Bone deformities, fetal hemorrhage, abortion, and abnormalities are the teratogenic effects of warfarin.
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 65-year-old woman presents to the emergency department with palpitations, lightheadedness, and shortness of breath. She explains she has experienced these symptoms for the past three days and that her symptoms are not associated with physical exertion. The patient has a history of hypertension, hyperlipidemia, and type 2 diabetes mellitus. The patient denies any loss of consciousness, muscle weakness, sensory deficits, chest pain, vomiting, fever, and chills. Examination shows that her temperature is 37℃ (98.6℉), blood pressure is 150/85 mm Hg, pulse is 160/min, respirations are 17/min, and oxygen saturation is 98% on room air. Cardiac examination reveals an irregularly irregular rhythm and tachycardia, which are both confirmed with an ECG. The patient promptly receives IV metoprolol and is closely monitored. Examination later reveals resolution of her dyspnea and a heart rate of 95 beats per minute. The consulting cardiologist reports that the patient is at moderate-high risk for thromboembolic events and recommends prophylaxis. Which of the following is the most appropriate next step in management?
Anticoagulant medications are used to prevent blood clots from forming.
These medications work by interfering with the normal function of plasma proteins called coagulation factors, which take part in secondary hemostasis.
Now, to understand how warfarin works, we first need to talk briefly about hemostasis-- where hemo refers to blood, and stasis meaning to halt or stop.
Hemostasis is divided into two phases: primary and secondary hemostasis.
Primary hemostasis involves the formation of a platelet plug around the site of an injured blood vessel, and secondary hemostasis reinforces the platelet plug with the creation of protein mesh called fibrin.
To get to fibrin, a set of enzymes called coagulation factors need to be activated.
These enzymes are activated via a process called proteolysis- which is where a portion of the protein is clipped off. In total, there are twelve coagulation factors numbered factors I-XIII, there’s no factor VI.
Then, vitamin K hydroquinone acts as a cofactor by donating its electrons to an enzyme called gamma glutamyl carboxylase, which converts the non-functional forms of coagulation factors II, VII, IX, and X into their functional forms.
Gamma glutamyl carboxylase adds a carboxyl group, a chemical group made up of one carbon, and two oxygens, onto the end of glutamic acid residues on the proteins.
In this fashion, a single molecule of vitamin K can be reused many times.
As it turns out, the drug warfarin, which was first used as a rat poison, blocks the function of this enzyme which blocks vitamin K from getting recycled and as a result factors II, VII, IX, and X don’t get activated.
The coagulation cascade begins via two pathways --the extrinsic and intrinsic pathways.
The intrinsic pathway starts when circulating factor XII comes into contact with the surface of activated platelets or collagen.
Activated factor XII, then activates factor XI, which activates factor IX which activates factor X.
Factor X starts the common pathway where it activates factor II, which activates factor I that builds the fibrin mesh.
When factor II gets activated it also activates 4 other factors: V, VIII, IX, and XIII.
Factor V gets activated and acts as a cofactor for X, factor VIII acts as a cofactor for factor IX, and factor XIII helps factor I, or fibrin, form crosslinks.
In the extrinsic pathway, exposed tissue factors on the damaged blood vessel activates factor VII, which activates factor X and starts the common pathway.
Next, levels of factor II, IX, and X also drop, causing inhibition of the intrinsic and common pathways.
Since factor VII drops first, warfarin’s efficacy is monitored using a blood test called prothrombin time, or PT, which is a measure of how well the extrinsic and common pathways are functioning.
To perform this test, blood is drawn and the plasma is separated out by centrifuge.
The plasma contains all the coagulation factors minus tissue factor, which is normally found within the blood vessel walls.
Next, tissue factor is added to the plasma to trigger coagulation, and the time until the sample clots is measured.
This time is called the prothrombin time, or PT and it’s compared to the control PT, which is the time it takes for the blood to clot in a healthy person.
Now the problem with PT is that there are multiple testing kits from different companies and they all cause clotting at different rates.
So it’s helpful to convert the PT into a standardized value called the INR, or international normalized ratio.
- "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
- "Rang and Dale's Pharmacology" Elsevier (2019)
- "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)
- "Overview of hemostasis" J.C. Aster, H. Bunn (Eds.), Pathophysiology of Blood Disorders, 2e. McGraw-Hill. (2016)
- "Nomograms" D. Nicoll, C. Mark Lu, S.J. McPhee (2017)
- "Ischemic Stroke: Risk Stratification, Warfarin Teatment and Outcome Measure" J Atr Fibrillation (2015)
- "Critical Issues and Recent Advances in Anticoagulant Therapy: A Review" Neurology India (2019)