Definition

Massive transfusion is arbitrarily definied as the replacement of a patient's total blood volume in less than 24 hours, or as the acute administration of more than half the patient's estimated blood volume per hour.

Aim of Treatment

The aim of treatment is the rapid and effective restoration of an adequate blood volume and to maintain blood composition within safe limits with regard to haemostasis, oxygen carrying capacity, oncotic pressure and biochemistry.

Complications of Massive Transfusion

The complications of massive transfusion are those of any blood transfusion plus :

• Blood Volume Replacement
  The complications of massive transfusion are exacerbated by inadequate or excessive transfusion. Traditionally transfusion of hypovolaemic patients has been directed towards maintaining a haemoglobin concentration of 10g/dl. The use of haemoglobin as the only indicator (or 'transfusion trigger') may result in unnecessary administration of blood products, with their concommittant risks.

  Transfusion requirements should be based on the patient's physiologic needs, defined by their oxygen demand (consumption).

  Oxygen consumption is given by :

  

  Where CO = Cardiac Output, CaO2 and CvO2 are arterial and venous oxygen content respectively.

  Oxygen delivery is :

  

  The extraction ratio (ER) is the ratio of oxygen consumption to oxygen delivery, normally around 25%.

  

  The most appropriate monitor of tissue oxygen supply is the tissue oxygen tension, reflected by the PvO2, or mixed venous partial pressure of oxygen (normally 6 kPa, 45mmHg). Patients with a low PvO2 can be classed as stable or unstable depending on haemodynamics, ventilation, acid base status and urine output. If they are stable, no therapy is indicated until a true critical level is reached (PvO2 around 3 kPa, 23mmHg). If unstable, treatment must be intituted.

  Thus transfusion should be guided by haemodynamic stability, PvO2 and ER. Obviously during trauma resuscitation, haemodynamic stability is the key indicator.

  In summary :

  • If Hb > 10g/dl transfusion is rarely indicated.
  • If Hb < 7g/dl transfusion is usually necessary.
  • With Hbs between 7 and 10 g/dl, clinical status, PvO2 and ER are helpful in defining transfusion requirements.
    
    

• Thrombocytopenia
  Dilutional thrombocytopenia is inevitable following massive transfusion as platelet function declines to zero after only a few days of storage. It has been shown that at least 1.5 times blood volume must be replaced for this to become a clinical problem. However, thrombocytopenia can occur following smaller transfusions if disseminated intravascular coagulation (DIC) occurs or there is pre-existing thrombocytopenia.

• Coagulation Factor Depletion
  Stored blood contains all coagulation factors except V and VIII. Production of these factos is increased by the stress response to trauma. Therefore only mild changes in coagulation are due to the transfusion per se, and supervening DIC is more likely to be responsible for disordered haemostasis. DIC is a consequence of delayed or inadequate resuscitation, and the usual explanation for abnormal coagulation indices out of proportion to the volume of blood transfused.

• Oxygen Affinity Changes
  Massive transfusion of stored blood with high oxygen affinity adversely affects oxygen delivery to the tissues. Evidence for this is as yet not forthcoming, but it would seem wise to use fairly fresh red cell transfusions (<1 week old). Use of fresh (<24 hours) blood is not indicated. 2,3 DPG levels rise rapidly following transfusion and normal oxygen affinity is usually restored in a few hours.

• Hypocalcaemia
  Each unit of blood contains approximately 3g citrate, which binds ionized calcium. The healthy adult liver will metabolise 3g citrate every 5 minutes. Transfusion at rates higher than one unit every five minutes or impaired liver function may thus lead to citrate toxicity and hypocalcaemia. Hypocalcaemia does not have a clinically apparent effect on coagulation, but patients may exhibit transient tetany and hypotension. Calcium should only be given if there is biochemical, clinical or electrocardiographic evidence of hypocalcaemia.

• Hyperkalaemia
  The plasma potassium concentration of stored blood increases during storage and may be over 30mmol/l. Hyperkalaemia is generally not a problem unless very large amounts of blood are given quickly. On the contrary, hypokalaemia is more common as red cells begin active metabolism and intracellular uptake of potassium restarts.

• Acid/Base Disturbances
  Lactic acid levels in the blood pack give stored blood an acid load of up to 30-40mmol/l. This, along with citric acid is usually metabolised rapidly. Indeed, citrate is metabolised to bicarbonate, and a profound metabolic alkalosis may ensue. The acid-base status of the recipient is usually of more importance, final acid/base status being dependent on tissue perfusion, rate of administration amd citrate metabolism.

• Hypothermia
  Hypothermia leads to reduction in citrate and lactate metabolism (leading to hypocalcaemia and metabolic acidosis), increase in affinity of haemoglobin for oxygen, impairment of red cell deformability, platelet dysfunction and an increased tendency to cardiac dysrhythmias.

• Acute Respiratory Distress Syndrome (ARDS)
  The aetiology of ARDS is as yet not fully understood, but various risk factors have been identified. Both under- and over-transfusion are associated with an increased risk of ARDS, as is albumin < 30g/l. Microaggregate filters should be used during massive transfusion except when giving fresh whole blood or platelets.