Ch. 2: Basic Principles of Drug Action and Drug interactions

Antagonist Effect

one drug interferes with the action of another

Desired Effect

the expected response when a drug is absorbed and distributed

Additive Effect

two drugs with similar actions are taken for an increased effect

Duration of Action

How long the drug has a pharmacologic effect

Drug Stages After Administration

LADME - Liberation - Absorption - Distribution - Metabolism - Excretion

Enteral

Via the gastrointestinal tract by the oral, rectal, or nasogastric routes

Percutaneous

- Absorbed through the skin and mucous membranes - Inhalation, sublingual, or topical

Drug Interactions

- Additive effect - Synergistic effect - Antagonistic effect - Displacement - Interference - Incompatibility

Changes in Distribution

- Affects the binding of a drug to an inactive site - Drug is usually transported throughout the body bound to plasma proteins - A drug that is highly bound (>90% bound) to a protein-binding site may be displaced by another drug that has a higher affinity for that binding site

Responses to Drugs

- Desired effect - Side effects - Adverse effects - Idiosyncratic effects - Allergic reactions - Teratogen - Carcinogen

Interference

first drug inhibits the metabolism or excretion of the second drug, thereby causing increased activity of the second drug

Unbound Drugs

- Drugs are bound to proteins in the blood during transportation - The more a drug is "bound" to a protein, the less available it is to the circulation - Some medications can cause binding or unbinding of another medication, increasing or decreasing its effect

Changes in Excretion

- usually act in the kidney tubules by changing the pH to enhance or inhibit excretion

Drug Blood Level

a blood sample drawn to view the amount of drug present

Which route of administration has the fastest rate of distribution? a) Subcutaneous b) Intramuscular c) Transcutaneous d) Intravenous

d) Intravenous Rationale: The IV route places the drug directly into the bloodstream and isn't slowed down by the need to first travel from the GI tract into the bloodstream.

Idiosyncratic Reaction

occurs when something unusual or abnormal happens when a drug is first administered

Pharmacodyamics

study of the interactions between drugs and their receptors and the series of events that result in a response

Synergistic Effect

the combined effect of two drugs is greater than the sum of the effect of each drug given together

Displacement

the displacement of the first drug from protein-binding sites (i.e., bound drugs are inactive) by a second drug increases the activity of the first drug because more unbound drug is available

Therapeutic Response

the drug concentration is targeted to be in the middle of the range (minimum effective response and the toxic response)

Adverse Effects

any noxious, unintended, and undesired effect of a drug, which occurs at dosages used in humans for prophylaxis, diagnosis or therapy Side Effects/Common Adverse Effects: mild Serious Adverse Effects: can lead to toxicity

When the nurse administers a 50-mg dose of a drug with a half-life of 6 hours, how many milligrams will remain in the body at 24 hours? a) 25 mg b) 12.5 mg c) 6.25 mg d) 3.13 mg e)1.56 mg

d) 3.13 mg Rationale: The half-life of a drug is the time required for 50% of the drug to be eliminated from the body. In this example at 24 hours, 6.25% or 3.13 mg of the drug would remain in the body.

Changes in Metabolism

- Drug interactions are caused by an alteration in metabolism, by inhibiting or stimulating the enzymes that metabolize a drug - Some medications increase another medication's actions, contributing to toxicity - Some medications decrease another medication's actions, decreasing effectiveness

Changes in Absorption

- Most drug interactions that alter absorption take place in the GI tract Antacids increase the gastric pH and can inhibit the dissolution of ketoconazole tablets - Many alterations in absorption can be managed by separating the times of administration

Absorption

- depends on route of administration - Drug is transferred from entry site into the body's circulating fluids - Absorption rate depends on route, blood flow, and solubility of the drug - Subcutaneous and intramuscular injection absorption is affected by circulation - Intravenous medications are absorbed fastest - Topical medications applied to the skin can be influenced by skin thickness and hydration

Half-Life of Drugs

- amount of time required for 50% of the drug to be eliminated from the body - When the half-life of a drug is known, dosages and frequency of administration can be calculated.

