Acute Lowering of Plasma Oncotic Pressure Increases Filtration Fraction and Sodium Excretion in Conscious Sheep

Abstract

We examined the effect of acutely lowering the colloid osmotic pressure by removing plasma (36.2 ± 3.1 ml/kg) and replacing it with Hartmann’s solution (93.0 ± 8.2 ml/kg) in 6 conscious merino sheep. The colloid osmotic pressure was reduced significantly (p < 0.05) from 20.3 ± 0.9 to 8.5 ± 2.5 mm Hg 0 h after plasmapheresis and to 15.2 ± 0.8 mm Hg 20 h after treatment. The filtration fraction increased from 0.16 ± 0.02 to 0.27 ± 0.02 at 0 h (p < 0.05 vs. control) and to 0.20 ± 0.02 at 20 h after treatment (p < 0.05 vs. control). At 0 h after plasmapheresis there was an increase in both sodium excretion from 187 ± 69 to 459 ± 82 µmol · min^-1(p < 0.05) and in fractional sodium excretion from 1.6 ± 0.4 to 4.4 ± 1.0% (p < 0.05). The results are consistent with the hypothesis that acute alteration of Starling forces produce clinically significant effects on both glomerular filtration and tubular reabsorption.

Understanding urine output in critically ill patients

“…Therefore, the oncotic pressure becomes the limiting factor of glomerular filtration [26]. In this line, the natriuresis and diuresis response to crystalloids infusion are in part mediated by the changes of intraglomerular oncotic forces following plasma protein dilution [27,28], an effect that is not observed after hyperoncotic colloids administration. When hydraulic permeability is altered (decreased of glomerular surface area as in chronic kidney disease) glomerular hydraulic capillary pressure becomes the major determinant of the glomerular filtration rate (Figure 1) [29].…”

Prevention of Acute Renal Failure - Fluid Repletion and Colloids

“…Mechanism of colloidinduced renal impairment include increased intraglomerular oncotic pressure with decreased glomerular filtration (15,16), induction of tubular backflow (17)(18)(19), and induction of intra-cellular colloid-conglomerates (20,21). Alterations in glomerular oncotic pressure probably plays a key role, since acute lowering of plasma oncotic pressure increases filtration fraction and sodium excretion (23). When colloids are compared to each other, pharmacokinetic effects, especially the intra-vascular half-life, and adverse effects are of main interest.…”