Glossary
A quick reference for the cardiac rhythm and ECG terminology used throughout Cardiac Cadence. Every definition is written to match how the term is taught in the chapters.
Aberrant Conduction (Aberration)
A supraventricular impulse that conducts through the ventricles with a wide QRS because one bundle branch is still refractory from the preceding beat. The beat is supraventricular in origin but appears ventricular on the ECG. Most common with PACs and in atrial fibrillation after a long-short R-R sequence.
Chapter 11 →Accelerated Idioventricular Rhythm (AIVR)
A regular ventricular rhythm at 41-100 bpm. Often seen during reperfusion after myocardial infarction or with certain medications. Usually transient and benign, but warrants monitoring.
Chapter 7 →Accelerated Junctional Rhythm (AJR)
A regular junctional rhythm at 60-100 bpm. The AV node has assumed pacemaker control at a rate faster than its intrinsic escape rate. P waves are absent, inverted, or follow the QRS.
Chapter 6 →Accessory Pathway
An abnormal extra electrical connection between the atria and ventricles that bypasses the AV node. The substrate for AVRT and Wolff-Parkinson-White syndrome.
Chapter 10 →Agonal Rhythm
A slow, irregular ventricular rhythm of widely spaced wide complexes, typically 20 bpm or fewer - a dying heart in the final stages before asystole. Treatment focuses on advanced cardiac life support and reversible causes.
Chapter 7 →Asystole
Complete absence of ventricular electrical activity on the ECG. Can present as a flatline or as P waves with no QRS complexes (ventricular standstill). A medical emergency requiring CPR and advanced cardiac life support.
Chapter 7 →Atrial Fibrillation (AF, AFib)
An irregularly irregular supraventricular rhythm with no discernible P waves and a fibrillatory baseline, caused by chaotic atrial electrical activity. Carries significant stroke and pulmonary embolism risk from atrial blood pooling.
Chapter 5 →Atrial Flutter (AFL)
A rapid, regular atrial rhythm at 250-350 bpm with characteristic sawtooth flutter waves, typically best seen in the inferior leads. Ventricular response depends on the AV conduction ratio.
Chapter 5 →Atrial-First Rule
When an abnormal P wave appears before a QRS, consider an atrial origin (PAC, ectopic atrial rhythm) before concluding a junctional rhythm. Junctional rhythms typically show absent P waves, P waves buried in the QRS, or P waves following the QRS - not abnormal P waves preceding it.
Chapter 6 →AV Block
A delay or interruption of electrical conduction between the atria and ventricles at the AV node or bundle of His. Classified as First-Degree, Second-Degree (Type I, Type II, or High-Grade), or Third-Degree (Complete).
Chapter 8 →AV Dissociation
A state in which the atria and ventricles are depolarized by independent pacemakers with no consistent relationship between P waves and QRS complexes. Seen in Third-Degree AV Block and ventricular tachycardia.
Chapter 8 →AVNRT
Atrioventricular Nodal Reentrant Tachycardia - a reentrant supraventricular tachycardia using two functional pathways within the AV node. P waves are typically buried in or fused with the QRS. Rate 150-250 bpm.
Chapter 10 →AVRT
Atrioventricular Reentrant Tachycardia - a reentrant supraventricular tachycardia using the AV node as one limb and an accessory pathway as the other. In orthodromic AVRT the QRS is narrow and retrograde P waves appear after the QRS.
Chapter 10 →Bundle Branch Block (BBB)
A delay or block in conduction through one of the bundle branches, producing a wide QRS complex (>0.12 s). Can be fixed (constant) or rate-dependent.
Chapter 7 →Bundle of His
The shared electrical trunk that carries the signal from the AV node before it splits into the right and left bundle branches.
Chapter 1 →Capture Beat
An occasional narrow QRS complex within a wide QRS tachycardia, indicating a sinus impulse momentarily captured the ventricles via the normal pathway. A finding that supports VT over SVT with BBB.
