Colloids

Introduction

A colloid solution contains large molecules that do not easily cross a semi permeable membrane. Colloids can be either natural or synthetic. They are used to help maintain colloid osmotic pressure (COP), correct hypovolemia and with plasma (see below) clotting factors. It is the number of particles within the colloid that influences the osmotic effect, not the size of the particles. They are highly efficient at expanding vascular volume, compared to crystalloids, which easily diffuse out of the circulation, meaning that less volume is required.

Types of Colloid

• Dextran
• Hetastarch
• Gelatin Solutions
• Blood products
  • Whole Blood
  • Packed Red Blood Cells
  • Plasma

Dextran

Dextran is a molecule produced by certain strains of bacteria to give this linear polysaccaharide as either a high or low weight molecule. It is found in a sodium chloride or dextrose solution. It is metabolised to glucose within the patient. Both high and low weight molecules produce the same plasma volume expansion per gram, although the low weight molecule it thought to achieve this more rapidly. There are concerns about it's interaction with fibrin, making clots weaker and decreasing factor VIII and von Willebrand's factor.

Examples include Dextran 40 and 70.

Hetastarch

Hetastarch is a synthetic glucose polymer, most commonly in a sodium chloride solution. Again, it is broken down into glucose in the patient. It is not as long in duration of action as dextran products but is still a very effective plasma volume expander. It can be used to draw fluid out of intersitium in patients with peripheral oedema or ascites. Again, there is some concern about prolonged bleeding times in patients who have received hetastarch products.

Examples include Voluven.

Gelatin Solution

Gelatin solutions are derived from degradation of bovine collagen. It is found in several forms. Their advantage over other colloid solutions are their minimal effects on coagulation and minimal antigenic reactions. They have a lower molecular weight also.

Examples include Haemaccel.

Blood Products

Whole Blood

Whole blood is administered when a patient requires all the components of whole blood. It contains clotting factors and active platelets and so requires correct storage. It is important to monitor patients receiving blood products closely in case of an anaphylactic reaction. Another concern with whole blood is the effect of citrate on the availability of calcium, which may require dosing also through a different line.

Plasma

Plasma is available as either a fresh or frozen product. Fresh plasma contains platelets and clotting factors, while fresh frozen plasma contains no platelets but does have clotting factors as long as it has been thawed correctly. Fresh frozen plasma can be used to expand the plasma volume, as long as the packed cell volume of that patient is within normal limits. However, there is always a concern when administering blood products of an anaphylactic reaction and so patients receiving these products should be closely monitored.