Page 494 - Problem-Based Feline Medicine

486 PART 7 SICK CAT WITH SPECIFIC SIGNS

factor X (the start of the common system). The ● Neoplasia. Neoplasia may disrupt blood vessels,
extrinsic system is the main pathway for the ini- promote inflammation, promote angiogenesis and
tiation of coagulation in vivo, but it cannot sus- promote fragile vasculature (e.g. endothelium with-
tain coagulation. out a vessel wall).
– The intrinsic system (contact activation path-
Bleeding is less often caused by a disorder of pri-
way) is activated by exposure to subendothe-
mary or secondary hemostasis. Such disorders may
lial tissue following vascular injury. Activation
cause subclinical abnormalities in test results or
of contact activating factors (factors XII
abnormal clinical bleeding.
[Hageman], high-molecular-weight kininogen,
prekallikrein), XI, IX and VIII occurs. Contact Abnormal bleeding is spontaneous or excessive.
activation by factor XII is probably of minimal ● Spontaneous bleeding occurs as a result of the
importance in initiating normal coagulation minor trauma of daily activity.
in vivo. However, while the extrinsic system is ● Similar to other organ systems, hemostasis has
most important for initiating coagulation, the a large reserve capacity – a severe deficiency
intrinsic system is important in sustaining coagu- is required for spontaneous bleeding to occur,
9
lation. In a simplified view, thrombin resulting for example, a platelet count < 10–20 × 10 /L
from extrinsic system activation ultimately (10 000–20 000/μl), or a factor VIII level < 10% nor-
results in factors IX and VIII (together with cal- mal is required for spontaneous bleeding to occur.
cium and platelet phospholipid) forming a – Bleeding diatheses are less common in cats than
“tenase complex” on the surface of activated in dogs.
platelets, which activates factor X. The contact ● Excessive bleeding occurs following trauma or
activation factors are important in inflammation development of other local lesions. The degree of
and fibrinolysis. bleeding is inappropriate for the severity of the
– The common system is activated by the end- lesion.
products of the extrinsic and intrinsic systems
Bleeding disorders may be inherited or acquired.
(i.e. the tissue-factor-activated factor VII and
● Inherited disorders are as common in mixed-breed
tenase complexes, respectively). In a simplified
cats as in purebreds.
view, the initial step is activation of factor X,
which then activates factor V. Activated factors Multiple mechanisms may contribute to bleeding in an
X and V (together with calcium and platelet individual patient.
phospholipid) form a “prothrombinase complex”
Acute blood loss causes acute anemia and hypo-
on the surface of activated platelets, which acti-
volemia. Signs include lethargy, anorexia, hypother-
vates thrombin (factor II), which in turn converts
mia, pallor, tachycardia, weak pulses and dyspnea.
fibrinogen (factor I) to fibrin. Factor XIII then
stabilizes the fibrin clot, converting the fibrin Chronic blood loss causes iron-deficiency anemia.
monomer to a fibrin polymer.
Regardless of cause, acute substantial bleeding may
● Fibrinolysis refers to clot dissolution by the plas-
cause signs due to a space-occupying effect and/or
minogen-plasmin system.
inflammation incited by the presence of blood. Signs
Bleeding is most often caused by local vascular include mucosal and cutaneous swellings (hematomas);
injury. Normal hemostasis ensues and the degree of neurologic signs from bleeding into or around the cen-
bleeding is appropriate for the injury. Causes of vas- tral nervous system; lameness, muscle pain, muscle
cular injury include: stiffness, joint swelling and joint pain (hemorrhage into
● Sharp and blunt trauma. This includes clinical muscles and joints); uveitis; dyspnea from pulmonary
procedures (e.g. venepuncture, dentistry, surgery). or pleural hemorrhage, or tracheal compression by a
● Tissue inflammation. Inflammation disrupts blood jugular hematoma; acute cardiac tamponade from peri-
vessels and causes vasodilation. The latter increases cardial hemorrhage (causing tachycardia and hypoten-
blood flow to the area and promotes extravasation sion similar to hypovolemia), abdominal distention
of red blood cells. and abdominal pain (presumably due to hemorrhage