The Hemodynamics of Inferior Vena Cava Caliber Changes from Rest to Exercise.

Objective: Venous Return is a major determinant of Cardiac Output and is modulated in a large part by conduit vein hemodynamics. We aimed to study changes in Inferior Vena Cava (IVC) cross-sectional area morphology, which is a major conduit vein, from rest to exercise with the hypothesis that sympathetic reflex mechanisms cause IVC dilation during exercise, thereby preventing an increase in venous resistance. Design: Uncontrolled longitudinal study. Setting: Single urban academic medical center. Subjects: Adult subjects free of any known cardiovascular disease, without a history of abdominal surgery that might interfere with CT scan measurements of IVC size and no contraindications to performing near-maximal levels of exercise. Interventions: We studied 14 healthy subjects (age 31 ± 10 y., BMI: 26 ± 4 kg/m2) at supine rest, passive leg-raising, and supine exercise (50% and 90% maximal effort) and measured blood pressure, heart rate (HR), peripheral and central venous pressures (PVP and CVP), and cardiac output (CO). Venous resistance (VR) calculated as VR=(PVP-CVP)/CO and cross-sectional areas (CSA) of the aorta and IVC were measured at each stage. Measurements and Main Results: Measurement instruments consisted of exercise equipment, hemodynamics, a thoracic bioimpedance monitor, Computed Tomography, Body Mass Index, and blood pressure. CVP increased with leg-raising from baseline 3.2 ± 2.3 to 5.2 ± 2.4 cm H2O (p<0.001). IVC CSA increased from 386 ± 216 mm2 at baseline to 449± 231 mm2 with passive legraising (p<0.001), and to 459 ± 171 mm2 at 50% exercise (p<0.05). VR did not change with leg raising or exercise. BMI showed a negative correlation with IVC CSA at baseline that tended to persist with leg raising and exercise (r=-0.35 to -0.52, p=0.028 to 0.109). Conclusions: The unchanged VR with exercise suggests that IVC dilation stabilizes VR despite increased CO. IVC size negatively correlates with BMI inferring that the IVC caliber is modulated by obesity and/or cardiovascular fitness