Chapter 11 →Compensatory Pause
The full pause following a PVC, in which the next normal sinus beat occurs at the expected time as if the PVC had not happened. The interval flanking the PVC equals exactly twice the R-R interval. PVCs typically produce compensatory pauses; PACs and PJCs typically produce non-compensatory pauses.
Chapter 7 →Conduction System
The specialized electrical tissue of the heart - SA node, AV node, bundle of His, bundle branches, and Purkinje fibers - responsible for coordinated cardiac depolarization.
Chapter 1 →CRAT
Certified Rhythm Analysis Technician - a credential offered by Cardiovascular Credentialing International (CCI) that validates competency in cardiac rhythm interpretation.
CRT-Paced Rhythm
Cardiac Resynchronization Therapy paced rhythm - a biventricular paced rhythm in which both ventricles are paced simultaneously to improve coordination in heart failure with dyssynchrony. The QRS may appear narrower than a standard right-ventricular-paced QRS.
Chapter 9 →Decision Tree
A systematic interpretive framework that branches on observed ECG features - QRS width, P wave presence, PR relationship, rate, and clinical context - to arrive at a rhythm diagnosis.
Chapter 11 →Delta Wave
A slurred upstroke at the beginning of the QRS complex, characteristic of pre-excitation through an accessory pathway in Wolff-Parkinson-White syndrome. Visible during sinus rhythm, not during AVRT itself.
Chapter 10 →Ectopic
Originating from a location other than the SA node. An ectopic beat or rhythm starts in atrial, junctional, or ventricular tissue rather than the heart's normal pacemaker.
Chapter 3 →Ectopic Atrial Rhythm (EAR)
A regular rhythm originating from an atrial focus outside the SA node. P waves are present but morphologically different from sinus P waves. Rate is typically within the sinus range.
Chapter 5 →Failure to Capture
A pacemaker malfunction in which a pacing spike fires but does not produce a P wave or QRS complex. The myocardium failed to respond to the stimulus.
Chapter 9 →Failure to Pace
A pacemaker malfunction in which the device does not deliver a pacing spike when one is expected. No spike, no beat.
Chapter 9 →Failure to Sense
A pacemaker malfunction in which the device fails to detect intrinsic cardiac activity and delivers a pacing spike when it should have inhibited. The underlying mechanism is typically undersensing.
Chapter 9 →Fascicular PVC (Narrow-Complex PVC)
A premature ventricular contraction originating high in the ventricular conduction system. Because the impulse uses the normal His-Purkinje system, the QRS remains narrow despite the ventricular origin. Often misidentified as a junctional ectopic beat - true PJCs from the AV node are uncommon.
Chapter 7 →Feedback Loop
A reentrant circuit in which an electrical impulse repeatedly traverses the same pathway, sustaining itself - the mechanism behind AVNRT and AVRT. Analogous to audio feedback between a microphone and speaker.
Chapter 10 →First-Degree AV Block
A consistent prolongation of the PR interval beyond 0.20 s, with every P wave conducted to a QRS. The signal is delayed but not blocked.
Chapter 8 →High-Grade AV Block (Second-Degree)
A Second-Degree AV Block in which conduction occurs in a fixed ratio of two or more P waves per QRS (such as 2:1, 3:1). Cannot be classified as Mobitz Type I or II without two consecutive conducted beats.
Chapter 8 →Idioventricular Rhythm (IVR)
A regular ventricular rhythm at 20-40 bpm originating from a ventricular focus. Acts as an escape mechanism when higher pacemakers fail. QRS is wide; no P waves or AV dissociation.
Chapter 7 →Isoelectric Line
The flat baseline of the ECG between waveforms, representing periods of no net electrical activity.
Chapter 2 →Junctional Escape Rhythm (JR or JER)
A regular rhythm at 40-60 bpm originating from the AV junction. Acts as a backup pacemaker when the SA node fails. P waves are absent, inverted, or follow the QRS.
Chapter 6 →Junctional Tachycardia (JT)
A regular junctional rhythm at rates above 100 bpm. P waves are absent, inverted, or follow the QRS. Causes include enhanced automaticity, digitalis toxicity, and post-cardiac surgery.
