Changes in biventricular volumes during respiration: assessment of the respiratory pump using real-time cardiac magnetic resonance imaging during exercise

Abstract

Background: Real-time cardiac magnetic resonance (CMR) imaging has emerged as an accurate and reproducible tool for biventricular volume assessment during free-breathing and whilst exercising. The aim of this study was to examine the degree to which the respiratory pump influenced cardiac filling and performance at rest and during exercise. Methods: Nine healthy subjects (8 male, 1 female) underwent CMR during free-breathing using an ungated real-time CMR sequence. ECG and respiratory movements were retrospectively synchronized enabling compensation for cardiac cycle and respiratory phase and cardiac volumes were determined for the left and right ventricles (LV and RV) from stacks of bi-plane cine images using customized software. End-diastolic and end-systolic volumes (EDV and ESV) as well as diastolic and systolic eccentricity indexes were measured for the LV and RV during peak inspiration and peak expiration, both at rest (heart rate 58±10 bpm) and whilst cycling at low (110±17 bpm), moderate (138±18 bpm) and strenuous (153±20 bpm) workload intensities. Results: As compared with expiration, there was an increase in RV volumes during inspiration (RVEDV +11%, RVESV +12%, RVSV +10%, p<0.05) In contrast, LV EDV, LV ESV and LV SV reduced during inspiration (-6%, -6%, -6%, p<0.05). LVEF and RVEF did not change with respiration, at rest or during exercise. During inspiration, RV SV tended to be larger than LV SV (p=0.064), whilst LV SV was significantly larger than RV SV during expiration (p<0.0001). Mean SV, defined as the average of peak-inspiratory and peak expiratory SV, was slightly greater (5%, p=0.047) for the LV than for the RV, possibly reflecting the expected physiological excess due to the bronchial and thebesian circulation. The interventricular septum pushed toward the LV during inspiration as evidenced by a greater eccentricity index during inspiration than during expiration, which was greatest in diastole (+ 16%, p<0.0001) but also evident during systole (+ 6%, p=0.019). Conclusions: Ventricular volumes oscillate with respiratory phase such that RV volumes and LV volumes are maximal during inspiration and expiration, respectively. Thus, instantaneous measures of stroke volume should not be expected to be equal. Real-time CMR provides a novel means of quantifying the degree to which pulmonary disorders may impact on cardiac performance.