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HOW DO YOU CHOOSE YOUR AGENT? Choosing anesthetic agents is something that is not taken lightly. In addition to your minimum data base of your history, physical exam, and diagnostics, the following affect how an agent should be chosen CHOOSING YOUR AGENT AVAILABILITY -An anesthetic agent is chosen by what is available to the veterinarian/clinic. -Most clinics will not have the ability to choose from every anesthetic option. -An anesthetic machine would not be practical to take on a farm call. Injectables are used. FAMILIARITY -The safest anesthetic protocol is the safest one that the doctor is familiar with. -A doctor wouldn’t use the drug that he/she just read about on a Class V anesthetic risk. CHOOSING YOUR AGENT PROCEDURE -A short-acting anesthetic should not be chosen for a long procedure. An animal that is undergoing major surgery requires more than a sedative. -The anesthetic must be evaluated for duration of action, amount of pain control, and facilitation of restraint. -Look at what the procedure is.. Drugs that are acceptable for a spay may not be acceptable for a C-section because of its affect on the fetuses CHOOSING YOUR AGENT COST -Some anesthetics are pricey. It is ok to use a cheaper anesthetic, as long as it is as safe as the other, more expensive options. -Don’t substitute safety for cost. SPEED -If an emergency surgery is about to be performed, it is not wise to choose a premedication that will take 20 minutes to take affect. *** The anesthetic protocol, dose, and route are chosen by the veterinarian. *** A vet/clinic may have a routine protocol, but ALL PATIENTS ARE DIFFERENT and the best anesthetic for a patient may not be the one that is used frequently *** Point out all of your findings – they will help the veterinarian with drug selection PREANESTHETIC AGENTS Drugs that are administered to an animal before general anesthesia. Can either be a single drug or a combination of drugs. Dosages are adjusted according to the patient’s need/health. SO WHY ARE WE GIVING AN ANIMAL DRUGS BEFORE IT IS ANESTHETIZED? 1) To calm or sedate an excited, fractious, or vicious animal This makes the patient more comfortable and your job a lot easier The dose is adjusted according to the patient’s need and health status -some animals require minimal dosing, however some are unaffected by a high dose that was given to them. 2) To counteract the effects of other injectable or inhalant anesthetics If the ideal agents being used for anesthesia produce effects of hypersalivation (ketamine) or bradycardia (opiods), atropine can be given because it can treat both of these side effects 3) To reduce the amount of general anesthesia required The animal should not be so sedate from it’s premedications that it can be intubated, but it will not need a large amount of anesthetics to produce surgical anesthesia. To use low doses of preanesthetics and general anesthetics in combination to reduce the effects on the animal from each is called BALANCED ANESTHESIA. 4) To reduce pain/discomfort If the anesthetic procedure is brief, the premedications may still be in effect during recovery BESIDES SURGERY AND ANESTHESIA, PREANESTHETICS CAN BE USED IN OTHER WAYS: Calming nervous patients for travel, reducing the aggressiveness during a physical exam, minor procedures, thunderstorm anxiety, antiemetics, cough suppressants. OTHER THINGS TO NOTE: Preanesthetics are usually given about 20 minutes before the induction anesthetic is given. If you try to induce anesthesia too soon, you may not see any effect from the pre-meds Usually given IM or SQ. Many patients do not have catheters in place until they are sedated, depending on the protocol of the clinic. Most preanesthetics are too potent to be given IV Every preanesthetic has a side effect All anesthetics are not safe for all animals CLASSES OF PREANESTHETIC AGENTS ANTICHOLINERGICS TRANQUILIZERS AND SEDATIVES OPIODS ANTICHOLINERGICS SO WHAT IS AN ANTICHOLINERGIC? The anticholinergics are also called parasympatholytics, meaning they block acetylcholine at the muscarinic receptors in the parasympathetic portion of the autonomic nervous system. The muscarinic receptors are in the heart, GI tract, bronchi, glands, and eye. When the parasympathetic system is activated by anesthesia, bradycardia, GI stimulation, salivation, and pupil constriction may occur. Anticholinergics like ATROPINE and GLYCOPYRROLATE can help to stop these effects. Both ATROPINE AND GLYCOPYRROLATE can be given SQ, IM (preanesthetic purposes) or IV (emergency bradycardia/cardiac arrest). The IV dose is much lower than the SQ, IM doses Atropine comes in a variety of concentrations that vary from 0.4mg/mL to 1/120 grain/mL to 15 mg/mL. When calculating and drawing up doses, be sure you are using the