Chapter 6 →Lead
A view of the heart's electrical activity from a specific angle. The standard 12-lead ECG includes six limb leads (I, II, III, aVR, aVL, aVF) and six chest leads (V1-V6).
Chapter 2 →Mobitz Type I (Wenckebach)
A Second-Degree AV Block with progressive PR interval lengthening over consecutive beats until a P wave is non-conducted, then the cycle resets. Usually a benign block at the AV node level.
Chapter 8 →Mobitz Type II
A Second-Degree AV Block with intermittent non-conducted P waves and constant PR intervals on conducted beats. Typically reflects disease below the AV node and carries a higher risk of progression to complete heart block. What defines Type II is constancy, not duration.
Chapter 8 →Multifocal Atrial Tachycardia (MAT)
An irregular atrial tachycardia above 100 bpm with at least three different P wave morphologies. Often associated with severe pulmonary disease.
Chapter 5 →Non-Conducted PAC (NCPAC)
A premature atrial complex that occurs early enough to find the AV node refractory and is not followed by a QRS. Mimics a dropped beat in AV block. The distinguishing feature is the early, abnormally shaped P wave, often hidden in the preceding T wave.
Chapter 5 →Normal Sinus Rhythm (NSR)
The heart's default rhythm: SA node origin, regular, 60-100 bpm, upright P waves before each QRS, normal PR interval (0.12-0.20 s), and a narrow QRS (0.06-0.12 s).
Chapter 2 →Orthodromic AVRT
AVRT in which the reentry circuit conducts down the AV node and back up the accessory pathway. Produces a narrow QRS tachycardia with retrograde P waves following the QRS.
Chapter 10 →PAC (Premature Atrial Contraction)
An early beat originating from atrial tissue outside the SA node, with an abnormal early P wave followed by a typically narrow QRS. Usually produces a non-compensatory pause.
Chapter 5 →PAC Pair
Two consecutive PACs. Three or more consecutive PACs are typically classified as a run of atrial tachycardia.
Chapter 5 →Paced Rhythm
A cardiac rhythm in which an artificial pacemaker initiates one or more chambers of the heart. Identified by pacing spikes preceding the affected waveforms.
Chapter 9 →Pacemaker
An artificial device that delivers electrical impulses to maintain rate and rhythm when the heart's native pacemakers or conduction pathways fail. Types include single-chamber, dual-chamber (AV-paced), and biventricular (CRT-paced).
Chapter 9 →PJC (Premature Junctional Contraction)
An early beat originating from the AV junction, with a narrow QRS and an absent, inverted, or post-QRS P wave. Usually produces a non-compensatory pause.
Chapter 6 →PR Interval
The time from the beginning of the P wave to the beginning of the QRS complex. Normal range 0.12-0.20 s. Reflects the time for the impulse to travel from the SA node through the atria and AV node into the ventricles.
Chapter 2 →Purkinje Fibers
The terminal branches of the cardiac conduction system that distribute the electrical impulse rapidly across the ventricular myocardium for coordinated contraction.
Chapter 1 →PVC (Premature Ventricular Contraction)
An early beat originating from a ventricular focus, with a wide, bizarre QRS and no preceding P wave. Usually followed by a compensatory pause.
Chapter 7 →PVC Pair
Two consecutive PVCs. Three or more consecutive PVCs are typically classified as a run of ventricular tachycardia.
Chapter 7 →QRS Complex
The series of deflections representing ventricular depolarization. Normal duration 0.06-0.12 s. A QRS wider than 0.12 s indicates ventricular origin or aberrant supraventricular conduction.
Chapter 2 →Rate-Dependent BBB
A bundle branch block that appears only when the heart rate exceeds a critical threshold and resolves when the rate falls below it. Distinguished from VT by upright P waves preceding each wide QRS.
Chapter 11 →Reentry
A self-sustaining electrical circuit in which an impulse re-excites tissue it has already depolarized, producing a sustained tachycardia. The mechanism behind AVNRT, AVRT, and most atrial flutter.
