Clinical evaluation of a bleeding patient (Proceedings)

Abnormal bleeding or bruising is frequently encountered in veterinary clinical practice. Accurate diagnosis and treatment of the bleeding patient requires a basic understanding of the pathophysiology of hemostasis. A working knowledge of the hemostatic mechanism along with a complete medical history, thorough physical examination, and specific laboratory tests are integral steps in the diagnostic process. The most appropriate treatment options can only be identified after the cause of bleeding has been determined.

Hemostasis

Hemostasis, the body's balancing mechanism of arresting hemorrhage while simultaneously maintaining blood flow within the vascular compartment, occurs through a complex series of events involving the vessels, platelets, plasma coagulation factors and the fibrinolytic system. The role that each component plays in hemostasis is dependent on the size of the vessel and the amount of damage that has occurred. Bleeding in smaller vessels may be controlled by a simple response involving the vasculature and platelets (e.g., normal wear and tear on capillaries), whereas incorporation of the plasma coagulation factors are necessary for hemorrhage control involving larger damaged vessels.

The first response to blood vessel injury is vasoconstriction, which allows for diversion of blood flow around the injured area. Once the endothelial lining of the vessel is disrupted, the subendothelial connective tissue (i.e., collagen fibers) is exposed. Circulating platelets pool to the area of injury and, with the help of certain adhesive proteins (i.e., collagen, fibrinogen, fibronectin, von Willebrand factor), adhere to the endothelial lining to arrest the initial episode of bleeding. This process is known as platelet adhesion. Once the platelets adhere to the subendothelium, they change shape and secrete certain biochemical substances that enhance platelet layering in the injured area. The platelets form a complete but unstable plug. This portion of the hemostatic process involving the vasculature and platelets is referred to as primary hemostasis and is usually adequate to stop bleeding in smaller vessels.

With greater damage to larger vessels, coagulation factors are needed to form a stable fibrin clot, a process known as secondary hemostasis. Blood coagulation involves a complex process by which the multiple coagulation factors contained in blood interact in three major pathways: the intrinsic, extrinsic and common pathways.

Plasma coagulation factors (denoted by Roman numerals) are produced in the liver, many with the help of vitamin K. They circulate in the blood in the inactive form and become activated only when exposed to certain substances. Initiation of the extrinsic and intrinsic clotting pathways leads to subsequent activation of all factors in a cascade-like effect.

Tissue factor (thromboplastin) is released from the injured vessel wall and initiates the extrinsic clotting pathway. This is an extravascular process in that tissue thromboplastin is not normally found in blood and must gain entry to the vascular system. Clotting via the intrinsic pathway begins when blood comes into contact with a foreign substance or surface (i.e., damaged endothelium). Activated platelets release a phospholipid allowing coagulation factors in this pathway to activate one another. In the intrinsic pathway, all factors necessary for clot formation are within the intravascular compartment. Both the extrinsic and intrinsic pathways merge into the common pathway, where the end result is the creation of fibrin, a threadlike protein. The fibrin threads form an insoluble meshwork over the site of the platelet plug, consolidating and stabilizing the clot.

**For simplicity of presentation, the pathways have been reviewed as divided processes. The reader must realize that classic cascade presentation of fibrin formation has many underlying complexities and interrelationships that go beyond the scope of this paper.

The fourth and final step in the hemostatic process is fibrinolysis. Once the vessel is healed, fibrinolytic enzymes digest the clot that has been formed, restoring normal blood flow. Clot lysis produces small pieces of fibrin, referred to as fibrin split products (FSP) (or fibrin degradation products FDP), which are cleared from circulation by the liver. Small levels of FSPs always appear in the circulation as a result of bleeding and clotting secondary to normal wear-and-tear on vessels. FSP levels increase during episodes of excessive bleeding with diffuse coagulation (i.e., disseminated intravascular coagulation {DIC}) and in patients with compromised liver function. Following clot digestion, vessel wall endothelium is reestablished and returned to its original state.

