Ventricular Rhythms, AV Blocks & Conduction Defects

Objective 1.1.4 — Identify ventricular rhythms, atrioventricular blocks, intraventricular conduction delays, and paced rhythms; recognize life-threatening rhythms and their treatments.

This lesson finishes the dysrhythmia catalog. Most rhythms here are wide-complex (QRS ≥ 0.12 s) or feature a broken AV relationship between P waves and QRS complexes. Several are emergencies — V-tach, V-fib, asystole, complete heart block — and decisive treatment depends on quick recognition.

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Ventricular Rhythms — General Characteristics

The ventricles are the heart's least efficient pacemaker. When an ectopic ventricular site assumes pacing, the impulse bypasses the normal intraventricular conduction pathway, so ventricular stimulation occurs asynchronously. Ventricular beats and rhythms may originate from any part of the ventricles.

The ventricles assume responsibility for pacing the heart if:

• The SA node fails to discharge
• An impulse from the SA node is blocked
• The SA node rate of discharge is slower than the ventricles
• An irritable site in the ventricles produces an early beat or rapid rhythm

ECG hallmarks of ventricular origin:

• P waves are retrograde or missing
• Wide and bizarre ventricular complexes measuring > 0.12 seconds

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Premature Ventricular Complex (PVC)

A "premature" beat from an irritable ventricular site that occurs earlier than the next expected sinus beat and interrupts the underlying rhythm (typically sinus). PVCs occur in healthy people with apparently normal hearts and no apparent cause, and frequency increases with age.

Types of PVCs

Premature-Beat Patterns (review)

Common causes:

• Age
• Hypoxia
• Stress / anxiety
• Stimulants
• Myocardial ischemia

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Ventricular Escape Beat & Idioventricular Rhythm (IVR)

Unlike PVCs, ventricular escape beats occur late — after the next expected sinus beat fails. IVR exists when 3 or more ventricular escape beats occur in a row at a rate of 20 – 40 bpm.

Common causes — supraventricular pacemaker fails, runs slower than the ventricular rate, or its impulses are blocked:

• MI
• Digitalis toxicity
• Acute coronary syndromes

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Accelerated Idioventricular Rhythm (AIVR)

Three or more ventricular beats in a row at a rate of 41 – 100 bpm. Considered a benign escape rhythm that appears when the sinus rate slows and disappears when the sinus rate speeds up.

Common causes:

• Acute myocarditis
• Digitalis toxicity
• Cocaine toxicity
• Dilated cardiomyopathy

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Ventricular Tachycardia (V-Tach)

Three or more PVCs in a row at a rate > 100 bpm.

• Non-sustained VT — short run lasting less than 30 seconds
• Sustained VT — persists for more than 30 seconds

V-Tach morphologies

Common causes:

• Acid–base imbalance
• Acute coronary syndromes
• Cardiomyopathy
• Electrolyte imbalance — hypokalemia, hyperkalemia, hypomagnesemia

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Ventricular Fibrillation (V-Fib)

Chaotic rhythm originating in the ventricles with no organized depolarization. The ventricular myocardium quivers, producing no effective contraction and no pulse. The rhythm is irregularly irregular, with chaotic deflections varying in shape and amplitude. No normal-looking waveforms are visible.

Common causes:

• Other dysrhythmias (e.g., V-Tach)
• Acute coronary syndromes
• Ventricular hypertrophy
• Electrolyte imbalance
• Severe heart failure

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Treatment — Ventricular Rhythms

• Correct the underlying cause
• Consult cardiology
• Provide supplemental O₂ for symptoms
• Stable but symptomatic V-Tach → antiarrhythmics (e.g., amiodarone)
• Pulseless V-Tach and V-Fib → CPR + defibrillation

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Asystole (Cardiac Standstill)

Total absence of ventricular electrical activity. No ventricular rate or rhythm, no pulse, no cardiac output. May occur temporarily following termination of a tachycardia with medications, defibrillation, or synchronized cardioversion. If atrial activity is present, the rhythm is called "P wave" asystole.

Common causes:

• MI
• Pulmonary embolism
• Tension pneumothorax
• Cardiac tamponade
• Severe hypoxia

Treatment for Asystole

• CPR
• Supplemental oxygen
• Vasopressors (e.g., epinephrine, vasopressin)
• Do NOT defibrillate asystole

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Heart Blocks — Atrioventricular (AV) Blocks

A dysrhythmia in which a delay or interruption of impulse conduction from the atria to the ventricles occurs because of a transient or permanent anatomic or functional impairment.

Types

• First-degree AV block
• Second-degree AV block (incomplete)
  • Type I (Mobitz I / Wenckebach phenomenon)
  • Type II (Mobitz II)
  • 2:1 AV block
• Third-degree AV block (complete block)

The Block Jingle

A widely-taught mnemonic — quick recognition at a glance:

If the R is far from P, then you have a first degree. If longer, longer, longer, drop — then you have a Wenckebach (Type I). If some P's don't get through, then you have a Mobitz II. If P's and Q's don't agree, then you have a third degree.

