My Clinical Notes

Clotting system

 

• Involves a biological amplification system – sequential proteolytic cascade of circulating precursor proteins which result in the generation of thrombin, which in turn converts soluble fibrinogen into fibrin which enmeshes the platelet aggregates at the site of vascular injury and converts the unstable platelet plugs into stable haematostatic plugs
• The factors required for coagulation are found in the circulation as co-factors and pro-enzymes and with the exception of Factor XIII, are all serine proteases that hydrolyse peptide bonds
• The coagulation factors adhere to activated platelets, the platelet surface functions as a catalytic membrane required for the speed and magnitude of the response

 

The extrinsic pathway

• Initiated by Tissue factor (TF), which is a glycoprotein on the surface of fibroblasts
• This is known as the extrinsic pathway because the circulation is not normally exposed to TF
• TF forms a complex with Factor (F) VII
• This complex activates FX and FIX
• In the ABSENCE of co-factor FVa, FXa can only covert a little prothrombin to thrombin
• Thrombin;
  • Coverts fibrinogen to fibrin
  • Activated FV, FXI and FXIII
  • Cleaves FVIII from vWF
  • Causes further platelet activation

 

• The extrinsic pathway is rapidly inactivated by tissue factor pathway inhibitor (TFPI) which forms a complex with TF, FVII and FXa
• At this point thrombin generation is dependant on the intrinsic pathway which has been primed by the small amounts of thrombin produced during inititation
• The FIX activated by TF, that hasn’t been inhibited by TFPI, complexes with FVIII which activates sufficient FX to combine with FVa to form thrombin from prothrombin
• It is the FIXa/VIIIa complex which is responsible for the continuous FXa formation and subsequent fibrin formation

 

The ‘intrinsic’ classical pathway

• The classical pathway was put forward to explain in vitro clotting
• It requires reaction with FXII, kallikrein and high molecular weight kininogen (HMWK) leading to the activation of FXI
• However individual with hereditary deficiencies in these factors do not suffer from abnormal bleeding questioning the physiological role
• FXI role is supplementary in the activation of FIX. It may be important at major sites of trauma and surgery

 

Fibrin

• The final step is the degradation of the plasma protein fibrinogen into small fibrin monomers
• These monomers polymerise spontaneously by H-bonds to form long fibres
• FXIIIa further stablises the clot by forming covalent cross-links between fibrin monomers

 

Vitamin K

• Factors, II, VII, IX and X undergo post-transcriptional γ-carboxylation of glutamic acid residues which allows them to bind to calcium and platelet phospholipids membranes
• This process requires Vit K as a co-factor
• Vit K is fat soluble and deficiency is often due to malabsorption

 

Calcium

• Adequate levels of calcium are required to allow clotting by activating the Vit K dependant co-factors
• Reducing calcium can prevent coagulation of blood samples. This is the principle behind EDTA

 

Physiological limitation of coagulation

• TFPI – produced by endothelial cells
• Antithrombin – inactivates serine proteases by combining with them by peptide bonds to form HMW stable complexes. It inhibits FIX, X and XI. The activity of antithrombin is potentiated by heparin
• Protein C and protein S – inhibits of the coagulation factors V and XIII. Both are Vit K dependant. Protein C is activated with thrombin binds to thrombomodulin on endothelial cell. Protein S acts as a co-factor by binding it to the platelets surface. Protein C also inhibits fibrinolysis by inhibiting tissue plaminogen inhibitor (TPI)

 

Coagulation tests

 

Prothombin time (PT)

• Measures factors V, VII, X, prothrombin and fibrinogen – therefore the extrinsic pathway
• Tissue thromboplastin and calcium are added to citrated plasma
• Normal clotting time is 10-14 secs
• Value is normally given as an international normalised ratio
• The most common causes of a disorder in PT are;
  • Liver disease
  • Warfarin therapy
  • DIC

 

Activated partial thromboplastin time (APTT)

