The QT interval is inversely related to the heart rate. A prolonged QT interval conveys adverse outcomes. A Bazett's corrected QT of 0.54 conveys a 1.7-fold increased risk of cardiac events, while a QTc of 0.64 conveys a 2.8-fold increased risk.

Bazett's formula:
Most studies involving QT interval corrections in the literature use Bazett's formula, defined as the observed QT interval divided by the square root of the R-R interval in seconds. A corrected QT interval of > 440 msec is defined as abnormal. However, linear correction formulae provide a better fit of the data than the Bazett's formula. Therefore, Hodges' correction, a linear correction formula, is used in the nomogram below, with data derived from MacFarlane & Veitch-Lawrie.

Use of the QT nomogram (Hodges correction)
The best leads in which to measure the QT interval are either V2 or V3, because the end of the T wave can usually be clearly distinguished from the beginning of the U wave, and these leads also normally contain the largest U waves. If the rate is regular, use the mean rate of the QRS complexes. If the rate is irregular, calculate the rate from the immediately prior R-R cycle, because this cycle determines the subsequent Q-T interval. Use the numbers ;; you have obtained to determine the classification of the QT interval from the nomogram below. A useful number to remember is that at heart rates of 40 bpm, an observed QT interval > 480 msec is abnormal.

The four major causes of a prolonged QT interval:

1. Electrolyte abnormalities:
   Hypokalemia and hypocalcemia
2. Drugs: (also associated with torsades de pointes)
   • Class Ia antiarrhvthmic agents: quinidine, procainamide, disopyramide
   • Class Ic agents: propafenone
   • Class III agents: amiodarone, bretylium, dofetilide, n-acetylprocainamide, sematilide, sotalol
   • Psychotropic agents: tricyclic antidepressants, tetracyclic antidepressants, phenothiazines, haloperidol
   • Antihistamines: astemizole, terfenadine
   • Antibiotics: erythromycin, trimethoprim-sulfamethoxazole
   • Antifungals: ketoconazole, itraconazole
   • Serotonin antagonists: ketanserin, zimeldine
   • Chemotherapeutics: pentamidine, possibly anthracyclines
   • Miscellaneous: bepridil, cisapride, prednisone, prenylamine, probucol, chloral hydrate
   • Toxins and poisons: organophosphate insecticides, anthopleurinn-A, liquid protein diets, some herbs
3. Congenital long Q-T syndromes:
   While congenital long QT syndromes are rare, identification of a patient with this problem may allow for life-saving therapy to be instituted. It should be searched for in any young patient who presents with syncope or presyncope.
4. A miscellaneous group, including patients with:
   • Third-degree and sometimes second-degree A-V block
   • At the cessation of ventricular pacing
   • Left ventricular hypertrophy (usually minor degrees of lengthening)
   • Myocardial infarction (in the evolutionary stages where there are marked repolarization abnormalities)
   • Significant active myocardial ischemia
   • Cerebrovascular accident (subarachnoid hemorrhage)
   • Hypothermia

The four causes of a short QT interval:

1. Hypercalcemia
2. Digitalis
3. Thyrotoxicosis
4. Increased sympathetic tone

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