Tim Davidson
senior lecturer in surgery
University College London Medical School
London W1P 7LD

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Ever been in this situation? "Right, student doctor, this patient needs fluid. What solution would you give? Colloid, crystalloid, blood product?" Shukri Shami and Tim Davidson provide the answers

Maintaining the correct fluid balance is an important part of treating patients. To do this you not only need to be able to assess how much fluid a patient needs but what type of fluid to give. We have already covered the assessment of fluid requirements,1 2 and in this article we focus on the different types of solutions to maintain or replace the intravascular compartment. Intravenous nutritional fluids will be covered in a further article.

Intravenous fluids may be classified into crystalloids, colloids, and blood products (box). Often patients are given both colloid and crystalloid. If fluid loss is undercorrected the patient will develop hypovolaemia, which may lead to renal impairment and other complications. Overcorrection can lead to pulmonary oedema and heart failure. In seriously ill patients it is often best to insert a central venous pressure catheter while giving fluids. This allows you to measure the central venous (right atrial) pressure and so you can assess if too much fluid is being given.

Crystalloid solutions
Crystalloids are substances that form a true solution and pass freely through a semipermeable membrane. They contain water and electrolytes and stay in the intravascular compartment for a shorter time than colloids. Around two to three times more crystalloid than colloid is therefore needed to achieve an equivalent haemodynamic response (fig 1). The expansion in intravascular volume achieved by crystalloid is transitory; colloid osmotic pressure is reduced and as fluid accumulates in the interstitial spaces pulmonary oedema may occur. However, crystalloid solutions are cheap, convenient to use, and have no side effects.

Crystalloids are useful in maintaining fluid balance - for example, postoperatively when a patient is not drinking - or in conjunction with colloids to replace intravascular volume rapidly after sudden blood loss. The type of crystalloid given as fluid maintenance needs to be tailored to the need of the patient and has to take into account the daily requirement, insensible losses, and measured losses of fluid and electrolytes.

Types of intravenous fluids commonly used in hospital for replacing blood loss

Crystalloids - Normal saline, dextrose 5%, Hartmann's solution (sodium lactate) Colloids - Dextran, gelatin (Gelofusine, Haemaccel) Blood products - Whole blood, fresh frozen plasma, albumin solution Saline solutions

Normal saline (0.9% saline) is the most commonly used saline solution. It contains water and 154 mmol/l of sodium and chloride ions. It is useful for maintaining the daily requirement of salt and water or for replacing gastrointestinal losses from nasogastric suction, vomiting, or enterocutaneous fistulae. Saline solutions can also be obtained with added potassium NaCl 0.9%, KCl 0.15%) or glucose (dextrose-saline). Table 1 shows the crystalloid solutions usually available in hospital and their composition.

Normal saline is usually inappropriate in the early postoperative period because of the salt and water retention caused by the metabolic response to stress. However, if there is salt loss - for example, from the gastrointestinal tract - normal saline can be used to compensate this loss. As a rule of thumb, replacement of the gastrointestinal volume lost with an equal volume of normal saline is sufficient. However, you will also have to account for potassium loss and compensate for it.

Dextrose solutions
The most commonly used dextrose solution is 5% dextrose, which contains water and 837 kJ/l energy in the form of glucose. Dextrose or dextrose-saline (which also provides 30 mmol/l of sodium chloride) are ideal to use in the early postoperative period since they do not cause salt and water overload and also provide some energy for the patient (albeit about 10% of the average daily requirement per litre). Be careful to alternate with solutions that contain enough sodium.

Hartmann's solution (sodium lactate)
Sodium lactate solution has the advantage of containing less chloride and contains 29 mmol/litre of bicarbonate ions, which are present in solution as lactate and later converted in the liver to bicarbonate. It also contains 5 mmol/l of potassium and so has an electrolyte profile similar to plasma. Some prefer to use this solution in place of normal saline (0.9% saline), claiming that it is more physiological.