Chapter 10 →Refractory Period
The brief interval following depolarization during which cardiac tissue cannot be re-excited. The basis for many rhythm phenomena including aberrant conduction and reentry.
Chapter 11 →Retrograde Conduction
Electrical conduction in the reverse of the normal direction - typically from the ventricles or AV junction back up to the atria. Produces inverted P waves in the inferior leads.
Chapter 6 →RP Interval
The time from a QRS complex to the following P wave. Useful in differentiating types of supraventricular tachycardia. AVRT typically shows a short RP.
Chapter 10 →SA Node (Sinoatrial Node)
The heart's primary natural pacemaker, located in the wall of the right atrium. Initiates the electrical impulse for each normal heartbeat at an intrinsic rate of 60-100 bpm.
Chapter 1 →SBAR
Situation, Background, Assessment, Recommendation - a structured communication framework used in healthcare to convey clinical findings clearly and concisely.
Chapter 13 →Sinus Arrest
A failure of the SA node to fire for an extended pause, typically longer than two complete cardiac cycles. May require an escape rhythm to maintain cardiac output.
Chapter 4 →Sinus Bradycardia
A sinus rhythm with a rate below 60 bpm. Common in athletes, during sleep, or with certain medications. Symptomatic when cardiac output is compromised.
Chapter 4 →Sinus Tachycardia
A sinus rhythm with a rate of 101 bpm or greater. A normal physiologic response to exertion, fever, hypovolemia, anxiety, and other stressors.
Chapter 4 →ST Segment
The portion of the ECG between the end of the QRS complex and the beginning of the T wave. Elevation or depression is clinically significant for ischemia and injury.
Chapter 2 →Supraventricular
Originating above the ventricles - in the atria or the AV junction. Supraventricular rhythms typically produce narrow QRS complexes because they use the normal ventricular conduction pathway.
Chapter 3 →Third-Degree AV Block (Complete Heart Block)
A complete failure of conduction between the atria and ventricles. P waves and QRS complexes occur independently at their own rates with no PR relationship. A medical emergency requiring pacemaker placement.
Chapter 8 →Torsade de Pointes (TdP)
A polymorphic ventricular tachycardia with QRS complexes that twist around the baseline. Typically pause-dependent, R-on-T initiated, and associated with QT prolongation. Treatment includes IV magnesium and correction of the QT prolongation.
Chapter 7 →U Wave
A small, low-amplitude deflection following the T wave on some ECGs, thought to represent late repolarization of the Purkinje fibers. Most visible at slow heart rates and in leads V2-V3. Prominent U waves are associated with hypokalemia and bradycardia.
Chapter 2 →Undersensing
A pacemaker malfunction mechanism in which the device fails to detect intrinsic cardiac activity because the signal is too small or the sensitivity threshold too high. Results in inappropriate pacing spikes during natural beats (failure to sense).
Chapter 9 →Ventricular Fibrillation (VF)
A chaotic, disorganized ventricular rhythm with no recognizable QRS complexes and no effective cardiac output. A medical emergency requiring immediate defibrillation and CPR.
Chapter 7 →Ventricular Standstill
A form of asystole in which P waves continue but no QRS complexes occur. Atrial activity is preserved; ventricular activity is absent. A medical emergency.
Chapter 7 →Ventricular Tachycardia (VT)
A regular wide QRS rhythm at 101-250 bpm originating from a ventricular focus. May be sustained or non-sustained. Requires immediate clinical attention based on patient stability.
Chapter 7 →Wandering Atrial Pacemaker (WAP)
An irregular rhythm at 100 bpm or below with at least three different P wave morphologies, indicating shifting atrial pacemaker sites. Typically benign.
Chapter 5 →Wenckebach
See Mobitz Type I - a Second-Degree AV Block with progressive PR lengthening until a beat is dropped.
Chapter 8 →Wolff-Parkinson-White Syndrome (WPW)
A pre-excitation syndrome caused by an accessory pathway between the atria and ventricles. Characterized in sinus rhythm by a short PR interval and a delta wave. Predisposes to AVRT and dangerously rapid conduction during atrial fibrillation.
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