In summary, bleeding disorders can be categorized into three groups: disorders of platelet function or number, disorders of clotting factors, and a combination of both.

Clinical assessment

An accurate history, thorough physical exam, and certain laboratory tests must be performed in order to properly evaluate a bleeding patient, determine a diagnosis, and define a therapeutic plan.

History

A complete history is critical in beginning a work-up for a hemostatic defect. In veterinary medicine, all pertinent information regarding patient history must be gathered from the owners. Obtaining and assessing a complete and detailed history will help define the nature, severity, and duration of clinical signs and aid in making a correct diagnosis. This attention to detail allows the clinician to establish probability for each possible differential early in the diagnostic process.

Questions should be very clear and thought provoking. Devising a list of questions for owners to review will hopefully help stimulate them to think of some very important, most likely not obvious facts. Does the animal have any previously diagnosed diseases? Is the animal currently on any medication? A list of any prescription or over-the-counter medications should be included as many drugs have potentially harmful or complicating side effects, resulting in a toxic effect on red blood cells, white blood cells, and platelets. Complete vaccination history should not be overlooked as a relationship between recent vaccination and onset of immune-mediated hemolytic anemia (IMHA) has been questioned. The animal's environmental history may suggest potential exposure to toxic or organic substances such as anticoagulant rodenticide poisons or lead. Tick exposure should also be investigated.

It is vital to evaluate the current bleeding episode and characterize the bleed as localized or multifocal. Is this the animal's first bleeding episode, or is there a history of bleeding tendency? These facts may help differentiate between an acquired or hereditary disorder. Specific breeds may suggest specific coagulopathies. Any information the owner may have regarding breed history could provide helpful clues.

Physical exam

A complete physical examination and multiple monitoring procedures may be required to properly assess the patient in a bleeding crisis. Optimal assessment cannot be based on the result of a single parameter, but is based on the results of several physical exam findings and monitored parameters which should always be evaluated in relation to one another.

Certain clinical signs found on physical exam may help determine the origin of the bleeding episode. Small surface bleeds (e.g., petechiation, ecchymosis, epistaxis, hematuria) are usually suggestive of platelet or vascular abnormalities. Larger bleeds or bleeding into body cavities (e.g., hematoma formation, hemarthroses, deep muscle hemorrhage) are suggestive of clotting factor deficiencies. A combination of these clinical signs is not uncommon.

In anemic patients, the development and progression of clinical signs depends on the rapidity of onset, degree, and cause of anemia, as well as the animal's physical activity. Common physical findings are those associated with a decrease in red cell mass: lethargy, weakness, pale mucous membranes, tachycardia, tachypnea, and bounding pulses. The cardiovascular and respiratory system should be carefully evaluated. Assessment of perfusion is based on mucous membrane color, capillary refill time (CRT), heart rate, and pulse rate, strength, and character. In a severe anemic state, a low-grade systolic flow murmur may occur secondary to decreased blood viscosity. Mucous membrane color can be used to monitor the patient's response to therapy or indicate the development of a problem. Prolonged CRT is suggestive of compromised tissue perfusion and shock, but may be difficult to assess in an anemic patient. Weak and rapid pulses suggest severe dehydration and poor perfusion; bounding pulses suggest anemia. Assessment of respiratory rate and effort, as well as careful auscultation, may help differentiate between decreased oxygen carrying capability and possible pulmonary thromboembolism. Monitoring all parameters in unison with one another will lend information regarding bleed severity and potentially life-threatening complications.

Laboratory tests

Although information obtained from the history and specific clinical signs may suggest a diagnosis, certain laboratory tests are necessary for definitive diagnosis. Laboratory tests should be performed ASAP and therapy instituted promptly after test samples are obtained.

