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Body Fluids and Salt Metabolism Peruzzo et al. Italian Journal of Pediatrics 2010, 36:78 http://www.ijponline.net/content/36/1/78 ITALIAN JOURNAL OF PEDIATRICS REVIEW Open Access Body fluids and salt metabolism - Part II Mattia Peruzzo1, Gregorio P Milani2, Luca Garzoni1, Laura Longoni3, Giacomo D Simonetti4, Alberto Bettinelli3, Emilio F Fossali2, Mario G Bianchetti1* Abstract There is a high frequency of diarrhea and vomiting in childhood. As a consequence the focus of the present review is to recognize the different body fluid compartments, to clinically assess the degree of dehydration, to know how the equilibrium between extracellular fluid and intracellular fluid is maintained, to calculate the effective blood osmolality and discuss both parenteral fluid maintenance and replacement. Introduction Hyponatremia The first part of this review, published some months ago, Introduction outlined the physiology of the body fluid compartments, Hyoponatremia [4,6] is classified (Figure 1 left and mid- dehydration and extracellular fluid volume depletion [1]. dle panel) according to the extracellular fluid volume The second part will focus the causes underlying dysna- status, as either hypovolemic (= depletional) or normo- tremia and, more importantly, both the parenteral hydra- hypervolemic (= dilutional). Vasopressin is released both tion and the management of dysnatremia. in children with low effective arterial blood volume, by far the most common cause of hyponatremia in Dysnatremia everyday clinical practice, as well as in those with Under normal conditions, blood sodium concentrations normo-hypervolemic hyponatremia [8]. In hypovolemic are maintained within the narrow range of 135-145 hyponatremia vasopressin release is triggered by the low mmol/L despite great variations in water and salt intake. effective arterial blood volume (this condition has been Sodium and its accompanying anions, principally chlor- called by some syndrome of appropriate anti-diuresis). ide and bicarbonate, account for 90% of the extracellular In dilutional hyponatremia [8] the primary defect is effective osmolality. The main determinant of the euvolemic, inappropriate increase in circulating vaso- sodium concentration is the plasma water content, itself pressin levels (this condition is also termed syndrome of determined by water intake (thirst or habit), “insensible” inappropriate anti-diuresis). losses, and urinary dilution. The last of these is under Assessing the cause of hyponatremia may be straight- most circumstances crucial and predominantly deter- forward if an obvious cause is present (for example in mined by vasopressin. In response to this hormone, con- the setting of vomiting or diarrhea) or in the presence centrated urine is produced by water reabsorption of a clinical evident extracellular fluid volume depletion. across the renal tubules. Dysnatremias produce signs Sometimes, however, distinguishing hypovolemic from and symptoms secondary to central nervous system dys- normo- hypervolemic hyponatremia may not be function. While hyponatremia may induce brain swel- straightforward. In such cases, further laboratory investi- ling, hypernatremia may induce brain shrinkage, yet the gations are warranted [4,6,8]: clinical features elicited by opposite changes in tonicity a) the urine spot sodium and the fractional sodium are remarkably similar [2-7]. clearance are helpful in patients in whom volume status is difficult to assess, as patients with dilutional hypona- tremia have a urinary sodium > 30 mmol/L (and frac- tional sodium clearance > 0.5 × 10-2), whereas those with extracellular fluid volume depletion (unless the * Correspondence: [email protected] source is renal) will have a urinary sodium < 30 mmol/L 1Department of Pediatrics, Bellinzona and Mendrisio, and University of Bern, (and fractional sodium clearance < 0.5 × 10-2). Since Switzerland Full list of author information is available at the end of the article effective blood osmolality is mostly low in hyponatremia, © 2010 Peruzzo et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Peruzzo et al. Italian Journal of Pediatrics 2010, 36:78 Page 2 of 8 http://www.ijponline.net/content/36/1/78 and, later, solutes like glutamate, taurine, myoinositol, and glutamine from intracellular to extracellular com- partments. This induces water loss and ameliorates brain swelling, and hence leads to few symptoms in sub- acute and chronic hyponatremia [4,6,8]. Evaluating the cause In normovolemic subjects, the primary defense against developing hyponatremia is the ability to dilute urine and excrete free-water. Rarely is excess ingestion of free-water alone the cause of