44. BLOOD TRANSFUSION

Blood should only be given after careful clinical and laboratory assessment. It can be life saving or prevent significant morbidity, but there is often a shortage of blood and a blood transfusion can cause complications.

There are other techniques to reduce blood loss and minimise the patient’s need for transfusion (e.g. positioning, haemodilution, vasoconstrictors, tourniquet, and blood pressure control).

The anaesthetist must consider the patient’s age, haematocrit (Hct), ongoing blood loss, rate of blood loss, cardiac and respiratory disease, and availability and efficiency of screening of blood before giving blood. Having considered the risks and benefits the anaesthetist should record their decision and reason for giving blood in the patient’s chart.

Blood Transfusion Therapy

Blood is transfused to correct anaemia, thombocytopenia or coagulopathy.

Anaemia

The main reason for transfusion of red blood cells is to maintain the oxygen carrying capacity of the blood.

Oxygen (O2) is carried in the blood in two ways. A very small amount is dissolved in blood (0.003 ml/dL of blood/mmHg) and the majority is bound to haemoglobin (Hb). When 100% saturated, each gram of haemoglobin contains 1.34 ml of oxygen. Arterial blood is usually 97% saturated. Therefore, arterial blood contains 19.8ml of oxygen per decilitre (0.29 ml dissolved and 19.5 ml bound to haemoglobin).

The quantity of oxygen made available to the body’s tissues each minute (oxygen delivery) is equal to the cardiac output times the amount of oxygen in the blood.

(The quantity of oxygen dissolved in the blood is tiny and not available to the tissues so may be excluded from calculations.)

Oxygen delivery (ml O2/min) = Cardiac output (litre/min) x Hb (g/litre) x 1.34 (ml O2/g of Hb) x % saturation.

[In the normal adult this is 5000 ml blood/min x 15 g/l x 1.34 x 0.97 = approximately 1000 ml O2/min].

If the patient’s cardiac function is normal and the circulating blood volume is kept normal (normovolaemia) then acute falls in haematocrit to 20% or 25% are well tolerated. Oxygen delivery is maintained by a 2.5 fold increase in cardiac output. An increase in heart rate, stroke volume and a decrease in blood viscosity increase cardiac output. It is essential that the patient be kept normovolaemic. A patient who has blood loss and is allowed to become hypovolaemic will suffer significant complications.

A healthy adult may tolerate a loss of 30% of their blood volume (Hb 7 to 8 g/dL or haematocrit 21 to 24%) if normovolaemia is maintained. A less healthy patient may only tolerate a 20% loss of blood volume, and a patient with poor health may only tolerate a 10% loss.

Patients who are unable to increase their cardiac output, who have decreased respiratory function, who have limitation of flow to vital organs (e.g. coronary artery disease) or pre-existing anaemia will not tolerate a large fall in haematocrit. They must remain normovolaemic and will need an earlier blood transfusion.

The anaesthetist should decide preoperatively how much blood loss is acceptable before giving a blood transfusion.

Transfusion is rarely indicated if the Hb is greater than 10 g/dL and is almost always indicated if the Hb is less than 6 g/dL, especially if the anaemia is acute.

To maintain normovolaemia, intravenous crystalloids need to be given at 3 times the estimated blood los, and colloids given in an amount equal to the volume of blood lost. 5% dextrose produces little effect on blood volume. The anaesthetist must estimate the blood lost. Maintenance intravenous fluid should be at least 5mls/kg/h for an adult.

One unit of packed red blood cells (Hct 70%, volume 250 ml) will usually raise the haematocrit of the adult patient by 2 to 3%.

Thombocytopenia

Spontaneous bleeding is unusual with platelet counts above 20,000/ml. Platelet count of above 50,000/ml is preferred for surgery. One unit of platelets increases platelet count by 5,000 to 10,000/ml.

Coagulopathy

Blood usually coagulates appropriately when coagulation factor concentrations are at least 20 to 30% of normal and when fibrinogen levels are greater than 75 mg/dL. Replacement of an entire blood volume usually reduces coagulation factors to approximately one third of normal. Fresh frozen plasma in a dose of 10 to 15 ml/kg generally increases plasma coagulation factors to 30% of normal. Fresh frozen plasma should be used for massive transfusion with active bleeding; urgent reversal of warfarin or to treat inherited or acquired coagulopathy.

