Peripheral vascular adaptation and orthostatic tolerance in Fontan physiology.
BACKGROUND
The Fontan circulation is critically dependent on elevated venous pressures to sustain effective venous return. We hypothesized that chronically increased systemic venous pressures lead to adaptive changes in regional and peripheral vessels to maintain cardiac output, especially when patients are upright.


RESULTS
Nine post-Fontan procedure patients (aged 13 to 24 years) and 6 age- and sex-matched controls were compared with techniques to measure circulatory responses (peripheral and compartmental blood flow, venous capacity, and microvascular filtration). Parameters studied included strain-gauge plethysmography measures of peripheral circulatory function, regional blood volume distribution by impedance plethysmography, and head-up tilt testing. Important differences between Fontan patients and controls were seen in several vascular compartments: (1) Calf capacitance was lower (median, 3.5 versus 5.5 mL/100 mL tissue; P=0.005), and resting venous pressure was higher (13.0 versus 10.5 mm Hg; P=0.004); (2) higher leg arterial resistance was observed (32.1 versus 22.2; P=0.03); (3) microvascular filtration pressures and threshold for edema were elevated; and (4) with head-up tilt testing, splanchnic flow was not reduced in Fontan patients versus controls (fractional change, +4% versus -32%; P=0.004), and splanchnic arterial resistance did not increase as expected (fractional change, +8% versus +79%; P=0.003).


CONCLUSIONS
Reduced venous compliance and increased filtration thresholds may act as adaptive mechanisms in maintaining venous return in Fontan circulation. Well-compensated Fontan subjects demonstrate superior orthostatic tolerance resulting from decreased compartmental fluid shifts in response to head-up tilt and higher vascular resistance. This results from increased venous stiffness and decreased splanchnic capacitance and may also be an adaptive mechanism to maintain venous return in these patients while standing.
Krishnan
Usha S
US
Department of Pediatric Cardiology, New York Medical College, Valhalla, NY 10595, USA. usha_krishnan@nymc.edu
Taneja
Indu
I
Gewitz
Michael
M
Young
Richard
R
Stewart
Julian
J
D000222:Adaptation, Physiological; D000293:Adolescent; D001775:Blood Circulation; D001808:Blood Vessels / Q000502:physiology*; D016022:Case-Control Studies; D004244:Dizziness; D018729:Fontan Procedure; D006439:Hemodynamics; D006801:Humans; D054971:Orthostatic Intolerance; D010991:Plethysmography; D011187:Posture; D012039:Regional Blood Flow; D055815:Young Adult