Anemia

Chapter 120 Anemia



Urs Giger, PD, Dr. Med. Vet., M.S., F.V.H., DACVIM, DECVIM



KEY POINTS



• Anemia is one of the most common problems in the emergency setting and despite not always being acute, it often requires immediate supportive care.

• Although the clinical signs may suggest anemia, routine and specific laboratory tests generally are needed to further define the severity, type, and cause.

• Although (peracute) acute external blood loss anemias are generally easy to identify, internal hemorrhage, hemolysis, and bone marrow failure are other important mechanisms that may not be readily evident.

• The specific underlying cause(s) of any anemia should be determined to formulate an accurate prognosis and therapeutic plan, albeit supportive care is started immediately.

• Whenever possible, appropriate blood samples (ethylenediaminetetraacetic acid, serum, and citrate tubes with adequate volumes) should be collected before any therapeutic intervention, to help reach a definitive diagnosis.

• A low packed cell volume and total protein level classically are seen following external blood loss, and animals with hemolysis typically have a normal to high serum total protein level and hyperbilirubinuria, with or without hemoglobinemia or hemoglobinuria.

• A microscopic evaluation of a blood film can readily provide clues regarding the cause of the anemia and should always be part of an emergency minimum database (see Chapter 121, Acute Hemolytic Disorders).

• Although there is no specific trigger, transfusions are given based on the assessment of clinical signs, rapidity of onset, severity, progression, and underlying cause(s) of the anemia (see Chapter 66, Transfusion Medicine).

• Severely anemic animals with serious clinical signs will likely benefit from blood type–compatible packed red blood cells (RBCs), whole blood transfusions, or alternative oxygen carrying fluids.


INTRODUCTION


Anemia is defined as a reduction in the oxygen carrying capacity of blood due to decreases in hemoglobin (Hb) concentration and red blood cell (RBC) volume. It is certainly one of the most common laboratory test abnormalities in small animals, particularly in an emergency setting. However, anemia is not a diagnosis and thus further clinical investigations are indicated to define the underlying cause. The three mechanisms leading to anemia are blood loss, hemolysis, and reduced erythropoiesis (Figure 120-1).1-3


image

Figure 120-1 Classification of anemias (also see other chapters in this section for more details). DIC, Disseminated intravascular coagulation; GI, gastrointestinal; NSAIDs, nonsteroidal antiinflammatory drugs.


Although anemia may result from primary hematologic disorders, it is much more often associated with other organ disorders. This chapter will discuss the clinical approach to, and general therapeutic principles of, an anemic animal for the critical care clinician, and specific hematologic disorders or problems are addressed in more depth in other chapters (see section titled Hematologic Disorders).



SIGNALMENT AND HISTORY


The breed of the patient may be of particular importance now that many hereditary blood diseases and genetic predispositions in certain breeds have been recognized (e.g., immune-mediated hemolytic anemia [IMHA] in Cocker Spaniels). Although many hereditary erythrocyte defects,2,3 coagulopathies (e.g., hemophilia A and B, factor VII or XI deficiency), von Willebrand disease, and thrombopathias lead to anemia in juvenile animals, some may be recognized only in the older animal after acute, and possibly recurrent, episodes of bleeding have occurred.2,4,5 Furthermore, most hereditary disorders are observed in equal proportions in both genders, although hemophilia A and B, two serious coagulopathies, affect only male animals (X-chromosomal recessive trait).


A carefully taken history and review of previous laboratory test results can often provide clues as to the duration and cause of the anemia. For instance, depending on the geographic location and travel history, exposure to a certain infectious agent such as Babesia, Ehrlichia, Leishmania, and Leptospira spp in dogs and various viral (feline leukemia virus, feline immunodeficiency virus, and feline infectious peritonitis) and bacterial (Mycoplasma haemofelis, other Mycoplasma spp) and parasitic (Cytauxzoon felis) infections in cats (as well as other emerging infectious diseases) may require diagnostic testing by serology, antigen assay, or polymerase chain reaction test. There are heavy metals (zinc and copper) and other chemicals (anticoagulant rodenticides), drugs (e.g., antithyroid drugs [cat], estrogens [dog], heparin, warfarin, aspirin), and even food components (onions, garlic) that represent known triggers of anemia (see Intoxications section). A history of anemia, hemorrhage, icterus, or requirement for transfusion therapy may indicate a recurrent problem. Finally, some concurrent chronic illnesses and organ disorders, such as renal and hepatic failure, diabetes mellitus, or adverse effects of medical therapy (e.g., chemotherapeutics and many other drugs, hypophosphatemia induced by insulin administration, or hyperalimentation) may lead to severe anemia.



