Veterinary drugs for anesthesia and sedation

Diseases in dogs are not uncommon. Some of them can be cured in the shortest possible time and with minimal losses, but to treat others it is necessary to resort to surgical operations of varying levels of complexity. The problem is that the dog cannot be persuaded to lie still while the surgeon does his job. Even “minor” operations require anesthesia for dogs, and not only the success of the surgical intervention, but also the entire future life of the pet largely depends on the quality of its implementation.

What is anesthesia

Anesthesia comes from an ancient Greek term that can literally be translated as “lack of sensation.” This “lack” is achieved through anesthetic drugs that temporarily “turn off” the nerve fibers responsible for transmitting pain sensations.

In addition, anesthesia is characterized by a partial loss of muscle rigidity (that is, their relaxation), which also facilitates surgical intervention. All types of pain relief can be divided into two large groups: local and general (anesthesia).

General anesthesia for dogs

Regardless of the type of anesthesia, it always begins with premedication. This is the name of the “event”, accompanied by the administration of light sedatives. Premedication is needed to calm the dog and prepare its body for deep, “full” anesthesia.

The absence of this stage is an almost 100% guarantee of the development of severe complications. General anesthesia can be divided into two broad types:

Mononarcosis (monovalent).
Polynarcosis.

In the first case, only one drug is used for anesthesia (occasionally two, if it is necessary to ensure their combined effect). As a rule, such a simple option is resorted to in the case of simple and short operations (many of which can be performed using only local anesthesia).

Accordingly, general polyvalent anesthesia involves the use of several drugs at once. It can be extremely difficult to take into account all the nuances of their interaction, but there is simply no other way out in cases where a complex and lengthy operation is ahead.

Parenteral anesthesia

This is the most common type of anesthesia, in which the active substance is administered intravenously into the dog’s body. The advantage of such anesthesia is the possibility of ideal dosing of the drug depending on the current condition of the animal being operated on. In addition, when administered intravenously, the medicine begins to act very quickly.

Important! Almost all drugs for this type of anesthesia can cause hypoventilation.

Simply put, the process of gas exchange in them slows down greatly, which is why the operated dog may well die by suffocating. It is for this reason that intubation is always performed (i.e., a special tube is inserted into the trachea through which air flows directly into the lungs).

Opioid dosages for dogs and cats

A drug	Dose	Saline volume (ml/kg)	Onset of action (min)	Duration of action (hours)
Morphine	0.1 mg/kg	0,13-0,26	30-60	10-24
Fentanyl	1.0-5.0 µg/kg	0,26	15-20	3-5
Butorphanol	0.25 mg/kg	0,26	10-20	3-4
Morphine + Bupivacaine 0.5%	0.1 mg/kg 1.0 mg/kg	Volume of local anesthetic	10-15	16-24

Morphine is the drug of choice for epidural analgesia. Opioids can be administered epidurally without local anesthetics and act well on type C (sensory) fibers and poorly on type A (motor) fibers without causing motor block. With such EA, animals can walk, but slight ataxia of the pelvic limbs may be present. Sometimes local anesthetics in the spinal canal spread more cranially than expected.

Factors influencing cranial extension

patient's size, age, body type (including obesity);
increased intra-abdominal pressure (pregnancy, gastric dilatation);
volume of administered drugs;
dose of administered drugs; speed of drug administration; needle cut direction;
patient position.

In older animals, the volume and distensibility of the ED decrease, therefore, when the same dose is administered to elderly patients, the anesthetic spreads more cranially than to young ones. To prevent the blockade from becoming too high, older patients are given a lower dose of anesthetic.

Obese patients have more fatty tissue in the ES, which leads to a decrease in ES volume. Reducing the volume of space requires reducing the dose of anesthetic.

Due to an increase in intra-abdominal pressure, the volume of the epidural venous plexus increases, which leads to a decrease in the volume and distensibility of the ES and, accordingly, to a higher level of blockade compared to expected.

Positioning the patient upside down should be avoided. Block with local anesthetics up to the T1 level leads to paralysis of the intercostal muscles, and up to C5-C7 - paralysis of the diaphragm.

Possible complications of epidural anesthesia:

Bleeding and hematoma formation in the spinal canal. Trauma to the epidural veins with a needle often causes minor bleeding in the spinal canal; it is usually benign and resolves without treatment. Clinically significant hematoma can occur after EA due to bleeding disorders.
Incorrect position of the needle in the spinal canal can lead to:
b) intravascular administration of a local anesthetic (complications from the central nervous system - convulsions, loss of consciousness; from the cardiovascular system - arterial hypotension, arrhythmia, shock);
c) to unintentional subarachnoid blockade (in this case, it is necessary to reduce the dose of anesthetic by 50-75% of the original).
Penetration of infection into the spinal canal. Meningitis and epidural abscesses are extremely rare complications. It is necessary to follow the rules of asepsis when performing the blockade.
High blockade. Causes: excessive dose of anesthetic; use of a standard dose of anesthetic in those patients for whom it should be reduced; unusually high sensitivity to anesthetic; excessive spread of the drug in the cranial direction. High blockade can cause hypotension and hypoventilation due to visceral sympathetic blockade and paralysis of the intercostal muscles. Patients with this complication will most likely require ventilatory and hemodynamic support, which includes oxygen therapy, mechanical ventilation, fluid resuscitation, and vasopressors.
Urinary retention. Epidural blockade at the lumbosacral level reduces the tone of the bladder and inhibits reflex urination. It is necessary to monitor the presence of urination in the postoperative period. In the absence of the latter, it is recommended to divert urine by squeezing or catheterization of the bladder.

