PDF → INTERACTIVE • STEP 1

ECG Arrhythmia Lab

Read every rhythm in the same order: rate → regularity → P waves → PR → QRS. Tap a rhythm, watch the strip change, then prove it in the checkpoint.

Tap-to-interpret monitor

Normal sinus rhythm
Regular; one upright P before every narrow QRS; constant PR. Start by proving sinus origin before naming an abnormality.

1 • RATEFast >100/min; slow <60/min in adults. The PDF’s sinus tachycardia example is ~150/min.

2 • REGULARITYIrregularly irregular strongly suggests atrial fibrillation; patterned irregularity suggests ectopy or block.

3 • P WAVESAbsent, hidden, dissociated, premature, or sawtooth? This often localizes the rhythm.

4 • PR + QRSPR tells AV conduction; QRS width tells ventricular activation pathway.

Source anchors: PDF pp. 2–8, 21, 33–37.

The AV block ladder

1°Every P conducts; PR >0.20 sDelay only

Mobitz IPR gets longer → QRS drops → resetAV node

Mobitz IIFixed PR with sudden dropped QRSHis–Purkinje

3°P waves and QRS march independentlyAV dissociation

Mechanism map

Exam trap: a 2:1 block cannot always be confidently typed as Mobitz I vs II from one strip because you cannot see a PR trend.

Stokes–Adams logic

AV conduction abruptly stops.

Overdrive suppression delays the ventricular escape rhythm by 5–30 s.

Cerebral perfusion fails after several seconds → syncope.

Distal pacemaker escapes, often 15–40/min; pacing prevents recurrence.

PDF pp. 6–10.

Premature beat localizer

PACEarly abnormal P; usually narrow QRSOften incomplete pause

Junctional premature beatP absent/hidden or retrogradeUsually narrow QRS

PVCNo preceding P; wide, bizarre, high-voltage QRSOften compensatory pause

Pulse deficit: an early beat may generate too little stroke volume to produce a palpable peripheral pulse.

Why is a PVC wide?

BigeminyEvery normal beat alternates with a PVC.

R-on-T riskA PVC during vulnerable repolarization can trigger VT/VF.

PDF pp. 11–18.

Narrow vs wide tachycardia

SVTUsually narrow, regular; P waves hidden/abnormalAbove ventricles

VTWide-complex tachycardia; consecutive ventricular beatsTreat as VT if unsure

VFChaotic electrical activity; no organized QRSNo effective output

A-fib vs flutter

A-fibNo discrete P; irregularly irregular R–RMany wandering wavelets

FlutterSawtooth atrial activity, often 2:1 or 3:1 conductionMacro-reentry

PDF anchor: atrial flutter is commonly 200–350 atrial beats/min; its example shows 250/min with 2:1 conduction producing a ventricular rate of 125/min.

Reentry: the engine beneath many tachyarrhythmias

A self-sustaining circuit needs a pathway long enough, slowed conduction, and recovered excitable tissue ahead of the wavefront. Multiple reentrant wavelets produce fibrillation.

PDF pp. 19–37.

15-question checkpoint

Answer before revealing feedback. The distractors target classic Step 1 ECG traps.

1. PR 0.28 s; every P conducts. Diagnosis?

First-degree block is delayed AV conduction without dropped beats.

2. PR progressively lengthens, then a QRS drops.

Wenckebach = longer, longer, longer, drop.

3. Fixed PR intervals with unpredictable dropped QRS complexes.

Mobitz II is infranodal and can progress to complete block.

4. P waves and QRS complexes have independent regular rhythms.

AV dissociation is the hallmark of complete block.

5. No discrete P waves and irregularly irregular R–R intervals.

A-fib has disorganized atrial activity and unpredictable AV conduction.

6. Sawtooth atrial waves at 250/min with ventricular rate 125/min.

Two atrial cycles per QRS gives 2:1 conduction.

7. Early wide bizarre QRS without preceding P wave.

A ventricular focus activates myocardium slowly outside Purkinje fibers.

8. Normal beat–PVC–normal beat–PVC pattern.

Bigeminy means every other beat is premature.

9. Why is a PVC QRS wide?

The impulse bypasses the rapid His–Purkinje network.

10. Regular narrow tachycardia with hidden P waves in a young adult.

AV nodal SVT commonly hides P waves within the QRS/T complex.

11. Repeating wide ventricular complexes without normal beats between.

VT resembles consecutive PVCs.

12. Chaotic waveform without organized QRS and no pulse.

VF produces no coordinated ventricular pumping and requires defibrillation.

13. Syncope after abrupt complete AV block is delayed by what mechanism?

Escape foci may remain suppressed for 5–30 seconds after AV conduction stops.

14. Which second-degree block most strongly suggests His–Purkinje disease?

Mobitz II is typically infranodal and higher risk.

15. Best first organizing question for any ECG rhythm strip?

A fixed reading sequence prevents premature pattern-matching errors.