Pharmacokinetics: The Kinetics behind Anesthetics

Kinetics plays a large role in understanding how the body eliminates anesthetics and therefore is imperative in determining the rate of administration during surgery that will keep the patient sedated.  This also applies to all drugs to ensure that the drug concentration remains within the therapeutically defined parameters.  This specific study of kinetics is called pharmacokinetics.

One key principle behind pharmacokinetics is the use for monitoring for determining the quantity of drug present in comparison to the quantity required for the desired effect.  The monitoring of these drugs in the body reveals how the body metabolizes these compounds.  The body absorbs, distributes, bio-transforms, and excretes these drugs and monitoring can allow for observations of the the concentration of the compound to allow for conclusions to be reached.  These enzymatic reactions in the body demonstrate that most drugs are broken down according to first-order kinetics.  The rate of metabolism in this order reaction depends on the concentration of the drug in such a way that a higher concentration is metabolized at a higher rate while a lower concentrations is metabolized at a slower rate.  This property is beneficial in that it allows the body to better prevent overdosage through the accumulation of the drug; however, this property makes it difficult to maintain drug concentrations especially when relatively high concentrations are required since the body counteracts it.

The other possible order in pharmacokinetics is a zero-order reaction.  Zero-order reactions occur at a constant rate and are independent of the concentration of the drug being metabolized.  Although most drugs follow first-order kinetics, there are several notable exceptions.  One exception to this rule is phenytoin.  Phenytoin, also known as sodium 5,5-diphenyl-2, 4-imidazolidinedione, is an injectable drug used in the treatment of seizures.  However this drug poses a serious threat of toxicity in the blood due to its metabolization being a zero-order reaction and therefore has the warning stating that the intake of this drug should be less than 50 mg/min.

Still, these kinetic models do not perfectly explain how the human body reacts with these compounds.  One major building block in pharmacokinetics deals with the first-pass effect.  This effect essentially explains how when compounds enter the human body through certain methods, the compound passes through the liver.  On the initial passing, the liver substantially metabolizes the drug before it even gets into circulation which is the foundation of the first-pass effect.  The next passes of the liver may not be as efficacious as the initial pass.

Overall, this understanding of kinetics plays a crucial role in the safe use of anesthesia for medical purposes.  This understanding can then be applied to almost all drugs and compounds that enter the body.


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