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Image : “Blood” by Lee Haywood . License: CC BY-SA 2.0

Etiology of Acute Blood Loss

Causes of Acute Blood Loss

Blood is constantly circulating throughout our bodies, supplying vital organs with oxygen and nutrients. Damage to blood vessels disrupts the integrity of the vessel walls. Usually, the body’s clotting mechanism helps repair and counteract the damaged vessel walls. However, when the damaged amount exceeds the clotting time, severe bleeding results.

Acute blood loss occurs both internally and externally. Internal bleeding occurs due to damage within tissues, organs, or cavities. It is not easily identifiable and is therefore often undiagnosed. If the bleeding is significant, blood may escape the compact heme system and collect in cavities of the body, forming hematomas.

Internal bleeding usually arises from upper GI bleeds, including bleeding ulcers, varices, and diverticuli; bone damage; subarachnoid hemorrhage arising from a ruptured aneurysm; a ruptured pregnancy; or malignancy. Other causes include physical damage to the body that does not always result in external wounds or lacerations. These are harder to identify and bleeding may occur internally without any visual defects.

External bleeding, which is easier to identify, is the second major cause of acute blood loss. It results from any damage or trauma to the body that creates identifiable blood loss from an open wound. These include lacerations, accidents, severed tissue and organs, lower gastrointestinal bleeds, hematochezia, and hemorrhoids.

Pathogenesis of Acute Blood Loss

Anemia Evoked by Acute Blood Loss

Acute blood loss creates anemia due to the loss of red blood cells and depletion of iron. This type of anemia is sometimes referred to as posthemorrhagic anemia. Hypovolemia is the largest threat, particularly to organs that have a large vascular supply. Hemoglobin is not usually affected, however, so lab tests will not indicate anemia.

The body’s baroreceptors will initiate the release of vasopressin, shifting extravascular fluid to intravascular compartments. This results in hemodilution, which changes hypovolemia to anemia. The plasma that was lost will be replaced by retaining volume in the kidneys. The anemia may be revealed by diluting the red blood cell count. Bone marrow response usually takes 5 to 7 days to respond with reticulocytosis.

In the worst cases, the loss of >20 % of blood leads to hypovolemic shock. Cardiac output is soon decreased, as the volume of circulated blood is significantly reduced. In response, catecholamines, antidiuretic hormone, and angiotensin II are released and lead to increased peripheral vascular resistance. Although the blood supply to the brain and heart stays stable, the perfusion of the extremities (later on even inner organs) is reduced and results in impaired oxygenation.

The affected extremities and organs use anaerobic metabolism to produce lactate. Precapillary dilatation and postcapillary constriction direct the blood toward the capillary bed, contributing to increased hypovolaemia.

Symptoms of Acute Blood Loss

Signs of Acute Blood Loss

Symptoms of anemia

Image: “Main symptoms that may appear in anemia.” by Mikael Häggström. License: Public Domain

Acute blood loss presents with a number of signs and symptoms. Initially, anemia is not present, as blood plasma is lost with red blood cells. However, as plasma and blood volume fall, anemia results. Primary symptoms include dyspnea at rest or exertion, fatigue, bounding pulses, palpitations, lethargy, and confusion. Volume depletion due to bleeding can result in cramping, vertigo, syncope, and hypotension. The most serious complication of acute blood loss is volume depletion due to bleeding, which can result in cramping, vertigo, syncope, and hypotension. This leads to hypovolemic shock. Inadequate blood and volume load result in unperfused organs and decreased cardiac output, causing hypotension and end-organ failure. Angina and even cardiac arrest may occur.

Diagnosis of Acute Blood Loss

Lab work and diagnosis suggest a hemoglobin level <7 g/dl as symptomatic and suggestive of acute anemia. Mean corpuscular volume is usually in the normal range, from 80 to 100 fL. Ultrasound and magnetic resonance imaging/computed tomography imaging can be performed to rule out bleeding locations if not visibly identifiable or if the damage is hidden. Tagged red blood cell studies may also be performed to identify sources of gastrointestinal bleeding.

Treatment and Management of Acute Blood Loss

Blood Transfusion

Image: “Blood Transfusion” by BruceBlaus. License: CC BY 3.0

Identifying the source of the bleed and eliminating hemorrhaging is the first priority. Stabilization is achieved by providing oxygen and cauterizing any bleeds. Blood may also need to be replaced via a transfusion if hematocrit levels are <20%. Red blood cell transfusion is the most efficient way to regulate heme concentration; however, increases in mortality have been reported in patients receiving transfusions.

A hemoglobin level <6.8 g/dl is usually the threshold for transfusions. Packed red blood cells may also be used to raise hematocrit levels.

Patients with acute blood loss should be monitored until stable; this includes monitoring their oxygen saturation levels, placing them on telemonitors, and inserting intravenous lines. Vasopressors are contraindicated. Pregnant patients who are RH-negative and have acute blood loss must be given RhoGAM due to the risk of the fetus’s interaction with the mother’s blood.

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