Drug-Induced QT Prolongation

QT Interval & TdP - Electric Heartbeat Hiccups

• QT Interval: ECG: Start of QRS to end of T wave. Represents ventricular repolarization; duration varies with heart rate.
• Corrected QT (QTc): Adjusts QT for heart rate.
  • Bazett's Formula: $QTc = QT / \sqrt{RR}$.
  • Prolonged QTc thresholds (risk of TdP):
    • Men: > 450ms
    • Women: > 470ms
  • Very High Risk for TdP: QTc > 500ms.
• Torsades de Pointes (TdP):
  • Life-threatening polymorphic ventricular tachycardia.
  • Triggered by QT prolongation.
  • ECG: 'Twisting of points' appearance.

  ⭐ Torsades de Pointes (TdP) is a polymorphic ventricular tachycardia specifically associated with QT prolongation, appearing as a 'twisting of points' on ECG.

Causative Drugs - Pharma's QT Stretchers

Many drugs prolong QT, risking Torsades de Pointes (TdP). Key classes:

• Antiarrhythmics (Class IA, III)
• AntiBiotics (Macrolides, Quinolones)
• AntiCychotics (e.g., Haloperidol)
• AntiDepressants (TCAs, SSRIs)
• AntiEmetics (e.g., Ondansetron)

⭐ Many non-antiarrhythmic drugs, such as macrolide antibiotics (e.g., erythromycin), fluoroquinolones, antipsychotics (e.g., haloperidol), and ondansetron, are notorious for causing QT prolongation.

Risk Factors - Dodging Danger Zones

• Patient-Specific Vulnerabilities:
  • Female gender, advanced age (>65 yrs)
  • Genetic predisposition (e.g., congenital LQTS variants)
  • Underlying structural heart disease (e.g., MI, HF, LVH)
  • Baseline bradycardia or AV block
• Modifiable/External Factors:
  • Polypharmacy: multiple QT-prolonging agents
  • Drug interactions (e.g., CYP3A4 inhibitors)
  • Rapid IV administration of offending drugs
  • Renal or hepatic impairment (↓ drug clearance)

  ⭐ Electrolyte abnormalities, particularly hypokalemia and hypomagnesemia, are major predisposing factors that significantly potentiate drug-induced QT prolongation and risk of TdP.

Clinical Picture & Management - Taming Torsades Twists

• Presentation: Often asymptomatic. Palpitations, dizziness, syncope; can lead to Sudden Cardiac Death (SCD).
• ECG: QTc >450ms (males), >470ms (females); >500ms = high risk. Torsades de Pointes (TdP): characteristic polymorphic VT, "twisting of points".
• Management Algorithm:

⭐ Intravenous Magnesium Sulfate is the first-line treatment for Torsades de Pointes, even in patients with normal serum magnesium levels.

• Key Interventions: Discontinue culprit agent. Aggressively replete K⁺ (target 4.5-5.0 mEq/L) and Mg²⁺. Monitor ECG continuously during acute phase.

• QT prolongation indicates delayed ventricular repolarization, risking Torsades de Pointes (TdP).
• Critical risk factors: female sex, hypokalemia, hypomagnesemia, bradycardia, structural heart disease.
• Common drug classes: Antiarrhythmics (IA, III), antipsychotics, macrolides, fluoroquinolones, azole antifungals, ondansetron.
• Main mechanism: Blockade of the IKr (hERG) potassium channel.
• TdP treatment: Discontinue drug, correct electrolytes (K+, Mg2+), IV magnesium sulfate.
• QTc monitoring is crucial; Bazett's formula (QTc = QT/√RR) is often used.