Prescribing for paramedics: pharmacodynamics

Understanding the nature of drug actions that can occur in the body allows the paramedic to make informed decisions regarding medicines that may be offered, the likely outcomes and any possible risks. Despite often being considered a complex subject, this article presents a set of principles that are applicable to almost all drugs along with relevant case studies to support theory and provide context where appropriate.

Many drugs exert their effect on the body via an action on cells. The net effect of this can be to:

Drugs can do this in a number of ways, but the most common methods are to chemically bind to the surface of a cell or, in the case of steroids, simply pass through the cell wall. Before discussing these methods in detail, we must acknowledge that these processes occur naturally in the body; our pharmacological interventions simply mimic the endogenous neurotransmitter or hormone. The importance of these endogenous chemicals cannot be understated. Indeed, before acetylcholine was the first neurotransmitter to be identified (Dale, 1914), synapses were widely considered to occur via an electrical process. The matter was largely put to rest after Loewi (1921) carried out his well known frog experiment which initially came to him in a dream. He stimulated the heart of a frog with electrical impulses and had it pump a small amount of nutrient solution. When the fluid was transferred to another heart, it operated similarly, proving that chemical substances convey nerve signals to organs.