• Measures factors, VII, IX, XI and XII in addition to factors V, X, prothrombin and fibrinogen
• Three substances, phospholipids, a surface activator and calcium are added to citrated plasma
• The normal clotting time should be 30-40 sec
• Defective APTT is seen in;
  • Haemophilia
  • Christmas disease (deficiency in FIX)
  • Liver disease
  • Warfarin therapy
  • DIC
  • Heparin therapy

 

Thrombin time (TT)

• Sensitive to deficiencies in fibrinogen and inhibition of thrombin
• Diluted bovine thrombin is added to citrated plasma
• Clotting time should be 10-12 secs
• Defects seen in;
  • DIC
  • Heparin therapy
  • Afibrinogenaemia

 

Fibrinogen quantification

• Fibrinogen deficiency is seen in DIC and liver disease

 

Hereditary coagulation disorders

• Hereditary deficiencies of each of the coagulation factots have been described
• The most common are Haemophilia A, Haemophilia B and von Willebrand disease

 

Haemophilia A

• Most common hereditary coagulation disorder
• Prevalence in males is 1 in 5000
• Inheritance is sex-linked by around 1/3 have no family history and result from spontaneous mutation
• Cause by a deficiency in Factor XIII

 

Clinical features

• May be mild moderate of severe with 70% of patients having severe disease
• Haemarthroses – bleeding, generally into weight bearing joints. This can lead to progressive joint deformity and disability
• Muscle haematomas
• Operative and post traumatic haemorrhage may be life threatening
• Increased risk of intra-cerebral haemorrhage
• Ear, nose and throat bleeding is also common
• Some of the more serious consequences of these bleeds;
• Chronic damage – fixed flexion deformities of limbs with muscle atropy, hearing impairment and neurological damage
• Acute problems – compartment syndrome, compression of the respiratory tracts, loss of circulating volume

 

Lab findings

• Bleeding time and PT time are normal
• APTT is abnormal
• Abnormal Factor VIII clotting assay

 

Treatment

• Factor VIII can be replaced with;
  • Fresh-frozen plasma
  • Cryoprecipitate
  • Factor VIII concentrates
  • Recombinant factor VIII
• These can be given during bleeding episodes to raise the level of Factor VIII to between 30-50% of normal
• For major surgery, serious post-traumatic bleed or a haemorrhage occurring at a dangerous site, the level of Factor VIII should be raised to 100% and maintained at 50% until healing has occurred
• Prophylactic treatment is recommended in children with severe disease (less than 2% normal FVIII)
• Desmopressin can be used to increase plasma Factor VIII in mild haemophiliacs, results in release of FXIII from the endothelial cells. The rise is proportional to the resting level

 

 

Haemophilia B

• Also called Factor IX deficiency and Christmas disease
• Clinical features and inheritance is the same as Haemophilia A, can only be distinguished by specific coagulation factor assays
• The incidence is 1/5 that of Haemophilia A
• Treatment is similar to that of haemophilia A but is with Factor IX replacement instead of FVIII
• Factor IX has a longer half life than Factor VIII so fewer infusions are required
• Lab findings are the same as Haemophilia A, although it is the Factor IX clotting assay that is abnormal

 

Von Willebrand disease

• Deficiency or defect in VWF resulting from a point mutation or major deletion
• Affects 1% of the population
• The most common inherited bleeding disorder
• Inheritance is autosomal dominant with varying expression
• VWF is produced by endothelial cells and megakaryocytes and has 2 roles;
• Promotes platelet adhesion to the damaged epithelium
• It is the carrier molecule for factor VIII, protecting it from premature destruction
• It is synthesized as a large protein which goes on to form multimers
• Elevation of VMF is part of the acute phase response
• Three types of VWD have been classified, with type 2 being divided into 4 depending on the type of functional defect
• Type I accounts for 75% of cases

 

Â	Â	Platelet associated function	Factor VIII binding capacity	High MW VWF multimers
Type 1	Quantitative partial deficiency	Â	Â	Â
Type 2	Functional abnormality	Â	Â	Â
Type 2A	Â	Decreased	Normal	Absent
Type 2B	Â	Increased affinity for GP1b	Normal	Usually reduced/absent
Type 2M	Â	Decreased	Normal	Normal or ultra-large
Type 2N	Â	Normal	Reduced	Normal
Type 3	Complete deficiency	Â	Â	Â