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First-Degree AV Block

Despite the name, the sinus impulse is not blocked but instead delayed for the same period before reaching the ventricles. All components of the cardiac cycle except the PR interval are within normal limits.

ECG hallmark: consistent PR intervals > 0.20 s.

Common causes:

• Can be benign, especially in athletes
• Acute MI
• Acute endocarditis or myocarditis

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Second-Degree AV Block, Type I (Mobitz I / Wenckebach)

Impulse delay/blockage above the bundle of His. Impulses generated by the SA node take longer and longer to conduct through the AV node until finally a sinus impulse is blocked ("dropped beat"). Pattern repeats — Wenckebach phenomenon.

ECG hallmark: progressively lengthening PR intervals followed by a dropped beat.

Common causes:

• Can be benign (especially in athletes)
• Acute MI
• Acute endocarditis or myocarditis

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Second-Degree AV Block, Type II (Mobitz II)

Conduction delay/blockage occurs below the AV node (within the His–Purkinje system). The SA node generates impulses normally, so each P wave occurs at a regular interval. However, not every P wave is followed by a QRS ("dropped" beat). Conducted impulses travel at the same rate with consistent, usually normal PR intervals until an impulse is suddenly blocked.

ECG hallmark: consistent PR intervals with sudden dropped beats. QRS may be narrow or wide (> 0.11 s).

Common causes:

• LCA disease (LCA supplies blood to the bundle branches)
• Acute myocarditis
• Cardiomyopathy
• Fibrosis of the conduction system

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2:1 AV Block

A second-degree AV block in which two consecutive conducted P waves never occur — every other P wave is blocked. Because you can't watch the PR interval lengthen across a sequence (only one conducted beat at a time), classification depends on QRS width:

ECG hallmark: consistent 2 P waves before each QRS (2:1 conduction).

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Third-Degree AV Block (Complete Heart Block)

Impulses from the SA node are blocked before reaching the ventricles — no P waves are conducted. Block may occur at the AV node, bundle of His, or bundle branches. Atria and ventricles beat independently of each other. A secondary pacemaker (junctional or ventricular) takes over for the ventricles, so QRS may be narrow or wide depending on the location of the escape.

ECG hallmark: no relationship between P waves and QRS complexes.

Common causes:

• Acute MI
• Acute myocarditis
• Side effects of drugs (e.g., amiodarone, beta-blockers, diltiazem)
• Fibrosis of the conduction system

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Intraventricular Conduction Delay (Bundle Branch Block, BBB)

A disruption of impulse conduction from the bundle of His through either the right or left bundle branch.

ECG hallmark: rSR' or "M" complexes ("rabbit ears").

Right Bundle Branch Block (RBBB)

Impulse travels through the AV node and down the left bundle branch into the interventricular septum.

• Septal depolarization moves left to right → produces a small initial R wave in V1.
• As the left bundle continues to conduct, the entire left ventricle is depolarized right to left → signal moves away from V1 → negative S wave.
• Typically results in right axis deviation (RAD).
• Produces an rSR' pattern ("rabbit ears").
• Normal R-wave progression is reversed in the chest leads.

Left Bundle Branch Block (LBBB)

The septum is depolarized by the right bundle branch (as is the right ventricle).

• Produces negative QRS complexes with small R waves and deep S waves in V1 and V2.
• Produces tall R waves in V5 and V6.

RBBB vs LBBB — The "WiLLiaM MaRRoW" Shortcut

A classic memorization trick using the leads V1 and V6:

Read the first and last letters of "WiLLiaM" and "MaRRoW" to recall the V1 → V6 morphology of each block.

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Paced Rhythms

In temporary or paced rhythms, pacing spikes appear before the QRS complex (or before a P wave for atrial pacing). This is electrical capture — the alteration of the ECG rhythm by proper temporary pacing impulse delivery, successfully depolarizing the atria and ventricles.

ECG hallmark: pacing spikes (sharp vertical lines) preceding the captured complex.

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High-Yield Recap

Wide-QRS rhythms by rate

V-tach flavors

V-fib visible amplitude

• Coarse V-fib ≥ 3 mm
• Fine V-fib < 3 mm

Emergency treatment shortcuts

AV block — pattern recognition

Block Jingle — at a glance

R far from P → 1st degree · longer-longer-drop → Wenckebach · some P's don't get through → Mobitz II · P's and Q's don't agree → 3rd degree.

Bundle branch blocks

• Complete BBB: QRS > 0.12 s. Incomplete: 0.10 – 0.12 s.
• rSR' / "rabbit ears" = BBB hallmark.
• WiLLiaM MaRRoW — V1/V6 morphology tells you which side: W-then-M = LBBB, M-then-W = RBBB.

Paced rhythms

• Pacing spike before each captured complex = electrical capture confirmed.