Author + information
- Christopher J. Clarke, MD, MSc⁎ (, )
- Matthew J. Gurka, PhD and
- Andrew W. Hoyer, MD
- ↵⁎Department of Cardiology, Children's Hospital at Dartmouth, 100 Hitchcock Way, Manchester, New Hampshire 03110
We thank Drs. Fratz and Stern for their interest in our recent publication (1) and for the opportunity to discuss their comments.
With respect to their first point regarding our definition of a clinically significant difference in right ventricular (RV) stroke volume, we agree that a value of 10 ml/m2 is a relatively large difference given the observed stroke volumes of patients included in the study. However, it is important to keep in mind that the reported biases in RV stroke volumes are mean values calculated from the entire study population with RV stroke volumes that ranged from 17 ml/m2 to 141 ml/m2. Given that smaller biases in RV stroke volumes were generally observed among patients with smaller volumes (Fig. 2 from our paper ), a worst case scenario in which this mean bias is considered only among patients with the smallest RV stroke volumes substantially overestimates a potential difference when translated into a percentage value. This said, we agree that these results expressed as percentages provide a standardized scale to account for the magnitude of differences relative to RV stroke volumes. Therefore, we include the following results. The RV stroke volume mean bias for phase contrast imaging versus axial contours was 0% (95% confidence interval [CI]: −7% to 6%), and for phase contrast imaging versus short-axis contours, it was 2% (95% CI: −4% to 7%). The difference in mean biases was 2% (95% CI: −1% to 5%) (p = 0.202). Additionally, because the initial report includes absolute values of the biases and differences observed, readers can draw their own conclusions about whether these values are clinically meaningful with respect to mean RV volumes and ejection fractions.
With regard to the second comment regarding the phase of each cine image chosen as end-diastole, we agree that this is an important point and one we considered when planning this study. Defining the phase of end-diastole visually is likely to avoid underestimating end-diastolic measurements in some cases. Conversely, this may decrease reproducibility because it increases the number of variables defined by the reader. Therefore, this trade-off must be weighed by practitioners of cardiac magnetic resonance, particularly for patients who require repeated studies. The decision to define end-diastole as the first phase of each cine image is unlikely to affect conclusions regarding accuracy drawn in our report because differences in time between the computed trigger signal and the upstroke of the QRS complex should be consistent for each patient regardless of the imaging plane. Thus, any underestimation in RV stroke volume resulting from this technique should be consistent in the 2 methods for any 1 subject.
- American College of Cardiology Foundation