RATIONALE AND OBJECTIVES: The aim of this study was to evaluate the effect of mean heart rate (HR) and HR variability on image quality in low-dose computed tomographic coronary angiography (CTCA) using prospective electrocardiographic (ECG) triggering. MATERIALS AND METHODS: One hundred thirty-six consecutive patients were scheduled for low-dose CTCA using prospective ECG triggering. The image quality of all coronary segments was rated on a 5-point scale by two independent readers (scores of 1-3 were considered diagnostic, and scores of 4 and 5 were considered nondiagnostic). Intravenous beta blockers were administered targeting HR < 65 beats/min before scanning, but not if HR increased during scanning. RESULTS: After the exclusion of seven patients because of arrhythmia (n = 4) or mean HRs > 65 beats/min despite using beta blockers (n = 3), 129 patients underwent computed tomographic scanning. The estimated mean effective radiation dose was 2.2 +/- 0.7 mSv (range, 1.1-3.5). The mean HR during scanning was 58.4 +/- 6.6 beats/min (range, 44.2-80.1), with a variability of 1.6 +/- 1.0 beats/min (range, 0.2-5.3). Mean HR (r = 0.49, P < .001) but not mean HR variability (r = 0.14) was related to image quality. Nondiagnostic image quality on CTCA was found in 5% of the coronary segments in 21 of 129 patients. However, on receiver-operating characteristic analysis, a cutoff HR of 62 beats/min was determined, below which nondiagnostic segments were significantly less frequent (2% vs 14%, P < .001). CONCLUSION: Prospective triggering allows low-dose CTCA but requires a low HR. Because a low HR offers a prolonged diastole, widening the optimal phase for scanning, HR variability seems to have a negligible impact on image quality.