Dysrhythmia Recognition Part 2: Ventricular and Heart Block Rhythms

Dysrhythmia Recognition Part 2: Ventricular and Heart Block Rhythms

Recognizing life-threatening cardiac rhythms quickly and accurately is one of the most critical skills an EMT or paramedic develops. This lesson builds on basic rhythm recognition by focusing on atrioventricular (AV) block rhythms and ventricular dysrhythmias—the rhythms most likely to demand immediate, high-stakes decisions in the field.

Atrioventricular Block Rhythms

AV blocks occur when conduction between the atria and ventricles is delayed or interrupted. The degree of block describes how severely conduction is impaired.

First-Degree AV Block is the mildest form. Every P wave conducts to the ventricles, but the PR interval is prolonged beyond 0.20 seconds (one large box on standard ECG paper). The rhythm is otherwise regular, QRS is typically narrow, and the patient is usually asymptomatic. First-degree block alone does not require intervention but signals underlying conduction disease. Prehospital significance: monitor and transport; no electrical or pharmacological intervention needed for the block itself.

Second-Degree AV Block, Type I (Mobitz I / Wenckebach) shows a PR interval that progressively lengthens with each beat until one P wave fails to conduct and a QRS is dropped. After the dropped beat, the cycle resets. The hallmark is the "grouped beating" pattern. The ventricular rate is slightly irregular. This block is usually benign and originates in the AV node. Prehospital significance: monitor closely; if the patient is symptomatic (hypotension, altered mental status, chest pain), prepare for intervention.

Second-Degree AV Block, Type II (Mobitz II) is more dangerous. The PR interval is constant from beat to beat, but a P wave suddenly fails to conduct without warning. The QRS that follows a conducted beat is often wide, indicating infranodal disease. The ratio of P waves to QRS complexes may be 2:1, 3:1, or variable. This block can deteriorate rapidly to complete heart block and is considered high risk. Prehospital significance: treat as high priority; be prepared for transcutaneous pacing.

Third-Degree (Complete) AV Block means no atrial impulses conduct to the ventricles at all. The atria and ventricles beat independently—atrial rate and ventricular rate are completely unrelated. P waves march through the strip at their own rate; QRS complexes appear at a slower, regular escape rate. The QRS may be narrow (junctional escape) or wide (ventricular escape). Patients are often severely symptomatic. Prehospital significance: this is a true emergency. Transcutaneous pacing is the immediate intervention; atropine may be attempted for junctional escape rhythms but is unlikely to help wide-complex escape rhythms.

Safety Point: Never withhold pacing from a symptomatic patient with third-degree block while waiting for an IV or medication to work. Electrical intervention takes priority.

Ventricular Dysrhythmias

Ventricular rhythms arise below the bundle of His, producing wide, bizarre QRS complexes (generally greater than 0.12 seconds) with no consistent relationship to preceding P waves.

Premature Ventricular Contractions (PVCs) are early, wide, bizarre beats interrupting an otherwise normal rhythm. A full compensatory pause typically follows. Unifocal PVCs look identical beat to beat (one ectopic focus). Multifocal PVCs vary in shape (multiple foci). Couplets are two PVCs in a row. R-on-T phenomenon occurs when a PVC lands on the vulnerable repolarization period of the preceding T wave—this can trigger ventricular fibrillation. Isolated, infrequent PVCs in a stable patient may not require treatment; however, frequent, multifocal, or R-on-T PVCs in an ischemic patient deserve close monitoring and medical direction guidance.

Idioventricular Rhythm is a slow ventricular escape rhythm, typically 20–40 beats per minute, with wide QRS complexes. It appears when all higher pacemakers fail. It may be the only rhythm keeping a patient alive. Do not suppress it with antiarrhythmics—doing so may cause complete cardiac standstill. Treatment focuses on addressing the underlying cause and, if the patient is symptomatic, considering pacing.

Ventricular Tachycardia (V-Tach) is three or more consecutive PVCs at a rate typically above 100 beats per minute, often 150–250 bpm. QRS complexes are wide and regular. Monomorphic V-tach has uniform QRS morphology. If the patient has a pulse and is stable, treatment follows medical direction (often antiarrhythmics). If the patient is pulseless or unstable, immediate defibrillation (unsynchronized shock) is required. Pulseless V-tach is a shockable arrest rhythm.

Ventricular Fibrillation (V-Fib) is chaotic, disorganized electrical activity with no identifiable QRS complexes—only an irregular, undulating baseline. There is no effective cardiac output. V-Fib is always a cardiac arrest requiring immediate defibrillation, high-quality CPR, and ACLS protocols. Every minute without defibrillation significantly reduces survival.

Asystole appears as a nearly flat line—no discernible electrical activity. It is a non-shockable rhythm. Confirm in two leads before declaring asystole. Treatment is CPR and epinephrine; search for and treat reversible causes (the Hs and Ts).

Pulseless Electrical Activity (PEA) is organized electrical activity on the monitor with no detectable pulse. Any organized rhythm except V-Fib and pulseless V-tach can present as PEA. Treatment is CPR, epinephrine, and aggressive identification of reversible causes (hypovolemia, tension pneumothorax, cardiac tamponade, hypoxia, hypothermia, toxins, thrombosis, and others).

Safety and Scope of Practice

Always assess the patient, not just the monitor. A rhythm that looks alarming may be artifact; a patient in PEA looks normal on the monitor but has no pulse. Check for a pulse with any rhythm change. Defibrillation is indicated for pulseless V-tach and V-Fib—synchronized cardioversion is used for unstable patients with a perfusing tachyarrhythmia, not for pulseless rhythms. Paramedics perform defibrillation and pacing per local protocols; EMTs operate AEDs and assist. Know your scope and act within it.