Phenobarbital Case #2

Date: 8/9/95
Name: SweetPea
Signalment: Canine, Female, 10 yrs, 5.3 pounds
History:CBC / Chem / VA ---> WNL
Current Status: Improving
Dose:8 mg, PO Dose Interval: 12 h
Time on Dose:2 weeks -- started on 6/14/95, then owners stopped. Started again 2 weeks ago.
Serum Concentration #1: 10.2 mcg/mL @ 11 h #2: none submitted

Plot data

• Plot data from case on arithmetic graph paper
  • May use same graph as did for Case #1, if wish
  • make "y" axis 0 to 50 (mcg/mL serum)
  • make "x" axis 0 to 14 (hours)
  • plot concentration #1 (10.15 mcg/mL @ 11 h)
  • Note: there is no concentration #2

Questions

• Does this value more nearly approximate a "peak" or a "trough"?
• Will the concentration go lower during this dose interval?
  Given what you know about the speed at which phenobarbital is eliminated what is your prediction?
• How was this sample time probably chosen?
• Is it ideal? ...practical?
• Will it introduce significant error in dose adjustment?

Questions regarding the significance of the data

• The concentration is approximately 10.2 mcg/mL at 11 hours ... So what?
• Is the concentration within the target concentration range?
• Is "SweetPea" still having seizures?
• Should the dose be adjusted?
• How might you adjust the dose?

Adjust regimen

• Desired trough concentration is 20 mcg/mL serum
• Measured "trough" concentration was 10.2 mcg/mL serum
• Setup simple proportion formula
  

  New dose = Old dose x (Conc desired / Conc measured)

• Do the calculation

Questions about dosage adjustment

• New dose regimen is approximately 16 mg q12h
• What does q12h mean? bid? q6h? q1d? q1m?
• Will this new regimen result in the drug concentration being in the therapeutic window for the entire dose interval?
• Look at plot for case #1 and predict on the graph paper a Peak for this animal?
• Why can we get away with using only one measurement for phenobarbital in some cases?
• When would it be crucial to know the approximate peak concentration?

Advice to referring veterinarian

Concentration at the trough is approximately half the normally recommended minimum. Because SweetPea is still having seizures, advise increasing dose to 15 mg q12h. If this produces excessive sedation and/or incoordination for more than 3 to 4 days, the evening dose should be decreased to 7.5 mg. After two to three weeks on the new dose, advise submitting a "trough" sample for analysis if SweetPea is still experiencing seizures.

Questions about the recommendation

• Why might sedation and / or incoordination last only 3 to 4 days?
• Why not just decrease both AM and PM doses instead of only one?
  (Hint: what dose forms are there for the medication?)
• Why wait two to three weeks to submit another sample?