Help with a Pharm Question

Discussion in 'Pharmacy' started by Pythagoras, Dec 4, 2005.

  1. Pythagoras

    Pythagoras Senior Member
    7+ Year Member

    May 13, 2003
    Likes Received:
    I've been trying to figure this out for a while and i feel like an idiot for not being able to get the right answer, any help would be appreciated.

    A new aminoglycoside antibiotic (5mg/kg) was infused IV over 30 min to a 70kg volunteer. The plasma concentration of the drug was measured at various times after the end of the infusion, as recorded below.

    Time after dosing stopped (hr) Plasma concentration (ug/mL)
    0.0 18.0
    0.5 10.0
    1.0 5.8
    2.0 4.6
    3.0 3.7
    4.0 3.0
    5.0 2.4
    6.0 1.9
    8.0 1.3

    The elimination half life is ~3.5 hrs

    The apparent volume of distribution is approximately?

    The total body clearance is approximately?

    any help would be appreciated. thanks :)
  2. dgroulx

    dgroulx Night Pharmacist
    10+ Year Member

    Jan 10, 2003
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    First solve for k. You are given T1/2

    T1/2 = 0.693/k

    If you want to calculate it, then use
    k = ln(C*max/C*min)/delta T
    C*max = measured peak, C*min = measured trough, delta T = time between them.

    k = CL/Vd, so you need to solve for either CL or Vd.

    There are many ways that you can do this. Are you supposed to be calculating the true Vd using obtained peaks & troughs? If so, you'll need to calculate Cmax & Cmin from your measured peaks & troughs, then calculate the true Vd.

    Vd = (D/k*T) x (1-e^-k * T)/Cmax - (Cmin * e^-k * T)
  3. lisi

    lisi Member
    5+ Year Member

    Jul 22, 2004
    Likes Received:
    Graph either log of the plasma concentration (Cp) or lnCp (y-axis) versus time (x-axis). Although aminoglycosides typically follow one-compartment kinetics, it looks like you've got the typical two-compartment graph. If you look at the graph, its pretty fact the terminal portion of the curve has a slope of about 0.2 (slope = elimination k, 0.693/k gives you the actual half-life) the first part of the curve is steeper and has a slope of about 1.1. I'd suggest you check out your text/notes regarding two-compartment kinetics. That may help you get on track with the other parameters you need to find. Although aminoglycosides typcially follow a one-compartment model, in this particular patient, there may be a physiological reason for the aminoglycoside to follow a two-compartment model. Remember, population data for pharmacokinetics is a great place to begin dosing...after real patient data comes back by measuring actual Cp's throw all of the population data out the window and use the equations to figure out what's going on in this particular patient.

    Good luck!

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