Crystalloids

Crystalloid solutions generally contain sodium as their major osmotically active particle. As shown in T§b!e 1, there is marked variability in the amount of sodium and other electrolytes, pH, and osmolality with the different solutions available. Although the cost differential for the various types of crystalloids is small, there is a theoretical advantage to the utilization of lactated Ringer's solution, which has a more favorable pH and the advantage that the lactate is converted to bicarbonate by the liver. In situations where a metabolic acidosis may exist, this may be helpful in at least not exacerbating it.

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Table 1 Types of crystalloid

Table 1 Types of crystalloid

The volume of distribution of crystalloids also varies, depending on what is utilized. For instance, 5 per cent glucose distributes throughout the total body water. If the rough estimation for a 70-kg individual is that two-thirds of the total body water is intracellular, then 667 ml/l goes into the cells and 333 ml/l is distributed extracellularly. Of this amount, three-quarters is interstitial and a quarter is intravascular. Thus, approximately 83 ml of each liter administered remains intravascular. In contrast, 0.9 per cent saline or lactated Ringer's solution remains in the extracellular space, so that administration of 1 liter will lead to 250 ml or 25 per cent going intravascularly, which is preferred. In a trauma situation lactated Ringer's solution or 0.9 per cent saline are the crystalloids of choice, whereas in other clinical conditions, such as renal failure or pulmonary edema, electrolyte values may guide the type, rate, and amount of fluid. In patients that are hypernatremic, a hypotonic solution may be utilized.

Interest in hypertonic saline is based on the fact that smaller amounts of fluid can be given with an equivalent or even augmentated improvement in intravascular volume. However, the problem arises that the sodium content may increase rapidly and markedly with hypertonic saline, leading to the serious complications associated with hypernatremia. The intravascular volume increase is for the most part transient.

Hypertonic fluids act as plasma volume expanders as fluid is translocated from the interstitial and intracellular compartments to the intravascular compartment. This can lead to an augmentation of cardiac output, mean arterial pressure, and other hemodynamic variables, and may actually lead to a decrease in intracranial pressure in patients where this is raised.

The volume of crystalloid necessary to resuscitate patients adequately during hypovolemic hemorrhagic shock is three to seven times the blood loss. Advanced Trauma Life Support advocates the administration of 2 liters of crystalloid to adults initially as part of resuscitation. Burn patients follow one of a number of burn formulas for early therapy.

Peripheral edema is expected when crystalloids are used. The presence of edema in the interstitial compartment or soft tissues does not imply that intravascular repletion is complete. In fact, interstitial edema may lead to organ dysfunction if not corrected, and soft tissue edema may lead to extremity or abdominal compartment syndrome. The decrease in plasma oncotic pressure from crystalloid utilization may also contribute to pulmonary edema and acute respiratory distress syndrome. However, this is part of the ongoing controversy related to crystalloid use and remains to be proven.

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