Methemoglobinemia

characterized by the presence of a higher than normal level of methemoglobin (metHb) in the blood.

Methemoglobin is a form of hemoglobin that does not bind oxygen. When its concentration is elevated in red blood cells, tissue hypoxia can occur.

Normally, methemoglobin levels are <1%, of total blood hemoglobins.

 

Etiology:

1. congenital methemoglobinemia – NADH methemoglobin reductase mutation
2. acquired methemoglobinemia -

•  exogenous oxidizing drugs and their metabolites
• benzovaine
• dapsone
• nitrates
•  may accelerate the rate of formation of methemoglobin up to one-thousandfold, overwhelming the protective enzyme systems and acutely increasing methemoglobin levels.

Acanthocytosis

The presence of acanthocytes in the blood stream.

Acanthocytes are a mutant spiny form of red blood cell.

Anthocytes are rapidly removed from circulation, leading to a decrease of red blood cells in circulation and anemia.

Homans’ Sign

pain or discomfort behind the knee upon passive dorsiflexion of the foot, due to thrombosis in the calf veins.

Scarvelis and Wells Clinical Assessment tool for Risk of Deep Vein Thrombosis

Scarvelis and Wells Clinical Assessment tool for Risk of Deep Vein Thrombosis

1. Active cancer (treatment within last 6 months or palliative) – 1 point
2. Calf swelling >3 cm compared to other calf (measured 10 cm below tibial tuberosity) – 1 point
3. Collateral superficial veins (non-varicose) – 1 point
4. Pitting edema (confined to symptomatic leg) – 1 point
5. Swelling of entire leg – 1 point
6. Localized pain along distribution of deep venous system – 1 point
7. Paralysis, paresis, or recent cast immobilization of lower extremities – 1 point
8. Recently bedridden > 3 days, or major surgery requiring regional or general anesthetic in past 4 weeks – 1 point
9. Alternative diagnosis at least as likely – Subtract 2 points

Assessment scoring guide:

1. Score of 2 or higher - deep vein thrombosis is likely. Follow up with diagnostic tests – imaging the leg veins.

2. Score of less than 2 – deep vein thrombosis is unlikely.

Scarvelis, D and Wells, P, S. 2006. Diagnosis and Treatment of Deep Vein Thrombosis.Journal of the American Medical Association. 175 (9) 1087-1092.

Coagulation Disorders and Treatment

The clotting factor cascade

Diseases of Hemostatsis

 Thromboembolic disorders

• occur when undesirable clots are formed
• Thrombus = stationary clot

• Embolus = a travelling clot

• Deep vein thrombosis = thrombi in veins, usually form in leg veins

• thrombi can form in the atria during atrial fibrillation
• R atrium thrombi can dislodge and travel to lungs = pulmonary embolism
• L atrial thrombus can dislodge can cause CVA or arterial infarct elsewhere in the body

• arterial thrombi and emboli can result from procedures involving arterial punctures such as angiography

 Thrombocytopenia

• abnormal clot formation
• deficiency of platelets
• a result of bone marrow function is suppression
• Etiology: chemotherapeutic agents and immunosuppressant

 Hemophilias

• genetic clotting factor deficiency
• classified by prolonged coagulation times that result in persistent bleeding that can be acute

1. Hemophilia A

• lack of clotting factor VIII
• 80% of all cases

 2. Hemophilia B (Christmas Disease)

• deficiency of factor IX
• 20% of cases

Tx:

• administration of the absent clotting factor

 3. von Willebrand’s disease (vWD)

• decrease in the quantity or quality of von Willebrand factor, role in platelet aggregation
• Tx: factor VIII concentrate
• desmopressin, which promotes release of stored vWF
• infusion of plasma products containing vWF

Four Mechanisms of hemostasis modification

Drug Classification	Mechanism	Type of Modification
Anticoagulants	Inhibition of specific cloting factors	Clot formation prevention
Anticoagulant/antiplatelets	Inhibition of platelet actions	Clot formation prevention
Thrombolytic	Dissolution of clot	thrombolytics
Antifibrinolytic	Inhibition of the destruction of fibrin	Antifibrinolytics – Promotion of thrombosis by inhibiting the normal removal of fibrin, keeping the clot in place for a longer period of time

Anticoagulants

• prolong bleeding time in order to prevent clot formation
• Tx of thromboembolic disease

• Heparin
• Warfarin

Mechanism of action:

• exert a negative charge on the surface of the platelets
• cell aggregation is inhibited

Heparin:

• acts by enhancing the inhibitory actions of antithrombin III

Warfarin:

• acts by inhibiting the hepatic synthesis of coagulation factors II, VII, IX and X

Low Molecular weight heparins (LMWHs)

• inhibit active X factor
• possess same anticoagulant activity as heparin
• yet are less likely to thrombocytopenia
• last 2-4 times longer than heparin

Thrombin Inhibitors

• bind both clot-bound and circulating thrombin preventing the formation of fibrin clots

 Adams, Micheal, Patrick, Bostwick, Paula, Manuel, Holland, Leland, Norman jr, and King, Shirley, Linda. 2009. Pharmacology for Nurses: A pathological approach. Pearson Canada, Toronto.

Von Willebrand Disease

A genetic blood disease where there is a malfunction in the Von Willebrand clotting factor, a factor that works with factor VIII, is missing or does not function normally causing blood not to clot properly. It is the most common hereditary coagulation abnormality in people. It is characterized by a tendency to hemorrhage, and caused by a defect in blood platelet activity.

Clotting process:

Damaged cells display a surface protein called tissue factor (TF)

Tissue factor binds to activated Factor 7.

The TF-7 heterodimer is a protease with two substrates:

• Factor 10 and
• Factor 9

Factor 10 binds and activates Factor 5. This heterodimer is called prothrombinase because it is a protease that converts prothrombin (also known as Factor II) to thrombin.

Thrombin has several different activities. Two of them are:

• proteolytic cleavage of fibrinogen (aka “Factor I”) to form:
  • soluble molecules of fibrin and a collection of small
  • fibrinopeptides
• activation of Factor 13 which forms covalent bonds between the soluble fibrin molecules converting them into an insoluble meshwork — the clot.

 

 

Blood Clotting. Retrieved September 15, 2009 from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Clotting.html