3.3 Dysrhythmia Interpretation & Electrical Therapies

Dysrhythmia Interpretation & Electrical Therapies

Rhythm interpretation and its associated electrical and pharmacologic therapies are among the highest-yield skills on the PCCN exam. A systematic approach — rate, rhythm regularity, P waves, PR interval, and QRS width — lets you classify almost any strip and choose the correct intervention.

QTc & Lead Selection

Before interpreting rhythm details, confirm the monitoring lead is appropriate: lead II is preferred for rhythm monitoring because it typically shows clear P waves, while a modified chest lead (MCL1) is favored when distinguishing right- from left-bundle morphology or differentiating aberrantly conducted SVT from VT. The QTc (rate-corrected QT interval, most often calculated with Bazett's formula) should be checked routinely in any patient on antiarrhythmics, antipsychotics, or certain antibiotics; a QTc greater than 500 ms (or an increase of more than 60 ms from baseline) signals significant risk for torsades de pointes and should prompt medication review and electrolyte correction.

Atrial Dysrhythmias

Atrial fibrillation shows an irregularly irregular rhythm with no discernible, organized P waves; a chaotic fibrillatory baseline replaces them. Because the atria quiver rather than contract, blood stasis in the left atrial appendage raises stroke risk, assessed with the CHA2DS2-VASc score and managed with anticoagulation. Acute management balances rate control (diltiazem, beta-blockers, or digoxin) against rhythm control (amiodarone or synchronized cardioversion), guided by hemodynamic stability and duration of the arrhythmia.

Atrial flutter produces a distinctive sawtooth pattern of flutter waves at an atrial rate of roughly 250–350 beats/min, often with a regular ventricular response at a fixed conduction ratio (commonly 2:1). Management principles mirror atrial fibrillation.

Supraventricular tachycardia (SVT) is a narrow-complex rhythm with an abrupt onset, typically 150–250 beats/min, with P waves often buried in the preceding T wave. First-line treatment is a vagal maneuver (Valsalva, carotid sinus massage), followed by adenosine if that fails — 6 mg rapid IV push followed immediately by a rapid saline flush, escalating to 12 mg if needed. Warn patients they may feel a brief sense of impending doom, flushing, or chest pressure, and expect a short period of asystole or bradycardia as adenosine transiently blocks AV nodal conduction.

Ventricular Dysrhythmias

Ventricular tachycardia (VT) is a wide-QRS rhythm (> 120 ms) at a rate above 100 beats/min. Monomorphic VT has a uniform QRS shape; polymorphic VT varies beat to beat. Management depends entirely on the patient's condition:

• Pulseless VT — treat as cardiac arrest with immediate defibrillation (unsynchronized shock) and CPR per ACLS.
• VT with a pulse, unstable (hypotension, altered mental status, chest pain) — synchronized cardioversion.
• VT with a pulse, stable — antiarrhythmic therapy, typically IV amiodarone.

Torsades de pointes is a distinctive polymorphic VT associated with a prolonged QT interval, where the QRS complexes appear to twist around the isoelectric baseline. First-line treatment is IV magnesium sulfate, regardless of the patient's serum magnesium level, along with correcting potassium and discontinuing any QT-prolonging medications. Overdrive pacing is used for refractory or recurrent episodes.

Ventricular fibrillation (VF) shows chaotic, disorganized electrical activity with no identifiable QRS complexes and no cardiac output. It requires immediate defibrillation (unsynchronized), high-quality CPR, and epinephrine per ACLS protocol.

Heart Blocks

Antiarrhythmic Medications & Key Safety Points

• Amiodarone — a multi-channel blocker used for both atrial and ventricular dysrhythmias; monitor for QT prolongation, and with long-term use, thyroid dysfunction, pulmonary fibrosis, and hepatotoxicity.
• Adenosine — an extremely short half-life (seconds), used only for acute SVT termination; give as a rapid push through the most proximal IV site followed immediately by a flush.
• Diltiazem — a non-dihydropyridine calcium channel blocker used for rate control in atrial fibrillation/flutter; monitor for hypotension and bradycardia.
• Digoxin — has a narrow therapeutic index (0.5–2.0 ng/mL); toxicity presents with nausea, vomiting, visual disturbances (halos, yellow-green vision), and new dysrhythmias, and is worsened by hypokalemia.

Electrical & Device Therapies

Cardioversion delivers a synchronized shock timed to the R wave to avoid firing during the vulnerable T wave, and is used for organized rhythms with a pulse (unstable SVT, atrial fibrillation/flutter, stable but symptomatic VT). Defibrillation delivers an unsynchronized shock for pulseless VT or VF, where there is no organized rhythm to synchronize to — attempting to synchronize to a chaotic VF baseline would simply prevent the shock from ever firing. Pacemakers may be single-chamber (right atrium or ventricle only), dual-chamber (sequential atrial and ventricular pacing), or biventricular (cardiac resynchronization therapy, CRT) for patients with heart failure and ventricular dyssynchrony. When assessing a paced rhythm, the nurse evaluates for capture (a QRS or P wave following each pacer spike) and sensing (the device appropriately detecting the patient's intrinsic activity and withholding an unnecessary spike); failure of either signals a device or lead problem requiring prompt evaluation. Catheter ablation offers a curative option for recurrent SVT, atrial flutter, and select atrial fibrillation cases by destroying the abnormal conduction pathway, typically allowing patients to discontinue long-term antiarrhythmic therapy afterward.