Page 1397 - Small Animal Internal Medicine, 6th Edition

CHAPTER 84 Erythrocytosis 1369

the number of circulating RBCs. This can be accomplished
BOX 84.1 by performing therapeutic phlebotomies, in which a certain
VetBooks.ir Classification and Causes of Erythrocytosis in Cats volume of blood (≈20 mL/kg) is collected from a central vein
through a blood collection set. In cats, a 19-gauge butterfly
and Dogs
of heparin diluted in 3 to 5 mL of saline solution is typically
Relative Erythrocytosis (Pseudoerythrocytosis) catheter coupled to a 60-mL syringe containing 500 to 600 U
Hemoconcentration used to collect blood from the jugular vein under chemical
restraint; we use sevoflurane inhalant anesthesia. Interest-
Absolute Erythrocytosis ingly, leeches have been used in a cat with PRV (Nett et al.,
Primary 2001). Gradual phlebotomy (5 mL/kg, repeated as needed) is
PRV recommended for dogs and cats with right-to-left shunts and
Secondary erythrocytosis because an increased RBC mass appears to be
Appropriate (i.e., secondary to decreased tissue the body’s way of enhancing oxygen delivery to the tissues,
oxygenation) thereby compensating for the chronic hypoxemia in these
Pulmonary disease animals. Because sudden decreases in blood volume can
Right-to-left cardiovascular shunts result in marked hypotension, a peripheral vein catheter can
High altitude be used to administer an equivalent volume of saline solu-
Hemoglobinopathies? tion at the same time that blood is being collected. However,
Inappropriate (normal tissue oxygenation) collapsing episodes during or immediately after phlebotomy
Hyperadrenocorticism are extremely rare. As a result of its high viscosity in patients
Hyperthyroidism with erythrocytosis, it may be extremely difficult to obtain
Renal masses blood through a relatively small (e.g., 19-gauge) catheter.
Neoplasms in other areas Once the patient’s condition has been stabilized, the cause
of the erythrocytosis should be sought (Fig. 84.1). The fol-
PRV, Polycythemia rubra vera; ?, not well documented in cats or
dogs. lowing approach is recommended. The patient’s cardiopul-
monary status should first be evaluated by auscultation,
precordial palpation, thoracic radiography, or echocardiog-
Cardiopulmonary signs may occasionally be present. raphy (see Chapters 1 and 2). An arterial blood sample
Although the erythrocytosis usually develops gradually, should be obtained for blood gas analysis to rule out hypox-
most affected animals do not exhibit clinical signs until the emia and pulse oximetry used to evaluate oxygenation. In
RBCs have reached a critical mass or the PCV has reached some animals with erythrocytosis, the blood viscosity is so
a certain percentage. A PCV of 70% to 80% is relatively high that the blood gas analyzer, which is usually flow-
common in cats and dogs with absolute erythrocytosis. dependent, cannot generate results; in this case, a therapeu-
Physical examination and historical findings in dogs and cats tic phlebotomy should be performed before a sample is
with erythrocytosis may also include bright red mucous resubmitted for testing (i.e., the partial pressure of oxygen,
membranes (plethora), erythema, differential cyanosis, poly- PO 2 , does not change after therapeutic phlebotomy). If the
uria, polydipsia, splenomegaly, renal masses, and a neoplasm PO 2 is normal, excretory abdominal ultrasonography or
elsewhere. computed tomography should be performed to determine
Hematologic abnormalities are usually limited to the whether masses or infiltrative lesions are present in the
erythrocytosis and reticulocytosis (in light of a high HCT), kidneys. If no such lesions are found, the patient most likely
although thrombocytosis may be present in cats and dogs does not have renal secondary erythrocytosis, so a search for
with PRV. Microcytosis caused by relative iron deficiency an extrarenal neoplasm should be conducted. Determination
(i.e., the erythron is extremely active and is relatively iron of serum Epo activity is not currently available for dogs and
deficient) is common in dogs with erythrocytosis. cats. In my experience, bone marrow evaluation of cats and
dogs with erythrocytosis is unrewarding; in most cases the
Diagnosis and Treatment only abnormality is a decreased myeloid-to-erythroid ratio
Relative erythrocytosis (i.e., dehydration) should be ruled as a result of erythroid hyperplasia.
out first. This is done primarily on the basis of the serum (or If PRV is diagnosed, hydroxyurea (30 mg/kg PO q24h) is
plasma) protein concentration, which is typically high in administered for 7 to 10 days, after which the dosage can be
dogs and cats with this form of erythrocytosis. However, in gradually decreased or the dosing interval gradually length-
certain circumstances, such as HGE, dogs may have a high ened to fulfill the patient’s needs. Phlebotomy should be
HCT but a relatively normal serum protein concentration. repeated as dictated by the patient’s clinical signs. If the final
Radioisotopic RBC mass determinations are commonly per- diagnosis is secondary erythrocytosis, the primary disorder
formed in humans with erythrocytosis, but this test is usually should be treated (e.g., surgery for a renal mass). The
not done in small animals. hydroxyurea protocol can also be used successfully in dogs
The initial approach used in small animals with absolute with right-to-left shunts and secondary erythrocytosis
erythrocytosis is to decrease the blood viscosity by reducing (Moore and Stepien, 2001).