A 67-year-old man with hypertension and type 2 diabetes is brought in by ambulance. He says his chest has felt "like someone's sitting on it" for the past 90 minutes. His BP is 88/60 and he's diaphoretic and pale. EKG shows ST elevations in leads II, III, and aVF with ST depression in I and aVL. What is the most likely culprit artery?
Tap any segment to learn what it means
📷 NORMAL SINUS RHYTHM · tap to expand
Tap a segment above
Select a waveform segment
Each segment of the EKG tells a specific electrical story. Tap the P wave, QRS, ST segment, T wave, or intervals to see what is normal and what goes wrong.
How to Read Paper Speed
📏
Standard paper runs at 25 mm/sec. Each small box = 0.04s. Each large box (5 small) = 0.2s. One full EKG strip = 10 seconds. This math is on every single interval question.
🔋
Voltage: Each small box = 0.1 mV vertically. Each large box = 0.5 mV. A QRS that is 10 small boxes tall = 1 mV. LVH and RVH criteria use this.
Lead Geography; What Each Lead Sees
Territory
Leads
Culprit Artery
Inferior
II, III, aVF
RCA (80%), LCx (20%)
Anterior
V1-V4
LAD
Lateral
I, aVL, V5-V6
LCx / LAD diagonal
Posterior
V1-V2 (reciprocal)
RCA / LCx
Septal
V1-V2
LAD (septal perforators)
Memory: I, II, III point like a clock face. II sits at the bottom. The inferior leads look UP from the feet of the patient; they see the bottom of the heart.
Calculating Rate; The 300 Method
Find an R wave on a thick line. Count the thick lines to the next R wave: 300, 150, 100, 75, 60, 50. That is your rate. If the rhythm is irregular, count all QRS complexes in 10 seconds and multiply by 6.
💡
Mnemonic: "300 Mexicans Ate Hot Burritos Sadly" = 300, 150, 100, 75, 60, 50. Yes it is weird. No you will not forget it.
Systematic Rhythm Assessment
1️⃣
Regular or irregular? March out the R waves. Regular = NSR candidate. Irregular = think AFib, MAT, or PACs.
2️⃣
P waves present? No P waves = AFib or junctional. P before every QRS = organized atrial activity.
3️⃣
P-QRS relationship? Is there a P for every QRS and a QRS for every P? If not, think AV block.
4️⃣
QRS narrow or wide? Less than 3 small boxes (0.12s) = narrow = supraventricular. Greater than 3 = wide = bundle branch, aberrant, or ventricular.
The Major Rhythms at a Glance
📷 ATRIAL FIBRILLATION · tap to expand
Rhythm
Rate
P waves
QRS
NSR
60-100
Upright in II
Narrow
AFib
Usually fast
None (fibrillatory baseline)
Narrow, irregular
AFlutter
150 (2:1 block)
Sawtooth at 300
Narrow
VTach
100-250
Dissociated
Wide, regular
VFib
Chaotic
None
Chaotic
SVT
150-250
Buried in T wave
Narrow, regular
Junctional
40-60
Retrograde/absent
Narrow
Board Trap
Atrial flutter almost always presents at heart rate 150. Why? The atria fire at 300 bpm and the AV node blocks every other beat (2:1 block). If you see a narrow complex tachycardia at exactly 150, look hard for sawtooth flutter waves; they can hide in the QRS or T wave. Cardiovert it like AFib.
Rhythm Battle Cards · Tap to Flip
SUPRAVENTRICULAR
🌞
Atrial Fibrillation
Irregularly irregular
RateVariable (usually fast)
P wavesNone; fibrillatory baseline
QRSNarrow, chaotically irregular
No two RR intervals are the same
tap to flip
AFib: Board Locks
Trace It
Multiple chaotic reentry circuits in atria. No organized atrial contraction. AV node fires when it wants.
Atrial clots form in the LAA. Stroke risk. CHADS-VASC score drives anticoagulation decision.
Treatment
Rate control (beta-blocker, CCB) vs. rhythm control (cardioversion). Anticoag if >48h or unknown onset.
