Tall, peaked (>2.5 mm) in II	Right atrial enlargement
Absent P waves	No P waves at all	Atrial fibrillation, junctional rhythm
Retrograde P	Negative in II, positive in aVR	Junctional or ventricular origin

Step 5: PR Interval

Measure from the start of the P wave to the start of the QRS complex in the lead where both are clearly visible (usually II or V1).

Normal: 120-200 ms (3-5 small boxes)

PR finding	Value	Meaning
First-degree AV block	>200 ms, constant	Delayed AV node conduction
Mobitz I (Wenckebach)	Progressive lengthening then dropped beat	AV node fatigue
Mobitz II	Constant PR, then sudden dropped QRS	Infranodal block - dangerous
Complete heart block	P and QRS unrelated	Full AV dissociation
Short PR	<120 ms	WPW (pre-excitation), junctional rhythm

Step 6: QRS Complex

This is the most information-dense part of the ECG. Break it into four sub-checks:

6a. Duration

Measure from the start to the end of the QRS. Normal: 75-110 ms (<3 small boxes).

Wide QRS (≥120 ms) = Bundle branch block, ventricular paced rhythm, ventricular tachycardia, hyperkalemia, Na-channel blocker toxicity

BBB pattern recognition:

Pattern	V1 shape	Lateral leads (I, aVL, V6)
RBBB	rSR' ("rabbit ears")	Wide S wave
LBBB	QS or rS (all negative)	Tall, broad, notched R wave

Memory trick: WiLLiaM MaRRoW

W in V1 + M in V6 = LBBB
M in V1 + W in V6 = RBBB

6b. Voltage

LVH (Sokolow-Lyon): S in V1 + R in V5 or V6 ≥ 35 mm
RVH: Dominant R in V1 (R > S) + right axis deviation
Low voltage: all QRS amplitudes <5 mm in limb leads OR <10 mm in precordial leads → think effusion, hypothyroidism, infiltrative disease

6c. R-Wave Progression

The diagram below shows why R waves should grow as you move from V1 to V6:

Ventricular depolarization phases showing septal vector (phase 1) going right producing small r in V1, and dominant LV vector (phase 2) going left producing tall R in V6

Harrison's, 22nd Ed. - Ventricular depolarization vectors explain R-wave progression

Here's the chest lead placement for reference:

Precordial lead placement on chest - V1 through V6 positions

Harrison's, 22nd Ed. - Chest lead electrode positions

V1: predominantly negative (small r, deep S) - right side of septum
V3-V4: transition zone where R ≈ S
V5-V6: predominantly positive (tall R) - lateral LV

Poor R-wave progression (R remains small through V4): anterior MI, LBBB, LVH, misplaced leads

6d. Pathological Q Waves

A Q wave is pathological if:

Width ≥ 40 ms (1 small box)
Depth ≥ 25% of the R wave height in the same lead
Seen in ≥2 contiguous leads

Normal exceptions: Small q waves ("septal q") in I, aVL, V5, V6 are normal. aVR can have a large Q normally.

Pathological Q waves = electrical scar from old infarction. Match the leads to the territory:

Q waves in...	Territory	Artery
II, III, aVF	Inferior	RCA
V1-V4	Anterior	LAD
I, aVL, V5-V6	Lateral	LCx

Step 7: ST Segment

The ST segment connects the end of the QRS (J point) to the start of the T wave. It should be flat at the isoelectric baseline (same level as the PR segment).

Measure elevation or depression at the J point, then 60-80 ms after.

ST Elevation

Pattern	Appearance	Cause
STEMI	Convex upward ("tombstone"), in contiguous regional leads	Acute MI - emergent
Pericarditis	Diffuse saddle-shaped elevation in most leads + PR depression	Inflammation
Early repolarization	Concave upward ("smiley face"), notched J point, V4-V6	Benign variant (usually)
Brugada	Coved ST elevation in V1-V2 + RBBB morphology	Channelopathy, risk of SCD
LV aneurysm	Persistent elevation weeks after MI, no dynamic change	Scar

ST Depression

Pattern	Leads	Cause
Horizontal or downsloping	Regional	NSTEMI / active ischemia
Upsloping	Regional	Rate-related, less specific
"Strain pattern" (downsloping + T inversion)	V5-V6 or I, aVL	LVH strain
Reciprocal changes	Opposite to ST elevation leads	Confirms STEMI - look for these
Digoxin effect	Any	Classic "scooped" ST depression

Step 8: T Wave

T waves should be:

Same direction as the QRS in most leads (concordant)
Upright in I, II, V3-V6
Inverted in aVR (normal)
May be inverted in V1 and III (normal variant)

T wave change	Leads	Think
Tall, peaked, symmetric	Any	Hyperkalemia, hyperacute STEMI
Deep symmetric inversion	Regional	Ischemia, Wellens syndrome
V1-V3 inversion	Right precordial	RBBB (normal), RVH, PE, anterior ischemia
Diffuse T inversion	All leads	Myocarditis, raised ICP, CNS event
Flat/biphasic T	Regional	Early ischemia, electrolyte abnormality

Wellens syndrome (a pattern every med student must know): deeply inverted or biphasic T waves in V2-V3 in a pain-free patient = critical LAD stenosis. Do NOT stress test this patient.

