General Anesthetics – Anesthetic Drugs

by Pravin Shukle, MD

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    00:01 So, let's talk about all of these factors and induction speed.

    00:05 Let's talk about solubility. The greater the solubility, the slower the induction.

    00:11 So remember my diagram, it just takes a long time for the blood to fill up.

    00:17 Inspired gas partial pressure. The greter the partial pressure of that gas, the faster the induction.

    00:25 That just make sense. What we are essentially saying here, is that if the partial pressure starts out at 100 %, you'll have a faster induction, than if the partial pressure starts out at 50 %.

    00:37 Next one, ventilation rate. The greater the ventilation rate, the faster the induction.

    00:44 This make sense, if you're breathing in the gas really fast, you're going to have a faster induction.

    00:52 Blood flow. This one seems counterintuitive but remember how we filled up the river in Chicago on St. Patty's Day.

    01:00 We'd like to turn that river green. If the river is flowing fast, it takes us more dye, and it's harder to fill up that river, right? The same thing with blood flow.

    01:10 The greater the blood flow, the slower the induction.

    01:14 AV concentration gradient. We didn't talk about this very much, but essentially the greater the AV gradient, the slower the induction.

    01:22 And once again, this has to do with how quickly blood is being taken away.

    01:27 So there you have it.

    01:29 Let's do a question and let's go back to our question now. Which of the following is true regarding anesthetic gases? A, the more soluble, the faster the induction rate. Is that correct? The slower the ventilation, the faster the rise in alveolar pressure. Is that correct? The partition coefficient is directly proportional to induction rate.

    01:53 Blood partial pressures are directly proportional to inspired partial pressure.

    01:57 And at high pulmonary blood flow, the pressure rises quickly.

    02:01 The correct answer is partial pressures are directly proportional to each other.

    02:06 The other ones are incorrect.

    02:10 Let's talk about the effects on different organs of general anesthesia.

    02:15 We have central nervous system effects, cardiovascular effects and respiratory effects.

    02:20 Starting off with central nervous system effects. In the brain, it will decrease your brain metabolic rate.

    02:27 And it may also increase cerebral blood flow. This is going to be relevant in brain injury patients because you want to choose your general anesthetic very carefully because you may increase the intracranial pressure.

    02:41 Enflurane may cause twitching and spike and wave activities so this is an important consideration in our patients prone to epilepsy. And nitrous oxide causes analgesia and amnesia.

    02:54 This may be a beneficial thing in certain situations like dental anesthesia.

    02:59 What about cardiovascular effects? Cardiovascular effects are generally fairly moderate.

    03:05 It does cause a moderate blood pressure reduction. It does cause a decrease in cardiac output.

    03:11 And in particular, halothane and enflurane are culprits in this. It does cause reduce liver and renal blood flow and it can increase arrhythmia risk. This is usually a catecholamine induced phenomenon.

    03:25 And we see it more with halothane and isoflurane.

    03:30 What about respiratory effects? Respiratory effects, you can decrease ventilation and in fact you may halt ventilation which is why we put people on mechanical ventilators. The decreased ventilatory response to hypoxia is particularly concerning in this regard. Nitrous oxide, on the other hand, has the smallest effect on respiration, which is why it is commonly used or most commonly used in dental anesthesia where you don't have respiratory equipment.

    04:00 Now, it may cause bronchospasm and I'm specifically thinking about desflurane here.

    04:08 In terms of toxicity, there are some concerns about all of these agents.

    04:12 Clearly, hepatitis is one of them because all of these drugs are broken down in the liver.

    04:16 Hepatitis is more of a concern if there's also hypovolemic shock or stress.

    04:21 Now, we think that the hepatitis is being brought about by free radical metabolites and ketones in the blood.

    04:30 Renal insufficiency, the suspected culprit is fluoride. And it may be related to metabolism of methoxyflurane which does contain a fluoride ion.

    04:42 Megaloblastic anemia is also a concern when we use general anesthetics.

    04:48 Sometimes, patients may develop a methionine deficiency after prolonged nitric oxide use.

    04:54 This is not very common, and it is one of the reasons why we use nitrous oxide in very short burst or for short procedures.

    05:02 Finally, there's malignant hyperthermia. Let's go into this a little bit more detail.

    05:09 Okay, malignant hyperthermia. Malignant hyperthermia or MH is a rare life threatening hypermetabolic state.

    05:17 It's usually brought about by volatile anesthetics like halothane and succinylcholine.

    05:23 It is transmitted in an autosomal dominant fashion. This is why when you're doing a preop evaluation on a patient going for general anesthesia, you have to ask about malignant hyperthermia in the family history.

    05:39 We suspect that it's due to a defect in the ryanodine receptor in muscles.

    05:46 The treatment is dantrolene. The goal is to treat within one minute.

    05:51 So, I can't stress this enough for your exams and when you're out in practice.

    05:56 Malignant hyperthermia, dantrolene, under one minute.

    About the Lecture

    The lecture General Anesthetics – Anesthetic Drugs by Pravin Shukle, MD is from the course CNS - Pharmacology. It contains the following chapters:

    • General Anasthetics
    • Central Nervous System Effects
    • Cardiovascular Effects
    • Respiratory Effects
    • Toxicity
    • Malignant Hyperthermia

    Included Quiz Questions

    1. Greater partial pressure of the inspired gas
    2. Greater solubility of the inspired gas
    3. Slower ventilation rate
    4. Greater blood flow
    5. Greater AV gradient
    1. Decreased intracranial pressure
    2. Myocardial depression
    3. Respiratory depression
    4. Hypotension
    5. Nausea
    1. Dantrolene
    2. Midazolam
    3. Propranolol
    4. Increase in anesthetic dose
    5. Haloperidol
    1. Medical history
    2. Muscle biopsy
    3. A trial of anesthesia and monitoring
    4. A cold saline test
    5. Temperature monitoring

    Author of lecture General Anesthetics – Anesthetic Drugs

     Pravin Shukle, MD

    Pravin Shukle, MD

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