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We thank Drs Yoneyama and Akashi for their important reference to the confounding influence of respiration on biventricular volume assessment by real-time cardiac magnetic resonance imaging. As we noted in the Discussion, changes in biventricular volumes "may be caused by physiological differences in preload attributable to the fall in intrathoracic pressures during inspiration and cardiac translation through the imaging plane."1 Real-time cardiac magnetic resonance imaging provides a novel means of expanding on the eloquent work of Natarajan et al 2 on assessing the influence of the respiratory pump on cardiac filling and function. This interaction between breathing and cardiac function has received limited attention, but now we have an excellent noninvasive tool for very accurately detailing changes in both right and left ventricular volumes with various respiratory maneuvers. We hope to provide novel data in the peer-reviewed literature soon that will detail the importance of this respiratory pump in enhancing cardiac filling. We slightly disagree with the assertion that breath-hold imaging should be considered a gold standard. Breath-holding is a nonphysiological surrogate that has previously been necessary to enable gating of cardiac magnetic resonance images. Real-time imaging enables the investigator to apply the accuracy of cardiac magnetic resonance to real physiological settings. We can neither ignore the fact that our patients breathe nor ignore the fact that most of our patients develop symptoms with exertion. This new methodology finally enables us to accurately measure cardiac function when it really matters. DisclosuresNone. References

Background-Accurate measures are critical when attempting to distinguish normal from pathological changes in cardiac function during exercise, yet imaging modalities have seldom been assessed against invasive exercise standards. We sought to validate a novel method of biventricular volume quantification by cardiac MRI (CMR) during maximal exercise. Methods and Results-CMR was performed on 34 subjects during exercise and free-breathing with the use of an ungated real-time (RT-ungated) CMR sequence. ECG and respiratory movements were retrospectively synchronized, enabling compensation for cardiac cycle and respiratory phase. Feasibility of RT-ungated imaging was compared with standard exercise CMR imaging with ECG gating (gated); accuracy of RT-ungated CMR was assessed against an invasive standard (direct Fick); and reproducibility was determined after a second bout of maximal exercise. Ventricular volumes were analyzed more frequently during high-intensity exercise with RT-ungated compared with gated CMR (100% versus 47%; P<0.0001) and with better interobserver variability for RT-ungated (coefficient of variation=1.9% and 2.0% for left and right ventricular stroke volumes, respectively) than gated (coefficient of variation=15.2% and 13.6%; P<0.01). Cardiac output determined by RT-ungated CMR proved accurate against the direct Fick method with excellent agreement (intraclass correlation coefficient, R=0.96), which was highly reproducible during a second bout of maximal exercise (R=0.98). Conclusions-When RT-ungated CMR is combined with post hoc analysis incorporating compensation for respiratory motion, highly reproducible and accurate biventricular volumes can be measured during maximal exercise. (Circ Cardiovasc Imaging. 2013;6:329-338.)

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