Step 9: QT Interval

Measure from start of QRS to end of T wave. Use lead II or V5 where it's clearest.

Always correct for heart rate using Bazett's formula:

QTc = QT (sec) ÷ √RR interval (sec)

Normal QTc:

Males: 390-450 ms
Females: 390-460 ms

QTc	Interpretation
<350 ms	Short QT (hypercalcemia, digoxin, genetic)
390-450/460 ms	Normal
450-500 ms	Borderline prolonged
>500 ms	Markedly prolonged - high risk of torsades de pointes

Common causes of long QT: hypokalemia, hypomagnesemia, class Ia/III antiarrhythmics, antipsychotics (haloperidol, quetiapine), macrolides, fluoroquinolones, congenital long QT syndromes.

Putting It All Together: The 90-Second Script

When you pick up an ECG strip, say this out loud (or in your head):

"Rate is ___ bpm. Rhythm is [regular/irregular]. I see [P before every QRS / no P waves / variable PR]. Axis: Lead I is [up/down], aVF is [up/down] → [normal/LAD/RAD]. P waves are [normal/abnormal]. PR is ___ ms - [normal/prolonged/short]. QRS is ___ ms - [narrow/wide]. R-wave progression is [normal/poor]. [No / Pathological] Q waves in [leads]. ST segments are [isoelectric / elevated in ___ / depressed in ___]. T waves are [concordant / inverted in ___]. QTc is ___ ms."

"My interpretation is: [Primary diagnosis]. [Secondary findings]."

Quick Reference Card

STEP  | CHECK                    | NORMAL
------|--------------------------|---------------------------
1     | Rate                     | 60-100 bpm
2     | Rhythm                   | Regular, P before every QRS
3     | Axis                     | I ↑ + aVF ↑ = normal
4     | P wave                   | Smooth, <120ms, + in II
5     | PR interval              | 120-200 ms, constant
6a    | QRS duration             | <110 ms
6b    | QRS voltage              | Not too tall, not too small
6c    | R-wave progression       | r small V1 → R tall V5-6
6d    | Q waves                  | No pathological Q waves
7     | ST segment               | Flat at baseline
8     | T wave                   | Concordant, upright in I,II
9     | QTc                      | <450 ms men / <460 ms women

The real skill comes from repetition - reading 10 ECGs a week is worth more than any amount of reading. A great free practice resource is ECG Wave-Maven at BIDMC - hundreds of real strips with expert interpretations, recommended by Harrison's itself.

Want to try working through a specific abnormal pattern next - arrhythmias, bundle branch blocks, or MI localization?

Work through a STEMI ECG example step-by-step

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I have enough real ECG images from Rosen's Emergency Medicine to build the full walkthrough. Here it is:

Working Through a STEMI ECG: Step-by-Step

We'll work through two real examples pulled from Rosen's Emergency Medicine - first an Anterior STEMI, then apply the same logic to an Inferior STEMI. Follow each step exactly as you would in a clinical setting.

The Case

Patient: 58-year-old male. 45 minutes of crushing central chest pain radiating to the left arm. Diaphoretic, BP 100/70. This ECG is placed in front of you.

Step 0: Before You Touch the Waveforms

Check two things immediately:

Calibration box present (1 mV = 10 mm)? ✓
All 12 leads labeled and present? ✓

Then scan the whole strip in 3 seconds - don't fixate on one area. You're asking: "Is anything obviously dangerous?" Your brain will often spot STEMI before your logical analysis catches up.

Step 1: Rate

Look at the anterior STEMI ECG:

Anterior STEMI - ST elevation in V1-V4, with prominent ST changes across precordial leads

Fig. 64.6, Rosen's Emergency Medicine - Anterior wall STEMI

Count large boxes between two R waves in the rhythm strip (lead II, bottom row).

~4 large boxes between R waves → Rate ≈ 75 bpm

Normal rate. In STEMI, you can see sinus tachycardia (pain/anxiety/sympathetic surge), but also bradycardia in inferior STEMI (RCA supplies AV node).

