Injectable Induction Agents
Injectable induction can be attained with the use of one or multiple agents given IV or IM. The duration of anesthesia is usually less than 20 minutes, but it varies based on the drug(s) and route of administration used (McKelvey and Hollingshead 2000). Induction via IM routes is beneficial in animals that cannot be handled. The most common IM induction drugs are ketamine, tiletamine and zolazepam (Telazol), xylazine, neuroleptanalgesics, and opioids. The use of premedications will commonly reduce the amount of induction agents needed.
Benzodiazepines include diazepam, midazolam, and zolazepam (Greene 2002). They decrease anxiety and produce calming effects and profound skeletal muscle relaxation in most patients. The benzodiazepines are metabolized in the liver and excreted in the urine and feces. They do not produce much sedation (except possibly in pediatric or critically ill patients) because most animals are still alert and aware of their surroundings. These drugs can cause excitement more often in young healthy animals and cats, so they are not commonly used as sole agents. They are more effective when combined with cyclohexamines, tranquilizers, or opioids that will help decrease the occurrence of excitement.
Benzodiazepines are also used for anticonvulsant therapy, making them a good adjunctive induction agent in patients that have a history of seizures or other intracranial disease (McKelvey and Hollingshead 2000). They can also be used with other anesthetics that may lower the seizure threshold and during procedures that may cause seizure activity, such as myelogram, cerebrospinal fluid centesis, or brain surgery.
Diazepam is one of the most commonly used benzodiazepines. It is not water soluble and cannot be mixed in the same syringe with other medications, with the exception of ketamine. It can be irritating to tissues when injected IM because it contains propylene glycol; when used in this manner, its absorption rate is somewhat unreliable and IV dosing is the preferred route. It has very little depressant effects on the cardiovascular and respiratory systems, making it a safe choice for most patients. The propylene glycol component in diazepam may cause hypotension, bradycardia, or apnea after rapid IV administration; therefore, care should be taken to administer it slowly (Paddleford 1999).
Midazolam is the second most commonly used benzodiazepine; it is water soluble and can be mixed with other anesthetic agents. Midazolam is not as irritating to tissues and can be administered IM; absorption after IM injection is more reliable than it is with diazepam. The cardiovascular effects are similar to diazepam with rapid IV administration. Midazolam can, however, cause more respiratory depression than is seen with diazepam (Paddleford 1999).
Zolazepam is the benzodiazepine mixed with tiletamine, a cyclohexamine, and the mixture is sold as Telazol. As a sole agent, it produces little sedation but provides excellent skeletal muscle relaxation, as does diazepam.
Cyclohexamines (ketamine, tiletamine)
Cyclohexamines work in a different manner than most other agents by stimulating the CNS rather than depressing it. This stimulation results in what is termed dissociative anesthesia or catalepsy, in which animals are in a trancelike state unaware of their surroundings. Stimulation of the CNS produces muscle rigidity characterized by extension of the front limbs and head and neck flexed back, increased reflexes such as the palpebral and pharyngeal, and sensitivity to light, sound, and touch. These effects can be diminished by using a benzodiazepine, opiate, or tranquilizer in combination with the cyclohexamine. Cyclohexamines typically do not cause cardiovascular depression. More often, tachycardia and increased blood pressure are seen after administration of a cyclohexamine due to indirect sympathetic stimulation. Because of this, they are not recommended for use in animals with known cardiac disease. Apneustic respirations are commonly seen. This is a particular type of breathing pattern in which inspiration is followed by a long pause and short expiration; some animals may hold their breath but can usually be easily stimulated to breathe by stroking their chest. The apneustic breathing pattern is not detrimental, and oxygen levels typically stay within normal limits. Cyclohexamines can also produce an increase in intraocular pressure and are not recommended for use in animals with ocular disease (Lin 1996). The eyes do not rotate as they do with other anesthetic agents, and nystagmus is commonly seen. Cyclohexamines also cause an increase in intracranial pressure and should not be used in animals with known head trauma or other intracranial disease (Lin 1996).
