38: Transfusion in Critical Care

INTRODUCTION

Fresh, warm whole blood most effectively restores red cell mass, plasma volume, clotting factors, and platelets. However, given shortages of blood products, the use of whole blood transfusions is not realistic. The use of component product transfusion is the mainstay of blood banking and transfusion practice, effectively utilizing a scare resource while matching the components transfused to the specific needs of the patient. Whole blood is usually separated into packed red blood cells (PRBC), fresh frozen plasma (FFP), and platelet concentrate soon after donation. The plasma can be further separated into cryoprecipitate, cryo-poor plasma, or undergo further fractionation to individual plasma proteins.

Indications for blood component therapy can be divided into two main categories: (1) enhancement of oxygen carrying capacity by increasing red blood cell (RBC) mass and (2) replacement of coagulation components due to loss, dysfunction, or consumption.

ANEMIA AND PACKED RED BLOOD CELL TRANSFUSION

Anemia is one of the most common abnormal laboratory findings among critically ill patients. The effect of anemia on outcome and the determination of transfusion triggers has been the subject of much debate in recent literature.

Historically, the decision to transfuse has been guided by the hemoglobin (Hb) concentration, usually 10 mg/dL. However, given the risks associated with PRBC transfusion and literature supporting better or similar outcomes with lower transfusion triggers, the optimal Hb level at which to transfuse patients remains unclear.

Benefits of RBC Transfusion

The main function of RBCs is to transport oxygen from the lungs to the peripheral tissues. Oxygen delivery (DO₂) is calculated by multiplying the cardiac output (CO) times the arterial oxygen content (CaO₂):

where DO₂ is in mL/min, CO in dL/min, and CaO₂ in mL/dL. And CaO₂ is calculated by the following equation:

where Sao₂ is the arterial oxygen saturation (in %), 1.34 is the oxygen carrying capacity of hemoglobin in mL/g, [Hb] is the hemoglobin concentration (in g/dl), 0.0031 is the solubility of oxygen in plasma at 37°C, and Pao₂ is measured in mm Hg.

Under normal conditions, DO₂ exceeds oxygen consumption (VO₂) by three to five times. However, in situations where the VO₂ of the peripheral tissues is greatly increased, or DO₂ is decreased by anemia or decreased CO, VO₂ can exceed DO₂ and result in tissue hypoxia. Increasing the [Hb] is one of the ways to increase the blood's oxygen carrying capacity and therefore increase DO₂. Additionally, transfusion can increase blood volume for patients following acute blood loss or hemorrhage and alleviate ...