A frog's eye. . A butterfly's wing.
. . Hair of a snake's armpit.
. . Hey, fella.
Scoop half a litre of this stuff and make it an IV for me while I start the intro, will you? Adjuvant analgesics What's up NAVE crew! In today's video lesson from the Anaesthesia Unravelled webseries we will be talking about about adjuvant analgesics.
As we saw in previous lessons, pain involves is a very complex pathway, that goes from the painful peripheral stimuli to the perception in the brain. I suggest that you stop by later to watch any classes you missed, all right? The main pharmacological groups used for antalgic therapy are anti-inflammatory drugs and opioids.
However, when dealing with very intense or chronic pain, usually of neuropathic origin, these two groups may not be enough. In this case, it’s interesting that we use alternatives, that are known as analgesic adjuvants, in order to improve the multimodal therapy of our patient. The problem is that, today, the adjuvant analgesics are being trivialized.
There are people out there who keep inventing some magic potions, to try to bring quality analgesia to the patient. There must be people even injecting Whey Protein intravenously to see if it works. This reminds me of a friend of mine who recently asked if i knew the cure for distemper.
I even got a little scared, because as far as I know, there is only the vaccine. But he insisted that "no, there is a cure. " So I asked what it might be.
"Mastruz with duck eggs. " You heard me right. He said "mastruz with duck eggs.
" I kept staring disbelievingly while he was full of arguments, explaining why it works, why this and why that, and so I asked "how many animals were cured by your recipe? " And he answered. .
. "One. " Well, right.
There are several drugs that are said to be adjuvant analgesics. In this video lesson, we are going to address the drugs that have been scientifically proven, or that have a lot of interesting research about it. All scientifically.
Not mastruz with duck eggs. Also, we're not going to talk about acupuncture, physiotherapy, lasertherapy, nutraceuticals, herbal medicine, homeopathy and what else, because it is not the focus of the lesson, and I don't have the technical capacity to give you information on these therapies, okay? The first group are the drugs that are usually used in intravenous infusion, both in the transoperative and postoperative periods.
They have two purposes: The first is to decrease the amount of general anesthetic, thereby, decreasing the undesirable effects of these drugs and also promote analgesia. The ideal is that we use drugs that act in different places so that we can do a better multimodal therapy. Lidocaine Lidocaine is one of the main drugs used as adjuvant analgesics in the trans and postoperative periods.
The analgesic effect of lidocaine occurs by blocking sodium channels in A-delta fibers and C, that are those ascending fibers, and with that, there's inhibition of the excitability of these fibers. But lidocaine also acts on the potassium and calcium channels, and is also an NMDA and glycine receptor antagonist, all of this in the spinal cord. As main clinical effects, regarding analgesia, we have excellent visceral and somatic analgesia, in addition, there's a prokinetic effect, that increases intestinal motility, Therefore, lidocaine is one of the most used drugs for trans and postoperative analgesia in equine colic, for example.
Lidocaine is also widely used as intravenous infusion in dogs, which reduces the requirement for general anesthetics and also provides excellent post-operative analgesia, yet, its use in cats is questionable. The main bibliographic references advise against the use of lidocaine for intravenous infusion in cats, as we can see in this work from the UC, Davis. Even though lidocaine promoted a decrease in the anesthetic requirement, there was intense cardiovascular depression.
That caused the authors to advise against the use of lidocaine by intravenous infusion in cats. Here we can see rates of intravenous continuous infusion of lidocaine in dogs, horses and ruminants. Ketamine Ketamine is one of the most widely used anaesthetics in veterinary medicine, mainly due to its versatility.
We'll be covering further details during the dissociative anaesthesia video lesson. Here, we will address only the effect of this drug as an analgesic adjuvant. The analgesia promoted by ketamine is due to the non-competitive block of NMDA and AMPA receptors, that are mainly located in the spinal cord.
So ketamine works on pain modulation, being very efficient in chronic pain. The doses used are slight when compared to dissociative anesthesia, for example, because our intent is purely analgesia and not a dissociative state promoted by anaesthesia. The doses and routes of analgesic ketamine, both in small and large animals, are described in this table.
Alpha-2 adrenergic agonists The alpha-2 adrenergic agonists are also great analgesics when administered by intravenous infusion, as we covered during the premedication lesson. Just to remember, they act on the alpha-2 receptors that are located in the spinal cord and certain areas of the brain. I suggest that whoever hasn't seen it go check it out in order to better understand the mechanism of action of these drugs, OK?
These drugs have been widely used, both in small and large animals. We can see in this review several infusion rates of alpha-2 adrenergic agonists in horses, with the desirable and also undesirable effects. In horses, as you see here, we can use xylazine, romifidine, detomidine and dexmedetomidine.
In small animals, it is more usual today to infuse dexmedetomidine. Alpha-2 adrenergic agonists can also be administered epidurally and combined with local anesthetics in locoregional anesthesia. But this we will be covering during the locoregional lesson, OK?