Estimating Blood Loss

A preoperative Hb or Hct should be taken. Patients with preoperative anaemia should be investigated and treated before elective surgery. Oral iron (ferrous sulphate 200 mg thee times a day for an adult and 15 mg/kg/day for a child) will raise the haemoglobin level by about 2 g/dL within thee weeks in a patient with iron deficiency anaemia.

The decision to give a blood transfusion will depend on the patient’s health and the percentage of the patient’s blood volume that is lost. (Neonates and small infants need only lose a small volume of blood to lose 20% of their blood volume). The blood volume of a neonate is 90 ml/kg, children 80 ml/kg and an adult 70 ml/kg.

The anaesthetist must estimate the amount of blood loss and monitor for signs of blood loss.

Measure the amount of blood in suction bottles. (Remember to subtract the volume of irrigation fluid). Estimate the amount of blood on surgical drapes and the floor and estimate the amount of blood in swabs and packs. Swabs and packs can be weighed and the dry weight subtracted. 1ml of blood weighs about 1 gram. (The small approximately 4 inch swabs hold about 5 ml of blood. The small packs hold about 20 ml and the large packs about 50 ml).

The anaesthetist must continually assess the blood loss because many of the signs of blood loss will not be apparent under general anaesthesia (restlessness, confusion, sweating, thirst). There are many causes of hypotension but hypovolaemia is a very common cause.

The patient must be kept normovolaemic. Preoperative and intra-operative fluid and blood loss must be replaced. The patient should not be hypotensive or tachycardic. Urine output should be greater than 0.5 to 1 ml/kg/h.

When to Transfuse

The benefit of transfusion must outweigh the risk of transfusion for the patient. There should be specific clinical or laboratory indicators for the transfusion.

The decision to transfuse blood can be made in two ways. Calculate the patient’s blood volume and decide on the percentage of the blood volume that can be safely lost, depending on the clinical condition of the patient and provided normovolaemia is maintained. Alternatively the anaesthetist can decide on the lowest acceptable Hb or Hct that is safe for the patient and calculate the allowable blood loss before requiring transfusion. Blood loss up to the allowable volume must be replaced with crystalloid or colloid to maintain normovolaemia. Blood loss greater than the calculated allowable loss will need to be replaced with blood.

The allowable blood loss can be approximately calculated.

Allowable blood loss =

blood volume x (preop Hb – lowest acceptable Hb)/ preop Hb

The volume of blood to transfuse can be estimated

Blood volume to transfuse =

 (Hct desired – Hct present) X blood volume/Hct transfused blood

Minimising Blood Transfusion

Good anaesthetic and surgical technique can reduce blood loss.

The patient can be positioned with the operative site above the level of the heart. A tourniquet (inflated 100 to 150 mmHg above the systolic blood pressure) can be used on the operative limb. Vasoconstrictors can be infiltrated along the incision. Good surgical technique should stop bleeding points.

The anaesthetist must avoid hypertension. Ensure that the patient is adequately anaesthetised. Avoid coughing, straining and patient manoeuvres that increase venous pressure. Avoid hypercarbia that will cause vasodilatation. Use regional anaesthesia (spinal or epidural) where appropriate. Avoid hypothermia and give adequate analgesia.

Controlled hypotension reduces blood loss but if performed poorly can cause significant morbidity. Hypotension may cause organ ischaemia, particularly heart, liver, kidneys, brain and spinal cord. The patient must be normovolaemic. Deliberate hypotension is considered too dangerous by some anaesthetists.

Preoperative donation. Units of the patient’s own blood can be collected every 5 to 7 days up to 35 days before the surgery. The blood must be tested, labelled and stored. The patient is given oral iron supplements.

Normovolaemic haemodilution is appropriate if the surgical blood loss is expected to be greater than 20% of blood volume. The patient should have a haemoglobin greater than 10 g/dL. A volume of blood is removed immediately prior to surgery into a blood donation bag, labelled and stored at room temperature for reinfusion within 6 hours. At the same time the patient is kept normovolaemic by the infusion of crystalloid or colloid. This will haemodilute the patient. The anaesthetist should aim for a Hct of about 30%. Blood loss during the surgery will contain fewer red blood cells. The collected blood is reinfused, preferably after surgery. This fresh blood will contain near full concentrations of platelets and coagulation factors. Efficacy is greatest when substantial haemodilution is followed by significant blood loss.