correct concentration! Often used in a preanesthetic mixture (often with Butorphanol and Acepromazine) Onset of SQ is 20 minutes, IV is 30 sec – 2 min. Duration of action: 60-90 minutes. Will notice heart rate decreasing as it wears off. Can be mixed with most other premeds except Diazepam EFFECTS OF ATROPINE - Increases heart rate -blocks stimulation of the vagus nerve which causes increased parasympathetic activity, bradycardia, and reduced cardiac output. -things that can stimulate the vagus nerve during anesthesia and cause decreased heart rate: endotracheal intubation, handling of abdominal organs in surgery, and other anesthetic agents -Atropine protects the heart, but care should be taken because tachycardia can occur. Do not use if the patient has CHF or hyperthyroidism, or if an animal is tachycardic before anesthesia (140 bpm for dogs, 180 bpm for cats) -Decreases salivation (antisialagogic agent) -Some agents cause increased salivation (Ketamine). This can be problematic when trying to intubate the patient and can even compromise the airway. -Reduces GI activity -Inhibits peristalsis whereas other agents can cause flatulence, vomiting, and diarrhea -Don’t use if you know that the patient has ileus- will make it worse. - Pupil Dilation -More common in cats with preanesthetic dosages. Makes PLRs less reliable in assessing anesthetic depth. More sensitive to light. -Reduces Tear Secretion -Reduces tear production dramatically. Contraindicated with glaucoma. ALWAYS LUBRICATE THE EYES (no matter what anesthetic agent is used). Eyes don’t close while they are under anesthesia + decreased tears = dry corneas. -Bronchodilation -Causes increased dead space (parts of the respiratory system that contain air, but oxygen and carbon dioxide are not exchanged. If dead space is too large, animal can get hypoxic. -Increased airway mucous secretions in cats. -may cause airway blockage TOXICITY Overdose = drowsiness, excitement, dry mucous membranes, ataxia, muscle tremors, dilated pupils, hyperthermia, and tachycardia Dogs are more susceptible Treated with physostigmine GLYCOPYRROLATE Similar to Atropine Effects last much longer (up to 3 hours when giving SQ) Less tachycardia and arrhythmias seen Better antisialagogue More expensive Safer for pregnant animals Atropine is better at rapidly treating bradycardia TRANQUILIZERS AND SEDATIVES SO WHAT DOES A TRANQUILIZER/SEDATIVE DO? Tranquilizers (Diazepam) reduce anxiety, but may not decrease awareness Sedatives (Medetomidine) reduce mental activity and causes sleepiness The terms are often interchanged These drugs are given to calm the patient, facilitate handling, have a quieter induction, less stressful recovery Do not leave these patients unattended unless in a kennel. These animals can be very easily aroused or aggressive is stimulated suddenly Act on the central nervous system 3 CLASSES: PHENOTHIAZINES, BENZODIAZEPINES, ALPHA-2 AGONISTS May cause 3rd eyelid prolapse or ataxia Only Alpha-2 agonists have analgesic effects PHENOTHIAZINES Includes Acepromazine and Chlorpromazine Not a controlled substance Do not cause significant respiratory or cardiac depression Wide margin of safety May be combined with other agents (commonly Atropine, Ketamine, and Opiods) Can administer orally, SQ, IM, IV (not common and with caution) EFFECTS OF PHENOTHIAZINES Sedation Affect the reticular activating center of the brain May last up to 24 hours at higher doses Less sedation seen in cats Not as deeply sedating as medetomidine Useful to also sedate for thunderstorms, travel Antiemetic Help prevent vomiting (Chlorpromazine) Antiarrhythmic effect Can antagonize arrhythmias caused by other anesthetics Antihistamine effect Prevent the release of histamine and help decrease allergic reactions. *Don’t use with allergy testing Peripheral Vasodilation May get a reflex tachycardia to compensate, hypothermia may accompany vasodilation, hypotension may follow. Personality Effects Occasionally may see excitement that can take 48 hours to resolve. Make owners aware. Penile Prolapse Occurs in horses. Rarely permanent. NO Analgesia No reversing agent Manufacturer’s listed dose may be as much as 50% too high High doses do not cause increased levels in sedation, but can induce significant hypotension Reduce the dose in geriatrics, neonates, and those with liver dysfunction Caution in Boxers, hypotensive, anemic, or dehydrated animals BENZODIAZEPINES Includes Diazepam (Valium), Zolazepam (in Telazol), Midazolam Use of Diazepam is off-label Thought to exert their effects by releasing GABA, in inhibitory neurotransmitter in the brain Diazepam and Midazolam can be used as appetite stimulants in cats or to correct inappropriate urination. EFFECTS OF BENZODIAZEPINES Anti-anxiety, calming Don’t cause significant sedation in healthy animals