Body fluids and salt metabolism - Part I
© Bianchetti et al; licensee BioMed Central Ltd. 2009
Received: 21 October 2009
Accepted: 19 November 2009
Published: 19 November 2009
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 requirments and repair.
Body fluid compartments
Effective circulating volume
Effective circulating volume denotes the part of the intravascular compartment that is in the arterial system and is effectively perfusing the tissues. The effective circulating volume is biologically more relevant than the intravascular compartment and usually varies directly with the extracellular fluid volume . As a result, the regulation of extracellular fluid balance (by alterations in urinary sodium excretion) and the maintenance of the effective circulating volume are intimately related. Sodium loading will tend to produce volume expansion, whereas sodium loss (e.g., due to vomiting, diarrhea, or drug management with diuretics) will lead to volume depletion. The body responds to changes in effective circulating volume in two steps: 1. The change is sensed by the volume receptors, which are located in the cardiopulmonary circulation, the carotid sinuses and aortic arch, and in the kidney; 2. These receptors activate effectors that restore normovolemia by varying vascular resistance, cardiac output, and renal water and salt excretion. Briefly, the extrarenal receptors primarily govern the activity of the sympathetic nervous system and natriuretic peptides. On the other side the renal receptors affect volume balance by modulating the renin-angiotensin II-aldosterone system.
In some settings the effective circulating volume is independent of the extracellular fluid volume. Among patients with heart failure the extracellular fluid volume is increased but the patient is effectively volume depleted due to the low cardiac output.
Blood osmolality - measurement of sodium
Flame photometry, the traditional assay for circulating sodium, measures the concentration of sodium per unit volume of solution, with a normal range between 135 and 145 mmol/L. In fact, sodium is dissolved in plasma water, which normally accounts for 93% of the total volume of plasma, the remaining 7% consisting of protein and lipid. Ion selective electrodes, that have replaced flame photometry in most laboratories, determine the activity of sodium in plasma water, which ranges between 145 and 155 mmol/L. For convenience, laboratories routinely apply a correction factor so that the reported values still correspond to the traditional normal range of 135-145 mmol/L [5–9]. A kind of "pseudohyponatraemia" caused by expansion of the non-aqueous phase of plasma - for example, due to hyperlipidaemia or paraproteinemia - is no longer seen because determination by selective electrodes in undiluted serum, plasma or whole blood is unaffected by this [The recommended name for this quantity is ionized sodium.] . Although, strictly speaking, a sodium concentration outside the range of 135-145 mmol/L denotes dysnatremia, clinically relevant hypo- or hypernatremia is mostly defined as a sodium concentration outside the extended normal range of 130-150 mmol/L [5–9].
Dehydration and extracellular fluid volume depletion
The terms extracellular fluid volume depletion and dehydration are mostly used interchangeably. However, these terms denote conditions resulting from different types of fluid losses. Volume depletion refers to any condition in which the effective circulating volume is reduced. It is produced by salt and water loss (as with vomiting, diarrhea, diuretics, bleeding, or third space sequestration). Strict sense dehydration refers to water loss alone. The consequence of strict sense dehydration is hypernatremia. The elevation in serum sodium concentration, and therefore effective blood osmolality, pulls water out of the cells into the extracellular fluid. However, much of the literature does not distinguish between the two terms. Although dehydration in general usage means loss of water, in physiology and medicine, the word means both a loss of water and salt. Depending on the type of pathophysiologic process, water and sodium may be lost in physiologic proportion or lost disparately, with each type producing a somewhat different clinical picture, designated as normotonic (mostly isonatremic), hypertonic (mostly hypernatremic), or hypotonic (always hyponatremic) dehydration. Dehydration develops when fluids are lost from the extracellular space at a rate exceeding intake. The most common sites for fluid loss are 1. the intestinal tract (diarrhea, vomiting, or bleeding), 2. the skin (fever, burns, or cystic fibrosis) and 3. the urine (osmotic diuresis, diuretic therapy, or diabetes insipidus). More rarely, dehydration results from prolonged inadequate intake without excessive losses [1–3, 10].
