Module 7 Haematology & Coagulopathies Flashcards
Explain the cell-based model of coagulation
The cell based model of coagulation better describes how coagulation occurs in vivo. It has a better description of the potency of factors, cf the cascade model..Describes tissue factors and platelet factors. Better illustrates how intrinsic and extrinsic parts of the cascade model are not sufficient by themselves as deficiencies in either can result in serious bleeding. Goes further than merely listing factor interactions, but outlines the factors and how they are carried on cell membrane surfaces. Activations etc alter the membranes and structural changes are core to the explanation offered by the cell-based model of coagulation. Is not just a cascade but occurs in overlapping waves and has feedback controls also. Requires cells bearing Tissue factor (TF) and platelets.Can be thought of in the following steps:initiation, amplification, propagation, and
termination.
INITIATION;Injury occurs and endothelial cells bearing TF are exposed to blood.Fviia is present in circulating blood and binds to TF. (approx 1% of circulating Fvii is active). (a=active form). TF-fviia complex activates more fvii to fviia. Fviia can activate small amounts of fix and fx. Slowly, some fxa activates fv. Fxa complexes with fva to from prothrombinase. Prothrombinase cleaves prothrombin to form small amounts of thrombin.
(Note any fxa which moves along bloodstream rather than fixing to eg the TF bearing cell, is deactivated).
Platelets and most factors are too large to penetrate the intracellular junctions, and so coagulation can only occur outside of the vasculature when the injury has significantly damaged those junctions.
AMPLIFICATION; Thrombin from initiation phase diffuses and activates platelets which have leaked from vasculature injury.Thrombin-platelet binding radically alters platelet membrane creating pro-coagulant cell membrane (calcium is necessary here)and release of platelet granules. Platelet granules contain proteins, pro-coagulant agonists and other substances used in coagulation. The thrombin from initiation also cleaves fxi to fxia and activates fv to fva on platelet surface. In addition, thrombin cleaves vwf from fviii so fviiia. The released vwf can then mediate platelet adhesion.
PROPAGATION; Platelet granule release results in recruitment of more platelets. Platelets aggregate. Intrinsic tenase complex is fixa-fviiia and rapidly generates fxa on the platelet surface. The fxa binds to fva and cleaves prothrombin to thrombin. This prothrombinase activity results in a burst of thrombin, leading to cleavage of fibrinopeptideA from fibrinogen. When critical mass of fibrin occurs these soluble fibres spontaneously polymerize into fibrin strands which then form an insoluble fibrin matrix.
TERMINATION/LATTER BITS;
approx 95% of fibrin generation occurs after the initial detection of fibrin gel detected in aptt and pt clotting times. ie they cannot detect issues occurring after the initial fibrin clot forms. Thrombin generated after the fibrin gel forms, is very important in what happens next in the structure of the clot.Thrombin causes fviii to become fviiia. This then for ms crosslinks in the fibrin clot which greatly strengthens it. (haemophiliacs have factor viii deficiency).
Some thrombin will bind to TM. This can trigger fibrinolysis inhibitor.
TM bound thrombin may also activate protein C which forms complexes with cofactor ProS and this prevents further activation of fv and fviii, thus switching off coagulation.
Recall the pathophysiology of primary haemostasis
Primary Haemostasis can be considered in 2 parts:
i. Vascular haemostasis and
ii. Platelet haemostasis.
VASCULAR HAEMOSTASIS;
vessel injury causes smooth muscle contracture and vessel constriction. Contraction lasts approx 60 sec. Vasoconstriction increases sheer stress. There is sheer stress when there is a difference in blood flow rate between the laminar layers. Typically laminar flow is fastest in the centre of the vessel. Sheer stress is greatest at the edges of the vessel. Larger cells such as rbc’s and wbc’s tend to flow more in the centre, and smaller cells such as platelets more at the periphery. The increased sheer stress makes platelet clot more likely and is more important in arteries than veins.
(venous thrombosis occurs more due to slow blood flow allowing spontaneous fibrin thrombus formation).
PLATELET HAEMOSTASIS.; Damage occurs to endothelial cells, thus exposing platelets to van Willebrand’s factor (vwf). Vwf binds to collagen and then receptors on the pletelet surface, resulting in platelet adhesion. This then causes platelet activation.
Platelet activation results in release of many platelet granule factors;
eg adenosine diphosphate (ADP), activates other platelets
eg thromboxane A2 (TXA2) (synthesised by cyclooxygenase1) which increases vasoconstriction
The most common receptor on the platelet surface is GPiib/iiia. When activated, this receptor site is where fibrinogen attaches.