CLINICAL SIGNS


The clinical signs of anemia vary greatly depending on the rapidity of onset, type, and underlying cause. It is of utmost importance to determine if hemorrhage, hemolysis, or a hematopoietic production disorder is causing the anemia. Any form of anemia may be associated with pallor, and this may be the only sign in some animals. However, characteristic signs such as hemorrhage with blood loss–induced anemia (Table 120-1) or icterus or pigmenturia with hemolytic anemia may help the clinician determine the etiology.



Table 120-1 Hematologic and Physical Examination Changes With Anemia From External Blood Loss*

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In animals with peracute blood loss, the clinical signs are mostly related to hemorrhage and hypovolemia (hypovolemic shock), and animals with acute and chronic anemias display the more typical signs (e.g., lethargy, pallor, tachycardia, tachypnea). External blood loss is often readily evident, except for animals with gastrointestinal (GI) hemorrhage originating from the nasopharynx to the rectum. In contrast, internal hemorrhage may be hard to localize, especially in animals with retroperitoneal or deep muscle hemorrhage. A single site of hemorrhage may occur secondary to a local process such as trauma or surgery (e.g., a single lacerated vessel, hematomas), but may also be caused by an underlying bleeding tendency. Surface hemorrhage, such as petechiations and ecchymoses, suggest a platelet disorder (thrombopathia or thrombocytopenia), whereas multiple or recurrent hematomas and intracavitary hemorrhage indicate a coagulopathy. von Willebrand disease and hereditary coagulopathies occur commonly in dogs.


Icterus may be observed the day after substantial hemolysis and the serum bilirubin concentration often exceeds 2 mg/dl. However, pigmenturia due to hyperbilirubinuria, and less commonly hemoglobinuria, is noted earlier in the disease process, even with mild hemolytic disease. It should be noted that animals with IMHA may have a combination of cholangiohepatic and hemolytic processes that result in severe icterus. Finally, cyanosis is not a clinical sign of anemia except in the presence of methemoglobinemia associated with oxidative injury to the RBCs, such as acetaminophen toxicity in cats. There must be approximately 5 g/dl of unoxygenated Hb in the capillaries for the blue color of cyanosis to be appreciated clinically; therefore severely anemic animals with severe hypoxemia may not appear cyanotic despite severe concurrent hypoxemia when having concurrent cardiopulmonary disease. The tissue hypoxia caused by anemia also activates a series of compensatory mechanisms that result in the typical signs of anemia and include1:


• Immediate shunting of blood away from tissues with low oxygen demand (e.g., skin) and toward the vital organs (i.e., brain, kidney, and heart) is accomplished by selective peripheral vasoconstriction and splenic contraction (in dogs). This will contribute to pale mucous membranes and delayed capillary refill time in patients with acute or peracute blood loss anemia. In contrast, patients with chronic anemia appear vasodilated from adaptation to local tissue hypoxia.

• There is an augmentation of cardiac output, initially via an increase in heart rate and subsequently by cardiomegaly. This serves to increase the supply of well-oxygenated blood to hypoxic tissue. A mild systolic flow murmur may be heard as a result of changes in blood rheology.

• Oxygen delivery is enhanced by a reduction in Hb-oxygen affinity (i.e., a right shift in the Hb-oxygen dissociation curve) caused by increased metabolic acidity (Bohr effect), and also an increased concentration of 2,3-diphosphoglyceride in RBCs (dogs only).

• Animals with any kind of anemia, but particularly those with acute anemias, will show decreased activity levels, exercise intolerance, lethargy, and possibly even collapse; they should be handled gently because they may decompensate rapidly.

• Finally, the erythropoietin-mediated accelerated erythropoiesis takes its maximal effect after about 5 to 7 days (see Laboratory Tests later in this chapter).

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  4. Shock Fluids and Fluid Challenge

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Anemia

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