Measuring QT and QTc: Bazett formula and clinical significance
The QT interval measures the total duration of ventricular depolarization and repolarization — from the beginning of the QRS complex to the end of the T wave. QT is rate-dependent: at faster heart rates, QT shortens; at slower rates, QT lengthens. For this reason, QTc (corrected QT) is used to compare intervals across different heart rates.
Bazett formula: QTc = QT / √RR (where RR is in seconds). This is the most widely used clinical formula, though it overcorrects at very fast and very slow rates. Most clinical ECG machines calculate and report QTc automatically.
Normal QTc values: Normal is typically defined as QTc ≤440 ms in men and ≤460 ms in women (women have inherently longer QTc than men due to hormonal effects on repolarization). Borderline prolonged: 440–500 ms in men, 460–500 ms in women. Significantly prolonged: QTc >500 ms, regardless of sex — this threshold correlates with substantially increased torsades de pointes risk.
Drug-induced QT prolongation: high-risk medications and combinations
Drug-induced QT prolongation is the most preventable cause of torsades in hospitalized patients. High-risk medications include: antiarrhythmics (sotalol, dofetilide, ibutilide, quinidine — class IA and III agents), antibiotics (azithromycin, fluoroquinolones, azole antifungals), antipsychotics (haloperidol — especially IV, droperidol, ziprasidone, quetiapine), antiemetics (ondansetron — particularly IV doses >32 mg, metoclopramide), antidepressants (citalopram at doses >40 mg, escitalopram >20 mg, TCAs), opioids (methadone).
Risk is additive: two QT-prolonging drugs combined carry more risk than either alone. Three or more QT-prolonging drugs, particularly in a patient with baseline QT prolongation, concurrent electrolyte abnormalities, or structural heart disease, represents a high-risk combination requiring mandatory telemetry monitoring and aggressive electrolyte management.
Nursing intervention: Check QTc before administering new QT-prolonging medications. Alert prescriber when adding a second QT-prolonging agent. Ensure potassium is ≥4.0 and magnesium is ≥2.0 before initiating high-risk drugs. Document baseline QTc and monitor trend.