Identify the clinical signs, diagnostic tests and treatment of primary haemostatic disorders
CLINICAL SIGNS OF PRIMARY HAEMOSTATIC DISORDERS:
Often associated with surface and mucosal haemorrhages.
* Appear as petechiation and ecchymoses on skin and mucosal membranes.
* Epistaxis can also occur secondary to nasal mucosal bleeding, however animals with
secondary haemostatic disorder (e.g. rodenticide toxicity) can present with similar
clinical signs.
* Gastrointestinal haemorrhage will appear as haematemesis or melena and sometimes
haematuria can be seen.
* In some animals, particularly those with hereditary primary haemostatic disorders,
may not be apparent until invasive procedures have been performed (e.g. surgery or
venepuncture).
DISORDERS OF PRIMARY HAEMOSTASIS;
*Thrombocytopenia (low platelet numbers)
* Thrombocytopathia (abnormal platelet function)
* von Willebrand disease (vWD)
* Vascular dysfunction (v. rare)
DIAGNOSTIC TESTS OF PRIMARY HAEMOSTASIS;
1. Platelet count:
* CBC
* Blood smear
2. If platelet count abnormal → thrombocytopenia
* Primary vs secondary → rule out underlying disease
3. If platelet count normal → rule-out secondary haemostatic disorder
* ACT or
* PT/aPTT
4. If normal ACT or PT/aPTT → Perform BMBT
5. Abnormal BMBT → either thrombocytopathia, vWD or vasculopathy
6. Vasculitis: usually associated with other clinical signs such as:
* Erythema, swelling, purpura, plaques, haemorrhagic bullae, urticaria, necrosis and
ulceration
7. vWD→ perform vWD antigen/ collagen binding assay
* If normal → Platelet function analysis (such as platelet aggregation test or PFA-100
platelet analyser) to confirm thrombocytopathia
THROMBOCYTOPENIA
There are four major categories to the differential diagnosis of thrombocytopenia:
i) Decreased platelet production (abnormalities of bone marrow megakaryocytes).
Primary eg aplastic anaemia, myelodysplasia, myelofibrosis.
Seconday eg radiation, infection, drugs, neoplasia often leads to myelopthisis.
ii) Increased platelet consumption (ddx massive haemorrhage, dilution from massive blood transfusions, haemangiosarcomas and some other tumours, vascultis (eg Rocky Mt Spotted Fever),DIC.
iii) Platelet destruction. Whilst usually due to immune-mediated processes, idiopathic cause is less common than for some underlying secondary reason such as drugs (eg sulphonimides), infection (eg ehrlichia, anaplasma) or neoplasia (especially lymphoma. MUST CHECK FOR SECONDARY CAUSES.
iv) Platelet sequestration – rare cause of coagulopathy but usually involved in diseases causing enlarged spleen (eg haemangiosarcoma).
In general, primary thrombocytopenia (hereditary causes) is very rare, except for congenital
cyclic haematopoiesis of grey collies. Therefore, most thrombocytopenia is a secondary or an
acquired process to an underlying cause.
SPECIFIC DZ’S;
IMTP;always search for underlying cause.
dexamethasone 0.3mg/kg i/v then prednisolone 2mg/kg bid taper slowly over 4-6 months. (once platelet count normal, reduce by 20% every 2 weeks). Little evidence of benefit with other non steroidal immunosuppressives. May also need sucralfate, proton pump inhibitor and or H2 blocker.
VWF DEFICIENCY;
Most common inherited coagulopathic disorder in dogs rare in cats.
* vWD can be divided into quantitative and/or qualitative deficiency of von Willebrand
factor (vWF)
TYPE 1 vWD
* Accounts for 90% of vWD
* Partial quantitative deficiency of all vWF multimers
* Bleeding tendencies: variable
* numerous breeds affected but most notably Doberman
TYPE 2 vWD; <10% of cases.Low concentration of high molecular weight vWF multimers
Bleeding moderate to severe
* Breeds affected: German wirehaired and German shorthaired pointers
TYPE 3 vWD; <2%. Most severe form.Severe quantitative deficiency of ALL vWF multimers
* Breeds affected: Scottish terrier, Shetland sheepdog, Chesapeake Bay retriever, and
Dutch kooiker.
vWD CLIN SIGNS; may have none. Severe bleeding after trauma/surgery/venupuncture esp type2 &3.Usually no petecchiation.
vWD DX; BMBT insensitive. normal to prolonged.Platelet function analysis, vWF antigen elisa assay, vWF binding collagen assay (normal to increased-more severe types have more increased), genetic testing for carrier and non carrier.