Step 2: Rhythm

P wave visible before every QRS? Yes
Every P followed by QRS? Yes
All P waves same shape? Yes
Regular R-R intervals? Yes

→ Normal sinus rhythm

In STEMI, watch for: PVCs (irritable ischemic myocardium), VT/VF (catastrophic), heart block (especially inferior STEMI with RCA involvement).

Step 3: Axis

Look at leads I and aVF:

Lead I: QRS is positive (R wave dominates)
Lead aVF: QRS is positive

→ Normal axis (+60° range)

Left axis deviation in an inferior STEMI patient = think left anterior fascicular block or extensive infarct.

Step 4: P Waves

Smooth, upright in lead II, <120 ms. Normal.

No P wave changes in this case. If you saw PR segment depression (the PR dips below baseline), think pericarditis or atrial infarction.

Step 5: PR Interval

Measure from start of P to start of QRS in lead II.

PR ≈ 160 ms → Normal (120-200 ms)

In inferior STEMI: PR prolongation or even Wenckebach (Mobitz I) develops because the RCA supplies the AV node. Always check the PR in inferior MI.

Step 6: QRS Complex

6a. Duration: Narrow, ~80 ms. No bundle branch block.

Wide QRS in STEMI context = either pre-existing BBB, or rate-related aberrancy. A new LBBB with chest pain is a STEMI-equivalent (Sgarbossa criteria apply).

6b. Q waves: Look carefully at V1-V4...

Early in STEMI there may be no Q waves yet - infarction has just begun. Q waves form over hours as myocytes die. Their absence does not rule out STEMI.

6c. R-wave progression: This is where the clue reveals itself. In leads V1-V4, the R waves are smaller than expected and may be absent, replaced by QS complexes. This is the electrical scar forming.

Step 7: ST Segment - THE KEY FINDING

This is where STEMI declares itself. Apply the 4-question ST analysis:

Q1: Is there ST elevation?

Look methodically through all 12 leads:

V1: ST elevated above baseline? YES - ~1-2 mm
V2: Elevated? YES - ~2-3 mm
V3: Elevated? YES - ~3+ mm
V4: Elevated? YES - ~2 mm
V5, V6, I, II, III, aVF: Elevated? Less so in this case
aVL, I: May show ST depression (reciprocal)

→ ST elevation in V1-V4 (≥2 contiguous leads)

Q2: Does it meet diagnostic criteria?

Per the Fourth Universal Definition of MI (as cited in Rosen's):

Lead group	Threshold for STEMI
Most leads	≥1 mm in ≥2 contiguous leads
V2-V3, males ≥40 yr	≥2 mm
V2-V3, males <40 yr	≥2.5 mm
V2-V3, any female	≥1.5 mm

This ECG meets criteria.

Q3: What is the ST morphology?

Rosen's describes the spectrum:

Morphology	Appearance	Significance
Concave ("smiley face")	Bowl-shaped upward	Less specific; also pericarditis, early repolarization
Flat/oblique	Horizontal rise from J point	Common in STEMI
Convex ("frowning")	Dome-shaped	More specific for STEMI
"Tombstone"	Massive upright arc, obliterates QRS	Large MI, very bad

Tip: STEMI ST changes are dynamic - repeat the ECG in 15-30 min if you're unsure. Benign early repolarization doesn't evolve; STEMI does.

Q4: Are there reciprocal changes?

This is the confirmation step. True STEMI almost always has reciprocal ST depression in the mirror-image leads:

STEMI territory	Elevation in	Reciprocal depression in
Anterior (LAD)	V1-V4	II, III, aVF (inferior leads)
Inferior (RCA)	II, III, aVF	I, aVL
Lateral (LCx)	I, aVL, V5-V6	II, III, aVF
Posterior	V7-V9 (posterior leads)	V1-V3 (tall R + ST depression)

In this ECG: ST elevation in V1-V4 + reciprocal ST depression visible in the inferior leads (II, III, aVF). This reciprocal pattern increases specificity and positive predictive value for true STEMI.

Rosen's notes: "Reciprocal changes in the setting of STEMI increase the specificity and positive predictive value of the ECG in AMI and coincide with larger infarction, greater chance of cardiovascular adverse events."

Step 8: T Waves - Catch the Early Stage

Before full ST elevation develops, the very first ECG sign is the hyperacute T wave. Here is what it looks like - and how it rapidly evolves to classic STEMI:

Hyperacute T waves (ECG A, top) - tall broad T waves in V3-V4 with subtle ST rise; 30 minutes later (ECG B, bottom) - frank ST elevation in V1-V4

Fig. 64.1, Rosen's Emergency Medicine - ECG A (top): Hyperacute T waves in V3-V4, subtle. ECG B (bottom): Same patient 30 minutes later - frank anterior STEMI.

