Cardiac Rhythms Quiz to Recognize ECG Patterns Fast

Normal Sinus Rhythm requires a heart rate between 60 and 100 beats per minute with regular R–R intervals. Each QRS complex must be preceded by a consistent upright P wave, demonstrating proper sinoatrial node initiation. The PR interval remains constant, and QRS duration is under 0.12 seconds. These findings confirm intact atrial conduction, AV node transmission, and synchronized ventricular depolarization without ectopic or conduction abnormalities.

Sinus Tachycardia is identified when heart rate exceeds 100 beats per minute while maintaining normal conduction patterns. The rhythm remains regular, P waves precede each QRS, and QRS duration stays narrow under 0.12 seconds. The increase in rate is often physiologic, related to stress or hypovolemia. Calculation using 300 divided by large boxes confirms elevated ventricular response exceeding normal resting parameters.

Sinus Bradycardia occurs when the sinoatrial node fires below 60 beats per minute while maintaining normal conduction. The rhythm is regular, each P wave precedes a QRS complex, and QRS duration remains narrow. Using the 300 method, more than five large boxes between R waves indicates bradycardia. It may be physiologic in athletes or pathologic in hypothyroidism or increased vagal tone.

Atrial Fibrillation produces an irregularly irregular ventricular rhythm due to chaotic atrial depolarizations exceeding 350 impulses per minute. No distinct P waves appear; instead, fibrillatory waves replace organized atrial activity. The absence of coordinated atrial contraction leads to variable R–R intervals. Calculating R–R spacing confirms unpredictability. This arrhythmia increases stroke risk because atrial stasis promotes thrombus formation within the atria.

Atrial Flutter results from a reentrant atrial circuit generating atrial rates between 250 and 350 beats per minute. ECG shows sawtooth flutter waves, especially in inferior leads. Ventricular response may be regular if conduction ratio remains fixed, such as 2:1. Counting flutter waves between QRS complexes confirms atrial tachyarrhythmia. Unlike fibrillation, atrial activity remains organized and rhythmic despite high frequency.

First Degree AV Block is defined by a PR interval exceeding 0.20 seconds while maintaining one P wave for every QRS complex. Measurement across five small boxes confirms prolongation. Conduction through the AV node is delayed but not interrupted. Ventricular depolarization remains normal with narrow QRS complexes, distinguishing it from higher-degree blocks that demonstrate dropped beats or dissociation.

Mobitz I, or Wenckebach block, shows progressive prolongation of the PR interval until a QRS complex fails to conduct. This repeating cycle confirms AV nodal fatigue. Measuring sequential PR intervals demonstrates incremental extension beyond 0.20 seconds before a dropped beat. The QRS remains narrow because block occurs at the AV node. This pattern differentiates it from Mobitz II, where PR remains constant.

Third Degree Heart Block demonstrates complete atrioventricular dissociation. Atrial impulses do not conduct to ventricles, so P waves and QRS complexes occur independently. Measuring intervals shows regular P–P and R–R spacing but no relationship between them. Ventricular escape rhythm may be narrow or wide depending on origin. Cardiac output decreases because atrial contribution to ventricular filling is lost.

Ventricular Tachycardia is identified by three or more consecutive ventricular beats exceeding 150 beats per minute with QRS duration greater than 0.12 seconds. P waves are usually absent or dissociated. Using large box calculations confirms rapid rate. The wide morphology indicates ventricular origin. Sustained VT compromises cardiac output and may deteriorate into ventricular fibrillation without urgent intervention.

Ventricular Fibrillation presents as chaotic electrical activity without identifiable P waves, QRS complexes, or T waves. The waveform is irregular and disorganized, reflecting multiple ventricular reentry circuits. No measurable R–R interval exists. Because ventricles quiver instead of contract effectively, cardiac output becomes zero. Immediate defibrillation is required to restore organized depolarization and prevent irreversible cerebral hypoxia.

Premature Ventricular Contractions originate from ectopic ventricular foci and appear as early, wide, bizarre QRS complexes without preceding P waves. The compensatory pause following PVC restores rhythm timing. QRS duration exceeds 0.12 seconds because depolarization spreads cell-to-cell. Counting intervals confirms early occurrence relative to sinus cycle. Frequent PVCs may indicate myocardial irritability or electrolyte imbalance.

Junctional Rhythm arises from the AV junction when the sinoatrial node fails. The intrinsic junctional rate ranges from 40 to 60 beats per minute. P waves may be inverted, absent, or appear after QRS because atrial depolarization occurs retrograde. QRS complexes remain narrow as ventricular conduction follows normal pathways. Rate calculation confirms bradycardic yet regular ventricular response.

Accelerated Junctional Rhythm occurs when the AV junction fires between 60 and 100 beats per minute, exceeding its normal intrinsic rate. The rhythm is regular, and P waves are absent or inverted due to retrograde conduction. QRS complexes remain narrow under 0.12 seconds. Rate calculation confirms acceleration beyond typical junctional baseline yet below ventricular tachycardia threshold.

Torsades de Pointes is a polymorphic ventricular tachycardia associated with prolonged QT interval. The QRS complexes appear to twist around the isoelectric baseline. Measuring QT interval exceeding 0.44 seconds confirms predisposition. Rate typically exceeds 150 beats per minute. The changing amplitude reflects shifting ventricular depolarization vectors. Electrolyte imbalance or medication effects often trigger this unstable rhythm.

Asystole is defined by complete absence of measurable electrical activity on ECG, producing a flatline tracing. No P waves, QRS complexes, or ventricular contractions occur. Cardiac output is zero because depolarization ceases entirely. Verification requires checking leads and gain to rule out artifact. Asystole represents terminal cardiac arrest requiring immediate advanced life support interventions.