Use of phase-contrast magnetic resonance angiography to measure adaptations of aortic and pulmonary artery flow during supine aerobic exercise

Abstract

BackgroundCardiovascular adaptations to aerobic exercise are mediated through interactions of cardiac, hormonal, metabolic, and muscular mechanisms with a dominant role for increase in cardiac output to enhance oxygen delivery. However, increased demands for oxygen uptake and use during exercise depend on adequate adaptations of systemic and pulmonary vasculature. Assessment of exercise response is crucial in various cardiac and pulmonary diseases. Recent advances in MRI techniques allow for direct measurement of aortic and pulmonary blood flow with phase-contrast magnetic resonance angiography (PCMRA). We have studied aortic and pulmonary flow in healthy individuals to assess haemodynamic adaptations to exercise using PCMRA. MethodsNine adult healthy volunteers underwent PCMRA while doing heart-rate-targeted (180% of resting heart rate) aerobic supine leg exercise. Aortic flow was reassessed post exercise after 2 min rest. All flow variables were assessed by retrospective free-breathing pulse-gated PCMRA in the mid ascending aorta and main pulmonary artery (field of view [FOV] 320 mm, FOV phase 75%, base resolution 256, voxel 1·3 × 1·3 × 5·0 mm, TE 2·2 ms, TR 29·9 ms, slice thickness 5 mm, three averages, and 30 reconstructed phases, Venc 150 cm/s at rest, 300 cm/s during exercise). All images were assessed by three individuals trained in MRI flow evaluation and independently checked by an experienced cardiac radiologist. Flow sequence analysis was done with offline software (Argus, Siemens Medical Systems), by manually contouring the vessel of interest so that all flow was accounted for. Pulmonary and aortic blood flow was determined from the flow velocities in individual voxels in the region of interest. FindingsIncrease in heart rate during exercise (from a mean of 69 beats per min [SD 10] at rest to 120 [14] after exercise) resulted in increased cardiac output (mean 6·5 L/min [SD 1·4] to 12·4 [1·8]). All flow variables significantly increased with exercise compared with rest: aorta systolic peak velocity increased from mean 89 cm/s [SD 14] to 122 [34] (p=0·016); pulmonary artery systolic peak velocity 86 cm/s [18] to 140 [48] (p=0·007); aorta systolic peak flow rate 415 mL/s [83] to 550 [135] (p=0·002); and pulmonary artery systolic peak flow rate 410 mL/s [80] to 577 [180] (p=0·006). These variables showed a trend to normalisation at 2 min recovery. InterpretationUse of free-breathing pulse-gated PCMRA to measure aortic and pulmonary blood flow and velocity during exercise is feasible. Moderate aerobic exercise leads to a steep increase in blood flow and flow velocities in aorta and main pulmonary artery facilitating increased oxygen uptake. Exercise PCMRA could help in diagnosis, assessment of treatment response, and follow-up in many patient groups with pulmonary and cardiovascular disease. FundingBristol NIHR Cardiovascular Biomedical Research Unit, NIHR Academic Clinical Lectureship to GEP