Anesthesia Pharmacology: General Principles: Pharmacokinetics

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  • Extent of Absorption

    • Incomplete absorption following oral drug administration is common.

      • For example, only 70% of a digoxin dose reaches systemic circulation. Factors:

        • Poor GI tract absorption

        • Digoxin (Lanoxin, Lanoxicaps):  metabolism by gastrointestinal flora

    • Very hydrophilic drugs - not be well absorbed --cannot cross cell membrane lipid component

    • Excessively lipid-soluble (hydrophobic) drugs may not be soluble enough to cross a water layer near the cell membrane.

  • Ion Trapping

    • "Ion Trapping - Introduction"

    • Kidney:

      • Nearly all drugs filtered at the glomerulus:

        • Most drugs in a lipid-soluble form will be reabsorbed by passive diffusion.

        • To increase excretion: change the urinary pH to favor the charged form of the drug since charged form cannot be readily reabsorbed since this form (charged) cannot readily pass through biological membranes.

          • Weak acids: excreted faster in alkaline pH (anion form favored)

          • Weak bases: excreted faster in acidic pH (cation form favored)

    • Other sites:

      • Body fluids where pH differences from blood pH favor trapping or reabsorption:

        • Stomach contents

        • Small intestine

        • Breast milk

        • Aqueous humor (eye)

        • Vaginal secretions

        • Prostatic secretions

    • Ion Trapping: Anesthesia Correlation:  Placental transfer of basic drugs

      • Placental transfer of basic drugs from mother to fetus: local anesthetics.

      • Fetal pH is lower than maternal pH.

      • Lipid-soluble, nonionized local anesthetic crosses the placenta converted to poorly lipid-soluble ionized drug.

        •  Anesthetic concentratin gradient is maintained for continual transfer of local anesthetic from maternal circulation to fetal circulation.

        •  In fetal distress, acidosis contributes to local anesthetic accumulation.


  • Weak bases-- amines

    •  N + 1 carbon (R) and 2 hydrogens: primary amine (reversible protonation)

    •  N + 2 carbons (R) and 1 hydrogen: secondary amine (reversible protonation)

    •  N + 3 carbons (R): tertiary amine (reversible protonation)

    •  N + 4 carbons (R): quaternary amine (permanently charged)

  1. Stoelting, R.K., "Pharmacokinetics and Pharmacodynamics of Injected and Inhaled Drugs", in Pharmacology and Physiology in Anesthetic Practice, Lippincott-Raven Publishers, 1999, 1-17.

  2. Dolin, S. J. "Drugs and pharmacology" in Total Intravenous Anesthesia, pp. 13-35 (Nicholas L. Padfield, ed), Butterworth Heinemann, Oxford, 2000.

  3. Katzung, B. G. Basic Principles-Introduction , in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, pp 1-33