Osmolality intravenous fluids

Table 20.1 Electrolyte composition (mmol L-1) and osmolality (mosmol kg-1) of some intravenous fluids ...

Apprehension and fear caused an increase in blood levels, but the induction of anesthesia was not a strong stimulus. ADH levels are often raised in the preoperative patient owing to fiuid deprivation, and intravenous fluids will frequently cause a reduction in plasma ADH activity. Skin incision in a patient under general anesthesia constitutes a stimulus which can be abolished by the additional use of a local anesthetic in the skin (M6). Traction on the root of the mesentery of the small intestine was shown to be a distinct stimulus. Osmoreceptors are involved in the control of ADH release, which is inhibited when tonicity is low and is increased as tonicity rises (H12). However, after injury when the plasma is often hypotonic for many reasons and the urine concentrated, the promotion of further antidiuresis is paradoxical and unrelated to normal mechanisms of osmolality control. Plasma volume changes and associated deprivation of intake in the immediate post injury period take precedence over tonicity control mechanisms. Thus many stimuli which in themselves are not associated with blood volume changes can evoke an ADH response. 

Table 3.1 lists the osmolalities of commonly used intravenous fluids. 

Proximal tubular function is immature, and bicarbonate and glucose reabsorption is reduced. This leads to a low serum bicarbonate concentration. Glycosuria and aminoaciduria may be found in the normal neonate. A baby s ability to concentrate urine is poor. A urine osmolality of 600 mmol/kg is the maximum that can be produced. In the neonatal period, the kidney s ability to excrete water and electrolytes is limited. Thus, great care must be taken in the provision of intravenous fluids. 

If the osmolality of the intravenous fluid is inappropriate, this may cause hemolysis, with subsequent changes to plasma electrolytes. Dehydration may occur when food and water intakes are severely reduced, and plasma sodium, total protein, albumin, and osmolality values tend to increase. In dehydration states, such as those associated with gastrointestinal toxicity, and diarrhea causing additional fluid loss, it is not uncommon for oliguria to occur until rehydration has taken place. Plasma... 

Fluid restriction is generally unnecessary as long as sodium intake is controlled. The thirst mechanism remains intact in CKD to maintain total body water and plasma osmolality near normal levels. Fluid intake should be maintained at the rate of urine output to replace urine losses, usually fixed at approximately 2 L/day as urine concentrating ability is lost. Significant increases in free water intake orally or intravenously can precipitate volume overload and hyponatremia. Patients with stage 5 CKD require renal replacement therapy to maintain normal volume status. Fluid intake is often limited in patients receiving hemodialysis to prevent fluid overload between dialysis sessions. 

The fundamental treatment for DKA includes aggressive intravenous hydration and insulin therapy and maintenance of potassium and other electrolyte levels. Fluid and insulin therapy is based on the patient s individual needs and requires frequent reevaluation and modification. Close attention has to be given to hydration and renal status, the sodium and potassium levels, and the rate of correction of plasma glucose and plasma osmolality. Fluid therapy generally begins with normal saline. Regular human insulin should be used for intravenous therapy with a usual starting dose of about 0.1 IU/kg/h. 

Sodium When patients present with hypernatremia and elevated serum osmolality, they are suffering from severe fluid deficits. Depending on the patient s hemodynamic stability, fluid therapy should generally be instituted as a moderate-to-slow intravenous infusion of 0.9% normal saline over a period of 48-72 h to avoid cerebral edema. Patients with evidence of circulatory compromise will require more aggressive fluid resuscitation. Estimated plasma osmolality and corrected serum sodium concentrations are calculated using the following formulas ... 

Osmotic pressure is expressed as osmolarity or osmolality. Osmolality is the concentration of body fluids. Tonicity is the effect bodily fluid has on cellular volume and is used to measure the concentration of intravenous solutions. Serum osmolality instead of tonicity is used to indicate the concentration of solutes in body fluids. 

This includes patients with fluid losses caused by diarrhea, excessive sweating, and diuretics. This transient hypernatremic hyperosmolality results in osmotic release of ADH and stimulation of thirst. If sodium and water losses continue, more ADH is released as a result of hypovolemia. Patients who then drink water or who are given hypotonic fluids intravenously retain water and develop hyponatremia. Urine osmolality is generally greater than 450 mOsm/kg, reflecting the presence of ADH and formation of a concentrated urine. The urine sodium concentration is <20 mEq/L when sodium losses are extrarenal, as in patients with diarrhea, and >20 mEq/L in patients with renal sodium losses, as occurs in the setting of diuretic use or adrenal insufflciency. °... 

FIGURE 49-4. Diagnostic and treatment algorithm for hypernatremia. D5W, 5% dextrose in water ECF, extracellular fluid H2O, water Na, sodium Uosm, urine osmolality Uvol, daily urine volume. See text for guidelines regarding calculations of infusion rates for intravenous solutions. 

Fluid or water overloading may occur when excessive thirst is induced by a test compound, inappropriately high intravenous infusion rates, cardiac failure, or failure of renal excretion to act as a compensatory mechanism. This may cause reductions of plasma sodium, protein, albumin, and osmolality values. The osmolality of a test solution for intravenous administration may affect fluid balance when the injection volumes are relatively large compared to the ECF volume of the animal (Michel... 

NaCI is 0.9 g NaCI/100 ml, 1 equivalent to 9 g/L. NaCI has a molecular weight of 58 g/mole, so the concentration of NaCI in isotonic saline is 0.155 M, or 155 mM. If all of the NaCI were dissociated into Na+and Cl ions, the osmolality would be 310 mOsm/kg water. Because NaCI is not completely dissociated and some of the hydration shells surround undissociated NaCI molecules, the osmolality of isotonic saline is approximately 290 mOsm/kg HjO. The osmolality of plasma, interstitial fluids, and ICF is also approximately 290 mOsm/kg water, so that no large shifts of water or swelling occur when isotonic saline is given intravenously. 

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