Therapeutic IV fluids include crystalloid solutions, colloidal solutions, and oxygen-carrying resuscitation solutions. Crystalloids are composed of water and electrolytes, all of which pass freely through semipermeable membranes and remain in the intravascular space for shorter periods of time. As such, these solutions are very useful for correcting electrolyte imbalances, but result in smaller hemodynamic changes for a given unit of volume.
Crystalloids can be classified further according to their tonicity. Isotonic solutions (i.e., normal saline or 0.9% sodium chloride [NaCl]) have a tonicity equal to that of the ICF (approximately 310 mEq/L or 310 mmol/L) and do not shift the distribution of water between the ECF and the ICF. Because hypertonic solutions (i.e., hypertonic saline or 3% NaCl) have greater tonicity than the ICF (greater than 376 mEq/L or 376 mmol/L), they draw water from the ICF into the ECF. In contrast, hypotonic solutions (i.e., 0.45% NaCl) have less tonicity than the ICF (less than 250 mEq/L or 250 mmol/ L) leading to osmotic pressure gradient that favors shifts of water from the ECF into the ICF. The tonicity, electrolyte content, and glucose content of selected fluids are shown in Table 27-3.
The tonicity of crystalloid solutions is directly related to their sodium concentration. The most commonly used crystalloids include normal saline, hypertonic saline, and lactated Ringer's solution. Excessive administration of any fluid replacement therapy, regardless of tonicity, can lead to fluid overload, particularly in patients with cardiac or renal insufficiency. Glucose is often added to hypotonic crystalloids in amounts than result in isotonic fluids (D5W, D5^NS, and D5HNS). These solutions are often used as maintenance fluids to provide basal amounts of calories and water.
Normal Saline (0.9% NaCl or NS)
Normal saline is an isotonic fluid composed of water, sodium, and chloride. It provides primarily ECF replacement and can be used for virtually any cause of TBW depletion. Common uses of normal saline include perioperative fluid administration; volume resuscitation of shock, hemorrhage, or burn patients; fluid challenges in hypo-tensive or oliguric patients; and hyponatremia. Normal saline can also be used to treat metabolic alkalosis (also known as contraction alkalosis).
Table 27-3 Electrolyte and Dextrose Content of Selected Crystalloid Fluids
Half-normal saline is a hypotonic fluid that provides free water in relative excess when compared to the sodium concentration. This crystalloid is typically used to treat patients who are hypertonic due to primary depletion of the ECF. Because half-normal saline is hypotonic, serum sodium must be closely monitored during administration.
Hypertonic Saline (3% NaCl)
Hypertonic saline is obviously hypertonic and provides a significant sodium load to the intravascular space. This solution is used very infrequently given the potential to cause significant shifts in the water balance between the ECF and the ICF. It is typically used to treat patients with severe hyponatremia who have symptoms attributable to low serum sodium. Hypertonic saline in concentrations of 7.5% to 23.4% has been used to acutely lower intracranial pressure in the setting of traumatic brain injury and stroke. The literature is inconsistent for the appropriate hypertonic concentration, dosing, timing of replacement, and goals for use in this population. Serum sodium and neurologic status must be very closely monitored whenever given.
This isotonic volume expander contains sodium, potassium, chloride, and lactate in concentrations that approximate the fluid and electrolyte composition of the blood.
Ringer's lactate (also known as "lactated Ringer" or LR) provides ECF replacement and is most often used in the perioperative setting, and for patients with lower GI fluid losses, burns, or dehydration. The lactate component of LR works as a buffer to increase the pH. Accordingly, large volumes of LR may cause iatrogenic metabolic alkalosis. Because patients with significant liver disease are unable to metabolize lactate sufficiently, LR administration in this population may lead to accumulation of lactate with iatrogenic lactic acidosis.
5% Dextrose in Water (D5W)
D5W is a solution of free water and dextrose that provides a modest amount of calories but no electrolytes. Although it is technically isotonic, it acts as a hypotonic solution in the body. It is commonly used to treat severe hypernatremia. D5 W is also used in small volumes (100 mL) to dilute many IV medications or at a low infusion rate (10-15 mL/h) to "keep the vein open" (KVO) for IV medications.
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