KETAMINE Ketamine should always be administered in combination with benzodiazepines, acepromazine, or alpha 2 agonists. Mechanism of action Antagonism at the N-methyl D-aspartate (NMDA) receptor. Effects of Ketamine ● Has analgesic properties. ● Metabolized by the liver and excreted by the urine. ● Causes rapid onset about 60 to 90 seconds after IV injection and 10 minutes after IM injection. ● Increases cerebral blood flow and intracranial pressure. ● Increases excitement during recovery in dogs and cats. It is best to recover in a quiet, darkened room if able. ● Increases heart rate and cardiac output. ● Causes minimal respiratory depression. ● Causes bronchodilation. ● Increase intraocular pressure and eyes remain open, so there is a risk of corneal drying. Contraindications of Ketamine ● Do not use in patients with increase intracranial pressure such as head trauma. ● Do not use in patients with seizures. ● Do not use in patients with intraocular pressure is a concern. ● Do not use in patients with hepatic or renal disease. TILETAMINE-ZOLAZEPAM (TELAZOL) Tiletamine is very similar to ketamine in actions but more potent with a longer duration of action. Many times, Telazol is combined with alpha 2 agonists so that a much lower dose can be used. It can be given IM or IV, and cardiovascular and respiratory functions are well maintained. ● Induction of anesthesia after 7 to 10 minutes after IM injections and within 60 to 90 seconds after IV injection. ● The main drawback to Telazol is a prolonged recovery that may be accompanied by ataxia, which makes this drug combination not ideal for short procedures. Effects of Telazol ● Hypersalivation. Advantages of TIVA ● No exposure to anesthetic gas. ● Convenience for procedures involving the airways or mouth. ● Recovery may be smoother. Disadvantages of TIVA ● Must have a secure IV access. ● A syringe pump is needed for accurate dosing. ● Respiratory support is usually needed. ● Anesthetic depth may not be as rapidly reversible as with gas anesthesia. ● Airway is not protected unless an ET tube is inserted.
● Does not have any analgesic properties. ● Causes respiratory depressions and apnea post-induction. ● Can be used for induction and maintenance of general anesthesia by total intravenous anesthesia. ● It is recommended to give propofol slowly and give until the desired effect is reached to help avoid overdosage and apnea. Uses of Propofol in cats Cats metabolize propofol very slowly due to deficiencies in hepatic glucuronidation. Propofol can be used on repeated occasions in cats, but more care must be taken with the dose than in dogs. Repeated propofol anesthesia or prolonged propofol CRI can lead to Heinz body anemia. ETOMIDATE Etomidate is rapidly metabolized to inactive metabolites by liver and plasma esterases. The drug has a fast onset of action of about one minute and will recover from a single bolus within 10 to 20 minutes. ● Causes a decrease in intracranial pressures. ● No analgesic properties. ● Causes minimal cardiovascular depressant effects. ● Minimal effects on the sympathetic nervous system. ● The anesthetic agent of choice in patients with cardiovascular disease or critically ill patients where cardiovascular derangements are undesirable. ● Not usually found in general veterinary practices but is commonly used in referral and specialty practices as the agent of choice for induction of anesthesia in small animals with cardiovascular disease and any cause where changes in heart rate and cardiac output are not well tolerated. ALFAXALONE Alfaxalone causes rapid anesthesia within 30 to 60 seconds after IV injection and should last for about 10 to 15 minutes. Alfaxalone produces excellent muscle relaxation. Alfaxalone is very similar to propofol and causes vasodilation, decreases cardiac output, and decreases blood pressure. It also commonly causes respiratory depression and apnea. It is best to give this drug by slow IV injection until the desired effect is reached. In cats, myoclonus (muscle twitching) and opisthotonos are commonly seen during recovery, especially if you do not use any pre-medications. Total Intravenous Anesthesia (TIVA) TIVA refers to the induction and maintenance of anesthesia by intravenous drugs only. This can be achieved by intermittent boluses, but a constant rate infusion causes a more stable plane of anesthesia. This can be a useful technique when maintaining anesthesia using inhalational agents is not desirable or not possible. A common use for TIVA in small animals is an infusion of propofol for tracheal surgery, bronchoscopy, or bronchoalveolar lavage.
Summary In this chapter, we discussed the many different options for induction of general anesthesia with injectable medications. We also discussed the pros and cons of total intravenous anesthesia.
You should have a good understanding of these drugs and know when to use to them for different scenarios that you may face in a small animal clinic.
INHALATIONAL ANESTHETIC DRUGS
Learning objectives Discuss many of the commonly used inhalational anesthetics drugs. Explain the effect that the drug will cause the patient. Predict any side effect that you may see when using these drugs.
Identify the pros and cons of using isoflurane versus sevoflurane. Once a small animal patient has been anesthetized, they will need to be given inhalational anesthetic drugs to continue to maintain a plain of anesthesia. The two main inhalational anesthetic agents currently used are:
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