This chapter is designed to give the rehabilitation specialist an overview of commonly prescribed medications in the rehabilitative setting. It is not intended to be a complete compendium of drugs but reviews a select few commonly seen in practice. It begins with an overview of pharmacokinetics and pharmacodynamics so that the reader can apply these principles for a better understanding of how drugs affect physiology and function.
PHARMACOKINETICS AND PHARMACODYNAMICS PRINCIPLES
Optimum drug response depends on a complex array of pharmacokinetic and pharmacodynamic parameters, as well as external influences such as patient compliance, formulations, dose and dosing schedules, physical activity, and even ambient temperature. Pharmacokinetics (PK) describes what happens to a drug as it enters the body and encompasses the phases of absorption, distribution, metabolism, and excretion.1 Generally, this is the process by which an active agent is absorbed into the systemic circulation, travels around the body either as free drug or bound to plasma proteins, and activates a target organ or receptor, before being metabolized into an inactive compound and excreted. Pharmacodynamics refers to what the drug does to the body, mechanism of action, and drug–receptor interactions (Fig. 45–1).
The relationship between dose and effect can be separated into pharmacokinetic (dose concentration) and pharmacodynamic (concentration effect) components. Concentration provides the link between pharmacokinetics and pharmacodynamics and is the focus of the target concentration approach to rational dosing. The three primary processes of pharmacokinetics are input, distribution, and elimination. (Reproduced with permission from Holford NG. Pharmacokinetics & Pharmacodynamics: Rational Dosing & the Time Course of Drug Action. In: Katzung BG, eds. Basic & Clinical Pharmacology, 14e New York, NY: McGraw-Hill; 2018.)
The amount of drug necessary to reach a therapeutic level in the blood depends on a delicate balance between the amount entering the body (dose) and the amount excreted.2 Drug absorption refers to the process by which a drug reaches the systemic circulation from its site of administration. Drugs can enter the body in a variety of ways; orally, via enteral administration through the gastrointestinal (GI) tract, and parenterally, referring to topical, transdermal, subcutaneous, and intravenous mode.3 Other routes of administration include sublingual, rectal, inhalation, intramuscular, and via implantable reservoirs. The mode chosen depends on the physiochemical properties, for example, molecular weight, solubility, and degree of ionization.
The most convenient administration is oral, in which a tablet, capsule, or liquid is swallowed and then enters the GI tract. However, this mode of administration is very inefficient. Usually the drug is absorbed through the GI tract and then enters the hepatic circulation, where some drug is lost as it undergoes metabolism before moving on to the systemic circulation. This is known as the “first-pass effect”4 (Fig. 45–2).