Читать книгу Fundamentals of Pharmacology for Paramedics - Группа авторов - Страница 34

The interaction between any administered drug and the person it is administered to is dynamic. From the moment it is administered, the drug will be moving from its administration point to other compartments of the body, being absorbed into the bloodstream, and leaving the blood to enter other tissues or other body compartments, so the concentration of the drug in the blood and in various tissues and body compartments will be changing. As the drug passes through the liver, it will be acted on by metabolic enzymes which will convert it to a different form, which may be more or less pharmacologically active, but certainly more water soluble. The drug travelling in the blood will also be filtered by the kidneys, and the water‐soluble form of the drug will be trapped there and excreted in the urine.

As the drug is being carried around the body, some of it will arrive at and bind to its sites of action, producing its effects. Even the binding of the drug to its receptors is a dynamic process, akin to molecules playing musical chairs with the receptors – molecules of the drug will bind and detach and bind again rather than simply binding and remaining in place. Each time the molecule detaches from its binding site, it may be whipped away and metabolised, and its place on the receptors may be taken by another, competing molecule. This constantly changing relationship between the drug and the living system it has been introduced into explains a great deal about how drugs have their effects. The delay between administration and action of a drug, the duration of action of the drug, and the ability to reverse or overcome the actions of one drug by giving another drug are all the result of this dynamic interaction between drug and living system.

For the paramedic administering drugs into a system which may be free of other drugs but more likely already contains some pharmacological agents, this constantly changing effect of the drug on the patient will require you to have a good enough grasp on what these agents can do, either alone or in combination, to be able to predict and maintain some control over their actions.

One challenge we are always faced with is getting enough of a drug from its site of administration to its site of action for it to have a therapeutic effect. The drug is effectively in a race to reach its site of action and have its effect before it is chemically degraded and removed from the body. A drug which has a highly desirable therapeutic action may turn out to be useless from a clinical point of view if it cannot be delivered to its site of action. So, a drug that is going to stand a chance of being useful would usually possess characteristics which allow it to be easily absorbed into the bloodstream, preferably after oral administration, which in turn would mean that the drug would not be destroyed by the acid of the stomach or digestive enzymes. And although it would probably be subject to metabolism by the liver, the metabolism should not be so rapid that it is almost completely gone after a single pass through the liver (a phenomenon known as first‐pass metabolism), as this would mean that very little of the active drug remained in the bloodstream to circulate after absorption. Other routes of administration might avoid the problem of first‐pass metabolism, but each administration route will have its own advantages and disadvantages.