4.3 Scenario Practice for EKG Analysis and Interpretation

Reading Heart Blocks

Atrioventricular (AV) blocks are graded by how the PR interval behaves and whether QRS complexes are dropped. Walk every suspected block through the PR pattern:

The scenario clue for Mobitz I is the lengthening PR with grouped beating; for Mobitz II it is a constant PR with abrupt drops. In complete heart block, P-P intervals are regular and R-R intervals are regular, but the two are dissociated, so the PR appears to wander randomly. Mobitz II and third-degree block are dangerous and warrant urgent escalation.

Ventricular Rhythms

Ventricular beats arise below the AV junction, so the QRS is wide (0.12 s or greater) and bizarre with no normal preceding P wave. Grade ventricular activity from benign to lethal:

• Premature ventricular contraction (PVC): a single early, wide, bizarre QRS with a compensatory pause. Patterns include bigeminy (every other beat) and couplets (two in a row).
• Idioventricular rhythm: a ventricular escape pacemaker at 20-40 bpm with wide complexes and no P waves; accelerated idioventricular runs 40-100 bpm.
• Ventricular tachycardia (V-tach): three or more PVCs in a row producing a regular, wide-complex tachycardia over 100 bpm. Sustained V-tach is a medical emergency.
• Ventricular fibrillation (V-fib): a totally chaotic baseline with no organized QRS complexes; the heart quivers and produces no output, requiring immediate CPR and defibrillation.
• Asystole: a near-flat line with no electrical activity; confirm in a second lead and verify lead attachment before calling it.

For any wide-complex tachycardia, V-tach, V-fib, or asystole, the CET stays with the patient, ensures a clean tracing, and escalates at once.

Grading Ectopy and Escalation Priority

Not every abnormal beat is an emergency, so the technician must grade severity to decide how urgently to report. Order ventricular findings from monitor-and-note to call-for-help:

• Occasional unifocal PVCs: single early wide beats of identical shape; note them and trend the frequency.
• Frequent, multifocal, or paired PVCs: different shapes (multifocal), couplets, bigeminy, or the R-on-T pattern (a PVC landing on the prior T wave) are more concerning and warrant prompt notification, because they can trigger lethal rhythms.
• Sustained V-tach: a true emergency, especially if the patient loses a pulse.
• V-fib and asystole: immediate emergency response, CPR, and clinician escalation.

For heart blocks, the same triage applies: first-degree block and Mobitz I are usually stable and reportable, while Mobitz II and third-degree block can deteriorate to asystole and demand urgent escalation, often with pacing. The recurring exam theme is matching the right urgency to the rhythm: do not under-react to complete heart block, and do not over-react to a single benign PVC.

Keep a short escalation reference in mind while you read blocks and ectopy together. The immediately life-threatening group is ventricular fibrillation, pulseless ventricular tachycardia, asystole, and third-degree heart block; these trigger the emergency response. The promptly-report group includes Mobitz II, new or frequent multifocal PVCs, sustained V-tach with a pulse, and any new wide-complex tachycardia. The note-and-monitor group includes first-degree block, Mobitz I, occasional unifocal PVCs, and stable sinus brady or tachycardia.

Sorting each finding into one of these three buckets is faster than memorizing a long list and mirrors how scenario questions are scored.

Scenario Walk-Throughs

Scenario A: A monitored patient's strip shows the PR interval growing longer over four beats, then a P wave with no QRS, after which the cycle restarts. The lengthening-then-dropping pattern is the signature of Mobitz I (Wenckebach) second-degree block. Document the finding and notify the nurse; it is usually less ominous than Mobitz II but still reportable.

Scenario B: A patient becomes unresponsive and the monitor shows a wide, chaotic, disorganized waveform with no discernible complexes. This is ventricular fibrillation; the technician calls for help, starts the emergency response, and ensures the leads are connected so the rhythm is not artifact.

Scenario C: Regular P waves march out at 80 bpm while the QRS complexes march out independently at 38 bpm, with no fixed PR relationship. Independent atrial and ventricular activity is third-degree (complete) heart block, an emergency requiring immediate escalation because the ventricular escape rate is unreliable.

Scenario D: Every other beat is an early, wide, bizarre QRS with no preceding P wave, alternating with normal sinus beats. This regular alternation of one normal beat and one PVC is ventricular bigeminy; it is noted and reported, and frequent or multifocal PVCs deserve prompt notification because they can precede V-tach.

Scenario E: A patient's strip is regular at 38 bpm with a normal upright P before each narrow QRS and a normal PR. Because the P waves are sinus and one precedes each QRS, this is sinus bradycardia, not a block or escape rhythm. The technician documents it and reports if the patient is symptomatic (dizziness, hypotension), illustrating that a slow rate alone does not equal an emergency once the 5-step method confirms a sinus origin.