The right plasma volume expander....

Daniel O'Neill, BSc, RN, is a staff nurse, A&E, John Radcliffe Hospital, Oxford
Plasma volume expanders are used for the treatment of circulatory shock. They restore vascular volume, stabilising circulatory haemodynamics and maintaining tissue perfusion. Two general categories of expander are used: crystalloids or colloids, or a mixture of both (Baskett, 1994; Astiz and Rackow, 1999).

Plasma volume expanders are used for the treatment of circulatory shock. They restore vascular volume, stabilising circulatory haemodynamics and maintaining tissue perfusion. Two general categories of expander are used: crystalloids or colloids, or a mixture of both (Baskett, 1994; Astiz and Rackow, 1999).

The crystalloids most commonly used are normal saline (0.9% NaCl) or lactated Ringer's solution. Colloids include Haemaccel, Gelofusin and the naturally occurring plasma substances (albumin, plasma protein fraction). Debate on the preferred type of volume expander is ongoing (Holt and Dolan, 2000).

Albumin is normally present in the blood and constitutes 50-60% of the plasma proteins and 80-85% of the oncotic pressure. Plasma protein fraction consists of 88% albumin and 12% globulins. Plasma protein fraction is effective in maintaining blood volume but does not increase oncotic pressure.

How do plasma volume expanders work?
Plasma volume expanders increase the oncotic pressure in the intravascular space. Water moves from the interstitial spaces into the intravascular space, increasing the circulating blood volume. This increased volume leads to an increase in central venous pressure, cardiac output, stroke volume, blood pressure, urinary output and capillary perfusion, and a decrease in heart rate, peripheral resistance and blood viscosity.

Key functions
The administration of a volume of 25% albumin solution causes three-and-a-half times the administered volume to be drawn into the circulation within 15 minutes. A single infusion of dextran 40 increases the circulating blood volume to a maximum within a few minutes but the effect tails off as it breaks down more rapidly than dextran 70 or 75, which reaches maximum volume within an hour but maintains this for longer. Hetastarch (etherified starch) produces a volume expansion that is slightly greater than the amount administered, with maximum expansion occurring within minutes. With all these products, volume expansion lasts about 24 hours.

Dextran 40, unlike the higher molecular weight dextran products, also improves microcirculation independently of its volume-expanding effects. The exact mechanism of this activity is unknown, but it is believed to occur by minimising erythrocyte aggregation and/or decreasing blood viscosity.

Dextran is used clinically in the prophylaxis of venous thrombosis and pulmonary embolism in patients undergoing surgery that carries a high risk of thromboembolic complications.

The main features of crystalloids are that they have an intravascular half-life of between 30 and 60 minutes and must be given in amounts equal to three times the volume lost. Colloids such as Haemaccel last several hours and replace the volume of blood lost in a ratio of one to one.

Which plasma volume expander is best?
Some clinicians have strong opinions on the pros and cons of using a crystalloid versus a colloid plasma expander. A study of 26 A&E patients with hypovolaemia and septic shock compared the haemodynamic and respiratory effects of normal saline (NS), albumin and hetastarch (Rackow et al, 1983). Patients were given enough plasma expander to reach a target central venous pressure.

About two to four times greater fluid volume was needed when using NS compared with albumin and hetastarch. The only haemodynamic differences included a greater increase in cardiac output in the albumin and hetastarch groups compared with the NS group.

Colloid osmotic pressure decreased below baseline in the NS group, which is interesting as it could cause serious problems, especially if either function is impaired, resulting in a higher incidence of pulmonary oedema in the NS group compared with the other groups.

Both albumin and hetastarch groups maintained or increased the colloid osmotic pressure compared with the baseline. In general there were no significant differences between the albumin and hetastarch groups.

Potential complications
Nurses overseeing an infusion of plasma expanders must be aware of the possible complications. Anaphylaxis reactions can occur with hetastarch, albumin or any of the dextran preparations. Dextran is produced by a bacterium, Leuconostoc mesenteroides, which contributes to its antigenicity. However, improved preparation methods have resulted in the incidence of hypersensitivity reactions falling. Of the dextran products, dextran 40 has less potential to cause an adverse reaction. The risk of antigenicity is lower with hetastarch than with dextran. With albumin, anaphylactic reactions are more likely to occur with high doses or repeat administration than with low doses.

With any product, the patient should be closely observed during the first few minutes of administration. Allergic reactions include urticaria, nasal congestion, wheezing, tightness of the chest, nausea and vomiting, periorbital oedema and hypotension, which can be mild or severe. Volume expander therapy should be stopped at the first sign of an allergic reaction.

Substances with a molecular weight of 50,000 daltons or less can be filtered by the glomerulus, so dextran 40 could cause renal injury if tubular flow is decreased. Dextran 40 undergoes rapid urinary excretion, increasing the viscosity and specific gravity of urine. Patients with a reduced flow of urine are particularly susceptible to tubular stasis and blocking, so it is essential to maintain hydration. Renal failure does not occur with dextran 70 or 75, but input and output must be monitored as volume overload may lead to cardiovascular effects, as can fluid overload with a crystalloid.

Excessive administration of albumin, dextran or hetastarch can precipitate cardiac failure, pulmonary oedema and peripheral oedema of the lower extremities, hypertension or tachycardia. Concentrations in plasma protein fraction may cause a higher incidence of hypotension. Nurses need to monitor patients' haemodynamic state.

In acutely ill patients measurements other than central venous pressure, such as cardiac output studies, may need to be carried out. Fluid balance may need to be sustained in conjunction with inotropic/cardiosupportive drug therapy.

Bleeding is a serious concern with hetastarch therapy. Hetastarch appears to affect total platelet count and haemodilution can exacerbate this. A prolonged bleeding time, partial thromboplastin time and prothrombin time can result as a temporary adverse effect. However, at volumes less than 1,500ml, effects on coagulation are minor.

Conclusion
To be able to administer intravenous therapies, the nurse must have a thorough knowledge of the principles and their applications. Intravenous medications should never be given without full knowledge of immediate and late effects, toxicity and nursing implications (UKCC, 1992). Only by continually updating and reviewing practice can the nurse develop as a safe practitioner.