Hyperacute T wave characteristics:

Tall, broad-based, asymmetric
Appears within minutes of occlusion
ST segment starts to rise at the J point
Transient - rapidly evolves to ST elevation
Differential: hyperkalemia (but those T waves are narrow and "tented"), benign early repolarization, LVH

Clinical pearl: If a patient has typical chest pain and hyperacute T waves but no frank ST elevation yet - treat as STEMI-equivalent. Do not wait for ST elevation to appear.

Step 9: QTc

Measured from start of QRS to end of T wave.

In acute STEMI, QT is often at the upper limit of normal or prolonged due to ischemic repolarization changes. No action required at this point, but note it as a baseline.

Final Diagnosis and Localization

ECG Interpretation:

"Sinus rhythm at 75 bpm. Normal axis. Narrow QRS. ST elevation of 2-3 mm in leads V1-V4 with convex morphology, meeting diagnostic criteria. Reciprocal ST depression in II, III, aVF. No established Q waves yet. Findings consistent with acute anterior STEMI."

Culprit artery: Left Anterior Descending (LAD)

The STEMI Localization Map - All Territories

Territory	ST Elevation	Reciprocal Depression	Culprit Artery
Anterior	V1-V4	II, III, aVF	LAD (proximal)
Anterolateral	V1-V6, I, aVL	II, III, aVF	Proximal LAD
Lateral / High lateral	I, aVL (V5-V6)	II, III, aVF, V1	LCx or Diagonal
Inferior	II, III, aVF	I, aVL	RCA (80%) or LCx (20%)
Posterior	Tall R + ST depression V1-V3	(posterior leads V7-V9 elevate)	RCA or LCx
RV infarction	V1, V4R	-	Proximal RCA
Left main / multivessel	aVR + widespread ST depression	All leads	LM or proximal LAD

Inferior STEMI clue: If ST elevation is greater in lead III than lead II, it's almost certainly the RCA. If III ≈ II with no aVL depression, think left circumflex.

RV involvement clue: In inferior STEMI, if V1 is also elevated, get right-sided leads (V4R). RV infarction changes management - these patients cannot tolerate nitrates (preload-dependent) and need IV fluids instead.

The Temporal Evolution of STEMI

The ECG changes in STEMI are not static - they evolve in a predictable sequence:

Timeframe	ECG Change	Clinical Correlate
Minutes	Hyperacute T waves	Acute occlusion, viable myocardium
Minutes-Hours	ST elevation	Active transmural ischemia
Hours-Days	Q wave formation	Myocyte death (electrical scar)
Hours-Days	T wave inversion	Repolarization abnormality
Days-Weeks	ST returns to baseline	Healing phase
Permanent	Persistent Q waves	Completed infarct scar

Important: Successful reperfusion (PCI or thrombolysis) accelerates this sequence and can abort Q wave formation if done early enough.

STEMI Mimics - What Could Fool You

Always consider these before calling STEMI:

Mimic	Distinguishing Feature
Early repolarization	Concave ST ("smiley"), stable across serial ECGs, young male, V4-V6
Pericarditis	Diffuse (all leads except aVR/V1), saddle-shaped, PR depression, no reciprocal changes
LBBB	Wide QRS - use Sgarbossa criteria to look for STEMI underneath
LVH strain	Downsloping ST depression + T inversion V5-V6, large QRS voltages
Brugada	Coved ST elevation only in V1-V2, no chest pain, RBBB morphology
Takotsubo	Anterior-pattern changes, but no culprit lesion at cath, usually postmenopausal female after stress
Hyperkalemia	Peaked narrow T waves + wide QRS + sinusoidal pattern

Clinical Action Summary

When you identify STEMI on ECG:

Activate the cath lab immediately (door-to-balloon time target: <90 min)
Aspirin 300 mg loading dose
P2Y12 inhibitor (ticagrelor or clopidogrel)
Anticoagulation (heparin or fondaparinux)
Nitrates - but NOT in RV infarction, NOT if systolic BP <90 mmHg
Serial ECGs every 15-30 min if initial ECG is non-diagnostic
Check right-sided leads if inferior STEMI

Sources: Rosen's Emergency Medicine, 9th Ed. - Chapter 64 | Goldman-Cecil Medicine, 26th Ed.

Ready to go deeper? I can walk you through inferior STEMI vs. pericarditis differentiation, Sgarbossa criteria for STEMI in LBBB, or posterior MI recognition in detail.

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