Medication Administration and Pharmacodynamics

• Medications are administered to produce a desired effect in the body.
• Pharmacodynamics is the alteration of a function or process of the body as a medication is administered.
• Any medication can cause toxic effects.

Medication Administration Process

• Absorption
• Distribution
• Biotransformation
• Elimination
• Pharmacokinetics refers to the action of the body on a medication.

Principles of Pharmacodynamics

• Receptor sites exist in proteins connected to cells
• Receptors are activated by endogenous and exogenous chemicals, including medications.
• When a medication binds with a receptor site, channels may open or close, biochemical messengers activate, or normal cell functions are prevented or begin.
• Medications bind with particular receptor sites of target cells.
• Newer medications target only specific receptor sites.
• Agonist medications initiate or alter a cellular activity.
• Antagonist medications prevent agonist chemicals from reaching cell receptor sites.

Agonist Medications

• Affinity is the ability of a medication to bind with a particular receptor site
• Agonists bind with receptor sites to initiate or alter cell action.
• Threshold level is the level at which initiation of alteration of cellular activity begins.
• As concentrations of agonist medications increase, their effects increase until all receptor sites are occupied and the cell's maximum capacity is reached.
• Potency is the concentration of medication required to initiate a cellular response; as potency increases, the concentration or dose required decreases.
• Efficacy is the ability to initiate or alter cell activity in a therapeutic or desired manner.

Antagonist Medications

• Antagonists bind with receptor sites to prevent cellular response to agonist chemicals.
• They inhibit normal cellular activation and treat harmful agonist effects of exogenous medications.
• Competitive antagonists temporarily bind with cellular receptor sites, and their efficacy depends on concentration near the receptor sites and affinity compared with the affinity of the agonist chemicals present.
• Noncompetitive antagonists permanently bind with receptor sites and prevent activation by agonist chemicals, with effects continuing until new receptor sites or cells are created.
• Noncompetitive antagonists cannot be overcome by increased doses of agonist chemicals.

Partial Agonist Chemicals

• Partial agonists bind to a receptor site, but do not initiate as much cellular activity as other agonists.
• They also lower the efficacy of other agonist chemicals.

Alternative Mechanisms of Drug Action

• Medications can alter cell function without interacting with receptors by targeting microorganisms, lipids, water, and exogenous toxic substances.
• Antimicrobials may target specific substances present in the cell walls of bacteria or fungi
• Examples include antibiotics and antifungals.
• Chelating agents bind with heavy metals to create a compound that can be eliminated.
• Sodium bicarbonate is an example.
• Diuretics create osmotic changes to alter the distribution of fluids and electrolytes.
• Electrolyte-based medications change the concentration and distribution of ions in cells and fluids throughout the body.

Factors Affecting Response to Medications

• Choice of medication
• Dose
• Route
• Timing
• Manner of administration
• Monitoring
• Age affects response to medications
• Infants need higher weight-based doses than adults and elderly people for water-soluble medications.
• Elderly people need higher weight-based doses for fat- and lipid-soluble medications.
• Titration of medication should be considered for pediatric or elderly patients.
• Altered metabolism/elimination may prolong effects.
• Patients at extremes of age are disproportionately prone to paradoxical medication reactions.
• Weight-based medication dosing involves multiplying a quantity of medication by the patient’s weight in kilograms.
• The advantages include:
  • The amount of medication is proportional to the size of the patient.
  • Manufacturers have already calculated factors.
  • You can calculate the appropriate dose for all ages.
• Limitations:
  • The patient’s weight in kilograms is needed.
  • Does not consider alterations in distribution, metabolism, and elimination.
  • It is sometimes based on the patient’s ideal body weight.
• Hyperthermia may increase metabolism of drugs and reduce the amount of drug returned to circulation.
• Hypothermia impairs the effectiveness of medications used in traditional cardiac life support.
• Exercise caution when administering medications to patients with genetic disorders:
  • Primary pulmonary hypertension
  • Sickle cell disease
  • Glucose-6-phosphate dehydrogenase deficiency
• Changes in pregnancy can affect absorption, distribution, or elimination of medications.
• These changes include increased cardiac output, intravascular volume, respiratory tidal volume, minute volumes, renal blood flow, and urinary elimination, as well as decreased hematocrit, inspiratory/expiratory reserve volumes, and gastrointestinal motility.
• There are also changes in endocrine glands.
• Consideration should be given to the effect of the medication on the fetus.
• Psychosocial factors like pain, anxiety, and discomfort can vary among individual patients.
• It's important to be alert for verbal and nonverbal cues.
• Medication administration is complicated by the placebo effect.

Types of Medication Responses

• Therapeutic effects are the desired effects based on the patient's illness, injury, complaint, signs, and symptoms.
• The condition should match the use/indication listed on the medication profile.
• Medication is administered in a dose intended to produce a desired clinical response.
• Some doses require repeated dosing.
• These are capable of demonstrating cumulative action.
• Adverse medication effects are clinical changes that are not desired.
• They cause harm or discomfort to the patient.
• They should be considered in relation to the patient’s condition.
• Patients with chronic medical conditions are more susceptible.
• Patients may be unable to tolerate even mild adverse effects.
• May range in severity
• Side effects can be desirable in certain situations and harmful in others
• Idiosyncratic medication reactions are completely unexpected.
• Therapeutic index is the relationship between the median effective dose and the median lethal dose.
• The median lethal dose (LD50) is the dose that causes death in 50% of animals tested.
• The median toxic dose (TD50) is the dose at or above which causes toxic effects in 50% of animals tested.
• A large difference between the median effective dose and the median lethal dose means that the medication is safe.
• A small difference means that the patient needs to be monitored.
• Immune-mediated response occurs when genetically predisposed patients have an initial exposure/sensitization to an allergen.
• Medication sensitivity may occur following the first exposure to a medication or substance.
• Avoid administering medications to patients who have had a serious reaction to the medication.
• Medication tolerance occurs when certain medications have a decreased efficacy when taken repeatedly.
• Tolerance results from down-regulation, which reduces available cell receptors for a particular medication.
• Cross tolerance occurs when repeated exposure within a particular class has the potential to cause a tolerance to the class.
• Tachyphylaxis occurs rapid tolerance from repeated doses within a short time.
• Stimulants and depressants are prone to misuse and abuse.
• Repeated exposure can cause habituation.
• Prolonged or significant exposure can cause dependence.
• Medication interactions occur when medication interference is undesirable.
• A major concern is incompatibility during administration.
• Medication may increase, decrease, or alter the effect of another medication.

FDA Pregnancy Categories

• Category A: Studies in pregnant women show no risk to the fetus.
• Category B: Animal studies show no risk, but no well-controlled studies in pregnant women.
• Category C: Animal studies show potential risk, but benefits may outweigh risks in certain cases.
• Category D: Positive evidence of human fetal risk, but benefits may justify use in life-threatening situations.
• Category X: Contraindicated in pregnancy; risks clearly outweigh benefits.