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PHYSICOCHEMICAL PROPERTIES OF IV INDUCTION AGENTS
pKa
- Defined as the pH were a drug exists as 50% ionized and 50% unionised.
- If pKa - pH = 0, then 50% is ionised and 50% is unionised.
- Most drugs are either weak acids or weak bases. They are part ionised and part unionised.
- Drugs are better absorbed in unionised form (as ionised drugs have attracted particles and become bigger)
- Simplistically, the higher the pKa the less ionised it is at a body pH of 7.4. Propofol = 11, so it's almost entirely unionised.
Volume of Distribution (Vd = L/kg)
The volume into which a drug appears to be uniformly distributed at the concentration measured in plasma. Usually in a "Steady State".
Imagine your patient is a bucket of water (volume) and you inject a drug into it, it distributes evenly in the bucket.
Imagine your patient is a bucket of water (volume) and you inject a drug into it, it distributes evenly in the bucket.
The Vd is a theoretical notion to describe how well the drug would disseminate into the fluid (fat, blood, tissue etc)
Drugs that are less ionised with higher lipid solubility tend to have higher Vd's
Clearance
- Defined as the volume of blood or plasma from which the drug is removed completely in unit time.
- Rate of drug elimination divided by the plasma concentration of the drug
- Often adjusted for body weight.
- A large elimination rate constant (k) produces a short elimination half-life (t ½); this will result from a high (Cl) or a small Vd.
- If the concentration in blood is reduced by 20% in 1hr, the result is equivalent to removing the entire drug from 20% of the blood volume
(1000 ml), corresponding to a clearance of 1000 ml h−1 or 16.7 ml/min.
% Protein Binding
Only the unbound portion of the drug is available for diffusion to its site of action.
Common blood proteins that drugs bind to are human serum albumin, lipoprotein, glycoprotein, and α, β‚ and γ globulins.
Active Metabolites
Can have effects of the drug even once its metabolised, if they are active. Inert metabolites do not have any effects.
An acid in an acid solution WILL NOT ionize
An acid in a basic solution WILL ionize
A base in a basic solution WILL NOT ionize
A base in an acid solution WILL ionize
@Gas_Craic
Dr. David Lynesspropofology.com
and how they influence pharmacokinetics, pharmacodynamics and your patient
Typical adult blood volume is 5 litres.
Approximately 77ml/kg