Adult Tachyarrhythmia

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

Adult tachyarrhythmias are driven by two principal pathophysiological mechanisms: enhanced automaticity and reentry circuits. Enhanced automaticity occurs when myocardial cells spontaneously depolarize above their normal electrical threshold, often due to heightened sympathetic tone, ischemia, or toxic-metabolic stressors. Reentry, the mechanism for most sustained tachydysrhythmias, involves an initial depolarizing impulse traveling a closed-loop conduction path. This requires a slower conduction pathway, prolonged refractoriness in one limb of the loop, and retrograde excitation that returns the impulse to its point of origin.

At the mechanical level, a rapid ventricular rate (typically $\ge$150 beats/min) drastically shortens diastolic filling time. This loss of filling time critically reduces left ventricular end-diastolic volume (preload), which plummets stroke volume and cardiac output. As the rate exceeds 220–240 beats/min, the patient experiences profound systemic hypoperfusion, end-organ ischemia, and an escalating risk of deterioration into a malignant ventricular dysrhythmia or cardiac arrest.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Assess the airway, breathing, and circulation (ABCs). Place the patient on continuous cardiac monitoring to identify the rhythm, monitor blood pressure and oximetry, establish IV/IO access, and obtain a 12-lead ECG. Provide oxygen if the patient is hypoxemic.
• Determine Stability: You must immediately determine if the tachyarrhythmia is causing cardiopulmonary compromise. Instability is strictly defined by the presence of: hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure.
• The Unstable Pathway:
• Perform immediate Synchronized Cardioversion. Sedate the patient whenever feasible.
• Critical Update: For synchronized cardioversion of Atrial Fibrillation or Atrial Flutter, utilize a higher initial energy setting of $\ge$200 Joules for the first shock to maximize first-shock success .
• The Stable Pathway (Narrow QRS <0.12 seconds):
• Attempt vagal maneuvers if the rhythm is regular.
• Administer Adenosine if regular: 6 mg rapid IV push followed by a normal saline flush. If required, give a second dose of 12 mg.
• If ineffective or irregular, utilize a $\beta$-blocker or calcium channel blocker (e.g., Diltiazem 15–20 mg IV bolus) for rate control.
• The Stable Pathway (Wide QRS $\ge$0.12 seconds):
• Consider Adenosine only if the rhythm is regular and monomorphic.
• Initiate antiarrhythmic infusions: Procainamide (20–50 mg/min until arrhythmia is suppressed, hypotension ensues, QRS duration increases >50%, or max dose of 17 mg/kg is given. Avoid if prolonged QT or CHF) OR Amiodarone (150 mg IV over 10 minutes, followed by 1 mg/min maintenance).

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• "Can't-Miss" Differential Diagnoses:
• Compensatory Sinus Tachycardia: Do not block a tachycardia driven by an underlying physiological crisis such as severe hypovolemia, hemorrhagic shock, sepsis, tension pneumothorax, or pulmonary embolism.
• Pre-excited Atrial Fibrillation (AFib with WPW): An irregularly irregular, wide-complex tachycardia that can degenerate into ventricular fibrillation if AV-nodal blockers are administered.
• Electrolyte/Toxicologic Tachycardias: Severe hyperkalemia or sodium-channel blocker toxicity can present with dramatically widened QRS complexes mimicking ventricular tachycardia.
• Prioritized Diagnostic Workup:
• 12-Lead ECG: The gold standard to determine rate, regularity, and QRS duration (narrow vs. wide).
• Laboratory Panel: Obtain a basic metabolic panel to screen for electrolyte abnormalities (especially potassium and magnesium), and cardiac biomarkers (troponin) to evaluate for demand ischemia or acute coronary syndrome. Check arterial/venous blood gases to assess for acidemia.
• Imaging: Obtain a portable chest radiograph to evaluate for cardiac size, pulmonary edema, and underlying cardiothoracic pathology.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• 12-Lead ECG Checklist:
• Wide vs. Narrow: Assess QRS duration. A QRS $\ge$0.12 seconds is defined as wide.
• VT vs. SVT with Aberrancy: In a wide complex tachycardia, actively look for fusion beats or AV dissociation. The presence of these findings essentially rules out SVT and confirms Ventricular Tachycardia.
• Wolff-Parkinson-White (WPW): Look for a short PR interval combined with the slurring of the R-wave upstroke (the delta wave).
• Brugada Syndrome: Look for a right bundle branch block (RBBB) pattern with ST-segment elevation in leads V1-V2.
• Point-of-Care Ultrasound (POCUS): Perform a bedside echocardiogram to evaluate left ventricular contractility (hyperdynamic vs. hypokinetic), assess for pericardial effusions/tamponade, and examine for right ventricular dilation (suggestive of PE).

