Replacement of excess fluid losses

Fluids lost are invariably salt-containing body fluids, including the plasma component of blood. It is usual to replace excess losses of blood with blood and losses of plasma with plasma or plasma substitutes. However, saline solutions are often used as a temporizing measure. If there are excess losses from the gastrointestinal tract (Table., .2.) or via sweating, saline solutions are again appropriate. Ringer's lactate is often used for these losses but only offers advantages over 0.9 per cent saline (buffering capacity) in cases of massive replacement.

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Table 2 Ion content of gastrointestinal fluids

Table 2 Ion content of gastrointestinal fluids

Ringer's lactate may be useful to maintain acid-base balance if large volumes of crystalloid are exchanged (e.g. during continuous hemofiltration). Correction of hypovolemia

Colloid solutions are generally used for maintenance of plasma volume and acute replacement of plasma volume deficit. There are two schools of thought regarding the appropriate solutions to correct hypovolemia. Some believe that plasma volume should be maintained or replaced with colloid solutions since crystalloids are rapidly lost from the plasma. Others believe that larger volumes of crystalloid may be used to maintain plasma volume at a lower cost.

Smaller volumes of colloid are required for resuscitation with less contribution to edema. Resuscitation by smaller volumes is quicker and easier. Plasma substitutes maintain plasma colloid osmotic pressure, a useful effect not seen with crystalloids, but they contain no clotting factors or other plasma enzyme systems. Crystalloids are short acting compared with plasma substitutes so that treatment must be repeated more frequently.

It should be noted that plasma substitutes are carried in 0.9 per cent saline so that the majority of critically ill patients will not require 0.9 per cent saline infusions if colloid solutions are used.

The rational choice of colloid solution depends on the length of effect required and knowledge of the persistence of effect of available solutions ( Webb 1991 ) (Table

Table 3 Relative persistence of colloid effect

1. short-term volume expansion (gelatin, dextran);

2. medium-term volume expansion (albumin, pentastarch);

3. long-term volume expansion (hetastarch).

There is no evidence that maintenance of plasma albumin levels, as opposed to maintenance of plasma colloid osmotic pressure with artificial plasma substitutes, is advantageous.

Albumin 20 to 25 per cent and hydroxyethyl starch 10 per cent are hyperoncotic and used to provide colloid where salt restriction is necessary. This use is rarely necessary in intensive care where it has been shown that plasma volume expansion with a particular colloid is related to the weight of colloid infused rather than the concentration (Lamke...and LiJl§d.ah! 19.7.6.). Artificial colloids used with ultrafiltration or diuresis are just as effective in edema states.

Urea-bridged gelatin (polygeline) is a 3.5 per cent solution and contains calcium (6.25 mmol/l). The calcium content prevents the use of the same administration set for blood transfusions. Succinylated gelatin is a 4.0 per cent solution with a larger molecular weight range than polygeline, giving a slightly longer effect. This and the lack of calcium in solution make this a more useful solution than polygeline for short-term plasma volume expansion.

In patients with capillary leak there is considerable leak of albumin and lower-molecular-weight colloids to the interstitium. In these cases it is probably better to use larger-molecular-weight colloids such as hydroxyethyl starch.

Hetastarch is usually a 6 per cent solution with a high degree of protection from metabolism. The molecular weight ranges vary, but molecular sizes are large enough to ensure a prolonged effect. These are the most useful colloids in capillary leak. Pentastarch has a lower degree of protection from metabolism and therefore a shorter effect.

The volume required for correction of hypovolemia is determined by the hemodynamic effect of the fluid given. Most colloid solutions are subject to maximum limits suggested by the manufacturers on the basis of available research data. These limits have been exceeded when necessary although side-effects are more likely.

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