Bazett's formula:
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. Bazett's formula corrects or normalizes the measured QT interval for a heart rate of 60 BPM. Thus, the QT is measured at the given heart rate, and the QTc estimates what the QT interval would be if the heart rate were 60. Bazett's formula works reasonably well at "normal" heart rates, but is less accurate when the heartrate is slow or fast.
The four major causes of a prolonged QT interval:
Electrolyte abnormalities:
Hypokalemia and hypocalcemia
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
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.
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:
Hypercalcemia
Digitalis
Thyrotoxicosis
Increased sympathetic tone
The interval between the start of the QRS complex and end of the T wave is an useful index of the ventricular action potential duration (APD). However, APD varies directly with heart rate (an increase in heart rate shortens APD and vice versa). Thus, QT interval also varies with heart rate and must be corrected for this variable. One common formula to obtain a QT interval correct for heart rate (QTC) is Bazett's formula: QTC interval = Uncorrected QT / square root of RR interval (Normal range: 0.3 - 0.45 sec)
Bazett's formula works reasonably well at "normal" heart rates, but is less accurate when the heartrate is slow or fast.
Several formulas have been proposed to adjust the QT-interval for the heart rate.
The most commonly used QT correction (QTc) formula is the one postulated by Bazett in 1920 (QTc=QT/RR1/2).
Other common formulas include
1)the nomogram method (QTNc=QT+correcting number),
2) the Friderica formula (QTFc=QT/RR1/3)
3)and the linear regression equitation (QTLc=QT+0.154x[1-RR]).
Recently, the adequacy of Bazett's formula has been questioned because it seems the QTc overcorrects the measured QT-interval at fast heart rate and undercorrects it at low heart rates.
In a study, Karjalainen et al. measured QT-intervals in 324 rest ECGs of healthy young men. The sample was weighted for low and high heart rates. A curve relating QT-intervals and heart rates from 40 to 120 beats per minute was constructed. The QT-interval at 60 beats per minute was used as the reference value, and an adjusting nomogram for different heart rates was created. The reliabilities of the nomogram and three earlier QT correction equitations were tested in the study group and in 396 middle-aged men.
The nomogram method adjusted the QT-interval most accurately over the whole range of heart rates on the basis of smallest mean-squared residual values between measured and predicted QT-intervals.
The Friderica formula gave the best correction at low, but failed at high heart rates.
The linear regression equitation (Framingham Study) was reliable at normal, but failed at low and high heart rates.
The Bazett formula performed poorest at all heart rates.
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