5. Etomidate Class: Imidazole derivatives Indications:
Precautions/adverse side effects: ● Can cause myoclonus, paddling, and vocalization at induction but can be prevented with co-induction with a benzodiazepine (e.g. midazolam). (AAHA, 2020). ● Etomidate with propylene glycol is an irritant to veins, causes pain on injection, and perivascular injections causes tissue necrosis/phlebitis (AAHA, 2020). ● Etomidate should not be administered for anesthesia maintenance for prolonged periods because of its ability to induce adrenal suppression. Reversal agent : None Other important information: ● No analgesic properties ● Minimal effects on the sympathetic nervous system
Etomidate induces anesthesia by enhancing GABAergic inhibitory neurotransmitter activity (Javdani et al., 2020). It is the anesthetic agent of choice in patients with significant cardiovascular disease. Causes minimal to no cardiovascular effects but some respiratory depression with rapid IV administration. Can dilute with fluids running to decrease injection pain and vascular effects (AAHA, n.d.). Onset of action: Fast; about 1 minute; recovery from a single bolus within 10-20 minutes.
INHALATIONAL ANESTHETIC DRUGS
● Pollution Ideal properties of an inhalant anesthetic (Lee, n.d.):
Inhalation anesthesia is the method of choice for maintenance of general anesthesia for most prolonged procedures. The use of inhalational anesthetics enhances the safety and efficacy of prolonged surgical and diagnostic procedures. Inhalation anesthetics provide quicker changes of anesthetic depth than injectable anesthetics, and reversal of central nervous depression is more readily achieved (Lee, n.d.). Inhalational anesthetics are used in combination with injectable premedications, induction agents, and ancillary analgesics. This is known as “balanced anesthetic technique” (Lee, n.d.). Volatile anesthetic agents have many advantages compared to IV agents. Isoflurane and sevoflurane undergo pulmonary elimination, have a low level of hepatic metabolism, and a rapid onset and offset of action, which allows for quick sedation onset and awakening. They primarily act on the cerebral cortex and leave autonomic functions, such as temperature control, blood pressure regulation, and respiration, relatively undisturbed (Aseri, 2023). Advantages of inhalational anesthesia: ● Patent airway ● Better control of anesthetic depth ● Less risk of overdosing and less accumulation ● Reversal of central nervous depression is more readily achieved, which means a quicker and smooth recovery Disadvantages of inhalational anesthesia: ● More pronounced cardiovascular depression including myocardial depression, hypotension, and bradycardia ● Has a lower blood/gas solubility than other inhalant agents, potentially resulting in a faster induction of anesthesia, and changes in depth and recovery from anesthesia (Lee, n.d.). ● Less airway irritation and breath-holding than other inhalants. ● Anesthetic induction, recovery, and intraoperative modulation of anesthetic depths tend to be faster than halothane and isoflurane. ● Dose dependent cardiovascular depression.
● Non-explosive ● Non-flammable ● Non-toxic ● Safe with CO2 absorbent ● Potent ● Pleasant to inhale ● Minimal metabolism ● Low blood gas solubility
● Good analgesia ● Good shelf life ● Minimal organ depression ● Inexpensive Techniques of inhalant anesthesia induction: ‘incremental’ versus ‘crash’ induction (Lee, n.d.): ● ‘Incremental’ induction technique uses 3 min of pre-oxygenation and then introduction of 0.5 % vapor setting for 30-60 seconds, and then 0.5 % increment for the same period. ● ‘Crash’ induction is achieved with 3-5 % vapor set of isoflurane following pre-oxygenation. The dog will more likely struggle with the crash induction method. Minimum alveolar concentration (MAC) value: Is a measure of anesthetic potency; an inhalant having a low MAC value is more potent than an inhalant having a high MAC value. Factors that influence MAC include body temperature, age, and hypercabia (hypercapnia) (Florida International University, Research, n.d.). ● It is very insoluble in blood and a lot less soluble than halothane in fat. Thus, induction and recovery is faster. ● Causes peripheral vasodilation, which is responsible for a low arterial blood pressure, but tissue looks more bright and pinkish indicating better perfusion. ● Less prone to cause arrhythmia compared to halothane. ● Analgesia is poor and muscle relaxation moderate. ● Induces a dose-dependent respiratory and cardiovascular depression. If administered by re- breathing circuits (especially those with in circle vaporizers) the quantities used are reduced, therefore cost can be reduced. (Lee, n.d.) Isoflurane ● MAC: 1.28% in dogs; 1.63% in cats
Common inhalational anesthetics used for small animals Sevoflurane ● MAC: 2.4% in canines; 2.6% in felines.
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