Pharmacology for Anesthesia I Introduction. What is a Drug?
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Transcript of Pharmacology for Anesthesia I Introduction. What is a Drug?
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Pharmacology for Anesthesia I
Introduction
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What is a Drug?
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Pharmacokinetics (PK)
What the body does to the drug
• Absorption
• Distribution
• Metabolism
• Excretion
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Absorption
Skipped by administering drugs parenterallyInhaled agents require special considerations
The process of diffusion or transport of a drug from the site of administration to the plasma
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Fick’s Law
concentration gradient x surface area x diffusion coefficient
membrane thicknessRate of Diffusion =
Diffusion coefficient =Permeability
Size
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Ionization StateHenderson – Hasselbalch Equation
log concentration (protonated)concentration (unprotonated) = pKa - pH
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Trapping
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Distribution
The process of diffusion of a drug throughout the body
Generally governed by the same characteristics as absorption
Vd = volume of distribution
Protein Binding
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Metabolism
• The enzymatic modification of the drug molecule by the body– Often occurs in liver
– May occur elsewhere
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Hepatic Metabolism
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Example of Phase II prior to Phase I
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CYP Enzymes
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Pharmacogenetics of Drug Metabolism
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Examples of Drug-Drug Interactions
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Elimination
• The removal of the drug from the body– Renal
– Hepatic
– Respiratory
– Cutaneous
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Clearance• Used to describe our ability to eliminate the active ingredient
– Combination of metabolism and excretion
Example of Zero order kinetics
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First Order Kinetics• Single compartment model• Double compartment model• Three compartment model• Etc.
Distribution and Clearance
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Absorption and Clearance
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Effect Not Always Governed by Plasma Concentration
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Dosing Regimens
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Can speed accumulation time by administering a loading dose
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Routes of Administration
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ROUTE ABSORPTION PATTERN SPECIAL UTILITY LIMITATIONS AND PRECAUTIONS
Intravenous Absorption circumvented Valuable for emergency use Increased risk of adverse effects
Potentially immediate effects Permits titration of dosage Must inject solutions slowly as a rule
Suitable for large volumes and for irritating substances, or complex mixtures, when diluted
Usually required for high-molecular-weight protein and peptide drugs
Not suitable for oily solutions or poorly soluble substances
Subcutaneous Prompt, from aqueous solutionSuitable for some poorly soluble suspensions and for instillation of slow-release implants
Not suitable for large volumes
Slow and sustained, from repository preparations
Possible pain or necrosis from irritating substances
Intramuscular Prompt, from aqueous solutionSuitable for moderate volumes, oily vehicles, and some irritating substances
Precluded during anticoagulant therapy
Slow and sustained, from repository preparations
Appropriate for self-administration (e.g., insulin)
May interfere with interpretation of certain diagnostic tests (e.g., creatine kinase)
Oral ingestionVariable, depends on many factors (see text)
Most convenient and economical; usually more safe
Requires patient compliance
Bioavailability potentially erratic and incomplete
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Pharmacodynamics
• What the drug does to the body– Typically receptor mediated
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What factors affect the ability of a drug to interact with a receptor?
Drug size• Large enough to be specific• Not so large as to be unable to interact with the receptor
Drug Shape
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Some drugs do not appear to fit into these categories
• Osmotic agents
• Transport regulators
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Agonists
Antagonists Competitive Noncompetitive
Allosteric Activators Potentiators
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Partial agonists
Inverse agonists
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Antagonists
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Noncompetitive Antagonist and Spare Receptors
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Full and Partial Agonists
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Cellular Receptors
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Different Drugs Similar Effects
Potency vs. Efficacy
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Population Variation and Therapeutic Window