Local Anesthesia

00:00 Okay, the local anesthetic is a very favorite subject in the exam.

00:05 They have inevitably have to question somewhere in this topic.

00:15 Okay first slide, it's boring, just to give you an idea of when in your local, You know, local anesthetic for look infiltration, nerve blocks, spinals, epidural, IV administration fine.

00:28 Okay, very important this slide.

00:32 All local anesthetics contain a lipophilic aromatic group and hydrophilic groups.

00:37 So, basically fat and water.

00:40 Okay, a combination of fat.

00:44 So, I won't draw the the chemistry of this but you just think about it this way.

00:52 Fat.

00:53 Water.

00:54 They are either connected by an ester or amide.

01:01 That's it, that's the local.

01:06 So all local anesthetic contains a lipophilic aromatic group and hydrophilic group that are linked by either amide or ester.

01:14 Amide are stable and have a longer shelf of life.

01:19 Well, esters are more volatile.

01:22 Give me a some example of ester.

01:27 Sniffing.

01:27 Okay.

01:28 Okay, cooked cocaine, anything volatile is an ester.

01:33 So amides is a standard, Lidocaine, bupivacaine, prilocaine.

01:39 These are your standard amides.

01:41 So they have a longer shelf life.

01:43 Okay, and ester is got cocaine and amethocaine.

01:47 We really don't use that much on a day-to-day practice.

01:50 So you need to really know about amides.

01:53 Question has been asked about adrenaline concentration, 1 in 200,000.

01:57 Nothing less in a local anesthetic.

02:02 Okay, 1 in 200.

02:05 Okay, the important ones, llidocaine, short-acting.

02:09 How short? How many hours? When do you give patient Lidocaine? When do you do it, ask take some paracetamol after few hours.

02:19 2 to 4 hours, it maximum 2 to 4 hours Bupivacaine is slightly longer acting.

02:26 We know of the 68 hours, Bupivacaine is more cardiotoxic than Lidocaine.

02:34 Because it's more lipid soluble the hence, its safe dose of bupivacaine with or without adrenaline is the same.

02:43 Okay.

02:48 If at all they have asked about local anesthetic.

02:50 They will ask about the mechanism of faction.

02:53 And I have highlighted this because this is a statement they usually ask.

02:58 Prevent influx of calcium inhibition, potassium inhibition, sodium.

03:02 So that's all you need to remember.

03:03 It prevents sodium inhibition, inhibiting the propagation of the action potential, okay.

03:10 So this is the crucial physiology behind how a local anesthetic works.

03:16 It inhibits the influx of sodium and prevents it from giving a action potential.

03:24 Okay.

03:27 This is more of theory, but in their exam, they won't have to have the time to ask you so much theory.

03:33 It will be just one statement.

03:36 Okay, and it causes reversible blockade of motor and sensory nerves.

03:41 Potency of ALA is related to the dose and lipid solubility.

03:46 There's another MCQ question.

03:48 Dosage and how soluble it is in lipid.

03:51 I'll give an example.

03:52 Okay.

03:53 Now I'm going to spend a bit of time on this and you need to really get this right.

03:59 Calculation of the maximum dose.

04:01 You need, I'm sure you might have worked, learned about this in pharmacology.

04:06 100% of a drug, so if you have a vial of local, 100% of the drug has got one milligram per ml.

04:16 So if you get a 1 Ml ampule, if it is hundred percent concentration, that is got one gram of the drug.

04:25 Okay.

04:26 Therefore 10% concentration is 100 ml, 100 mg per ml, and 1% is 10 milligrams per ml.

04:32 This is what you need to remember.

04:47 I normally work it out at this point, 1% is 10 mg, 2% is 20 mg, .5% is 5 mg.

04:56 That's how I work it out.

04:58 But for your better understanding you can start from here.

05:02 A drug, with a hundred percent concentration has got one gram.

05:09 Per ml.

05:11 1 gram per 1 ml.

05:13 So, 1% is 10 mg per ml.

05:16 Did you get that? So if you understand that then you can work out the maximum safe dose.

05:23 So if you give with adrenaline, it limits the blood flow to the area so increases the total dosage provides a bloodless field, blanching of the skin is shown, the area of infiltration is Blanche, and clearly must not be used in any doctor is, this is standard thing which is said But nowadays, we are still given digital vessels If you feel confident and if you're working especially center then spine you you can use in and arteries but not for the for an exam.

05:58 Okay.

05:59 It's not a dose.

06:00 We remember this.

06:15 Okay, so you will be given a scenario, 70-year-old coming for extension of something.

06:22 What's the maximum safe dose of Lidocaine with adrenaline? So you need to have those values.

06:29 So this is normally we give it a 3 to 5, 5 to 7.

06:33 But in the exam, you may get it as, just keep it to 3.

06:43 Okay.

06:43 Now at this point I need to ask you, you need to know how to calculate the ml from the milligram.

06:49 So the question will be, 70 kilogram patient coming for local anesthetic.

06:55 You're giving one percent of Lidocaine, how many mil’s can give? 70-kilogram patient coming in for surgery for some surgery, and you are infiltrating 1% of Lidocaine without adrenaline without without adrenaline.

07:16 What's the volume you can give? 21 mil’s, 21 to 35 mil’s.

07:21 So 3 to 5 mil’s is 21 to 35, yeah.

07:24 If it was 2%.

07:30 10.5 average.

07:33 Okay.

07:35 So your exam will never ask you in milligram, they'll ask in milliliter, because that's what we've get in the hospital, right? We don't, we'll just get 1% Lidocaine given to us.

07:46 So you need to know how many, what's the volume you can give.

07:49 So but you need to work it out in the exam that calculation.

07:53 So remember, 1% of Lidocaine is 10 milligrams of the drug.

08:03 Okay, what about an infected sides? Can you use local? You can use but probably doesn't work.

08:09 Because infection has gotten acidotic environment and it won't penetrate the cell membrane for preventing the depolarization.

08:20 And it can also worsen cellulitis in local, so you probably don't use local anesthetic in an area of infection.

08:29 Signs of local anesthetic toxicity.

08:33 This one, circumoral tingling and paraesthesia, unconsciousness, coma, cardiac disturbances.

08:41 And I put up this slide because it's the normal questions it's been asked what they give.

08:46 The reason I put up with this, I never come across this lipid emulsion, but I've read in many books about this.

08:52 This is a standard management.

08:54 I don't know whether the hospital has it, but they say if you have toxicity you need to stop it and start a bolus of 20% lipid emulsion.

09:05 I don't know how it works but it works.

09:13 Is it? I haven't used it, so I'm not sure.

09:27 Is lipid emulsion tpn? I don't know.

09:31 Okay, that's right, yeah.

09:34 Anyway, so TPN, XML change it to TPN.

09:37 Okay.

09:37 Okay, so that's you need to remember lipid emulsion.

09:41 That's it.

09:41 So local anesthetic as I said, safe dose is probably the most important thing you need to remember.