SUPRAVENTRICULAR
🐇
Atrial Flutter
Regularly irregular
RateAtria 300, ventricles 150 (2:1)
P wavesSawtooth at 300 bpm in II, III, aVF
QRSNarrow, regular (or regularly irregular)
HR exactly 150 = flutter until proven otherwise
tap to flip
AFlutter: Board Locks
Trace It
Single large reentry circuit in RA (cavotricuspid isthmus). Organized but too fast. AV node protects by blocking in ratio (2:1, 3:1, 4:1).
The Trap
Flutter waves hide in QRS or T wave. Carotid sinus massage slows AV conduction temporarily and uncovers them.
Treatment
Same anticoag rules as AFib. Rate control. Ablation is curative (95% success at isthmus).
VENTRICULAR
⚡
Ventricular Tachycardia
Wide complex emergency
Rate100-250 bpm
P wavesAV dissociation (cannon A waves)
QRSWide (>0.12s), monomorphic or poly
Wide complex + fast + hemodynamic instability = VTach until proven otherwise
tap to flip
VTach: Board Locks
Trace It
Reentry circuit in ventricular scar (post-MI most common). Both ventricles not contracting in sync; cardiac output tanks.
Brugada Criteria
RS absent in precordial leads, RS interval >100ms, AV dissociation, morphology criteria. Any one = VTach.
Treatment
Unstable: synchronized cardioversion immediately. Stable: amiodarone or procainamide. Never verapamil (kills patients with VTach).
SUPRAVENTRICULAR
🔥
SVT (AVNRT)
Paroxysmal narrow complex
Rate150-250 bpm
P wavesBuried in T wave or retrograde
QRSNarrow, regular, abrupt onset/offset
Abrupt "flip switch" start/stop in a young healthy person
tap to flip
SVT: Board Locks
Trace It
Dual pathway reentry in the AV node (AVNRT). Fast and slow pathways create a loop. Triggered by a PAC hitting the fast path when it's refractory.
Termination
Vagal maneuvers (Valsalva, carotid sinus massage) first. Adenosine 6mg IV if vagal fails; briefly blocks AV node and breaks the circuit.
Long-term
Ablation of slow pathway curative. Beta-blockers or CCBs for prevention if recurrent.
Axis; Which Direction Is the Heart Pointing?
The cardiac axis tells you the average direction of ventricular depolarization. Normal is between -30 and +90 degrees. Use Lead I and aVF together as a two-lead shortcut.
Two-Lead Shortcut
✅
Normal: Lead I positive + aVF positive = normal axis (roughly -30 to +90)
⬆
LAD: Lead I positive + aVF negative = left axis deviation. Think: LVH, LBBB, inferior MI, LAFB
➡
RAD: Lead I negative + aVF positive = right axis deviation. Think: RVH, RBBB, lateral MI, LPFB, tall thin people, dextrocardia
⚡
Extreme axis (NW axis): Both Lead I and aVF negative. Think: VTach, severe RVH, lead reversal
Cause
Axis
See It
LBBB
LAD
Wide QRS + LAD together
RBBB
Normal or RAD
rSR' in V1, wide S in I
LAFB
LAD
Narrow QRS, LAD alone
Inferior STEMI
LAD
Loss of inferior forces
RVH / PE
RAD
S1Q3T3 pattern in PE
Axis Identifier; Pick Your Leads
Tap Lead I then aVF to identify the axis.
LEAD I
aVF
Normal Intervals; Know These Cold
Interval
Normal Range
Measured How
PR interval
0.12 to 0.20s (3-5 small boxes)
Start of P to start of QRS
QRS duration
Under 0.12s (less than 3 boxes)
Start to end of QRS
QTc
Under 0.44s men, 0.46s women
QT corrected for rate
AV Blocks; The Three Flavors
ECG: LONG QT · tap to expand
1
First-degree: PR greater than 0.20s. Every P conducts. No treatment needed. "The bouncer is slow but everyone gets in."
2
Mobitz I (Wenckebach): PR progressively lengthens until a QRS is dropped. Then resets. Regularly irregular. Usually benign; observe. Inferior MI, athletic hearts, increased vagal tone.
