4.4 Ventricular Dysrhythmias (PVCs, Ventricular Tachycardia, Ventricular Fibrillation, Torsades de Pointes)

Ventricular Dysrhythmias

Dysrhythmias originating in the ventricles are the most dangerous and clinically pressing. Because the impulse arises below the bundle of His, it cannot utilize the fast, efficient Purkinje network for initial depolarization. Instead, the electrical wave must travel cell-by-cell through the ventricular muscle. This slow, inefficient conduction produces the hallmark of all ventricular beats: a wide, bizarre QRS complex measuring 0.12 seconds or greater. P waves are generally absent, as the impulse rarely conducts retrogradely into the atria. When ventricular dysrhythmias occur, they can severely compromise cardiac output, rapidly leading to hemodynamic collapse if left unchecked.

Premature Ventricular Complexes (PVCs)

A PVC is an early ectopic beat originating in the ventricles. These are incredibly common and can occur in both healthy individuals and those with structural heart disease. The presence of frequent PVCs, however, often signifies underlying myocardial irritability.

Characteristics

• QRS Width: ≥ 0.12 seconds, usually wide and bizarre in shape compared to the patient's normal QRS complexes.
• T Wave: Usually opposite in direction to the major deflection of the QRS complex (discordant T wave). If the QRS is predominantly positive, the T wave will be negative, and vice versa.
• Pause: Typically followed by a fully compensatory pause (the distance between the normal beat before the PVC and the normal beat after the PVC equals exactly two normal R-R intervals). This happens because the PVC does not reset the SA node, allowing the next sinus beat to fall exactly where it was expected.

Patterns of PVCs

Recognizing the pattern of PVCs helps stratify the patient's risk. The more frequent and complex the PVCs, the higher the risk of progressing to sustained ventricular arrhythmias.

The R-on-T Phenomenon

A critical vulnerability period exists during the relative refractory period of the cardiac cycle (the peak to the end of the T wave). If a PVC falls directly on the T wave of the preceding beat, it can trigger Ventricular Fibrillation. This is known as the R-on-T phenomenon, and recognizing PVCs approaching this zone requires high vigilance.

Ventricular Tachycardia (VT), VF, and Torsades

These rhythms represent the extreme end of ventricular irritability. They are true cardiac emergencies.

Comparison Table: VT vs VF vs Torsades

Ventricular Tachycardia (VT)

Ventricular Tachycardia is defined as a run of three or more consecutive PVCs at a rate greater than 100 bpm.

• Monomorphic VT: All QRS complexes have the same shape and amplitude. It originates from a single ectopic focus or reentry circuit.
• Polymorphic VT: The QRS complexes vary in shape and amplitude from beat to beat.
• Treatment: Pulseless VT is treated exactly like Ventricular Fibrillation (Immediate CPR and Defibrillation). Unstable VT with a pulse requires immediate synchronized cardioversion. Stable VT with a pulse may be treated pharmacologically (e.g., Amiodarone).

Torsades de Pointes

Torsades de Pointes ("twisting of the points") is a specific, highly dangerous form of polymorphic VT uniquely associated with a prolonged QT interval (delayed ventricular repolarization).

• ECG Appearance: The QRS complexes appear to twist in a spiral pattern around the isoelectric baseline, gradually increasing and decreasing in amplitude.
• Treatment: The definitive treatment for Torsades de Pointes is IV Magnesium Sulfate, overriding the standard VT protocols. Identifying QT prolonging drugs is also paramount to prevent recurrence.

Ventricular Fibrillation (VF)

Ventricular Fibrillation is a state of total electrical chaos in the ventricles. Multiple ectopic foci fire rapidly and erratically. The ventricles merely quiver and do not contract.

• Result: There is zero cardiac output. The patient is clinically dead.
• Treatment: Immediate, high-quality CPR and rapid Defibrillation (unsynchronized shock) are the only effective treatments. Time is tissue; survival decreases by 7-10% for every minute without defibrillation.

Idioventricular Rhythm (IVR)

IVR is a ventricular escape rhythm. It is the absolute last resort pacemaker of the heart, kicking in when both the SA and AV nodes fail. It presents with a rate of 20-40 bpm and a wide QRS. It is often an agonal rhythm (a dying heart) requiring immediate pacing; suppressive antiarrhythmics are strictly contraindicated.