Colloids
Colloids are substances that do not dissolve into a true solution and do not pass through a semipermeable membrane. Colloid solutions tend to stay in the intravascular compartment for longer than crystalloids, and therefore less volume is needed. Colloids also increase colloidal osmotic (or oncotic) pressure, draining water out of the interstitial spaces into the intravascular compartment. However, when capillary permeability is increased colloids may leak across the capillary membrane and increase interstitial oncotic pressure, causing oedema. This may also happen if too much colloid is given. This is most dangerous in the lungs where pulmonary oedema can interfere with gas exchange.

Colloid solutions are most useful in situations where the intravascular compartment needs to be expanded rapidly - for example, in severe hypovolaemia after major trauma. It is useful in a patient with severe haemorrhage before blood is available. Colloid solutions are more expensive than crystalloid solutions, and some occasionally provoke allergic reactions and interfere with cross matching of blood.

Colloids can be classified into natural colloids (this will be covered under blood products), dextrans, gelatines, and hydroxyethyl starches (table 1).

Dextrans
Dextrans are polysaccharides in solution with either normal saline or 5% dextrose. Two types are available: dextran 40, in which the polysaccharides have an average molecular weight of 40000, and dextran 70, in which the average molecular weight is 70000. Both solutions are hypertonic and exert a powerful osmotic effect, drawing water from the extravascular compartment into the intravascular compartment. Dextran 40 is rapidly excreted by the kidney and therefore has a shorter half life than dextran 70. Both interfere with cross matching of blood, so you should take blood samples before infusing dextrans. If dextrans have already been infused tell the haematologist so that the red blood cells can be washed before cross matching.

Gelatine
The gelatines commonly used are Haemaccel and Gelofusine and have a molecular weight of 35000 and 30000 respectively. They are isotonic, but because they are readily excreted by the kidney their half life in the circulation is only two to three hours. They can also cause allergic reactions.

Hydoxyethyl starches (hetastarch)
The hetastarches most commonly available are Elohes 6% and Hespan and have a molecular weight of 200000 and 450000 respectively. They are more expensive than dextrans and gelatines but cheaper than blood products. They expand the intravascular compartment slightly in excess of the volume infused and have a half life of about six hours. They may improve the haemodynamic status for 24 hours or longer.

Blood products
We will discuss here only the commonly used blood products. Remember that the risks associated with the use of blood products include infection with HIV (1 in 3 million risk), hepatitis B virus (1 in 20000) and hepatitis C virus (1 in 13000). Other risks include allergic reactions, ABO incompatibility resulting in intravascular haemolysis of recipient red cells, fluid overload, antibody formation to donor granulocytes leading to leucocyte aggregation, and acute lung injury.

Blood
Transfusion with whole blood is required when the patient has a major bleed or is acutely anaemic - for example, after an operation where there has been significant blood loss. In emergency cases transfusion with uncrossmatched blood (group O, Rhesus negative) may be needed to save a patient's life, but it is usually possible to maintain the intravascular volume with plasma expanders until crossmatched blood is available.

Packed cells are concentrated suspensions of red cells prepared by removing most of the supernatant plasma citrate from the blood after settling of the blood or centrifugation. It is preferable to give packed cells if the patient is at risk of heart failure - for example, an elderly anaemic patient. In non-emergency situations you should give a diuretic with the blood to avoid fluid overload (normally 20 mg oral frusemide with every other unit of blood). A useful rule of thumb to remember is that in an average person one unit of blood will raise the haemoglobin concentration by roughly 1 g.

Case history

A 19 year old previously fit butcher is brought to the accident and emergency department with severe bleeding from his upper thigh following an injury while cutting meat. He is conscious, there is fresh blood spurting from the upper thigh in the region of the femoral vessels, he is sweaty and clammy, his pulse is 160/min, and his blood pressure is 80/40 mm Hg. How would you manage this case? The management consists of pressure on the area of bleeding, insertion of a large bore cannula (16 G) into an arm vein, withdrawal of blood for cross matching, and rapid infusion of 500 ml normal saline. It is difficult to estimate the blood loss in this case. However, since he is fit and young he must have lost at least one litre of blood for his pulse to have increased and blood pressure dropped to that extent. You should therefore request one to two litres of blood. Follow the normal saline with a colloid such as Gelofusine (2 x 500 ml solution), carefully monitoring the pulse and blood pressure. Once the crossmatched blood has arrived, replace the colloid with blood and continue at the same rate till the systolic blood pressure rises above 100 mm Hg. The infusion can then be slowed down to maintain the pulse and blood pressure. This man will have to go to theatre to have his wound explored and you should make arrangements for this during the resuscitation.

