Colloids

Colloids contain particles in suspension which are too large to pass through semipermeable membranes; thus they exert an oncotic pressure which draws fluid across the membrane into the intravascular space. By increasing intravascular oncotic pressure, fluid is drawn from the interstitial and intracellular spaces into the intravascular space. The oncotic pressure exerted by a specific fluid is dependent on the number of particles present in the plasma, their duration in the plasma, and the rate at which they are removed from the plasma.

Natural colloidal solutions are prepared from human plasma and include albumin, plasma protein fraction, and fresh frozen plasma. Plasma protein fraction is a mixture of proteins which comprises approximately 85 per cent albumin and 15 per cent other serum globulins. Since albumin is the primary component of plasma protein fraction, its volume expansion characteristics are similar to those of albumin.

As indicated in Xable...2, two concentrations of albumin are generally utilized, a 5 per cent and a 25 per cent solution. The only major difference is the volume infused. As it is more concentrated, the 25 per cent solution, which usually comes in a 50-ml vial, is much more oncotically active and capable of drawing more fluids intravascularly than the 5 per cent solution. However, in practice the 25 per cent solution is diluted up to a liter total of 5 per cent. Albumin is isotonic and a major component (80 per cent) of the plasma oncotic pressure, with 40 per cent of albumin residing within the intravascular compartment and 60 per cent in the interstitial compartment. Equilibration occurs between the intravascular and extravascular compartments with a plasma half-life for albumin of about 16 h. However, in a pathological state, when membrane integrity is lost, albumin will leak out of the intravascular space into the interstitial and intracellular spaces, drawing water with it.

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Table 2 Types of colloid

Table 2 Types of colloid

Solutions of albumin are heated to 60 °C for 10 h during processing to inactivate any viruses. Thus they are relatively risk free with respect to HIV and hepatitis. Although allergic reactions have been reported, with adverse reactions including urticaria, fever, and chills, they are rare.

The major artificial colloids in use are gelatins, starches, and glucose polymers. Owing to its effect on blood coagulation, including decreases in platelet adhesiveness, serum fibrinogen, and platelet factor 3, dextran should be avoided in the acute situation of hemorrhagic hypovolemic shock. Its utilization includes resuscitation of individuals who have microvascular thrombosis secondary to such conditions as polycythemia, since dextran reduces red cell aggregation.

Renal failure related to unrecognized hypovolemia may occur due to the osmotic diuresis induced by dextran along with a toxic effect on renal tubular cells. Anaphylactic reactions are rare, and dextran may result in a falsely elevated blood glucose level when certain methods of serological testing are used. The interference with cross-matching of blood due to in vitro aggregation of red cells is another reason for avoiding dextran when transfusions may be required. Gelatins and hydroxyethyl starch solutions form the mainstay of artificial colloid treatment worldwide.

The optimum hematocrit remains under debate (Lundsgaa.rd-H.§.DseD...1996). It has been suggested that patients over 40 undergoing elective procedures should have a hematocrit greater than 30 per cent unless silent myocardial ischemia has been excluded. However, the optimum level of transfusion also remains under debate, with concerns of viral transmission of hepatitis and HIV also needing to be considered ( Gie§n.w§J.t ®t..3/: 1988). Nonetheless, those patients who have large blood losses require blood replacement in a fastidious fashion. This is one of the advantages of the rapid transfusion systems which also warm the blood as quickly as it is transfused, thus preventing hypothermia. Adverse effects of hypothermia include platelet dysfunction, reversible inhibition of thromboxane synthetase, and decreased clotting factor production. In addition, hypothermia increases hemoglobin oxygen affinity, causing a decrease in oxygen release to the tissues by shifting the oxyhemoglobin curve to the left. Initial urgent blood transfusions may be given using O-negative blood, with roughly 15 min required for typing and another 15 min for cross-matching. Coagulopathy frequently accompanies massive blood transfusions and is exacerbated by hypothermia. The trend for blood component therapy administration initially using packed red blood cells contributes to this coagulopathy since it is deficient in platelets, deficient in coagulation factors 5 and 8, and leads to a dilution of other platelet factors. This can be treated with the administration of fresh frozen plasma and platelet concentrates, although there is no consensus as to the best time to give or how much to give. In the face of surgical lesions (e.g. transected femoral artery), early operation remains the procedure of choice ( Spence 1995).

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