DIMINISHED VENOUS VASCULAR CAPACITANCE IN PATIENTS WITH UNIVENTRICULAR HEARTS AFTER THE FONTAN OPERATION

Patients who have undergone Fontan's operation are known to have impai red cardiac output response to dynamic exercise, This may be due to ei ther poor cardiac function or a limited ability to mobilize blood from capacitance vessels due to increased resting venous tone, We tested t he latter hypothesis by determining venous vascular capacitance-at res t and during orthostatic stress produced by lower body negative pressu re (LBNP) in 6 subjects who had undergone the Fontan operation and 6 h ealthy age-, sex-, height-, and weight-matched controls, Resting blood volume was similar for Fontan and control subjects (79 +/- 6 vs 70 +/ - 3 ml/kg body weight, respectively), while central venous pressure (C VP) was elevated in Fontan subjects (18.4 +/- 1.0 vs 3.5 +/- 0.9 mm Hg , p <0.05). Forearm venous capacitance at a distending pressure of 40 mm Hg was less in Fontan subjects than in controls (2.6 +/- 0.1 vs 3.9 +/- 0.5 ml/100 ml), while resting plasma norepinephrine level was ele vated in Fontan subjects (255 +/- 28 vs 144 +/- 9 pg/ml, p <0.05). The increase in calf volume (1.6 +/- 0.2 vs 2.3 +/- 0.2 ml) and decrease in CVP (-5,0 +/- 0.5 vs -6.7 +/- 1.1 mm Hg) during -30 mm Hg LBNP were smaller for Fontan than control subjects (p <0.05). Reduced forearm v enous capacitance and diminished pooling of blood into capacitance ves sels of the leg during orthostatic stress indicated higher venous tone in Fontan than control subjects, We propose that increased resting ve nous tone in Fontan subjects may limit their ability to mobilize blood from capacitance vessels during exercise and may contribute to impair ed cardiac output response.