Blood transfusion
A blood transfusion is a medical procedure in which whole blood or specific blood components, such as packed red blood cells, platelets, or plasma, are transferred from a donor to a recipient via an intravenous line to restore blood volume, enhance oxygen transport, or supply clotting factors.[1] This intervention addresses acute conditions including hemorrhage from trauma or surgery, severe anemia, and hypovolemic shock, where empirical evidence demonstrates improved survival rates when hemoglobin levels fall below critical thresholds.[2][3] Transfusions utilize fractionated components rather than whole blood to minimize risks like volume overload, with packed red blood cells providing oxygen-carrying capacity, platelets aiding hemostasis in thrombocytopenia, and fresh frozen plasma supplying coagulation factors for deficiencies.[1] Compatibility testing based on ABO and Rh blood group systems, discovered in 1901, prevents acute hemolytic reactions by matching donor and recipient antigens.[4] The procedure's efficacy stems from causal mechanisms where transfused erythrocytes directly augment tissue oxygenation, as validated in clinical trials for perioperative and critical care settings.[1] Key historical milestones include the first successful human-to-human transfusion in 1818 by James Blundell for postpartum hemorrhage and the development of anticoagulants like citrate in the early 20th century, enabling safe storage and large-scale use during World War I.[4] Advances in pathogen inactivation and screening have reduced infectious transmission risks from historical highs, such as HIV outbreaks in the 1980s, to near-elimination today, though bacterial contamination and transfusion-related acute lung injury persist as notable complications.[1][5] Despite benefits in reducing mortality from exsanguination, transfusions carry inherent risks including immune-mediated hemolysis, allergic responses, and immunomodulation potentially exacerbating infections or cancer recurrence, with peer-reviewed data indicating febrile non-hemolytic reactions in up to 1% of units transfused.[1][6] Patient blood management strategies, emphasizing restrictive thresholds over liberal transfusion policies, reflect causal evidence that unnecessary transfusions confer net harm without proportional survival gains.[1]Fundamentals
Definition and Physiological Principles
Blood transfusion constitutes the intravenous administration of whole blood or its fractionated components—such as red blood cells, platelets, or plasma—to a recipient whose physiological homeostasis is disrupted by conditions including hemorrhage, anemia, or coagulopathy.[1] This process aims to replenish deficits in oxygen transport, primary hemostasis, or coagulation factors, thereby mitigating tissue hypoxia, uncontrolled bleeding, or hypovolemia through direct supplementation of functional blood elements.[7] Unlike synthetic alternatives, transfusions leverage the inherent biological compatibility of donor-derived cellular and acellular components to integrate into the recipient's circulation, though empirical constraints arise from antigenic mismatches that can precipitate immune-mediated rejection.[1] At the core of transfusion physiology lies the restoration of oxygen delivery, where red blood cells, laden with hemoglobin, bind molecular oxygen in the pulmonary capillaries and facilitate its diffusion to metabolically active tissues based on partial pressure gradients and the Bohr effect.[7] In states of acute anemia or blood loss, hemoglobin concentrations below critical thresholds—typically around 7 g/dL in stable adults, though varying by compensatory mechanisms like increased cardiac output—impair this transport, leading to lactic acidosis and organ dysfunction if unaddressed.[7] Platelets contribute to hemostasis by adhering to vascular endothelium via von Willebrand factor and aggregating through glycoprotein receptors, forming initial plugs that stabilize breaches; their transfusion counters thrombocytopenia where counts fall below 10,000–20,000 per microliter, reducing spontaneous hemorrhage risk via empirical correlations observed in clinical cohorts.[1] Plasma, comprising water, electrolytes, and labile clotting factors (e.g., fibrinogen, prothrombin), supports fibrin clot formation in the coagulation cascade; its administration addresses deficiencies from massive hemorrhage or liver dysfunction, empirically restoring international normalized ratios toward unity to prevent diffuse microvascular bleeding.[8] Compatibility hinges on antigenic matching to avert alloimmune responses, primarily governed by the ABO and Rh (D) systems, where surface glycoproteins on erythrocytes elicit pre-formed immunoglobulin M antibodies in non-matching recipients.[9] ABO incompatibility triggers acute intravascular hemolysis through complement activation and cytokine release, manifesting within minutes as fever, hemoglobinuria, and renal failure, with mortality rates exceeding 5% in severe cases per registry data.[10] RhD mismatches, lacking natural antibodies, induce delayed extravascular hemolysis via IgG-mediated splenic sequestration upon sensitization, underscoring the causal primacy of antigenic identity in preserving transfused cell viability and function.[9] These principles reflect the evolutionary adaptation of blood as a closed, self-regulating system intolerant to foreign intrusion without immunological tolerance.[10]Blood Components Transfused
Red blood cell (RBC) concentrates, prepared by centrifugation of whole blood to remove plasma and achieve a hematocrit of 55-65%, serve to restore oxygen-carrying capacity.[11] These units are suspended in additive solutions like AS-1 or CPDA-1, which inhibit glycolysis and hemolysis by providing nutrients such as adenine and dextrose.[12] Stored at 1-6°C, RBC concentrates maintain viability for 42 days, with FDA standards requiring less than 1% hemolysis at expiration and an average 24-hour posttransfusion recovery exceeding 75%.[13] [14] Platelet concentrates, obtained via apheresis from single donors or pooling from multiple whole blood units, provide hemostatic support by replenishing clotting platelets.[15] They are stored at 20-24°C with continuous agitation to preserve function and prevent clumping, yielding a shelf life of 5-7 days depending on bacterial testing protocols.[16] [17] Fresh frozen plasma (FFP) is separated from whole blood and frozen within 8 hours of collection to preserve labile coagulation factors.[18] Containing all plasma proteins including factors II, V, VII, VIII, IX, X, and fibrinogen, FFP is stored at -18°C or colder for up to 1 year.[19] Cryoprecipitate, derived by controlled thawing of FFP at 1-6°C followed by recentrifugation, concentrates fibrinogen, factor VIII, von Willebrand factor, factor XIII, and fibronectin into a smaller volume.[20] It is refrozen and stored at -18°C or below, with a shelf life of 1 year.[21] Whole blood, less commonly fractionated for transfusion in civilian settings but employed in military and prehospital trauma resuscitation, delivers balanced RBCs, plasma, and platelets in their native ratios.[22] Recent advances have expanded its use in select civilian trauma protocols to mimic physiological hemostasis.[23]| Component | Key Contents/Roles | Storage Conditions | Shelf Life |
|---|---|---|---|
| RBC Concentrates | Oxygen transport (hematocrit 55-65%) | 1-6°C | 42 days |
| Platelet Concentrates | Hemostasis (clotting) | 20-24°C, agitated | 5-7 days |
| Fresh Frozen Plasma | Coagulation factors, proteins | ≤-18°C | 1 year |
| Cryoprecipitate | Fibrinogen, FVIII, vWF, FXIII, fibronectin | ≤-18°C | 1 year |
| Whole Blood | Integrated RBCs, plasma, platelets | 1-6°C (limited use) | 21-35 days |