Tx vWD;pressure bandage, fresh frozen plasma or total blood. Desmopressin nasal spray slight benefit to type 1.
ERHLICHIA; Tick-borne bacteria in monocytes and multiplies in spleen and lymph noses etc. Signs 8-20 days post infection. Varies from acute,chronic, mild, severe. Spontaneous recovery after few weeks possible. Clin signs vary; fever, thrombocytopaenia, haemorrhage, anaemia, pancytopaenia, lympadenopathy, myositis, neuro signs,uveitis, monoclonal gammopathy, increased alt/alkp, glomerulonephritis, splenic enlargement etc.
Can take 7-28 days for antibodies to be detectable, therefore repeat testing advised after 3 weeks if necessary. Dx via indirect fluorescent antibody test or elisa or pcr and MUST go to biosecurity labs. Tx doxycycline 5mg/kg bid 6-8 weeks. Alternatively use enrofloxacin. If no response after 1 week, may try immunosuppressive doses steroids 1 week. Thrombocytopaenia should respond in 1-2 weeks. Recheck 1 month then 3 months as risk of relapse.
THROMBOCYTOPATHIA;(abnormal platelet function)
Diagnostic tests for platelet function include:
* Buccal mucosal bleeding time (BMBT)
o To perform a BMBT, you will need to have a patient in lateral recumbency
with the lip tied up on one side. Using a standardised 10mm long x 1mm
template, a cut is to be made into the buccal mucosa (inside lip). Using some
wicking paper, blood should be wicked away from the incision without touching the wound itself. The timing starts from the time the incision is made
and is stopped once the bleeding stops.
o Normal BMBT - < 4 mins in dogs, < 2.5 mins in cats
o Prolonged BMBT does not differentiate the cause of platelet function disorder,therefore it must be followed up with a vWF activity assay since this is the most common cause affecting platelet function.
▪ vWF assay measures the amount of vWF protein present in blood
(vWF antigen test) and determines how well the protein functions
(vWF activity).
o False negative BMBT can occur generally from significant bleeding post
invasive procedures.
o Theoretically azotaemia can compromise platelet function, however animals are
more likely to be unwell as a result of the azotaemia as opposed to abnormal
platelet function.
Recall the pathophysiology of secondary haemostasis
Secondary haemostasis is defined as the formation of fibrin through the coagulation cascade.
It involves coagulation factors which generates fibrin polymers that helps stabilise the platelet
plug to form a mature thrombus. For testing purposes, the coagulation cascade has been
traditionally separated into three pathways: intrinsic, extrinsic, and common pathways.
* Intrinsic pathway involves the clotting factors: XII, XI, IX and VIII
* Extrinsic pathway clotting factors: VII/ VIIa, tissue factor (TF)
* Common pathway factors: X, II/IIa (prothrombin/ thrombin), and I/Ia (fibrinogen/ fibrin)
Identify the clinical signs, diagnostic tests and treatment of secondary haemostatic disorders
CLINICAL SIGNS;
Secondary haemostatic disorders are associated with more significant bleeding and is
observed as cavitary bleeding and haematoma formations:
* Cavitary bleeding most commonly occurs in the joints (haemoarthroses), thorax
(haemothorax) and abdomen (haemoabdomen)
* Other unusual places include the brain and spinal cord leading to various neurological
signs depending on the location of the haemorrhage
* Haematomas from areas of minor trauma and venepuncture sites
* Respiratory tract bleeding from coughing and haemoptysis
* Mediastinal haemorrhage (seen on lateral views of the thoracic radiographs as
compression of the dorsal tracheal membrane) can be associated with coughing or
respiratory distress
* Gastrointestinal tract haemorrhage seen as haemetamesis, melena, haemetochezia, and
may even appear as a mass lesion on ultrasound when bleeding occurs within the
gastric mucosa.
DIAGNOSTIC TESTS;
* Activated clotting time (ACT)
ACT assesses both the intrinsic and final common pathway of the traditional coagulation cascade.
* Prothrombin time (PT)
Assess the extrinsic and final common pathways
First to be prolonged in early rodenticide toxicity as factor VII has the shortest
half-life of all the vitamin K dependent clotting factors.
* Activated partial thromboplastin time (aPTT)
aPTT assess both the intrinsic and final common pathways similar to ACT
Prolongation of the ACT only occurs when there is more than 90% loss of clotting factor
activity, making the ACT less sensitive to early coagulopathies. In contrast to PT and aPTT
where there will be prolongation of the results once there is >70% of clotting factor activity
lost.
