Formulas & Things to Remember Flashcards
Half-life pharm formula
T1/2 = (0.693 x Vd)/Clearance
Steady state concentration formula
Steady state (IV) = in / out
Steady state Css = (Bioavailability x Dose) / (Dose interval x Clearance)
Oral bioavailability formula
Bio = AUC oral / AUC IV
Renal dose adjustment for FU (fraction unbound)
Renal dose adjustment for FU of 1
= CrCl of patient / CrCl normal
Renal dose adjustment for FU <1
= (1 - FU) + FU (CrCl of patient / CrCl normal)
Theapeutic Index formula
Therapeutic Index = LD50 / ED50
ED50 = median effective dose i.e. 50% sample has effect
LD50 is the amount ingested that kills 50% of test sample
Potency vs Efficacy
Potency = dose required to produce 50% of maximal effect
- i.e. if 2 drugs have same maximum effect but Drug A achieves at a lower dose it is more potent than drug B
Efficacy = maximum effect expected from drug
HLA*B1502 drug interaction
Carbamazepine & SJS/TEN
HLA*B5701 drug interaction(s)
Abacavir hypersensitivity
Flucloxacillin drug induced liver injury
HLA*B5801 drug interaction
Allopurinol in Han Chinese populations - risk of SJS/TEN
Loading dose determinant
Volume of distribution the main factor
Loading dose = desired concentration x Vd
Odds ratio
OR = odds of exposure in cases / odds of exposure in controls
i.e. ad / bc
Describe the flow-volume curves for the following:
- Normal
- Fixed obstruction
- Variable extra-thoracic obstruction
- Causes? - Variable intra-thoracic obstruction
- Causes? - Restrictive disease
- Normal = upside down ice-cream cone
- Fixed obstruction = flat top and bottom (pancake)
- causes: goitre, tracheal stenosis, masses - Variable extra-thoracic obstruction = flat inspiration (stingy upside down ice cream cone)
- causes: laryngomalacia, vocal cord palsy
- Inspiration sucks in the upper airways - Variable intra-thoracic obstruction = flat expiration
( stingy upright ice cream cone)
- causes: tracheal masses, tracheomalacia
- Expiration increases pleural pressure worsening obstruction - Restrictive = small, low volume but normal shape
A-a gradient formula
List causes of:
Elevated gradient
Low gradient
(150 - 5/4 x paCO2) - paO2
or 1.2 x pCO2
High A-a gradient:
- V/Q mismatch (PE, pneumonia, ARDS, APO)
- Right to left shunt
- Alveolar hypoventilation (interstitial lung disease)
Low/normal A-a gradient:
- Hypoventilation (e.g. COPD, NMD)
- High altitude
Which drugs affect renin/aldosterone ratio testing?
- False negative
- False positive
False negative - increase renin
- ACEi
- ARB
- Diuretics
- Dihydropine CCBs (amlodipine)
False positive - decrease renin ABCD suppresses Alpha-methyldopa Beta blockers Clonidine Diclofenace (NSAIDs)
Hold diuretics (incl. spironolactone 6 weeks) Other interfering meds 2-4 weeks Verapamil, prazosin and hydralazine are ok to use
Describe Amiodarone effects on the thyroid
o Amiodarone has multiple mechanisms of interfering with thyroid function
All patients have a transient rise in TSH due to Wolff-Chaikoff effect (temporary increase in thyroid production due to iodine load)
o Hypothyroidism occurs due to interference with T4 synthesis and action (also interferes with peripheral conversion of T4 to T3
Treated with cessation +/- thyroxine
Thyrotoxicosis occurs by 1. Iodine load (type 1) and 2. Thyroiditis (type 2)
Can occur anytime after starting Amiodarone (idiosyncratic)
Amiodarone contains large amounts of iodine (200mg more than usual daily intake) enhanced thyroid hormone production (type 1)
• More commonly in patients with underlying multinodular goitre
• Jod-Basedow phenomenon
Amiodarone has a direct toxic effect on the thyroid follicular cells release of excess thyroid hormone (type 2)
Often mixed mechanism – AIT 1 more common early (e.g. <3 months) and AIT more common overall
• AIT 2 responds rapidly whereas AIT 1 responds slowly
o Management
1. Stop Amiodarone
2. PTU/Carbimazole (target type 1) AND steroids (target type 2)
3. Colestyramine if desperate (blocks enterohepatic iodine circulation)
4. Surgery if no response
Note: Iodine radio-ablation is not helpful