Plasma protein fraction (human albumin solution)
Human albumin solution is a good plasma expander, although it is more expensive than synthetic colloids. It has a half life of 5-15 days and may be used for plasma replacement in severely burnt patients. Because of the way it is produced there are no risks of transmission of disease.

Fresh frozen plasma
Fresh frozen plasma is usually given to patients with multiple coagulation defects associated with severe liver disease, intravascular coagulation, and massive blood transfusion. It has no place as a plasma expander. Patients with coagulation defects usually require four to six units over one to two hours. The transfusion should be repeated if the prothrombin time remains deranged.

Concentrates of specific factors
Concentrates prepared from plasma are available for factor VIII deficiency (haemophilia). Concentrates for other factor deficiencies such as antithrombin III and commercially made protein C are currently being assessed for clinical use.

Treatment of hypovolaemia
Acute hypovolaemia due to blood loss
In patients who are hypovolaemic and losing blood it is important to stem any external bleeding using pressure, set up a good peripheral intravenous line using the biggest cannula practicable, take blood for cross matching and baseline haemoglobin concentration, and start rapid infusion of 500 ml of normal saline followed by a colloid solution. If the bleeding is catastrophic request unmatched O negative blood (universal donor blood). Try to estimate the volume of blood lost and request that amount of whole blood (it is better to err on the side of caution and overestimate the blood loss). Figure 2 shows the intravenous strategy used depending on the estimated blood loss. If the bleeding is still persistent once the patient is stable (systolic blood pressure over 100 mm Hg) insert a central venous pressure line and a urinary catheter to monitor progress. Give whole blood in appropriate quantities as soon as it is available.

Gradual hypovolaemia
Gradual hypovolaemia usually arises in a patient whose fluid loss is persistent and its full extent not appreciated. The usual signs of this are tachycardia and reduced urine output. In severe cases hypotension may occur. In these cases a fluid challenge may be needed to determine if the reduced urine output is due to hypovolaemia. Give 200 ml of colloid rapidly (a central venous pressure line may be needed in patients with myocardial compromise). If this improves the urine output and blood pressure with only a transient or no improvement in the central venous pressure, then hypovolaemia is confirmed and further fluids are given as appropriate.

Case history

A 64 year old man with a history of angina is seen on the second day after a left hemicolectomy. His pulse is 110/min, urine output is 60 ml over the previous four hours, and blood pressure 110/80 mm Hg (preoperative blood pressure 140/90 mm Hg). Blood results from that morning show a haemoglobin concentration of 136 g/l, sodium 140 mmol/l, potassium 3.7 mmol/l, urea 12 mmol/l, and creatinine 145 mmol/l. What is the diagnosis and treatment? This patient is hypovolaemic with signs of renal compromise. A possible contributory factor to his hypovolaemia may have been the preoperative bowel preparation. This usually causes severe diarrhoea and may not have been accounted for in his perioperative fluid replacement. The correct treatment for this patient is to insert a central venous pressure line and give fluid challenge of 200 ml of colloid over a few minutes while monitoring his central venous pressure. The fluid challenge improves his blood pressure and produces a small transient rise in his central venous pressure, confirming that he has hypovolaemia. He therefore requires infusion of one litre of colloid over two hours while monitoring his central venous pressure. After this his condition needs to be reassessed and further fluid resuscitation given as required.

Conclusion
Fluid and electrolyte resuscitation need to be undertaken after calculation of the requirements of the patient based on the sum of the daily maintenance requirements and the estimated losses. In patients who are severely hypovolaemic it is
often helpful to insert a central venous pressure for rapid fluid resuscitation. You should also put in a urinary catheter because it is essential to monitor the outflow of urine.