2B
Mobitz II: Constant PR but sudden QRS drops without warning. Dangerous; can progress to complete block. Needs pacemaker. Associated with anterior MI and His-Purkinje disease.
3
Third-degree (complete): P waves and QRS march at completely different rates with no relationship. Escape rhythm keeps the patient alive. Pacemaker now.
Board Trap
Mobitz I vs Mobitz II matters enormously: Wenckebach (I) = observation. Mobitz II = pacemaker. The boards will try to make you confuse them. Key: in Mobitz I, the P-R interval GETS LONGER before the dropped beat. In Mobitz II, all conducted PR intervals are IDENTICAL; the drop comes out of nowhere.
Bundle Branch Blocks
⬅
LBBB: Wide QRS + broad notched R in I, aVL, V5-V6 ("William") + deep S in V1. Causes LAD. Invalidates STEMI criteria; any new LBBB with chest pain = cath lab.
➡
RBBB: Wide QRS + rSR' in V1 ("rabbit ears") + wide S in I, aVL, V5-V6 ("Marrow"). Can be normal variant. Does NOT invalidate STEMI.
🧠
WiLLiaM MaRRoW: LBBB = W in V1, M in V6. RBBB = M in V1, W in V6.
QT Prolongation; Drugs That Kill
Long QT is dangerous because it can degenerate into Torsades de Pointes (TdP); a polymorphic VTach that can become VFib. QTc greater than 500ms is a medical emergency.
Drug Class
Examples
Antiarrhythmics
Sotalol, amiodarone, quinidine, procainamide
Antibiotics
Azithromycin, fluoroquinolones
Antipsychotics
Haloperidol, quetiapine, ziprasidone
Antiemetics
Ondansetron, metoclopramide
Antifungals
Fluconazole
💊
TdP treatment: IV magnesium sulfate first. Then overdrive pacing or isoproterenol if needed. Stop the offending drug.
ST Elevation; STEMI vs Benign
ECG: HYPERKALEMIA · peaked T waves · tap to expand
Pattern
Shape
Key Clue
STEMI
Convex ("tombstone") or flat elevation
Reciprocal depression in opposite leads
Pericarditis
Concave ("saddle-shaped"), diffuse
PR depression, no reciprocal changes
Early repolarization
Concave, notch at J point
Young athletic person, V2-V5, benign
Brugada
Coved STE in V1-V2
Young man, syncope, especially at rest
LVH strain
Downsloping ST in V5-V6
Deep S in V1, tall R in V5-V6
STEMI Territories
❤
Inferior STEMI (II, III, aVF): RCA in 80%. Check right-sided leads (V4R) for RV involvement; if present, do NOT give nitrates or diuretics (preload-dependent).
⬆
Anterior STEMI (V1-V4): LAD. Worst prognosis. Can cause cardiogenic shock, new LBBB, papillary muscle rupture.
↙
Posterior STEMI: ST depression + tall R in V1-V2 + upright T. The posterior wall has no direct leads; you see it as a mirror image. Add V7-V9 (posterior leads) to confirm.
Board Trap
Wellens Syndrome: A patient with unstable angina and biphasic or deeply inverted T waves in V2-V3 when they are PAIN FREE. This means the LAD is critically stenosed and the patient is about to have a massive anterior STEMI. Do NOT stress test. Send urgently to cath lab.
T Wave Changes
🔺
Hyperacute T waves: Tall, peaked, symmetric T waves are the EARLIEST sign of STEMI; before ST elevation. Often missed.
🔻
T wave inversions: Ischemia, PE (V1-V4), Wellens, Takotsubo, increased ICP (deep symmetric inversions).
⚡
Peaked T waves with wide QRS: Think hyperkalemia first. Sine wave pattern = extreme hyperkalemia = cardiac arrest risk.
Prove It
Clinical Vignettes
Five questions. One shot each. Let's see what stuck.
out of 5
Board-Style Walkthrough
Board-Style Walkthrough
Original board-style vignettes. Shuffled, never-repeat, full explanations for every choice.
