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Pharmacokinetics is the branch of pharmacology that is concerned with the effects of the body on both endogenous ligands and drugs. Almost all drugs (except those delivered directly to the target tissue where the proposed receptors are located) are absorbed from the site of administration and transported by the circulation to various tissues before they arrive at the target tissue. At the same time, chemical reactions in the tissues attempt to convert drugs into forms that allow for easier removal from the body. This sequence of actions represents the absorption, distribution, biotransformation, and elimination of drugs.
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Chapter 2 defined the effective drug concentration as its concentration at the receptor site. However, the concentration of the drug in the blood is more readily measured. Except for topically applied agents, the effective drug concentration is usually proportional to its concentration in the plasma or whole blood. The plasma concentration is a function of the rate of input of the drug through absorption, distribution to the peripheral tissues (including the target tissue), and elimination from the body. These are all functions of time. If the rate of drug delivery is known, the remaining processes are well described by the pharmacokinetic parameters known as volume of distribution and clearance. Although these parameters are unique for a particular drug in a particular patient, average values in large populations can be used to predict drug concentrations. These parameters allow the calculation of loading and maintenance doses required for dosage regimens.
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Dosage regimens depend on the pharmacodynamics (Chapter 2) and pharmacokinetics of the drug (this chapter) as well as an individual’s specific comorbidities. While general guidelines for dosing regimens are available, certain comorbidities will affect drug clearance, or the rate at which active drug is removed from the body. Decreased clearance will increase how long the drug stays in the body and thus how long its effects—beneficial and adverse—will last. Renal disease or reduced cardiac output often decrease the clearance of drugs that depend on renal function. Altered clearance by liver disease is less common but can occur, especially if hepatic biotransformation of the drug is reduced. When liver blood flow is reduced, such as in heart failure, clearance for drugs that are extensively cleared from the blood by the liver also decreases. When clearance of a drug is reduced by such conditions, the dose (specific amount of medication taken at one time) and possibly the dosage (frequency of doses over a specific period of time) must be modified appropriately. During an episode of care, physical therapists often become aware of a patient’s new or progressive comorbidities as well as potential drug interactions. It is important for therapists to initiate and participate in discussion with other (prescribing) healthcare professionals to determine whether these factors may affect the dosing regimen and the rehabilitation treatment program for the patient. Ongoing interprofessional communication can improve dosing regimens by providing the prescriber a ...