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#1
03-28-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121
Calculating Drug Css (Steady State Concentration)

29 years female brought to ER after accident. Morphine continuous at dose of 0.1 mg/min, t 1/2 = 1.9 hour , clearance = 60L/hr , Vd= 230 L, then which Css (steady state concentration) will be attained??

a) 0.016 mg/L
b) 0.1 mg/L
c) 0.026 mg/L
d) 0.0016 mg/L
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#2
03-28-2013
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I think its b
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#3
03-28-2013
 USMLE Forums Veteran Steps History: Step 1 Only Posts: 283 Threads: 35 Thanked 204 Times in 115 Posts Reputation: 214

Maintenance dose = CL X Css

Css = Maintenance dose / CL

= 0.1 mg/min / 60 L/Hr

Do the conversion: 1 Hr = 60 min

= 0.1 mg/min / 1 L/min

Css = 0.1 mg/L

 The above post was thanked by: Meli (05-02-2014), neha_subh (03-28-2013)
#4
03-28-2013
 USMLE Forums Veteran Steps History: 1 + CS Posts: 209 Threads: 18 Thanked 111 Times in 74 Posts Reputation: 121

29 years female brought to ER after accident. Morphine continuous at dose of 0.1 mg/min, t 1/2 = 1.9 hour , clearance = 60L/hr , Vd= 230 L, then which Css (steady state concentration) will be attained??

b) 0.1 mg/L

for Css = Dosing rate "drug in" must be equal to rate of elimination "drug out'

MD = CL X Css / f
Css = dose / CL

dose = 0.1 mg/min
units should be same for other measure hence,
CL = 60L/hr then
CL = 1L / min

now

Css= 0.1 / 1 = 0.1 mg / L

Css= 0.1 mg/ L

formulas:
MD = CL X Css / f
Ko = CL X Css
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 The above post was thanked by: Doctor Ali (03-29-2013), dr,maps (03-29-2013), fadi551 (03-29-2013), nandish_m (03-29-2013), sami trailer (03-29-2013)
#5
03-29-2013
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 367 Threads: 0 Thanked 70 Times in 60 Posts Reputation: 80

What happened to the dosing interval value? When you initially setup up the equation to figure out the Css. Because the equation is MD=Css x cl x dosing interval divide by bioavailability.
#6
05-01-2014
 USMLE Forums Scout Steps History: Not yet Posts: 35 Threads: 14 Thanked 9 Times in 5 Posts Reputation: 19

Shouldn't the answer be D, the question mentions the infusion rate not the maintenance dose so using the formula for infusion rate,
infusion rate = clearance * steady state plasma concentration...
hence .1 = 60 * steady state plasma concentration
#7
05-01-2014
 USMLE Forums Scout Steps History: 1 + CK Posts: 44 Threads: 13 Thanked 4 Times in 3 Posts Reputation: 14

Quote:
 Originally Posted by Doctor Ali Maintenance dose = CL X Css Css = Maintenance dose / CL = 0.1 mg/min / 60 L/Hr Do the conversion: 1 Hr = 60 min = 0.1 mg/min / 1 L/min Css = 0.1 mg/L My answer is B.
its right calculation

 Tags Pharmacology-, Step-1-Questions

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