VTE Chronic Therapy Flashcards
(37 cards)
What is the half-life of warfarin
How long until 95% steady state
t-1/2: 48 hours
95% SS: 10 days
What day was INR found to be predictive of maintenance dose in A. fib
Day 3
Why can it take up to 3 weeks to reach steady state INR values
Factor II lasts the longest
Half-life of disappearance of factor II: up to 72 hours
95% SS = 15 days
How often should INR be checked for outpatients
twice weekly for the first month
What is the equation for maintenance dose if the 10mg initiation was used?
2.5 mg + 10% week 1 dose - INR day 8 + 1.5mg (if INR at D5 was less than 2)
What is the longest time INR tests should be monitored
4-6 weeks
however 12 weeks was non-inferior to 4 weeks in one study
How long can transition state last up to to reach maintenance phase
3 months
What are warfarin dose adjustment alternatives (3) Which one’s don’t work?
- Just respond to the INR (NEVER DO THIS)
- Use a validated dosing adjustment instrument
- computerized warfarin managements systems
- Hard copy algorithm (ex. mcmaster 2-step nomogram) - Use a systematic approach to patient evaluation
What is the systematic approach to warfarin dose adjustments (4)
- Try to figure out why
- Determine patient’s risk of thrombosis
- Consider how far INR is out of range
- Consider alternatives
(a. no changes, repeat INR sooner)
(b. one-time adjustment dose, then resume maintenance dose)
(c. change maintenance dose)
What are reasons that INR is fluctuating? (7)
- Non-adherence
- Changes in meds (addition/deletion)
- Acute illnesses (GI, diarrhea)
- Dietary habit changes
- Lifestyle habits (alcohol, exercise, travel)
- Stress
- Inaccurate INR test
What is the % chance of getting a clot again with no therapy after:
1 month
1-3 months
Recurrent VTE
1 month: 40% per month (>1% per day)
1-3 months: 10% per 2 months (>0.15% per day)
Recurrent VTE: 15% per year (0.04% per day)
How far can the INR be out of range? What do you do?
When do you adjust if not in range? (2)
If less than 0.5 units difference
- repeat INR in 1-2 weeks
Adjust if:
- over 0.5+ units difference
OR
- within the first 1-3 months
(assuming they missed a dose, just give back their dose)
How do you adjust if multiple INR tests are
+/- 0.5 units
over 0.5 units
+/- 0.5 units
- 5-10% of weekly dose
over 0.5 units
- 10-20% of weekly dose
Changes in INR seen within a day or 2 reflect changes in what factor?
Factor VII 7 concentrations
What are the mechanisms for warfarin drug interactions (5)
- Inhibition of platelet function
- these drugs inhibit primary hemo, warfarin inhibits secondary hemo –> addidtive effect –> BLEED RISK
- ASA, clopidogrel, NSAIDs - Reduced synthesis of vitamin K by gastrointestinal flora
some antibiotics - Alteration of warfarin metabolism
- especially inhibitors/inducers of CYP 2C9 (eg. cotrimaxazole, flucanozole, amiodorone) - Injury to GI mucosa
- NSAIDs have risk of stomach ulcer, passage for bleeding - Interference with vitamin K epoxide-reductase
- Acetaminophen
When is the mechanism of reduced synthesis of vitamin K by GI flora only an issue? (3)
- Antibiotic is inhibitor of CYP450 enzymes
- Patient is very ill with fever/reduced apetite or activity
- Antibiotic caused diarrhea as a side effect
Which of the 5 mechanisms would affect INR values (3)
- Reduced synthesis of vitamin K by GI (antibiotics)
- Alteration of warfarin metabolism (CYP 2C9 inhibitors/inducers, -azoles)
- Interference with vitamin K reductase (acetaminophen)
Recommendation for the following on warfarin:
interaction? INR? Monitor time?
Starting therapy today with acetaminophen 1g q6h
- Interacts w/ warfarin, inc INR
- 1-1.3g of tylenol requires INR testing
- Come back in 3 days to adjust warfarin dose
Recommendation for the following on warfarin:
interaction? INR? Monitor time?
Starting therapy with bisoprolol 5mg once daily
- No interaction
- Any new drug come back in 1 week for INR test
Recommendation for the following on warfarin:
interaction? INR? Monitor time?
Starting therapy with naproxen 200mg once daily
- Interaction, inc risk of bleeding
- No INR change
- try using topical NSAIDs, or cox-2 selective, or PPI
- can switch to tylenol if need be
- monitor INR
Recommendation for the following on warfarin:
interaction? INR? Monitor time?
planning to stop taking glucosamine after 4 months since it was not working
- unknown interaction
- follow up INR in 1 week
How much % does ASA increase bleed risk in patients taking warfarin?
50%
Which group of patients may require ASA in addition to warfarin (3)
- Recent acute coronary syndrome (MI)
- Recent coronary stent/coronary bypass procedure
- Mechanical heart valves
What is the risk for the following INR values
over 4
over 5
over 6
over 4
- risk of major bleeding
over 5
- risk of intracranial hemorrhage
over 6
- life-threatening bleeding in the next 2 weeks = 4%
