Crystalloid and Colloid Fluids

Crystalloid Fluids

Crystalloid fluids are sterile water solutions that contain ionized electrolytes, such as sodium and potassium, or small molecules, such as glucose and mannitol. The term “crystalloid” refers to the ability of the fluid to form crystals if left to evaporate.

These fluids easily pass through the capillary membrane and distribute from the plasma to the interstitial fluid space. The distribution is not immediate but requires about 30 min for completion. The slowness of this distribution is probably due to the inherent restriction of fluid movement in the interstitial fluid gel. Excretion occurs via the kidneys, with a half-life of 20–40 min in conscious humans, but is prolonged to several hundred minutes in anesthetized subjects and in others who have a low arterial pressure ( Fig. 50.1 ). The plasma volume effect is about 50% at the end of a 30-min infusion but is reduced to approximately 20% within 30 min after the infusion is ended.

Crystalloid fluids that contain only extracellular electrolytes (like sodium) do not enter the cells, provided that their osmolality is the same as that of the body fluids (295 mosmol/kg). Fluids having a lower osmolality enter the cells to some degree, while fluids having an osmolality higher than 295 mosmol/kg cause the cells to dehydrate and shrink. Mannitol also remains in the extracellular fluid space. In contrast, glucose solutions always hydrate the cells because the glucose molecules are taken up by the cells and bring along water by osmosis.

Isotonic (0.9%) saline is the most frequently used crystalloid fluid worldwide. This fluid is often used in brain trauma because the osmolality of 308 mosmol/kg ensures that there is no risk of ensuing cellular and brain edema. However, this fluid can also be called “unbalanced,” because no attempt has been made to mimic the electrolyte composition of the extracellular fluid. The surplus of chloride ions slightly inhibits kidney function and causes metabolic acidosis after infusion, particularly if the infusion fluid volume exceeds 2 L.

Ringer solution is a more physiological crystalloid solution than isotonic saline because it contains several extracellular electrolytes, such as calcium and magnesium. In most cases, a precursor to the bicarbonate buffer, in the form of lactate or acetate , is added to the solution. These ions permit reduction of the chloride content, thereby avoiding metabolic acidosis. Downsides to Ringer solution include the issue that both acetate and lactate are weak vasodilators. Ringer lactate may raise blood glucose in diabetic patients, and the use of large amounts of lactated Ringer interferes with assays used to monitor lactic acidosis. The use of Ringer lactate and Ringer acetate has also been questioned because they are slightly hypo-osmotic (280 mosmol/kg), which means that they should probably be avoided in brain surgery or in patients with brain trauma unless the intracranial pressure (ICP) is monitored. Finally, the calcium content makes all Ringer solutions unsuitable for infusion in the same intravenous line as erythrocytes that were treated with citrate as an anticoagulant. For this infusion, isotonic saline or Plasma-Lyte should be used.

Plasma-Lyte is a brand name for a crystalloid fluid that is marketed worldwide by Baxter Healthcare. Plasma-Lyte further refines the composition of buffered Ringer solutions to give an osmolality and chloride concentration that are both the same as that of an extracellular fluid. This is made possible by adding both acetate and gluconate as buffers; the latter is used widely as a taste improver in the food industry. Plasma-Lyte is superior to isotonic saline in terms of avoiding adverse effects on kidney function and postoperative outcome, although 2 L does not seem to be a sufficiently large volume to demonstrate a difference.

Glucose solutions hydrate both the extra- and intracellular fluid spaces, and they provide calories to prevent starvation. The most commonly used fluid is the isotonic 5% solution, to which electrolytes are added to prevent the development of hyponatremia. The recommendation for debilitated hospital patients is a volume of 1.2–1.3 mL/kg/h, and 1 mmol of sodium and 1 mmol of potassium are usually added per 24 h. The energy content of 5% glucose is only 200 calories, which is insufficient for long-term treatment.

The insulin resistance that develops in response to trauma and surgery greatly increases the likelihood of hyperglycemia in response to infusion rates of 5% glucose >100 mL/h. Therefore, glucose solutions intended for perioperative use are often diluted with Ringer lactate to form a 2.5% fluid. More highly concentrated solutions are also marketed (10% and 20%), but these are used mainly when more adequate nutrition is needed. Monitoring of plasma glucose can be conveniently performed with bedside equipment, and this monitoring is recommended as a precaution to avoid hyperglycemia. Cerebral damage in the ischemic brain becomes greater in the presence of marked hyperglycemia; therefore, caution should still be practiced when providing this type of fluid in association with neurosurgery. Glucose infusions should also be avoided in cases of acute stroke.

Hypertonic saline is used to raise the serum sodium concentration in hyponatremic states, to serve as an effective resuscitation fluid, and to dehydrate the cells. The capacity of hypertonic saline to dehydrate cells is of interest in neurotrauma, because the fluid can be used to control brain edema and to reduce the ICP. These fluids are usually prepared at the local hospital, as they are rarely available commercially. Typical concentrations include: 3, 7.5, and 20%. The more concentrated fluids often cause initial pain at the site of the infusion if given via a peripheral vein.

Hypertonic saline expands the plasma volume to a much greater extent than occurs with isotonic saline. A volunteer study demonstrated that the plasma volume expansion was 4 times larger with 7.5% saline than with isotonic saline. Note that the shift is not 8 times, as one might expect. The time course for the reduction of the plasma volume expansion depends mainly on the capacity of the kidneys to excrete the sodium load.

The osmotic shift across the cell membrane can be estimated mathematically for different strengths of hypertonic saline. If the osmolality of the body fluids is initially 295 mosmol/kg, then the translocated volume f (t) is given by the following mass balance equation, which assumes that the volume of the intracellular fluid is 40% and the volume of the extracellular fluid is 20% of the body weight (BW):

Monitoring of the serum sodium concentration during prolonged treatment with hypertonic saline is warranted, as serum sodium concentrations >155 mmol/L are associated with neurological disturbances.

Mannitol is a nonmetabolizable isomer of glucose that is eliminated by renal excretion with a half-life of approximately 2 h. In contrast to glucose, the mannitol molecules remain outside the cells; hence, it does not cause cellular swelling as long as the solution is at least isosmotic (5%).

The clinical indication for mannitol is primarily to increase the urinary excretion in failing, oliguric kidneys. Unlike furosemide, mannitol operates by inducing osmotic diuresis; therefore, it does not increase the renal oxygen consumption. For this purpose, mannitol (500–750 mL) is infused at concentrations of 10, 15, or 20%.

The use of hypertonic mannitol solutions not only decreases the total body water by inducing diuresis but also, like hypertonic saline, withdraws water from the cells and thereby shrinks them. Therefore, hypertonic mannitol has long been used for acute reduction of the ICP in patients with head trauma. A downside is that the mannitol solutions are typically devoid of electrolytes, despite the fact that osmotic diuresis brings along sodium and chloride ions that maintain the extracellular fluid volume. The net loss of extracellular ions causes an increase in the intracellular fluid volume at the expense of the extracellular volume, which in turn increases the ICP (“rebound effect”). This adverse effect cannot occur if the ICP is controlled with hypertonic saline, or if saline is provided after mannitol.