Intravenous combinations Usually, when we think about analgesia by continuous infusion, we don't think of a single analgesic, but maybe two up to three, administered simultaneously. The use of one or two combined drugs in the infusion depends a lot on the case, there is no cake recipe or a single standard for everything. Generally, we associate lidocaine with ketamine and an opioid or an alpha-2 adrenergic agonist, but remember that every drug has undesirable effects and these mixtures must be made properly, whilst you need to be very sure of what you're doing.
Don't forget the witch's cauldron, people throw a lot of things in there and, in the end, can't be sure of what's doing what anymore. Gabapentin Gabapentin started to be used as an anticonvulsant, but soon the doctors realized that it had a good effect in chronic pain, mainly of neuropathic origin. Gabapentin's mechanism of action that results in analgesia hasn't been well determined, but it's probably by acting on the calcium and sodium channels.
Pregabalin has a molecular structure structure that's very similar to gabapentin, but with a much higher affinity for the sodium channels, which is why we get an effect equivalent to gabapentin with smaller doses. The beneficial effects of gabapentin on acute pain are extremely questionable. There have been several reports and studies that administered gabapentin combined with some other drug, but it’s very difficult for us to find actual proof.
This is true for some reasons. The first is that, if we are going to do a study, usually with OH or orthopedics, it's very impossible for us to use only gabapentin during the post-operative period. We will invariably have to use other pharmacological groups, drugs that may overshadow some possible effect of gabapentin in this animal.
The second is that the ofset of the therapeutic response can take a few or even several days, so that we would need to start giving gabapentin days before the surgical procedure, so it has some effect during the immediate post-operative period. A bit complicated, right? Well, regarding this delayed effect of gabapentin and if we think of chronic pain, we have to start by titrating this medication so that it promotes desirable effects, like analgesia and not undesirable effects.
So, we starts with lower doses until you get the analgesic effect, without the appearance of undesirable effects. Speaking of it, the main undesirable effects are: sedation, xerostomia and ataxia. In the table we can see the doses and the administration interval for gabapentin in both small and large animals.
Amitryptiline Amitriptyline is an antidepressant generally used in the treatment of behavior disorders in cats. Never occurs, right? The effect of amitriptyline occurs by inhibiting the reuptake of serotonin, but there are also signs that it blocks NMDA receptors and sodium channels.
It's frequently the second choice in the treatment of neuropathic pain, usually in patients that do not respond to gabapentin. One detail about amitriptyline is that it takes two to four weeks to start producing some analgesic effect, then just as with gabapentin, we need to titrate amitriptyline so that it promotes analgesic effect without undesirable effects. An important effect of amitriptyline is given by its combination with tramadol, meperidine or fentanyl-like drugs, as fentanyl, remifentanil, sufentanil or alfentanil, because these drugs also work by inhibiting the reuptake of serotonin, so that we can have the occurrence of the serotonin syndrome.
Its main effects are tachycardia, tachypnea, hypertension, hypertonia and seizures, and can lead to death. Obviously, it is best not to associate amitriptyline to those opioids. Here we can see the doses and administration intervals of amitriptyline in dogs and cats.
There are currently no established doses for safe use in large animals. Amantadine Amantadine was originally used as an antiviral agent and also as a treatment for Parkinson's disease. Some time later, it was used in patients who had injuries to the central nervous system, amputation, other neuropathies and arthritis.
The analgesic effect of amantadine occurs by blocking NMDA receptors, but it also acts to inhibit the reuptake of dopamine and serotonin. There are few studies on the use of amantadine in chronic pain in small animals. In this study, the authors verified the additive effect of amantadine on analgesic therapy with NSAIDs in dogs with osteoarthritis and they realized that, after a month, amantadine made a difference in the treatment.
As it also acts to inhibit the reuptake of serotonin and dopamine, we have to watch out for the serotonin syndrome. The dose of amantadine in dogs and cats is 3 to 5 mg/kg, orally, every 12 or 24 hours. We should not forget to titrate this drug as well.
Maropitant We'll talk about maropitant because it has been the object of many studies lately. The main effect of this medication is antiemetic and it is very good for that. As this drug works by inhibiting neurokinin receptors and these are connected to the peripheric nociceptive stimuli, it is believed that it has some analgesic effect, mainly in the transmission phase, however, the studies are showing that it has no analgesic effect at all, especially in the post-operative period.
We even had some studies exhibited at the Brazilian Meeting of Veterinary Anaesthesiology in 2019 in both dogs and cats where the authors found that maropintant had no analgesic effect whatsoever in the postoperative period. The conclusion of this video lesson is that the analgesic adjuvants alone can not promote sufficient analgesia in patients, but can be great choices for multimodal therapy. Some of them can be used as intravenous infusion and are generally great choices in acute pain.
Others are mainly used in chronic pain and must be titrated. We need to be very careful with the combination of analgesics to promote multimodal therapy, since every drug is capable of promoting undesirable effects. Well, guys, with this video lesson we finished the pain and analgesia module, that consists of four video lessons.
I hope that the information has been relevant to you, OK? Please, leave your comments on our videos, suggest, criticize, also praise, because this feedback is very important. Don’t forget to subscribe on social media and also on our channel here, to always know when there is a new video, OK?
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