Pharmacokinetics in paramedic prescribing

Pharmacology is the study of how drugs affect the body and is divided into two fundamental components:

Pharmacokinetics are the set of principles that govern what the human body will do to a drug. What the drug does to the body is governed by the principles of pharmacodynamics (Loftsson, 2015), which will be covered in a subsequent article.

This article gives an overview of the principles of pharmacokinetics, which are broken down into four separate processes:

These principles—ADME—apply to a drug from the moment of ingestion to the excretion of its last remnants and will now be explored individually.

Absorption is the process a drug undergoes to enter the circulation. The most common route is the oral route, where drugs are ingested orally, and then absorbed from the gastrointestinal tract into the blood. This however means that the drugs are affected by the first pass metabolism in the liver. Other routes of administration include: sublingual; topical; transdermal; rectal; vaginal; inhaled and parenteral, which includes intravenous (IV), spinal, intramuscular and subcutaneous routes. In the case of the oral route, the drug is ingested, passes through the stomach, and is absorbed across the membranes of the intestine. This may be via diffusion of small drug molecules through the mucosa; molecules too large to diffuse are moved by active transport, and finally some lipid-soluble molecules are able to move freely through the lipid bilayer and into the circulation. Several factors will affect absorption via the oral route. A drug must be able to survive in the acid environment of the stomach first of all, but can be affected by excessive levels of gastric acids. If transit into the small intestine is impaired, this will affect the drug's ability to reach the intestinal mucosa. Equally, if intestinal blood flow is reduced, less drug will be moved from the gut through the hepatic portal vein to the liver and hence the rest of the body.