Blood volume, colloid osmotic pressure and F-cell ratio in children with the nephrotic syndrome

Abstract

The pathogenesis of edema in the nephrotic syndrome (NS) is classically explained by a decrease in blood volume secondary to low plasma colloid osmotic pressure () [1], resulting in stimulation of renal sodium reabsorption via neural [2] and endocrine [3] pathways. However, we have previously measured blood volume in a large group of adult nephrotic subjects and found values predominantly within the normal range [4]. Compensatory mechanisms such as a decrease in interstitial appear to play a major role in the defense of circulating blood volume in the NS [5]. Children with acute relapse of the NS often show symptoms such as abdominal pain, oliguria, cold extremities, anorexia and diarrhea that are rapidly ameliorated by infusing albumin, and hence are attributed to hypovolemia [6]. On the other hand, such albumin infusions are frequently complicated by hypertension, congestive heart failure and pulmonary edema [7]. Measurements of blood volume in nephrotic children are sparse. Both low and high volumes [8—10] have been suggested in preliminary reports. However, blood volume was usually computed from albumin distribution volume and hematocrit, which is relatively insensitive because the relation between whole-body hematocrit and largevessel hematocrit, the so-called F-cell ratio [11], is not constant between individuals. The F-cell ratio is sensitive to changes in distribution of the circulation between the microvasculature (where the hematocrit is low due to the Fahreus-Lindqvist effect) and the large vessels. Considering the large spontaneous variation found in blood volume measured by plasma volume and hematocrit in healthy adults [12] and children [131, it is questionable whether a decreased blood volume can actually be measured with this approach in nephrotic children. Simultaneous measurements of plasma volume and red cell volume, providing more reliable assessment of whole blood volume, are not available in normal children. Recently we found that children with or without symptoms of hypovolemia due to relapse of minimal change nephrosis had blood volumes that were not different from children in remission [14]. However, it is not certain whether the latter group have blood volumes that fall within the normal range, and can serve as a reference group. The