The effect of oral ciprofloxacin at conventional doses on the QT interval in a tertiary outpatient setting

Aims Ciprofloxacin has been demonstrated to prolong QT intervals at higher doses beyond those used in conventional practice. We investigated the effect of ciprofloxacin on the QT interval by examining the difference between mean QT intervals (raw and corrected) at baseline and after theoretical achievement of steady state levels in 88 patients receiving oral ciprofloxacin therapy. The utility of a nomogram to decide the cut point for QT interval prolongation was also examined. Patients and methods A retrospective study examined serial surface 12-lead ECGs in a sample of patents prescribed oral ciprofloxacin by physicians in the Department of Ambulatory Care and Hospital in the Home Service at Liverpool Hospital, a tertiary referral hospital in Sydney, Australia. All patients had a baseline ECG and a follow-up ECG after achievement of theoretical steady state. Results A total of 88 patients were included. No significant difference in either raw QT or corrected QT using Bazett’s formula was observed. Two methods were used to measure the QT interval, one using the median of six observations and one using a single lead. Poor inter-rater reliability for raw measures was observed. A high level of agreement for the cut point for QT prolongation was observed using a nomogram. Conclusion QT and QTc prolongation in a standard 12 lead ECG would not be expected after oral administration of ciprofloxacin at conventional doses. We recommend ECG monitoring to be undertaken only with increased clinical risk.