Children and especially infants are more susceptible to dehydration than adults. The risk is high for the following causes: a. infants and children are more susceptible to infectious diarrhea and vomiting than adults; b. there is a higher proportional turnover of body fluid in infants compared to adults (it is estimated that the daily fluid intake and outgo, as a propotion of extracellular fluid, is in infancy twice that of an adult); c. young children do not communicate their need for fluids or do not independently access fluids to replenish volume losses [1–3, 10].
Dehydration reduces the effective circulating volume, therefore impairing tissue perfusion. If not rapidly corrected, ischemic end-organ damage occurs, leading to serious morbidity.
Three groups of symptoms and signs occur in dehydration [1–3, 5–10]: a. those related to the manner in which fluids loss occurs (including diarrhea, vomiting or polyuria); b. those related to the electrolyte and acid-base imbalances that sometimes accompany dehydration; and c. those directly due to dehydration. The following discussion will focus the third group.
When assessing a child with a tendency towards dehydration, the clinician needs to address the degree of extracellular fluid volume depletion. More rarely the clinician will address the laboratory testing and the type of fluid lost (extracellular or intracellular fluid).
Degree of dehydration
It is imperative to accurately assess the degree of dehydration since severe extracellular fluid volume depletion calls for rapid fluid resuscitation [10, 11]. Dehydration is most objectively measured as a change in weight from baseline (acute loss of body weight reflects the loss of fluid, not lean or fat body mass; thus, a 1.2 kg weight loss should reflect the loss of 1.2 liters of fluid). In most cases, however, a previous recent weight is unavailable.
"4-item 8-point rating scale" clinical dehydration scale .
Thirsty, restless or lethargic but irritable when touched
Drowsy, limp, cold, or sweathy; comatose or not
Mucous membranes (tongue)
Laboratory testing and the type of fluid lost
is < 0.5 × 10-2 (< 0.5 percent) and the urine spot sodium concentration < 30 mmol/L (unless the source of dehydration is renal).
Dipstick measurement of specific gravity is very popular but unfortunately unreliable .
Furthermore, laboratory testing can detect associated electrolyte and acid-base disturbances but determination of circulating electrolytes and acid-base balance is typically limited to children requiring intravenous fluids. These children are more severely volume depleted and are therefore at greater risk for dyselectrolytemias. Laboratory testing is less useful for assessing the degree of volume depletion.
- Bicarbonatemia ≤ 17.0 mmol/L might be the most useful laboratory test to assess dehydration. The blood urea level reflects the severity of dehydration, the decreased glomerular filtration rate and the increased sodium and water reabsorption in the proximal tubule . Unfortunately the clinical usefulness of this test is limited, since this blood parameter can be increased by other factors such as bleeding or tissue breakdown (on the other side the rise can be minimized by a concurrent decrease in protein intake).
- The sodium concentration varies with the relative loss of solute to water. Changes in sodium concentration play a pivotal role in deciding the type of fluid depletion (figure 4):
Hyponatremic and hypotonic dehydration [5, 6]: The development of hyponatremia reflects net solute loss in excess of water loss. This does not occur directly, as fluid losses such as diarrhea are not hypertonic. Usually solute and water are lost in proportion, but water is taken in and retained in the context of hypovolemia-induced secretion of antidiuretic hormone. Since body water shifts from extracellular fluid to cells under these circumstances, signs of dehydration easily become profound.
Normonatremic and isotonic dehydration: In this setting, solute is lost in proportion to water loss.
Hypernatremic and hypertonic dehydration [7, 8]: This setting reflects water loss in excess of solute loss. Since body water shifts from intracellular to extracellular fluid under these circumstances, these children have less signs of dehydration for any given amount of fluid loss than do children with normonatremic (or normotonic) dehydration and especially those with hyponatremic dehydration.
Clinical assessment of dehydration may be difficult, especially in young infants. A large body of evidence suggests the use of a recently developped and validated "4-item 8-point rating scale". Mainly extracellular fluid losses occur in hypotonic dehydration, where signs of dehydration easily become profound; on the contrary, mainly intracellular fluid losses occur in hypertonic dehydration, where signs of dehydration tend to be less evident.
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