Common Pharmacokinetics

First Things First (assess & treat for the following)

Pharmacokinetics is a description of the absorption, distribution, metabolism & excretion of a drug.

• Common pharmacokinetic abbreviations
  • Ro: dose/(amount of time the drug infused over)
  • Vd: volume of distribution
  • ke: elimination rate constant
  • T1/2: half-life
  • ti: amount of time the drug infused over
  • ∆t: time between the level should have been drawn and the drawn level
  • τ: dosing interval

• Before you monitor a medication, you have to dose it. Below are listed some commonly used agents that require pharmacokinetic monitoring that need weight-based dosing.

• Aminoglycosides (Conventional Dosing)
  Indication	Gentamicin and Tobramycin Dose	Amikacin Dose
  Gram-positive synergy	1 mg/kg (gentamicin only)	NA
  Urinary tract infection	1-1.3 mg/kg	5-6 mg/kg
  Bacteremia/skin and soft tissue infection	1.5-1.7 mg/kg	6 mg/kg
  Pneumonia/septic shock	1.7-2.5 mg/kg	7-8 mg/kg
  Cystic fibrosis	2-3 mg/kg	7-8 mg/kg

• Aminoglycoside dosing weight: IBW + 0.4 (TBW - IBW)
  • Ideal Body Weight (IBW) for men in kg: 50 + 2.3 (# of inches above 5 ft)
  • Ideal Body Weight for women in kg: 45.5 + 2.3 (# of inches above 5 ft)
  • Total Body Weight (TBW) in kg: Patient’s actual body weight
• Aminoglycoside dosing interval determination
  • ke: 0.00293 x Creatinine Clearance + 0.014
  • τ: ((ln (Goal Trough/Goal Peak)/ (-ke)) + ti
    • Use 8, 12, or 24 hours.
    • If >24 hours, dose intermittently based on levels.

• Vancomycin
  
  • Dose
    • 15 mg/kg for most infections
    • 20 mg/kg for meningitis or pneumonia
    • Use TBW in most patients.
      • Use Adjusted Body Weight (ABW) in obese patients
      • ABC = IBW + 0.2 (TBW - IBW)
  • Dosing Interval
    • k: 0.00083 x Creatinine Clearance + 0.004
    • τ: ((ln (Goal Trough/Goal Peak)/ (-k)) + ti
    • Use 8, 12, 24, or 48 hours.
    • If >48 hours, dose intermittently based on levels.

• Phenytoin
  • Loading dose
    • IV: 20 mg/kg
    • PO: 20 mg/kg given in doses of 200-400 mg q2h
  • Maintenance dose
    • 5-6 mg/kg/day
    • Do not give single dose >400 mg; divide it and give bid.