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• AHA ACLS Tachycardia Algorithm: The critical decision node in the ACLS guideline strictly depends on the presence of cardiopulmonary compromise. If a patient with a heart rate typically $\ge$150/min exhibits hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure, they are categorized as Unstable and mandate immediate synchronized cardioversion.
• QRS Categorization: Risk stratification and pharmacological choices further divide based on QRS duration. A QRS $\ge$0.12 seconds is wide and carries a higher risk of being ventricular in origin, mandating cautious use of antiarrhythmics.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Cognitive Trap (Treating the Number): Administering rate-controlling agents to a patient with a severe compensatory sinus tachycardia (e.g., treating a heart rate of 140 bpm in a septic or hemorrhagic patient with a beta-blocker). Correction: You must treat the underlying cause (fluids, antibiotics, blood) rather than the heart rate.
• Cognitive Trap (Combining Antiarrhythmics): Administering both Amiodarone and Procainamide for a wide complex tachycardia. Correction: Do not give both. Both medications prolong the QRS and QTc intervals; stacking them severely increases the risk of polymorphic VT or cardiovascular collapse.
• Critical Action: You must recognize that a wide-complex, irregularly irregular tachycardia is highly suspicious for Atrial Fibrillation with WPW. Administering AV-nodal blocking agents (Adenosine, Beta-blockers, Calcium channel blockers) will force conduction down the accessory pathway and can precipitate fatal ventricular fibrillation.
• Critical Action: Do not delay synchronized cardioversion for an unstable patient to obtain a 12-lead ECG or trial medications.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzwords: "Fusion beats" or "AV dissociation" (Ventricular Tachycardia), "Delta wave" (WPW), "Irregularly irregular rhythm after binge drinking" (Holiday Heart Syndrome / AFib).
• Classic Distractor: A question describes a 65-year-old patient with a wide-complex tachycardia at 160 bpm who is hypotensive (BP 70/40 mm Hg) and confused. The distractor option will suggest "Administer Amiodarone 150 mg IV over 10 minutes." Explanation: Antiarrhythmic infusions are reserved for stable patients. Hypotension and altered mental status define this patient as unstable, making immediate synchronized cardioversion the only correct answer.
• Classic Distractor: A patient presents with a regular, narrow-complex tachycardia at 180 bpm. They are hemodynamically stable. The distractor offers "Synchronized cardioversion." Explanation: For a stable, regular, narrow-complex SVT, the first-line interventions are vagal maneuvers followed by rapid IV Adenosine (6 mg).

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Initial Approach: "This patient is presenting with a tachyarrhythmia. I will immediately assess their airway, breathing, and circulation, apply supplemental oxygen if they are hypoxemic, place them on continuous cardiac monitoring, establish IV access, and obtain a stat 12-lead ECG to evaluate the QRS duration."
• Assessing Stability: "I am explicitly assessing for signs of cardiopulmonary compromise—specifically evaluating for hypotension, altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure. Because the patient meets instability criteria, I am ordering immediate synchronized cardioversion, utilizing procedural sedation if time permits."
• Stable Wide-Complex Articulation: "The patient is hemodynamically stable, but the ECG shows a wide-complex tachycardia. I will treat this as ventricular tachycardia. I am ordering an IV infusion of Procainamide at 20 to 50 mg/min. I will instruct my team to stop the infusion immediately if the arrhythmia is suppressed, if the patient becomes hypotensive, or if the QRS duration increases by more than 50%."