We performed renal function tests in 18 young patients, 1.8–14.6 years of age, with cyanotic congenital heart disease (CCHD). Glomerular filtration rate was normal (116 ± 4.5 ml/min/1.73 m2), and renal plasma flow was decreased (410 ± 25 ml/min/1.73 m2) with a rise in the filtration fraction (29 ± 1.1%). The suggested pathophysiologic explanation of these findings is that the blood hyperviscosity seen in patients with CCHD causes an overall increase in renal vascular resistance with a rise in intraglomerular blood pressure. Despite a sluggish flow of blood in the glomerular capillary bed, the effective filtration pressure was adjusted to conserve the glomerular filtration rate. In addition to these renal hemodynamic parameters, we also studied renal acidification and tubular sodium and water handling during a forced water diuresis. Our data indicate that children with CCHD have a mild to moderate normal ion gap metabolic acidosis due to a low proximal tubular threshold for bicarbonate. Proximal tubular sodium and water reabsorption under these conditions were somewhat increased, though not significantly, probably due to intrarenal hydrostatic forces, in particular the rise in the oncotic pressure in the postglomerular capillaries in patients with high hematocrit values. The distal tubular functions such as sodium handling and acidification were not affected.
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