Recovery of cyclohexamine anesthesia can be rough because of the hypersensitivity to sound, light, and touch. Some animals will paw at their face or experience bizarre behavior. It is recommended that patients recover in a quiet dark environment to reduce stress, and occasionally additional sedatives are needed to smooth out the recovery.
Ketamine (ketaset, vetalar, ketalean)
Ketamine is a highly lipid soluble and has a fast onset of action. After IV administration, an endotracheal tube can usually be placed very quickly. After IM injection, intubation can be performed in 2–4 minutes. Recovery after IM administration can be prolonged and may take 3–5 hours (McCurnin 1994).
Ketamine in the dog is primarily excreted by hepatic metabolism and is not recommended for use in dogs with known liver problems. In the cat, it’s primarily excreted via the kidneys and should not be used in cats with renal disease or urinary obstruction (Lin 1996).
Tiletamine is purchased as a mixture with zolazepam; it is sold as Telazol and comes as a powder that must be reconstituted. Once in solution, it remains stable for 4 days at room temperature and 14 days when refrigerated (Paddleford 1999). After IV injection, intubation can be performed within 1–2 minutes, and after IM injection, intubation can be performed within 2–4 minutes. Recovery after IM administration can take 3–5 hours. It can also be given SQ or orally.
Tiletamine has similar effects as ketamine, but because it’s mixed with zolazepam, it is more like a ketamine-diazepam combination. It produces good muscle relaxation and less apneustic breathing patterns; however, respiratory depression is more commonly seen, especially when administered in conjunction with other tranquilizers or sedatives (Lin 1996).
Tiletamine is excreted via the kidneys in both dogs and cats and is not recommended for use in patients with known renal disease or urinary obstruction (Lin 1996).
Propofol (diprivan, rapinovet)
Propofol is unlike any other anesthetic agent. It’s an oil-in-water emulsion, and although it has a milky appearance, it is to be administered IV only (McKelvey et al. 2000). It is metabolized quickly, has a fast onset of action, and has short duration of effect, making it a relatively safe induction agent when administered correctly. It can also be used as a CRI or redosed through multiple low-dose injections given for a short procedure.
Propofol is not without negative side effects, including a transient decrease in cardiac contractility and hypotension due to vasodilation. Propofol is a potent respiratory depressant and if given too rapidly it can cause apnea following induction. This can be greatly diminished by slow injection (dose given over 90 seconds), titrating to effect, and the use of premedications that lower the dose of propofol required (Paddleford 1999). Transient tachycardia and bradycardia have also been noted but are typically not detrimental if the patient is healthy. Repeated doses of propofol over a short period of time in cats may cause oxidative injury to the red blood cells called Heinz body formations (Paddleford 1999). The clinical side effects of this are lethargy, vomiting, diarrhea, and anorexia.
Excitement may be seen during administration and is more common in younger healthy animals. The use of premedications or combining the induction dose with a benzodiazepine will decrease the excitement phase. Recovery is quick and usually smooth, especially when premedications have been used (Paddleford 1999).
Propofol also causes a decrease of intracranial and intraocular pressure, making it a good choice for animals with known intracranial disease, head trauma, or ocular disease (Branson 2007). It can also be safely used as an induction agent for cesarean sections as it has minimal depressant effects on the fetus, increasing survival rates (Paddleford 1999).
Propofol contains soybean oil, egg lichen, and glycerol, making it an easy environment for microbes to grow in. Left unopened the shelf life may be several years. The exact shelf life of an opened bottle is under debate; some studies suggest that keeping an opened bottle refrigerated will allow for a shelf life of 5–7 days, while others recommend that an opened bottle be discarded at the end of each day. Regardless of the debate over shelf life, everyone agrees that strict sterile technique should be used to prevent possible contamination of the bottle or ampule. Because of the potential for contamination there is some debate over this drug’s safe use in septic patients.
The induction dose for patients that have not received premedications is 8–10 mg/kg IV, if premedications have been given, the dose is 4–6 mg/kg IV (Thurmon et al. 1996). There are several techniques for the administration of propofol to reduce the occurrence of respiratory depression, apnea, and hypotension. The commonality of all techniques is slow administration and titrating to effect. Some techniques include the following: