Many drugs produce more than one effect.
This book discusses agents and adjuncts based on their chemistry.
Antagonists competitively bind to target tissues preventing the corresponding agonist from causing more stimulation.
Must use the proper antagonist when “waking” a patient from anesthesia.
Most anesthesia protocols use a combination of drugs to provide adequate analgesia and anesthesia.
Give either drug IM 20-30 minutes before anesthetic induction.
Secretions from the respiratory system, gastrointestinal system, salivary glands, and eye will be decreased.
Ophthalmic lubricating ointment will prevent corneal drying.
These two terms are often used interchangeably even though they are not the same.
Tranquilizer—reduces anxiety.
Sedative—reduces mental activity and causes sleepiness.
Make sure needle is in a vein, not in an artery.
Overdoses are treated with phenylephrine or norepinephrine.
Diazepam is the active ingredient in Valium.
Only zolazepam (as a component of Telazol®) is approved for use in animals in the United States and Canada.
Diazepam and midazolam are commonly used off-label in animals.
If a water-insoluble and water-soluble drug are mixed in a syringe, a precipitate may form.
Diazepam can also be administered concurrently with opioids, thiopental, and propofol.
Diazepam is light sensitive.
Water solubility means midazolam can be mixed with other commonly used water-soluble drugs without a precipitate forming.
Telazol® is discussed further with dissociative anesthetics.
Minor procedures include radiography, wound treatment, and bandaging.
Easy reversal means patients wake up quickly and can go home the same day.
Alpha2-agonists can be reversed with the proper antagonist.
Without reversal, complete recovery can take 2-4 hours.
The early phase cardiovascular effects are more pronounced if the drug is given IV.
By increasing the effect of other anesthetic agents, the amount necessary can be decreased.
Adverse effects are more severe when drugs are administered IV.
Don’t administer drug to an animal with any respiratory disease.
Small amounts of alpha2-agonists can cause effects in humans and animals if absorbed through skin abrasions or mucous membranes.
The agonist and antagonist are marketed together as a package.
doses are recommended for “sedation/analgesia in dogs” and “preanesthesia in dogs”
Painful when given IM.
Keep animal in a quiet environment 10-15 minutes after administration.
Alpha2-antagonists compete for binding sites on receptors with alpha2-agonists and will eventually replace the alpha2-agonists.
Classification depends on the predominant effect of the derivative.
Specific opioids and their uses in anesthesia are discussed later.
Endogenous opioid peptides are chemicals that are naturally present in the body.
The effect of an opioid depends on the dose, route of administration, agent used, species of patient, temperament, and pain status.
Narcosis is a sleep-like state induced by high doses of an opioid.
Most general anesthetics have limited analgesic properties.
The ceiling effect occurs when there is no greater respiratory depression with high doses than with low doses.
Any combination of opioid and tranquilizer is acceptable. Veterinarian’s preference.
Administer IM or slowly IV to prevent excitation.
An agonist-antagonist (butorphanol) can be used to partially reverse the effects of a pure agonist.
Opioid antagonists reverse the effects of opioids only.
Reversal is not necessary following routine anesthesia.
Intermediate and long-acting barbiturates are no longer used as anesthetics in veterinary medicine.
Vessel-rich tissues include the brain, heart, liver, kidneys, and endocrine glands.
High lipid-soluble drugs result in rapid tissue uptake. The brain has a high fat content.
Drugs will move from tissues with high drug concentration to tissues with lower drug concentrations. The animal shows signs of recovery as the drug concentration in the brain decreases.
Blood carries the drug to muscle, fat, etc.
Drug is metabolized by the liver and excreted in urine.
Intubation prevents aspiration of fluids or vomitus. It also allows the anesthetist to provide artificial respiration if necessary.
A dilute concentration would be 2% or 2.5%.
The veterinarian will decide the protocol for thiopental administration.
Lethal dose is only two to three times the anesthetic dose.
A longer recovery time is needed if multiple injections were used.
Phencyclidine has a high potential for abuse so it not used in veterinary medicine.
Ketamine may be given orally to restrain feral cats.
Ketamine is a controlled substance.
Use 5% to 10% solution in dextrose to minimize hemolysis.
Also desflurane, halothane, methoxyflurane, and enflurane.
Oxygen-anesthetic mixture delivered to patient through a breathing circuit.
Mechanism of action is not fully understood.
Bagging with 100% oxygen hastens the process of dropping the levels of anesthetic in the alveoli.
Isoflurane, sevoflurane, and desflurane are almost entirely eliminated through the lungs.
Older halogenated compounds are partially metabolized by the liver and eliminated through the urinary system.
Not all inhalant anesthetic agents are the same.
Inhalation anesthetic agents also differ in how they affect the cardiovascular, respiratory, and other vital systems.
Table 3-3 summarizes the physical properties and pharmacology of commonly used anesthetic agents.
Vapor pressure is measured at 20°C (68°F).
Isoflurane can be delivered with a nonprecision vaporizer if carefully monitored. A 5% mixture is the concentration needed for anesthesia. The concentration level can easily rise about that level because of the high vapor pressure of isoflurane.
Most precision vaporizers have a maximum 5% concentration delivery level.
A nonprecision vaporizer could be a glass jar with a wick.
Methoxyflurane is no longer available in the United States.
A low blood:gas partition coefficient means the agent will enter and leave the blood rapidly.
Sevoflurane has a low blood:gas partition coefficient so it is characterized by very rapid induction and recovery rates.
A high blood:gas partition coefficient means a low concentration gradient will be created and the agent will enter the blood slowly. It will also leave tissues more slowly.
Methoxyflurane has a high blood:gas partition coefficient.
MAC is measured as the lowest concentration of the agent at which 50% of patients show no response to painful stimulus.
A vaporizer setting of 1 × MAC will maintain light surgical anesthesia, 1.5 × MAC will maintain moderate surgical anesthesia, and 2 × MAC will maintain deep surgical anesthesia in most patients.
Isoflurane vapors are irritating to some animals so they will resist induction with a mask.
Must be careful not to turn off the vaporizer too soon as animals recover quickly.
Animals respond quickly to changes in the vaporizer setting during anesthesia.
A low rubber solubility means that little anesthetic agent will be lost by absorption into the rubber components of the anesthetic machine and breathing circuit.
The lack of preservatives means that there will be no preservative residue that accumulates in the vaporizer.
Because of its minimal cardiovascular effects, isoflurane is the agent of choice in patients with cardiac disease.
Isoflurane is used in patients with head trauma or brain tumors because it maintains cerebral blood flow.
There is little retention of isoflurane in body fat because it is almost completely eliminated from the body through the lungs after the vaporizer has been turned off. This also makes it the agent of choice for patients with liver or kidney disease, neonatal patients, and geriatric patients.
Unless given an analgesic after anesthesia, the patient may show signs of pain and excitement during recovery.
Carbon monoxide poisoning is characterized by cherry-red blood and mucous membranes and must be treated immediately.
Sevoflurane is nonirritating so it is the agent of choice for mask or chamber induction.
High controllability of depth of anesthesia is desirable for equine patients.
Sevoflurane is less potent than isoflurane and therefore higher concentrations are needed to induce and maintain anesthesia.
May cause surgical apnea lasting 30 seconds or longer.
Being the least potent agent means that the highest concentration of the agent must be used to induce and maintain anesthesia.
Known as the “one breath anesthesia” because of the rapid recovery (one breath).
The transient increase in heart rate and blood pressure (sympathetic storm) has not been reported in animals.
Appendix B contains detailed information on the use of nitrous oxide.
Doxapram stimulates the respiratory centers in the brainstem.