الفهرس 
PATHOPHYSIOLOGY OF RED BLOOD CELLSOnly 14 pages are availabe for public view
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Abstract
B
lood and blood products are a valuable commodity and treatment modality, and is at times life saving and may not be available when needed. The transfusion of blood and it’s products may also result in a number of avoidable complications. Therefore triggers for transfusion of blood and blood products have always been a controversy in medicine especially critically ill patients with their different comorbidities. Multiple trials have been made in order to come up with guidelines or triggers for this common practice, to conserve blood and minimize the complications that may arise from such a practice.
Anemia in adults has been defined using different morphological, physiological and etiological criteria. The most commonly used are roughly a Hb level <13 g/dL in males and <12g/dL in females, morphologically according to RBC size and Hb content, into microcytic, normocytic, macrocytic and hypochromic and normochromic respectively. The physiological effects of anemia will result in organ and tissue hypoperfusion, which in turn may result in rising lactate levels, rising renal functions, cardiac arrhythmias, disturbed conscious level and an increase in oxygen extraction. The causes of anemia are numerous are generally due to an increase in blood loss, or a decrease in production each with different etiologies and incidence rates.
Thrombocytopenia has been defined as a platelet count <150,000 /µL. The causes of thrombocytopenia are also generally categorized under causes of increased consumption/loss and decreased production. Although the platelet count may fall between normal ranges, bleeding may still occur due to thromboasthenia, which must be taken into consideration. New tests have been developed for platelet function testing, however the two most commonly used are the thromboelastography and platelet aggregometry.
Many causes may lead to a disturbance in the factor dependent coagulation cascade. The causes include diseases or drugs that cause factor consumption, or decreased production. Other causes or genetic disorders that manifest as a decrease or an error in the production of certain coagulation factors. The testing for the competency of the coagulation system widely used are still the commonly used PT, aPTT, INR and isolated factor assays.
Many trials have been made as attempts to determine and unify transfusion triggers that may be used with adult critically ill patients. A summary of these triggers to transfuse packed RBCs are as follows:
- It is generally recommended to keep the Hb level between 7-9 g/dL.
Transfuse blood when one or more of the following criteria are met:
- Hb level <7 g/dL.
- Evidence of tissue/organ hypoperfusion with a Hb level between 7 and 9 g/dL.
e) Rising lactate levels.
f) Rising renal chemistry with no other identifiable cause.
g) Disturbed conscious level with no other identifiable cause.
h) Increased peripheral oxygen extraction (more than 50%).
- Hb level <8 g/dL in patients with coronary artery disease.
- Hb level <10 g/dL in patients with sepsis with a peripheral oxygen extraction >30%.
- Active bleeding.
- Treat asymptomatic chronic anemia with pharmacological agents
- Hb level <6 g/dL Severe symptomatic chronic anemia
- Keep Hb >10 g/dL in pateints with sickle cell disease
It is important to keep in mind that 1 unit of PRBCs will raise the Hb level by approximately 1g/dL, and the aim is to keep the Hb level generally between 7-9 g/dL. Therefore, an estimate of the number of units to be transfused should be made accordingly. In the event of acute evident bleeding, a liberal transfusion protocol should be used according to the amount of blood loss.
Platelet Transfusion:
Perioperative/Periprocedural
Cardiothoracic Surgery: Counts <100,000/mm3 accompanied by major unexpected microvascular bleeding.
Other Surgical Procedures:
Counts <50,000/mm3 guided by risk factors for intermediate counts.
Neurologic or ophthalmologic procedures require a platelet count near 100,000/mm3
Transfusion may be required with apparently adequate counts when known or suspected platelet dysfunction results in microvascular bleeding.
Specific Procedures:
In the absence of other coagulopathy, major invasive procedures require platelet counts of at least 40,000 to 50,000/mm3 (including CVP placement, paracentesis/ thoracentesis, respiratory tract / GI biopsies, closed liver biopsy, lumbar puncture, sinus aspiration & dental extraction).
A threshold of 80,000/mm3 has been proposed for spinal epidural anesthesia.
Fiberoptic bronchoscopy without biopsy by an experienced operator may be safely performed in the presence of a platelet count <20,000/mm3. GI endoscopy without biopsy may be safely performed at platelet counts <20,000/mm3.
Platelet Function Defects
Use of Antiplatelet Agents
Massive Transfusion:
A transfusion target of >50,000/mm3 is recommended for acutely bleeding patients and >100,000/mm3 for those with multiple trauma or CNS injury. The platelet count may fall below 50,000/mm3 when >1.5–2 blood volumes have been replaced with red cells. In the presence of microvascular bleeding, transfusion may be appropriate when counts are known or suspected to be <100,000/mm3.
Disseminated/Local Intravascular Coagulation (DIC/LIC) and/or Sepsis: Microvascular bleeding is treated in adults with platelet counts <50,000/mm3.
Hematology/Oncology:
Acute Leukemia and Following High Dose Chemotherapy:
A prophylactic transfusion trigger of ≤10,000/mm3 may be used for stable patients. Therapeutic transfusion for major bleeding should maintain counts ≥50,000/mm3.
Chemotherapy for Solid Tumors:
The usual prophylactic transfusion trigger is ≤10,000/ mm3. The greater risk of bleeding from bladder neoplasms / necrotic tumors and the serious impact of even minor bleeding in patients with limited physiologic reserve may warrant a transfusion trigger of ≤20,000/mm3.
Idiopathic Thrombocytopenic Purpura (ITP):
Patients who experience major, life-threatening bleeding or intraoperative hemorrhage should receive high-dose platelet transfusions. Transfusion maybe considered before elective splenectomy with platelet counts ≤10,000/mm3.
Aplastic Anemia:
Transfuse stable patients prophylactically at counts ≤5,000/mm3 and patients with fever or minor hemorrhage at counts 6,000-10,000/mm3.
On average, a platelet concentrate contains 8 x 1010 platelets. The usual dose of pooled platelets is four to six concentrates per transfusion. Based on these average values, one platelet concentrate should increase the 1-hour posttransfusion platelet count by 7000 to 10,000 platelets/μL in a 75-kg man. The usual dose of pooled platelets or an apheresis collection will produce a CCI of 10,000 to 20,000.
The transfusion triggers for plasma and isolated coagulation factors are as follows:
- Isolated factor deficiency with factor concentration <25%.
- Bleeding with an INR >2.
- Prophylactically before a surgery or a major maneuver with an INR >2.
The complications of blood and blood product transfusion are numerous and can be avoidable. They range include and are not limited to transfusion reactions, infection transmission and iron overload. Some are life threatening and debilitating and could be avoided or treated. Therefore, it is of upmost importance to always keep them in mind and be alert of their significance and incidence and to treat them as they emerge.
Practices to minimize blood transfusion should be considered and implemented when considering blood transfusion in a critically ill patient. They include and are not limited to:
- Tranexamic acid or epsilon aminocaproic acid administration
- Aprotinin administration
- Desmopressin administration
- Recombinant activated factor VII administration
- Aritificial O2 carriers administration
- Post-Op bl. recovery techniques
- Closed bl. sampling techniques
- Small-volume sample tubes
- Point-of-care microanalysis
- Erythropoietin administration
- Restrictive red bl. cell transfusion trigger application