Ladies and gentlemen, the Aerial Demonstration Squadron of the Brazilian Air Force. Insane, right? The Smoke Squadron is known worldwide as one of the best air demonstration squadrons in the world and is based here in the city of Pirassununga.
We anaesthetists frequently make an analogy between the pilot and the aircraft and the anaesthetist and their patient, and that's what we're going to see in this video lesson. Bring it on! Anaesthetic premedication: benzodiazepines and phenotiazines What's up NAVE crew!
In today's Anaesthesia Unravelled episode we'll be talking about anaesthetic premedication, which is one of the first steps of the anaesthetic protocol. Just like a pilot, the anaesthetist must devise an anaesthetic protocol, check all equipment and also predict all possible complications before starting the anaesthesia. The first thing that we are going to do is the anaesthesia checklist.
The checklist is so important that we made a separate video for that. I suggest you watch it later or check this link right here, or here, or here, or here, or here. .
. Making an analogy or actually a joke about flying a plane, we notice that anaesthesia follows virtually the same steps. The first part, that is the taxiing and arrival at the departure point of the runway, represents the pre-anaesthetic medication.
Next, we have the takeoff, that represents the anaesthetic induction. The flight itself illustrates the anaesthetic maintenance. The start of landing procedures represents the interruption in the anaesthetics' supply, considering a patient in general anaesthesia, and the final taxiing represents the recovery period.
'Man, that is so lame'. 'You think? ' 'Definitely'.
We'll see during the course that there are various anaesthetic modalities but they all have something in common: the anaesthetic premedication. Premedication is present in almost all anaesthetic protocols. We need to understand that the phases of anaesthesia must be in tune with each other.
There's no way we can choose the premedication without taking the induction and then the maintenance into account, because there is an interaction between medications, so, we can't give the premedication without knowing what we'll be using next. We are going to plan the entire anaesthetic protocol, study its viability, and only then we will administer the premedication. But before we choose the premedication, we need to understand its purposes, which are basically: to help contain the patient, altering its behaviour; with that we will be reducing the animal's stress; potentiate anaesthetic induction drugs and also minimise their adverse effects; provide a swift anaesthetic induction; minimise autonomic reflex functions whether sympathetic or parasympathetic, during the surgical procedure; and promote analgesia and myorelaxation.
Therefore, we need to understand that the premedication doesn't serve to only get the animal quiet, because of that, the anaesthetic premedication is present in anaesthetic protocols in general, being avoided only in very specific situations. Another important point for choosing the premedication is knowing who our patient is, the physiological condition of our patient, we need to know if it's a healthy patient, if it's obese or, for example, polytraumatised, the metabolism of the patient, since we work with several species with completely different metabolisms, which is sure to influence the anaesthesia, and combinations, since drugs chosen for premedication will certainly have some influence over the maintenance drugs. The premedication can be done with one, two or more drugs, it will depend on the anaesthetic protocol of the anaesthetist.
The medications generally used for premedication are benzodiazepines, phenothiazines, alpha-2 adrenergic agonists, opioids and NSAIDs. But these two we are going to ignore for now, because we'll come back to them on the pain and analgesia lesson. The premedication lesson will be split into two parts: In the first, we'll talk about benzodiazepines and phenothiazines, and in the second, alpha-2 adrenergic agonists.
This way, we address the anxiolytics and tranquilisers in the first part and sedatives in the second. Benzodiazepines This group is mainly represented by two drugs: midazolam and diazepam. The benzodiazepines have both a great advantage and a great disadvantage.
The advantage is that they practically do not alter the physiological parameters, on the other hand, in domestic animals, they don't even promote tranquilisation, at most, a decrease in anxiety. The benzodiazepines act on GABA receptors, enhancing the action of the GABA. With that, a reduction on the cerebral activity occurs.
As we can see in this really cool illustration, the GABA neurotransmitters bind to a GABAergic receptor, facilitating the passage of the chloride ion into the cell and promoting depolarisation. The benzodiazepines bind to another portion of this receptor, leading to a greater influx of chlorine into the neuron, hyperpolarising it and increasing the activity of GABAergic neurons. For those who want to watch this entire video, you can find the link in the description.
Well, even though these drugs fail to promote tranquilisation in domestic animals, we see this effect in neonatal, geriatric and debilitated patients. We can see in this video that this foal, less than 15 days old, received a dose of midazolam and became well sedated after two minutes. Also interesting is that midazolam is a great sedative for birds and lizards.
No kidding! Another interesting effect of benzodiazepines is myorelaxation and because of that, they're much more useful in anaesthetic induction than premedication. Midazolam and diazepam will provide the same effect on the patient, the basic differences are that midazolam is very water-soluble, it can be given intravenously or intramuscularly, while diazepam has propylene glycol as medium, an oil, so when administered intramuscularly, promotes considerable pain.
Another difference between the midazolam and diazepam is the period of action. They have the same duration, two to three hours. The thing is that diazepam has two additional metabolites that are also active, so we usually see an effect of diazepam that lasts for 6, 8, up to 20 hours sometimes.