hyponatremia. It is also rare to develop hyponatremia from excess urinary sodium losses in the absence of free-water ingestion. In order for hyponatremia to develop it typically requires a relative excess of free-water in conjunction with an Figure 1 Mechanisms underlying hypotonic hyponatremia.In underlying condition that impairs the ability to excrete most cases (middle panel) hyotonic hyponatemia results from a low free-water. Renal water handling is primarily under the effective arterial blood volume and is termed hypovolemic (or control of vasopressin, which is released from the pos- depletional) hyponatremia. The term syndrome of appropriate anti- diuresis has also been used to denote this condition. In childhood terior pituitary and impairs water diuresis by increasing diarrhea, vomiting and febrile infections are the most common the permeability to water in the collecting tubule. cause of hypovolemic hyponatremia. Persistently high levels of There are osmotic, hemodynamic and non-hemody- vasopressin or, exceptionally, an increased renal response to namic stimuli for release of vasopressin. In most cases, vasopressin cause the syndrome of inappropriate anti-diuresis (left hyponatremia develops when the body attempts to pre- panel), which is less frequent than the syndrome of appropriate anti-diuresis (hypovolemic or depletional hyponatremia). A peculiar serve the extracellular fluid volume at the expense of form of depletional hyponatremia sometimes develops in patients circulating sodium (therefore, a hemodynamic stimulus with cerebral disease that mimics all of the findings in the for vasopressin production overrides an inhibitory effect syndrome of inappropriate anti-diuresis, except that renal salt- of hyponatremia). However, there are further stimuli for wasting is the primary defect with the ensuing volume depletion production of vasopressin in hospitalized children that leading to a secondary rise in release of antidiuretic hormone (right panel). The ultimate causes of the three different conditions are make virtually any hospitalized patient at risk for “bordered”. hyponatremia (Table 1). Some special causes of hypotonic hyponatremia deserve some further discussion. and urine is less than maximally dilute (inappropriately • Hospital-acquired hyponatremia is most often seen concentrated), blood and urine osmolalities, although in the postoperative period or in association with a usually measured, are rarely discriminant. reduced effective circulating volume [4,6]. More rarely b) in hypovolemic hyponatremia the urine spot hospital-acquired hyponatremia is seen in association sodium concentration and the fractional sodium clear- with the syndrome of inappropriate anti-diuresis [8], ance allow the distinction between extrarenal (sodium < which is caused either by elevated activity of vasopressin 30 mmol/L; fractional sodium clearance < 0.5 × 10-2) (80-90 percent of the cases) or by hyperfunction of its and renal (sodium > 30 mmol/L; fractional sodium renal (= V2) receptor (10-20 pecent of the cases), inde- clearance > 0.5 × 10-2) salt loss. pendently of increased effective blood osmolality and A decrease in sodium concentration and effective hemodynamic stimulus (i.e.: reduced effective circulating blood osmolality causes movement of water into brain volume). It is currently assumedthatthiscondition cells and results in cellular swelling and raised intracra- results not only from dilution of the blood by free-water nial pressure. Nausea and malaise are typically seen but also from inappropriate natriuresis [8]. The syn- when sodium level acutely falls below 125-130 mmol/L. drome of inappropriate anti-diuresis (Figure 1 middle Headache, lethargy, restlessness, and disorientation fol- panel) should be suspected in any child with hyponatre- low, as the sodium concentration falls below 115-120 mic hypotonia, a urine osmolality above 100 mosmol/kg mmol/L. With severe and rapidly evolving hyponatre- H20, a normal fractional clearance of sodium (> 0.5 × mia, seizure, coma, permanent brain damage, respiratory 10-2), low normal or reduced uric acid level, low blood arrest, brain stem herniation, and death may occur. In urea level and normal acid-base and potassium balance. more gradually evolving hyponatremia, the brain self The longstanding assumption that hypontremia [8-11] regulates to prevent swelling over hours to days by associated with meningitis and respiratory infectious dis- transport of, firstly, sodium, chloride, and potassium eases is caused by inappropriate anti-diuresis has not Peruzzo et al. Italian Journal of Pediatrics 2010, 36:78 Page 3 of 8 http://www.ijponline.net/content/36/1/78
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