Low fibrinogen (<75 mg/dL in dogs and <50 mg/dL in cats) will prolong PT (and APTT)
independent of other clotting factor deficiency.
*It is important that citrate tube is adequately filled, otherwise it will result in prolonged
clotting times.
*Only prolonged PT or APTT is a relevant finding
TREATMENT;
a) Anticoagulant rodenticide
* Non-life threatening;Vitamin K1 therapy: 2-5mg/kg PO q12h (with small fatty meal for
optimum absorption)
* Life-threatening;Vitamin K1 therapy: 5mg/kg PO or SC if vomiting (oral route given with
a fatty meal is preferable as there is rapid absorption from the GIT),followed by 2.5mg/kg PO q12h
o Duration of therapy:
▪ Warfarin 14 days
▪ Bromadiolones 21 days
▪ Brodifacoum 30 days
▪ 1st generation shorter length of Vit K required, versus 2nd /3rd generation
* Fresh frozen plasma transfusion (10-20ml/kg)
▪ Give if clinical evidence of bleeding
▪ Also, can give to patients at high risk of haemorrhage
▪ Recheck clotting time after each 10ml/kg bolus – stop once
normal clotting time has been achieved
b) Disseminated Intravascular Coagulopathy (DIC)
17
* Characterised by the presence of a disease or illness that causes severe
inflammation or infection.
* In overt DIC, there is clinical evidence of bleeding and at least three
abnormal test results (thrombocytopenia, prolonged PT, prolonged aPTT,
increased D-dimer, decreased fibrinogen levels and decreased
antithrombin III)
* *MUST treat underlying disease
* Supportive therapy
* Blood products:
▪ Be aware that there is often a concurrent widespread
microvascular thrombosis
▪ FFP (10-20ml/kg)
o 10ml/kg boluses
o Pre-emptively prior to surgery
o If life-threatening bleeding, administer until clinical improvement
of haemorrhage
c) Snake envenomation-tx specific to region/ snake.Antivenene, oxygenation, anti-seizure, pain relief, anti-emetic, fresh frozen plasma.
Recall the main disorder of tertiary haemostasis (fibrinolysis)
Tertiary haemostasis is defined as the formation of plasmin. It is the main enzyme responsible
for fibrinolysis (breakdown of clot). Tissue plasminogen activator (tPA) is released from the
endothelial cells during the activation of coagulation cascade. tPA binds to the plasminogen
within the clot to convert it to plasmin. Plasmin lyses fibrinogen and fibrin in the clot,
producing fibrin degradation product.
HYPERFIBRINOLYSIS SYNDROME
* Can be congenital (up to 30% Greyhounds predisposed) or acquired (trauma, liver disease etc)
* Results in dysregulation of the normal fibrinolytic system
o In a normal animal, the fibrinolytic system helps to maintain haemostasis, controlling
localisation of clot formation and dissolution of clots throughout the normal healing
process
o Dysfunction results in premature breakdown of clots, leading to clinically significant
haemorrhage in times of trauma or peri-operatively
* Diagnosis: TEG or ROTEM testing – not readily available in Australia at current
o PT, aPTT and ACT generally unremarkable
* Treatment:
o Tranexamic acid (TXA)
▪ Initial dose - 10-20mg/kg slow IV (over 15-20 minutes)
▪ Subsequent doses:
* 10mg/kg slow IV q 8hrs for 24 hours
* OR 10mg/kg/hr CRI over 3-6 hours, repeated every 8 hours or 24 hours
▪ Rapid administration or higher doses can cause emesis.
▪ 500mg/greyhound PO for 3 days
o Or ε-Aminocaproic acid
▪ 500-1000mg PO q6-8 (or up to 100mg/kg q6-8h)
o Empirical treatment with antifibrinolytics up to 48 hours pre-operatively, to be
continued for 5-7 days post-operatively in high-risk animals (i.e. greyhounds)
undergoing any invasive procedures
Recall the basic overview of traumatic coagulopathy
TRAUMATIC COAGULOPATHY
Acute traumatic coagulopathy (ATC) is an endogenous hypocoagulable condition identified in up to 38% of human trauma patients during the immediate (<1 hour) post traumatic period. It
manifests as a state of systemic hypocoagulanility and hyperfibrinolysis. It is thought to be a
manifestation of severe tissue injury, shock-induced hypoperfusion, systemic inflammation and
endothelial damage.
The incidence of ATC is up to 33% in dogs and up to 14% in cats, although most studies found that
patients can present in hypo-, normo- or hypercoagulable state.