 

• The severity of clinical symptoms is very variable
• Typically there is mucous membrane bleeding
• Excessive blood loss from superficial cuts and abrasions
• Operative and post-traumatic haemorrhage
• Haemarthrosis and muscle haematomas are rare expect in Type 3 disease

 

Lab findings

• Prolonged bleeding time
• Factor VIII levels are often low
• The APTT may be prolonged
• VWF levels are low
• Defective platelet aggregation by patient plasma in the presence of ristocetin. Aggregation to other agents e.g. ADP, thrombin or adrenalin is usually normal
• Collagen binding function is generally reduced
• The platelet count is normal expect for type 2B disease

 

Treatment

• Local measures and antifibrinolytic agent e.g tranexamic acid for mild bleeding
• Desmopressin for those with Type I disease
• High purity VWF concentrates for patients with very low VWF levels. Factor VIII may also be given for more rapid correction

 

Disseminated Intravascular coagulation

• Caused by wide spread in appropriate intravascular deposition of fibrin and the consumption of coagulation factors and platelets
• Is a consequence of many disorders which release procoagulant material into the circulation or cause widespread endothelial damage or platelet damage

 

Causes of DIC include;

• Infections e.g. septicaemia
• Malignancy
• Obstetric complications – amniotic fluid embolism, premature separation of the placenta, eclampsia, septic abortion
• Hypersensitivity reactions – anaphylaxis, incompatible blood transfusion
• Widespread tissue injury – following surgery or trauma or after severe burns
• Vascular abnormalities – cardiac bypass surgery, vascular aneurisms
• Miscellaneous – liver failure, pancreatitis, hypothermia, heat stroke, acute hypoxia, massive blood loss

 

Pathogenesis

• Coagulation can be activated in 2 ways;
  • Release of tissue factor from damage tissues, monocytes or red blood cells
  • Activation of factors XI and XII by damaged vascular endothelium
• There is generalized fibrin deposition on vascular endothelium with extensive consumption of platelets and coagulation factors
• Small vessels are obstructed leading to tissue damage and multiple organ dysfunction
• Fibrin deposition activates the fibrinolytic pathway resulting in the formation of fibrin degradation producs. This inhibit fibrin polymerization and impair coagulation
• The net result is a bleeding disorder due to a lack of platelets and clotting factors and inhibition of fibrin polymerization by FDP

 

Clinical features

• Bleeding – particularly from venepuncture sites or recent wounds
• There may be generalized bleeding in the GI tract, oropharynx, lungs, urogenital tract and vagina
• Less frequently microthrombi may cause skin lesions, renal failure, gangrene or cerebral ischaemia

 

Lab findings

• Platelet count in low
• Fibrinogen concentration is low
• Thrombin time is prolonged
• The PT and APTT are prolonged
• High levels of fibrin degradation products such as D-dimers are found in the serum and urine
• Blood film may show a haemolytic anaemia

 

Treatment

• Correct underlying disorder
• Supportive care of bleeding involves FFP, platelets and cyroprecipiate (more concentrated source of fibrinogen). Red cell transfusion may also be required
• In patients with thrombotic disorders, heparin and antiplatelet drugs can be used

 

Thrombotic thrombocytopenic purpura (TTP) and Haemolytic uraemic syndrome (HUS)

• TTP is a rare but serious condition which generally affects young adults
• It is marked by a fever, transient neurological defects and renal failure
• Microthombi are deposited in arterioles and capillaries causing thrombocytopenia and a microangiopathic haemolytic anaemia
• The exact aetiology is unknown but immune mediated endothelial damage and synthesis of abnormal formed of vWF causing platelet hyperaggregability has been proposed to contribute

 

• Haemolytic uraemic syndrome is similar to TPP affecting infants and young children
• In HUS however the platelet-fibrin microthrombi are limited to the kidneys
• In many patients the disease follows recent infections with E.coli (particularly verotoxin 0157) or other enteric pathogens such as shigella

 

This entry was posted on Monday, January 28th, 2008 at 9:45 am and is filed under Haematology. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.