Blood recovery is the aseptic collection of blood from the wound or body cavity and its reinfusion to the patient. The blood is anti-coagulated, washed, filtered and stored. Systems may be automatic (cell saver) or in the simplest form blood is collected with a small bowl or low pressure suction and filtered though at least 8 layers of sterile gauze into a sterile bottle containing anticoagulants

Complications of Blood Transfusion

Blood should be kept refrigerated. It can be stored for 35 to 42 days depending on which anti-coagulant/preservative solution is used. Blood at room temperature should be used within 4 hours. After 4 hours the blood should be discarded.

Whole blood consists of approximately 450 ml of blood with 65 grams of haemoglobin. Packed red blood cells contain about 200 ml with a haematocrit of 80 percent.

A massive blood transfusion in an adult may be considered as more than 10 units within 6 hours or more than 5 units in 1 hour or more than one blood volume within 24 hours. These patients are at risk of complications.

There are several possible complications though it is often the underlying cause of the haemorrhage and the end result of major haemorrhage that cause complications rather than the transfusion itself.

Massive transfusion of refrigerated blood will cause hypothermia. Hypothermia will increase oxygen consumption and increase bleeding. A blood warmer should be used.

There will be dilution of platelets and coagulation factors. Stored blood has no platelet activity after 24 hours and there is progressive loss of coagulation factors, especially factors V and V111. Platelets should be given if the platelet count falls below 50,000 and there are signs of microvascular bleeding or if the platelet count falls below 20,000. Fresh frozen plasma (10 to 15 ml/kg) should be given to correct bleeding due to reduced coagulation factors.

Citrate toxicity and hypocalcaemia is rare. Citrate is usually rapidly metabolised to bicarbonate (which neutralises the acid load of transfusion). Hypocalcaemia in combination with hypothermia and acidosis may cause decreased cardiac output and arrhythmias. Hyperkalaemia is rarely of clinical significance. Acidosis is usually the result of inadequate resuscitation rather than the transfusion.

Disseminated intravascular coagulation (DIC) may occur with massive transfusion.

Complications may also occur with routine blood transfusion. These may be immune or non-immune.

Non-immune complications include infection, hypervolaemia, iron overload and hypothermia.

Numerous different viruses, bacteria and protozoa can be transmitted though blood transfusion (including hepatitis B, hepatitis C, HIV, CMV, syphilis, malaria and Chagas’ disease). The efficiency with which different countries reduce the risk of transmission though screening donors for risk factors and laboratory tests varies.

Iron overload will only occur with chonic transfusions.

Transfusion Reactions

Immune complications include acute haemolytic transfusion reactions, delayed haemolysis and febrile non-haemolytic reactions.

Acute haemolytic reactions may occur in 1 in 6,000 to 1 in 30,000 transfusions. Most are due to clerical errors. The most common cause of severe transfusion reactions is the patient being given the wrong blood. Even a small volume (10 to 50 ml) of the incorrect blood can cause a severe reaction. Patients may complain of chest pain, flank pain, headache, dyspnoea, chills and fevers. They show signs of anxiety and agitation. Under general anaesthesia there are fewer signs (fever, hypotension, tachycardia, bleeding, haemoglobinuria). Transfusion reactions occur during or shortly after transfusion.

Treatment of Transfusion Reaction

Stop the transfusion.

Send the unused donor blood and a fresh sample of the patient’s blood for re cross matching.

Send blood samples for free Hb, haptoglobin, Coombs test and DIC screening if available.

Replace the infusion set with normal saline.

Be prepared to maintain the blood pressure and oxygenation. Give 100% oxygen. Give intravenous fluids. Give adrenaline

Preserve renal function. Monitor the urine output. Maintain normovolaemia.

Be alert for DIC.

Pre-Transfusion Check List

Before giving blood always check

The identity of the patient against notes and transfusion form. Is it the correct patient?

The label on the blood and the transfusion form. Is it the correct blood?

The donor blood group and the patient blood group. Is the blood compatible?

The expiry date on the blood.