Hepatic metabolism plays important role in this first pass effect. When taken orally lidocaine is absorbed into enterohepatic circulation through GIT and reaches to the liver. Since it is metabolised before reaching to the target, the effect is called as "First pass effect". Due to this hepatic clearance very less amount of drug is available in the blood which is not sufficient to produce therapeutic response.
Hence lidocaine can be given by IV infusion which bypasses enterohepatic circulation preventing its first pass effect. Even though IV route doesn't show first pass effect, still lidocaine can be metabolised by hepatic system. There is a difference between first pass metabolism and systemic metabolism. In first pass effect the drug is metabolised before reaching the target. But in systemic metabolism the drug already reaches systemic circulation and some of the drug enters into the target organ and remaining portion of drug is distributed , metabolised and excreted.
Hepatic metabolism is the main mechanism by which drug is cleared from the systemic circulation. Sometimes only a small amount of drug leaves the liver through the hepatic vein and makes it to the bloodstream.
Eventually the vasculature redistributes the drug back to the liver through the hepatic artery. First pass metabolism determines what fraction of an oral dose will reach the circulation - the bioavailable fraction. Examples of drugs exhibiting first pass metabolism are aspirin and lidocaine. Some drugs, such as lidocaine, which have relatively low bioavailability are not given orally because of concern of metabolite toxicity.
Lidocaine would be completely metabolized by first pass metabolism before it reached the blood stream if it were given orally. Therefore, it is always administered IM or IV. First Pass Hepatic Metabolism. How to Subscribe. Need multiple seats for your university or lab?
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