unless used in combination with other drugs Doesn’t cause drowsiness, is still alert but is less anxious. May get more effect in a geriatric or debilitated animal. Enhances sedation of other drugs Relatively safe NO ANALGESIA Skeletal muscle relaxation Can be used to counteract rigidity seen with other agents (ketamine) Anticonvulsant Excellent Used often, good for animals with a seizure history Can be combined with drugs that lower the seizure threshold to counteract (NOT WITH SEIZURE PATIENTS) Minimal Adverse Effects High margin of safety Good for geriatrics or high risks Flumazenil can reverse Not good for animals with liver dysfunction or Csections DIAZEPAM Preferred route of administration is slowly IV (can cause arrhythmias if too quickly). (can also give per rectum for seizing animals) CAN ONLY BE MIXED WITH KETAMINE -Equal volumes in same syringe, prolonged storage is not recommended (absorbed by syringes and IV bags) -Not water soluble, may precipitate out with other meds Can’t induce anesthesia by itself -usually administered with Ketamine CONTROLLED DRUG MIDAZOLAM Can be mixed with other anesthetic agents More readily absorbed after IM or SQ routes See effects in 3 minutes Isn’t used as the only anesthetic agent -Animal is not inducible or easy to restrain Combined with Ketamine, Thiopental, or an opiod Minimal cardiovascular effects ALPHA 2 AGONISTS Xylazine and Medetomidine most commonly used (xylazine often in large animals) Also called thiazine derivatives Fast onset, good effect, REVERSIBLE, ANALGESIA produced, not controlled Stimulate alpha 2 adrenoreceptors on sympathetic nerves in the brain and spinal cord causing decrease in norepinephrine. Also affects receptors in the heart and blood vessels. Result is sedation, analgesia, muscle relaxation Standard recommended doses are only given to young, healthy patients EFFECTS OF ALPHA 2 AGONISTS If combined with other agents, surgical procedures can be performed (usually minor procedures) Sedation may last hours, but analgesia can be as short as 20 minutes if another analgesic is not used with it Humans should be careful when administering Can be absorbed through skin abrasions and mucous membranes (causing hypotension and sedation) Metabolized in the liver, excreted in the urine. Be sure both systems are functioning adequately before administering. XYLAZINE 2% solution for small animals, 10% solution for large animals If used with another anesthetic, the dose of the other medication can be reduced up to 80% IV or IM gives best effect – avoid SQ MEDETOMIDINE Usually called DOMITOR Approved for dogs, but used in many other species Usually combined with Butorphanol to be given IV (preferred) or IM Allow the drug 20 minutes to take effect – let the animal be in a quiet environment Great sedative effect. Often combined with a local anesthetic for minor surgical procedures TOO AWAKE TO INTUBATE (but you can always provide oxygen in the mask) Can be suddenly aroused (especially with deep ear cleanings or as medication is wearing off) For a major surgery, a general anesthetic is also used but at a much lower dose than usual ADVERSE EFFECTS Usually seen when given IV BRADYCARDIA, reduced cardiac output, arrhythmias. DON’T use in animals that are debilitated or have heart disease -usually best treated with reversal agent. Atropine may not be as effective and may make the animal hypertensive Blood pressure initially rises, and then falls making the animal potentially hypotensive RESPIRATORY SUPPRESSION is common. Don’t use with respiratory diseases Cyanosis is sometimes seen (careful in Brachycephalics) Can cause vomiting in a large # of animals (especially cats and more so with Xylazine). Atropine may help to prevent this Xylazine depresses GI activity. Can cause bloat in ruminants and GI stasis in dogs. Use with caution or not at all in deep chested breeds that are prone to GDVs. Temporary behavior changes Reduces insulin secretion. May cause transient hyperglycemia. Xylazine can increase intrauterine pressure in cattle and may cause abortion in the last trimester. REVERSAL AGENTS Commonly performed Yohimbine reverses Xylazine. Administered IV and results are seen within a few minutes. Tolazoline also reverses Xylazine. Administered IV and is more effective than Yohimbine in cattle. May cause vasodilation and hypotension. Both Tolazoline and Yohimbine should be administered slowly. Tremors, excitement may result. Medetomidine is reversed with Atipamezole (Antisedan). Given IM. Dose is 5 times greater than Medetomidine, but the drug is 5 times more concentrated, so the volume amounts are equal. Cats are more sensitive, so they are given ½ of the reversal dosage. Reversal occurs within 20 minutes. Bradycardia and analgesia should no longer exist. An analgesic is usually administered with it.