Electrocardiography (ECG) "altered waves" sometimes occur during data acquisition when computed tomography coronary angiography (CTCA) is performed with the prospectively ECG-triggered high-pitch (Flash spiral) mode using a second-generation dual-source CT. The aim of this study was to assess the effect of the ECG altered waves on image quality. Seventy-three consecutive patients with stable sinus rhythm ≤ 65 beats per minute were retrospectively enrolled in this study. CTCA was performed using the Flash spiral mode in which the data acquisition was prospectively triggered at 60 % of the R-R interval and completed within one cardiac cycle. The ECG waves before and during data acquisition were analyzed for grouping purposes. Image quality was evaluated using a four-point scale (1 = best, 4 = unevaluatable). Thirty patients (group 1) were found to have ECG altered waves during data acquisition, while 43 patients (group 2) had ECG "stable waves." The altered waves were seen as the baseline drifting; the broad, erected, or inverted P wave or QRS complexes; and a new wave. However, the length of the R-R interval did not change during the data acquisition. There were no significant differences in image quality scores between the two groups on the per-patient (2 ± 0.87 vs. 2.2 ± 0.74, P = 0.273) or per-segment (1.27 ± 0.54 vs. 1.32 ± 0.55, P = 0.577) basis. There were no significant differences in coronary evaluatability as well (per-patient; 93.3 vs. 95.3 %, P = 0.352; per-segment; 99.4 vs. 99.6 %, P = 1.0). CTCA image quality is not affected by ECG altered waves during data acquisition using the Flash spiral mode in low and stable heart rate patients. Thus, the ECG altered waves are considered artifacts.