Contraindications to epidural anesthesia:

Coagulopathy and treatment with anticoagulants.
Severe hypovolemia.
Presence of infection (sepsis, skin infection at the puncture site).
Neurological dysfunctions.
Diseases of the central nervous system.
Anatomical disorders that complicate the puncture technique.

Local anesthesia for dogs

The most common type. In veterinary medicine, local anesthesia is used daily and very actively. Divided into several varieties.

Application anesthesia

The simplest method. It consists of applying “freezing” substances directly to the surface of the skin or mucous membranes. Contrary to popular belief, the lethality of this type of anesthesia is quite high. So, today on the shelves of pharmacies you can find a lot of pain-relieving “sports” sprays, the effect of which lasts several minutes. This may be quite sufficient for a simple surgical intervention.

Dosage for small animals

Use of propofol in dogs

In dogs, propofol is administered at a dose of 6.6 mg/kg intravenously slowly over 30-60 seconds.

If necessary, an additional dose of 0.5-1 mg/kg may be prescribed for intubation. Although the typical propofol dose range required for induction is 5–7 mg/kg IV, most inductions can be performed at a dose of 3.7 ± 1.5 mg/kg. When premedicated with alpha2-agonists (eg, dexmedetomidine) or other premedications, reduce the dose by 20-30% (eg, a lower dose of 2-3 mg/kg may be used). After initial induction, maintenance doses of 1–3 mg/kg IV may be used.

Using propofol as a constant rate infusion:

5 mg/kg IV slowly, then 100-400 mcg/kg/min (or 6-24 mg/kg/hour).

Use of propofol for status epilepticus in dogs: 1-6 mg/kg IV (until effect) followed by infusion at a constant rate of 0.1-0.6 mg/kg/min.

Use of propofol in cats

Induction of anesthesia:

4-8 mg/kg intravenously slowly (if propofol is used alone). After the starting dose, additional boluses of 1-3 mg/kg IV can be administered if necessary. If other preanesthetics and sedatives are used, the initial dose of propofol should be reduced by 16-24%. When combined with midazolam, the need for propofol can be reduced by 25%.

Constant rate propofol infusion in cats:

6 mg/kg IV slowly, then 200-300 mcg/kg/min (0.2-0.3 mg/kg) IV. For short-term procedures, the total dose is 15 mg/kg for a 30-minute protocol.

Constant rate infusion of propofol when used in combination with ketamine in cats: propofol 0.025 mg/min/kg + ketamine 23-46 mcg/kg/min.

Short-term operations: 10 mg/kg intravenously over 1 minute (duration of anesthesia 10-20 minutes).

Contraindications for local anesthesia

It should be remembered that local anesthesia, for all its attractiveness (simplicity and low cost), is not always used. The reason is the presence of serious contraindications:

Local anesthesia is not used in cases where the operation (at least theoretically) may last longer than the duration of the painkiller.

The animal has a tendency to develop allergic reactions (however, in this case, you need to be extremely careful with general anesthesia).

Choleric character type. Simply put, it is better to immediately immerse “pocket” dogs with a quarrelsome and quarrelsome disposition into general anesthesia.

The same applies to cases where the dog is large and strong. It is far from certain that the owner will be able to hold him and calm him down.

Risks and likelihood of complications

What are the possible complications of anesthesia? It is believed that sudden allergic reactions and intolerance to anesthetic drugs under normal conditions occur once in 100,000 uses. These reactions can range from mild swelling at the injection site to anaphylactic shock or death.

Remember! Always and under all conditions, there is some chance that the body of a particular animal will react inadequately to a particular drug, even if the latter has been used for medical and veterinary purposes for decades.

In almost 100% of cases, problems will arise during general anesthesia (quite possibly fatal) if the dog has not “fasted” for at least 12 hours before the operation. A well-fed dog may die during surgery. The problem is that with general anesthesia, the principle of muscle innervation changes greatly.

Monitoring during surgery

A prerequisite for general anesthesia is monitoring the vital functions of the body .

When the animal enters the operating room, the anesthesiologist performs premedication (pharmacological preparation for anesthesia) and administers anesthesia. In parallel with this, vital functions of the body are monitored - pulse rate, ECG, respiratory rate, blood pressure and blood oxygen saturation are measured.

During short and low-traumatic operations, pulse and respiration rates and blood oxygen saturation are measured. For operations lasting more than 10 minutes, the animal must be intubated and given pure oxygen through a ventilator.

To monitor the condition of the animal during surgery, our clinic uses a Mindray MEC 1000 cardiac monitor, which allows us to determine the pulse rate, blood oxygen saturation, respiratory rate and evaluate blood pressure and ECG.

Heart monitor Mindray MEC 1000.

During short and low-traumatic operations, pulse and respiration rates and blood oxygen saturation are measured. For this purpose, the Best veterinary clinic uses Mindray PM 60 pulse oximeters.