Serial hematocrit determinations may help demonstrate progression or stabilization of bleeding, taking into account the body takes a certain amount of time to equilibrate following an acute bleeding episode. Anemia is suggested when one or more of the red cell parameters are below normal for the age, sex and breed of the species concerned. Of these parameters, PCV provides a simple, quick, and accurate means of detecting anemia, and allows classification of the anemia as mild, moderate, or severe. Dehydration and splenic contraction may mask anemia, whereas hemodilution may cause a temporary reduction in red cell parameters. Evaluating both PCV and total plasma protein (TPP) levels may help in differentiating these variables. Dehydration is associated with increases in both PCV and TPP, while PCV elevation alone is seen with splenic contraction . Decreases in both PCV and TPP are associated with hemodilution following acute blood loss or fluid therapy, whereas a reduction in PCV only is usually associated with hemolytic anemias.

Normal platelet count is 150,000-400,000/µl. Abnormal bleeding may occur with platelet counts below 40,000/µl, although each patient varies and some animals may not exhibit clinical signs associated with bleeding with a platelet count of 2,000/µl. The thrombocytopenic patient requires special care (e.g., extra cage padding, avoidance of central vessels for blood collection, extended application of pressure to venipuncture sites). In an animal exhibiting signs of surface bleeding with a normal platelet count, consideration should be given to the function of the platelets.

Certain tests are available to monitor coagulation in patients with suspected coagulopathies. Prothrombin time (PT) measures extrinsic and common clotting pathway activity, whereas activated partial thromboplastin time (aPTT) measures intrinsic and common pathway activity. Prolongation of PT/aPTT will be seen when clotting factors are depleted below 30% of normal. PT and aPTT samples must be collected and processed carefully to avoid potential sample errors. Atraumatic venipuncture and smooth blood flow into collection tubes are necessary to avoid extraneous clotting mechanism activation. Samples should be processed immediately after collection and frozen if being sent to an outside laboratory.

Elevation in FSPs occurs with excessive bleeding and fibrinolysis, and in animals with severe liver dysfunction. Interpreted in conjunction with the PT, aPTT, and platelet count, elevated FSP levels are useful as a diagnostic indicator of DIC.

Practical hemostatic tests

The following are simple, in-house tests requiring no specialized equipment. They are quick, inexpensive, practical tests that allow recognition and characterization of hemostatic defects. These tests are often referred to as "cage-side", in that they provide results almost immediately.

Platelet estimation

A quick, reasonably accurate estimation of platelet numbers can be made from a stained blood smear is much quicker than an actual platelet count. After routine preparation and staining, the blood smear is scanned to ensure even platelet distribution and that there is no evidence of platelet clumping. The average number of platelets in approximately 5-10 oil immersion fields is counted to estimate platelet numbers. The count is ranked as very low, low, normal, or high. One platelet per oil-immersion field represents approximately 20,000 platelets. Approximately 8-12 platelets per oil-immersion field are considered normal.

While platelet estimation helps determine the presence of thrombocytopenia in an emergency situation, a true platelet count is necessary to classify the severity of depletion. Ongoing platelet quantitation is helpful in monitoring the course of a disease or the patient's response to certain therapies.

Buccal mucosal bleeding time

Bleeding time is the time it takes for bleeding to stop after severing a vessel. The bleeding time test most often used in veterinary medicine today is the buccal mucosal bleeding time (BMBT).

The BMBT assesses platelet and vascular contribution to hemostasis, thereby evaluating primary hemostasis. A disposable template with two spring-loaded blades is used to produce standardized incisions in the buccal mucosal surface of the upper lip. The blades create 5mm long X 1 mm deep incisions. The duration of bleeding from these incisions is monitored.

Buccal mucosal bleeding time

Materials

• bleeding time device
• gauze strip
• filter paper or gauze sponges
• timing device

Procedure

1. Place animal in lateral recumbency.

2. Expose mucosal surface of upper lip. Position a gauze strip around the maxilla to fold up the upper lip. Tie the strip gently, just tight enough to partially block venous return.

