3 Ventricular Rhythms and Conduction Disorders
Learn a systematic approach to ECG rhythm recognition, conduction disorders, pacing problems, and the treatment priorities determined by pulse and clinical stability.
A systematic approach to assessment
Begin with the patient, not the rhythm label: assess responsiveness, pulse, blood pressure, perfusion, breathing, and symptoms. A rhythm interpretation does not replace clinical assessment, and instability—such as hypotension, shock, altered mental status, ischemic chest discomfort, or acute heart failure—changes treatment urgency.
Confirm patient identity, calibration, and whether artifact is present. Standard calibration is usually and ; one small box represents .
Determine rate and regularity by comparing R–R intervals. Then assess P waves and whether each P wave is followed by a QRS. Measure the PR interval, normally –, and the QRS duration: narrow is below , while wide is at or above . Assess QT/QTc and compare with prior ECGs when available.
Review all leads for axis, QRS morphology, conduction delay, ST–T changes, and whether findings are new. A wide QRS may reflect ventricular origin, bundle-branch block, pacing, or pre-existing conduction disease. A -lead may reveal a cause or consequence of a rhythm disturbance, including ischemia, but obtaining one must not delay urgent stabilization.
Consider reversible causes, including ischemia, hypoxia, electrolyte abnormality, medication or toxin effects, and structural heart disease.
Ventricular rhythms
Ventricular rhythms differ in rate, regularity, and QRS appearance. A is one early ventricular beat that differs from the underlying rhythm; assess its symptoms, frequency, morphology, and the presence of underlying disease. A is slow, usually regular, and wide-complex, without a consistent preceding P wave; it may appear when higher pacemakers fail or conduction is blocked, so assess perfusion and address the cause.
consists of repeating ventricular complexes at a rate typically faster than an escape rhythm but slower than most VT. It is often transient, including during reperfusion; distinguish it from VT using rate, context, and clinical state.
is usually a regular wide-complex tachycardia with a consistent QRS shape. In an adult, treat an unexplained wide-complex tachycardia as VT until proven otherwise. AV dissociation, beats, or fusion beats support VT, but may be absent.
Polymorphic VT has rapid wide complexes whose shape or axis varies. appears to twist around the baseline and is associated with prolonged QT; check QT/QTc and causes of QT prolongation. Sustained polymorphic VT requires an immediate shock. Torsades recurrence with prolonged QT may respond to magnesium and correction of contributing factors.
appears as disorganized electrical activity without identifiable repeating QRS complexes and is a cardiac-arrest rhythm with no effective pulse.
Treatment priorities for tachycardia
First determine whether a pulse is present. Pulseless VT or VF requires immediate CPR and unsynchronized defibrillation according to the current resuscitation algorithm. For tachycardia with a pulse, an unstable tachyarrhythmia generally requires prompt synchronized cardioversion.
Sustained polymorphic VT cannot be reliably synchronized, so use an immediate unsynchronized shock. For stable regular wide-complex tachycardia, obtain expert help and follow the current algorithm; adenosine is considered only in selected regular, monomorphic rhythms. Do not give verapamil or diltiazem for an undifferentiated wide-complex tachycardia.
Wide-complex tachycardia with a pulse may be VT, supraventricular tachycardia with aberrant conduction, or a paced rhythm. Clinical instability determines urgency; when uncertain, presume VT and seek expert help.
Recognizing atrioventricular blocks
Classify AV block by examining P waves, PR intervals, and dropped QRS complexes. In , every P wave conducts, but the PR interval is longer than ; this is conduction delay, not a dropped beat.
In , the PR interval progressively lengthens until a P wave is not followed by a QRS, and then the cycle repeats. The block is often at the AV node. In , conducted beats have a stable PR interval, but intermittent P waves fail to conduct; this can progress to complete block and merits urgent clinical assessment.
In , every other P wave is blocked, so the tracing alone usually cannot distinguish Mobitz I from Mobitz II. Do not assign a type from the 2:1 pattern alone. involves multiple consecutive nonconducted P waves, although some AV conduction remains.
In , P waves and QRS complexes occur independently, with no consistent PR relationship; a slower escape rhythm maintains ventricular activity. Compare atrial and ventricular rates and look for AV dissociation. A narrow escape QRS often suggests a junctional source, while a wide escape QRS often suggests a ventricular source; these clues are not definitive.
Mobitz II, high-grade, and complete AV block need urgent evaluation, and pacing may be required. Permanent pacing decisions depend on the cause, symptoms, and level of block.
Responding to symptomatic bradycardia
For symptomatic bradycardia with cardiopulmonary compromise, support airway and breathing, monitor the patient, and address reversible causes. The adult bradycardia algorithm recommends atropine; if it is ineffective, use transcutaneous pacing and/or an infusion such as dopamine or epinephrine. Consider expert consultation and transvenous pacing, and follow current local protocols and the current algorithm.
Bundle-branch and fascicular patterns
Bundle-branch block widens the QRS because ventricular activation is delayed. A typical has a terminal positive deflection in V1 and a broad terminal S wave in leads I and V6. A typical has a broad, often notched R wave in lateral leads and a predominantly negative QRS in V1.
Fascicular blocks alter the frontal-plane axis, and combinations may be described as bifascicular block. Interpret new conduction abnormalities in clinical context and compare them with previous tracings. Bundle-branch block and ventricular pacing can make ischemia assessment more difficult; use appropriate criteria and clinical evaluation rather than assuming the excludes ischemia.
Paced rhythms and device problems
A pacing spike is a brief vertical mark. Determine whether it is followed by atrial depolarization (a P wave), ventricular depolarization (a QRS), or both. Ventricular pacing commonly produces a wide QRS; its morphology depends on lead location and the patient’s conduction system. A spike followed by the expected chamber response indicates .
Important pacing problems include:
Failure to : a pacing spike is not followed by the expected P wave or QRS.
Failure to pace (output failure): no pacing spike appears when one is expected based on device settings and the patient’s intrinsic rhythm.
Failure to sense: pacing occurs at an inappropriate time because the device does not correctly recognize intrinsic activity; oversensing can inhibit needed pacing.
A pacing spike or appearance alone cannot confirm that a device is functioning safely. Assess the patient, check perfusion and , and urgently escalate suspected malfunction, especially when symptoms or instability are present.
, PEA, and the role of the pulse
has no discernible ventricular electrical activity. Confirm the tracing and leads; it is not a shockable rhythm. Provide CPR and follow the nonshockable-arrest algorithm.
is organized electrical activity without a palpable pulse. Treat it as cardiac arrest with CPR and management of reversible causes; do not defibrillate solely because an organized rhythm is present.
Treatment follows the patient’s pulse and stability: pulseless VT or VF calls for CPR and defibrillation, unstable tachycardia with a pulse generally calls for cardioversion, and unstable bradycardia may require medication and pacing. Use current resuscitation algorithms and local protocols.