Blood flow conditions in the proximal pulmonary arteries and vena cavae: Healthy children during upright cycling exercise

Diagnostic testing in patients with congenital heart disease is usually performed supine and at rest, conditions not representative of their typical hemodynamics. Upright exercise measurements of blood flow may prove valuable in the assessment of these patients, but data in normal subjects are first required. With the use of a 0.5-T open magnet, a magnetic resonance-compatible exercise cycle, and cine phase-contrast techniques, time-dependent blood flow velocities were measured in the right (RPA), left (LPA), and main (MPA) pulmonary arteries and superior (SVC) and inferior (IVC) vena cavae of 10 healthy 10- to 14-yr-old subjects. Measurements were made at seated rest and during upright cycling exercise (150% resting heart rate). Mean blood flow (1/min) and reverse flow index were computed from the velocity data. With exercise, RPA and LPA mean flow increased 2.0 ± 0.5 to 3.7 ± 0.7 (P < 0.05) and 1.6 ± 0.4 to 2.9 ± 0.8 (P < 0.05), respectively. Pulmonary reverse flow index (rest vs. exercise) decreased with exercise as follows: MPA: 0.014 ± 0.012 vs. 0.006 ± 0.006 [P = not significant (NS)], RPA: 0.005 ± 0.004 vs. 0.000 ± 0.000 (P < 0.05), and LPA: 0.041 ± 0.019 vs. 0.014 ± 0.016 (P < 0.05). SVC and IVC flow increased from 1.5 ± 0.2 to 1.9 ± 0.6 (P = NS) and 1.6 ± 0.4 to 4.9 ± 1.3 (P < 0.05), respectively. A 56/44% RPA/LPA flow distribution at both rest and during exercise suggests blood flow distribution is dominated by distal pulmonary resistance. Reverse flow in the MPA appears to originate solely from the LPA while the RPA is in relative isolation. During seated rest, the SVC-to-IVC venous return ratio is 50/50%. With light/moderate cycling exercise, IVC flow increases by three-fold, whereas SVC remains essentially constant.