OBJECTIVE To compare QT dispersion in patients with impaired left ventricular systolic function and in matched control patients with normal left ventricular systolic function.

DESIGN A retrospective, case-control study with controls matched 4:1 for age, sex, previous myocardial infarction, and diuretic and β blocker treatment.

SETTING A regional cardiology centre and a university teaching hospital.

PATIENTS 25 patients with impaired left ventricular systolic function and 100 patients with normal left ventricular systolic function.

MAIN OUTCOME MEASURES QT and QTc dispersion measured by three methods: the difference between maximum and minimum QT and QTc intervals, the standard deviation of QT and QTc intervals, and the “lead adjusted” QT and QTc dispersion.

RESULTS All measures of QT/QTc dispersion were closely interrelated (r values 0.86 to 0.99; all p < 0.001). All measures of QT and QTc dispersion were significantly increased in the patients with impaired left ventricular systolic functionv controls (p < 0.001): 71.9 (6.5) (mean (SEM)) v 46.9 (1.7) ms for QT dispersion, and 83.6 (7.6) v 54.3 (2.1) ms⁻¹−2 for QTc dispersion. All six dispersion parameters were reduced in patients taking β blockers (p < 0.05), regardless of whether left ventricular function was normal or impaired—by 9.4 (4.6) ms for QT dispersion (p < 0.05) and by 13.8 (6.5) ms⁻¹−2 for QTc dispersion (p = 0.01).

CONCLUSIONS QT and QTc dispersion are increased in patients with systolic heart failure in comparison with matched controls, regardless of the method of measurement and independently of possible confounding factors. β Blockers are associated with a reduction in both QT and QTc dispersion, raising the possibility that a reduction in dispersion of ventricular repolarisation may be an important antiarrhythmic mechanism of β blockade.