Time-response curve

- demonstrates the relationship between the administration of a drug and the associated response - If the drug level does not reach the minimum effective concentration, there will be no pharmacologic effect - If the peak level exceeds the toxicity threshold, toxic effects will result

Excretion

- depends on GI tract and kidneys - Elimination of drug metabolites and some of the active drug from the body - Kidneys are the major organ of excretion; some excreted in feces

Metabolism

- depends on enzyme systems - The process whereby the body inactivates drugs - Primary organ of metabolism is the liver; other sites are GI tract and lungs

Liberation

- drug must be released from dosage form and dissolved in body fluids before it can be absorbed - Process of converting oral drug can be influenced by food and water in the stomach

Distribution

-depends on circulation to be transported throughout body - Drugs are transported throughout the body by body fluids to the sites of action -Protein binding and fat solubility affect distribution - Organs with largest blood supply receive the distributed drug most rapidly - Some drugs cannot pass through the blood-brain barrier or the placental barrier

Drug Interaction

1. Agonist 2. Antagonists 3. Partial Agonists

Routes of Drug Administration

1. Enteral 2. Parenteral 3. Percutaneous

Drug Actions

1. Onset of action 2. Peak action 3. Duration of action

Peak action

A drug reaches the highest concentration on target receptor sites, inducing maximal pharmacologic response

Principles of Drug Action and Drug Interactions

An understanding of the human body's processes is important to grasp drug actions and drug interactions in the body A: Drugs act by forming a chemical bond with specific receptor sites, similar to a key and lock. B: The better the "fit," the better the response, called agonists. C: Drugs that attach but do not elicit a response, called antagonists. D: Drugs that attach and elicit a small response, but also block other responses, called partial agonists.

Parenteral

Bypasses the GI tract by using subcutaneous, intramuscular, and intravenous injection

Onset of action

Concentration of a drug is sufficient to start a pharmacologic response

All drugs are processed in the body through pharmacokinetics. What is the correct order that drugs pass through the body? a) Absorption, distribution, metabolism, excretion b) Distribution, metabolism, absorption, excretion c) Biotransformation, distribution, absorption, excretion d) Excretion, distribution, absorption, metabolism

a) Absorption, distribution, metabolism, excretion Rationale: All drugs go through four stages after administration, which can be remembered with the acronym ADME.

A patient is being started on a new drug that has been used safely by many people for years. The patient has no known allergies, and the nurse administers the drug correctly. Suddenly the patient experiences cardiac arrest. What is this type of reaction called? a) Allergic b) Mutagenic c) Idiosyncratic d)Therapeutic

a) Allergic Rationale: Because the drug was a known safe drug and the patient had no known reason not to receive it, the response to the drug in this case was totally unexpected, or idiosyncratic. An allergic response is typically preceded by such reactions as rash, hives, tingling, or swelling.

A patient reports postoperative pain, and the nurse administers morphine (a narcotic analgesic) intravenously to ease the pain. Fifteen minutes later, the nurse notes that the patient is very drowsy, respirations are slow and shallow, and oxygen saturation is low. The nurse administers another drug that decreases the action of the morphine. What is this effect called? a) Displacement b) Antagonistic c) Interference d) Synergistic

b) Antagonistic Rationale: The morphine had a greater-than-desired effect on the patient. Because the drug was given intravenously, it is impossible to remove the drug from the patient's bloodstream. Therefore, the nurse does the next best thing and administers another drug that interferes with the action of the first, otherwise known as an antagonistic effect. The result is a decrease in the action of the original drug. The second drug is sometimes referred to as the antidote to the first.

Antagonists

drugs that attach to a receptor but do not stimulate a response

Partial Agonists

drugs that interact with a receptor to simulate a response but inhibit other responses

Agonist

drugs that interact with a receptor to stimulate a response

Incompatibility

first drug is chemically incompatible with the second drug, thereby causing deterioration when the drugs are mixed in the same syringe or solution or are administered together at the same site. Signs include haziness, formation of a precipitate, or a change in the color of the solution when the drugs are mixed

Allergic Reactions

hypersensitivity occurring in patients who have previously been exposed to a drug and whose immune systems have developed antibodies to the drug anaphylactic reaction: respiratory distress and cardiovascular collapse