Outline the primary mechanisms of anaemia
- INCREASED LOSS;Blood loss can be categorised as to whether it issecondary to abnormal haemostasis or in the
presence of normal haemostasis.
Normal haemostatic blood loss can then be further divided into internal and external blood loss: - INCREASED DESTRUCTION;
Non-immune mediated causes include:
o Mechanical (e.g. DIC), hypophosphataemia, chemical or drug induced (zinc, copper, anti-thyroid, sulfa-drugs)
* Inherited and immune-mediated causes include:
o Infectious, envenomation (e.g. red-bellied black snake), primary immunemediated haemolytic anaemia, and neoplasia (e.g. haemangiosarcoma and histiocytosis). - REDUCED PRODUCTION;Decreased erythropoiesis is either refractory or non-refractory, depending on their underlying
causes.
* The term refractory anaemia (RA) is part of the heterogenous group of disease that affects
normal blood cell production in the bone marrow and a category of myelodysplastic
syndrome. In RA, marrow blood cells fail to mature properly and are unable to work
properly.
* The most common non-refractory anaemia is anaemia of chronic disease.There are three pathophysiological mechanisms that have been identified to
cause this.
1) Red blood cell survival is shortened because of the release of
inflammatory cytokines, especially in the case where cancer or
granulomatous infections are present.
2) Erythropoiesis is impaired because there is both a reduction in
erythropoietin (EPO) production as well as a reduced bone marrow
responsiveness to EPO.
3) Finally, iron metabolism is altered due to an increase in hepcidin, which
inhibits iron absorption and recycling, leading to iron sequestration and
reduced availability for RBC production.
o On a blood smear, anaemia of chronic disease is characterised by microcytic
or normocytic anaemia with low reticulocyte counts.
Identify the clinical signs, diagnostic tests and the treatment of anaemia
CLINICAL SIGNS;Acute blood loss is associated with signs of hypovolaemic shock:
o Mucous membrane pallor, prolonged capillary refill time, hypotension,
tachycardia, dull mentation, lethargy and cold periphery, increased serum
lactate (transfusion triggers)
o PCV can initially be normal with hypovolaemic shock and may not be
apparent until fluid therapy has been instituted.
* Chronic blood loss is more commonly associated with:
o Mucous membrane pallor, tachypnoea, tachycardia and lethargy or otherwise a
relatively haemostatically stable patient with normal mentation
* If all the signs above collectively remain after the underlying shock/pain or other
potential causes have been addressed, then a red blood cell transfusion may be
indicated. The above clinical signs are referred to as “transfusion triggers”. The
combination of clinical signs and laboratory parameters suggest a red blood cell
transfusion is required. Generally, a transfusion is indicated when a combination of
triggers are present.
* TRANSFUSION INDICATORS:
o PCV <15% with transfusion triggers (exercise intolerance, tachycardia,
tachypnoea, dyspnoea, weakness, hypotension, syncope and stupor)
o Rapidly PCV drop to <20% in dogs and 15% in cats
o PCV <25% and need to do surgery or anaesthesia
DIAGNOSTICS
* PCV/TP, serum colour, blood gas and electrolytes, coagulation profile, complete
blood count, blood smear and serum biochemistry.
* PCV/TP:
o Can be normal or slightly reduced in cases of acute external blood loss. This is
very different to anaemia caused by haemolysis where PCV is typically low and
TP is normal to high.
* Serum colour:
o If either haemolysis or icterus is present it can suggest either haemolytic
anaemia or severe hepatopathy.
* SLIDE AGGLUTINATION:
o A simple test that if macroscopic autoagglutination is seen is highly suggestive
of IMHA. Should also always assess under microscopy with slide cover to
ensure true agglutination rather than insufficiently separated rouleaux.
Slide Agglutination Test
* A single drop of EDTA-anticoagulated blood is placed onto a microscope slide and
mixed with saline (1–2 drops in dogs; 3–4 drops in cats due to their greater
propensity to develop rouleaux).
* The slide is rocked back and forth; then evaluated for the formation of
macroagglutination (obvious agglutination to the naked eye) (Figure 1).
* A coverslip can then be placed on the mixture, and the slide evaluated under a
microscope for microagglutination (4 or more RBCs in a cluster) (Figure 2).
* True agglutination appears as “clusters of grapes” while rouleaux appear as “stacks of
coins” (Figure 3).
* Rouleaux can further be differentiated from autoagglutination by adding additional
saline and by RBC washing techniques; extra saline often disperses rouleaux but will
not disperse true autoagglutination.
TX; dependent upon underlying cause. O2 therapy of minimal use without adequate haemoglobin. Haemorrhage control. Fluids.