3. The incision site should be void of surface vessels and slightly inclined so that shed blood from the incision can flow freely toward the mouth. Place bleeding time device flush against mucosal surface, applying as little pressure as possible, and press tab to release scalpels.

4. Let stab incisions bleed freely, undisturbed, and time until bleeding stops. Excessive blood should be blotted as often as necessary so as not to have blood flow into patient's mouth. Place either filter paper or gauze sponge approximately 3-4 mm below the incision, taking care not to disturb the incision site and any clot that may be forming.

5. The end point is recorded when the edge of the filter paper/sponge does not soak up free-flowing blood. The bleeding time is the mean bleeding time for the two incisions. Normal bleeding time is less than 4 minutes.

The BMBT is a screening test. As with any screening test, it is not 100% sensitive and, therefore, not all primary hemostatic defects will be discovered. This test also will not differentiate between vascular defects or platelet function defects. The BMBT is prolonged in cases of thrombocytopenia/pathia, von Willebrand's disease, uremia, and aspirin therapy. Obviously, BMBT should not be performed on any patient that is known to be thrombocytopenic. Although it does have limitations, there are several advantages to this test. Commercial bleeding time devices are readily available. The templates are standardized and therefore, results are reproducible. It is a simple and quick test to perform, and the results are almost immediately available. Patients seem to tolerate the procedure well, eliminating need for chemical restraint. The incisions produced are well above the concentrated pain fibers in the lip. Sometimes the animal will reflex upon hearing the noise the scalpels make when released from the device, but the procedure itself is not painful.

The CBT is another bleeding time test. The CBT is useful for evaluating overall hemostasis. It is sensitive to defects in vascular integrity, platelet function, and coagulation.

Activated clotting time

The activated clotting time (ACT) is a simple, inexpensive screening test for severe abnormalities in the intrinsic and common pathways of the clotting cascade. It evaluates the same pathways as aPTT. Some argue that ACT is less sensitive at detecting factor deficiencies than aPTT in that factors must be decreased to less than 5% of normal in order to prolong ACT, whereas the aPTT will be prolonged with factor deficiency less than 30% normal.

Activated clotting time test

Materials

• vacutainer sleeve
• vacutainer single collection needle
• ACT tube containing diatomaceous earth
• 370 C electric heat block (can substitute hot water bath or hold in hand)

Procedure

1. Warm ACT tube in heat block to 370 C for approximately 3 minutes.

2. Perform clean venipuncture on an unthrombosed vessel. Discard the first few drops of blood to eliminate tissue thromboplastin, the tissue factor responsible for activation of the extrinsic pathway.

3. Puncture the ACT tube with the distal needle and collect approximately 2 milliliters of blood. Begin timing as soon as blood enters the tube.

4. After collection, invert the tube several times to mix with diatomaceous earth and place in heating block.

5. After thirty seconds from start of timing, gently tilt the tube and examine for clot formation. Return tube to heat block and repeat procedure every ten seconds.

6. The ACT time is the time from collection of blood in the tube to initial clot formation. In the dog, the normal is 60-110 seconds. In the cat, the normal is 50-75 seconds.

Prolongation of ACT occurs with severe factor deficiency in the intrinsic and/or common clotting pathway (e.g. Hemophilia), in the presence of inhibitors (e.g. heparin, warfarin), or in cases of severe thrombocytopenia due to the lack of platelet phospholipid (mild prolongation of 10-20 seconds). The ACT is inexpensive, easily learned, quick to perform, reproducible, and provides immediate results. It is a very useful measurement of coagulation in emergency situations. When compared with the aPTT, the role that technical and laboratory error can have on the test results must be taken into consideration. This is not to suggest that one should rely solely on the ACT. In most situations, the ACT should be followed up with an aPTT.

References available upon request