"So it doesn't matter if I choose one or the other". Yeah, alright, I just don't usually use diazepam in case of equine orthopaedic surgery, because I want this animal to have no lasting effects of myorelaxation on return from anaesthesia, to avoid a bad recovery. I think that's the only case that I prefer to use midazolam instead of diazepam.
But really, for me, it's all the same. The posology is practically the same for both, although the literature says that midazolam can be used in a slightly lower dose, but clinically we use a dose of 0. 1 to 0.
5 mg/kg in small animals and 0. 05 to 0. 2 mg/kg in large animals.
An important tip that applies not only to benzodiazepines but for all the other drugs that we are going to address in premedication as well, is the route of administration. In small animals, we always prefer the intramuscular administration to reduce the impact of the medication on the patient's physiology. In large animals, on the other hand, this impact is not so evident and we need the effect to be more intense, because sometimes the animal does not collaborate due to its size and nature, so we prefer to do it intravenously.
It is worth mentioning that the benzodiazepines have an antagonist, flumazenil. Its dose ranges from 0. 07 to 0.
1 mg/kg. It can be given intramuscularly or intravenously, but we usually use this drug in emergencies, overdoses, something of the sort, so we end up doing it intravenously. Phenothiazines The phenothiazines are part of a group that has been extensively used in veterinary medicine, and still is.
The main drug in this group is acepromazine. Acepromazine promotes a tranquilising effect, blocking serotonergic, dopaminergic and catecholinergic pathways. As we can see in this picture, there's a competitive antagonism for the receptor by dopamine, serotonin or norepinephrine.
Regarding the tranquilisation, we need to understand that the effect of acepromazine is not dose-response, and there is no point in increasing the dose of acepromazine in order to promote a sedative effect instead of tranquilisation. It doesn't work like that. On the other hand, we'll end up with excitation, catalepsy and muscle tremors, all extrapyramidal symptoms.
An important effect of acepromazine that we need to remember is the depression of the cardiovascular system. Acepromazine promotes hypotension and decreased contractility of the myocardium. The hypotension is caused by the block on alpha-1 receptors.
With this, the animal usually has a drop in blood pressure up to 30%. Additionally, there's the myocardial depression, decreasing the cardiac afterload and interfering on the arterial pressure. This effect can last for up to 6h, as is the duration of acepromazine.
So, we have to avoid as much as possible using acepromazine in patients who have some cardiovascular instability. The activation of the alpha receptors will also affect the animal's haematocrit. The spleen is innervated by adrenergic fibres, and when these fibres are blocked, splenic dilation and sequestration of red blood cells occur, affecting the haematocrit.
Other interesting effects that acepromazine promote is an antisialagogue effect, the animal has a decrease in the flow rate of saliva; antiarrhythmic effect, usually, acepromazine decreases the possibility of tachyarrhythmias especially those originating from the sinoatrial node; antiemetic effect, acepromazine tends to decrease the possibility of the animal vomiting; thermoregulatory centre block, acepromazine frequently promotes hypothermia, because we put the patient in a slightly colder environment, but if we use acepromazine on a horse and leave that horse in the sun, it may have hyperthermia. Acepromazine is used in dogs at the dose of 0. 02 to 0.
05 mg/kg, preferably by intramuscular route. An interesting fact is that brachycephalic dogs are extremely sensitive to phenothiazines, so we lower the doses up to 50% when compared to those used in other breeds. In cats, phenothiazines do not work very well, it’s even possible they'll become excited instead.
In horses, the dose is of 0. 03 to 0. 05 mg/kg, and we usually do it intravenously.
Like with cats, it’s of no use in ruminants, because the effect is not good. Regardless of the species, the latency period is from 5 to 10 min, but the peak occurs after half an hour. And as I said, this effect can last for up to 6 hours.
Adriano, how was that thing with acepromazine and stallions? Alright my friend, here's the thing. In every veterinary book we'll find at least one paragraph saying that phenothiazines promote permanent penis exposure, usually in stallions.
In fact, this exposure occurs because the retractor penis muscle has only adrenergic innervation, so, as there's an adrenergic block, the exposure of the penis occurs. But this permanent exposure is controversial, there are some reports saying that the penis can be relaxed for up to 10 hours, which can be followed by an oedema and the animal can’t retract the penis. We can see in this retrospective study from 2011, in which the authors evaluated 575 anaesthesias in males, that only 2.
4% had penile exposure for up to 4 hours, and 0. 2%, up to 18 hours, but there were no irreversible cases. Well, better safe than sorry, so is better to avoid phenothiazines in males, especially stallions.
As a conclusion of this video lesson, we have that premedication must be chosen according to the patient and the procedure to be performed. It must also take into account induction and maintenance agents. Benzodiazepines are great myorelaxants, but with little tranquilisation effect in most animals.
Phenothiazines are good tranquilisers, but recommended only for patients without hemodynamic disorders, and remembering that these two groups do not promote analgesia. Folks, take a look on this video's description, there's more information about phenothiazines and benzodiazepines, I suggest you watch the second part of this lesson, that addresses the alpha-2 adrenergic agonists. And, before leaving, give us like, share and subscribe to the channel, OK?
See you soon! There are no jokes here today! Go and watch the video on alpha-2 agonists!
