Small Animal Anesthesia, Parts I And II
William W. Muir, III, DVM, PhD, DACVECC, DACVA
“There are no safe anesthetic agents; there are no safe anesthetic
procedures; there are only safe anesthetists.” Robert Smith
Anticholinergics are competitive antagonists of acetylcholine at post ganglionic parasympathetic muscurinic receptors. Stimulation of muscurinic receptors induces salivation, pupillary constriction, bronchoconstriction, gastric acid secretion, gastrointestinal motility, and slowing of the heart rate. The two most commonly used anticholinergics in veterinary medicine are atropine and glycopyrrolate (Table 1). These two drugs differ in their duration of action, with the effects of glycopyrrolate lasting at least twice as long as atropine. Furthermore, glycopyrrolate does not cross the intact blood brain barrier or the placenta. Anticholinergics increase myocardial oxygen consumption by increasing heart rate. and can precipitate cardiac arrhythmias and decrease the threshold for ventricular fibrillation. For these reasons anticholinergics should not be used as a routine part of the anesthetic regimen, since many critically ill patients can not tolerate increases in myocardial oxygen consumption or alterations of cardiac rhythm. Anticholinergics are indicated for specific situations. For example, animals with high resting vagal tone, or high vagal tone resulting from narcotic or alpha-2 administration, those with excessive upper airway secretions, or in those animals in which cardiac output is dependent on maintenance of a normal heart rate. This includes neonates or individuals with cardiac tamponade.
Diazepam and midazolam are benzodiazepines frequently used in small animals. These two drugs are similar except that midazolam is water soluble while diazepam is solubilized in 40% propylene glycol. Intravascular injection of diazepam can be associated with pain at the injection site, furthermore the propylene glycol diluent prevents rapid absorption following intramuscular injection. The elimination half life of midazolam is shorter than diazepam. Benzodiazepines act by facilitating the actions of GABA and glycine, two inhibitory neurotransmitters. They can unmask suppressed behavior and can cause increased agitation and restlessness, particularly in cats. Debilitated and depressed dogs or cats often respond to a relatively small IV dose of diazepam with profound CNS depression. Endotracheal intubation can often be accomplished following their use alone. Benzodiazepines produce minimal cardiac and respiratory depression and initiate antiarrhythmic effects. They are anticonvulsants and skeletal muscle relaxants, and when used prior to barbiturates or inhalation anesthetics they decrease the dose of drug necessary to induce and maintain anesthesia. Both diazepam and midazolam can be combined with opioids to produce neuroleptanalgesia, with ketamine to produce short-term general anesthesia or with inhalant anesthesia to improve muscle relaxation. They are also useful when used just prior to anesthetic induction drugs such as the thiobarbiturates, etomidate or propofol.
Opioid Analgesics and Neuroleptanalgesics
The opioid (narcotic) analgesics can be used as adjuncts to general anesthesia as well as for postoperative pain control. As adjuncts to general anesthesia, they can be used alone for pre-anesthetic analgesia and behavior modification while in depressed animals they can be used for anesthetic induction either alone or combined with other drugs. Combined with sedatives (alpha-2 agonists) or tranquilizers (acepromazine) they induce profound sedation and analgesia. This is referred to as neuroleptanalgesia. Opioids act by binding to specific opioid receptors in the central nervous system. The opioid receptors present within the CNS are termed mu, kappa, sigma, and delta, and there is evidence suggesting their existence in peripheral afferent nerves. Opioids are usually classified as agonists, agonist-antagonists, partial agonists, or antagonists depending on their activity at the various receptors. Stimulation of the mu receptor induces analgesia, respiratory depression, miosis, bradycardia, hypothermia, and euphoria. Stimulation of the kappa receptor produces sedation, analgesia, and miosis. Stimulation of the sigma receptor induces mydriasis, tachycardia and excitement. The function of the delta receptor is not well understood. Morphine, meperidine, oxymorphone, and fentanyl are agonists at the mu and kappa receptors. Naloxone is an antagonist at all three receptors, and butorphanol, pentazocine, nalbuphine, and buprenorphine are partial agonists at mu and kappa receptors or antagonists at mu receptors and agonists at kappa and sigma receptors. The overall effect of opioids on behavior in normal conscious dogs and cats varies from sedation to euphoria and excitement. Administration to debilitated or depressed patients usually results in sedation.
In general, opioids are powerful respiratory depressants. Experimentally, respiratory depression is characterized by a delayed response (altered threshold) and decreased sensitivity to increased in carbon dioxide concentration. Some opioids such as butorphanol demonstrate little or no respiratory depression due to their receptor specificity. Panting is a common side effects of opioids and is probably related to drug effects in the thermoregulatory center in the hypothalamus.
Cardiovascular function is well maintained following opioid administration. Left ventricular contractility, cardiac output, and systemic blood pressure are minimally changed. Some opioids such as oxymorphone, fentanyl, and morphine can induce bradycardia. Morphine and meperidine initiate histamine release with resulting peripheral vasodilatation. Histamine release is dose dependent and low doses induce only mild vasodilatation.
Other notable opioid effects include decreased urinary output due to stimulation of ADH release. The gastrointestinal effects are variable among species but in the dog generally include vomiting due to stimulation of the chemoreceptor trigger zone. Dogs initially defecate and there is an increase in flatulence following opioid administration. Opioids induce an initial GI hypermotility in the dog with an increase in non-propulsive rhythmic contractions and an increase in smooth muscle tone. This is followed by a period of GI stasis and constipation.
Opioid agonists that are the most useful in anesthesia of the critically ill patient include morphine, oxymorphone, and fentanyl. These three drugs can be used alone or as a part of a neuroleptanalgesic combination (Table 1).
Oxymorphone is 10 to 15 times more potent than morphine. An IV bolus of oxymorphone can induce anesthesia in depressed dogs, however used alone in healthy alert dogs it does not produce loss of consciousness. Diazepam can be administered IV just prior to oxymorphone for additional relaxation and sedation. Alternatively midazolam and oxymorphone can be administered IM for pre-anesthetic neuroleptanalgesia. Like morphine oxymorphone can induce vomiting.
Fentanyl is approximately 50-100 times more potent than morphine and has a 15-30 minute duration of action. It minimally depresses cardiovascular function. A small IV bolus (0.005-0.01 mg/kg) of fentanyl administered every 15-30 minutes during inhalation anesthesia provides additional analgesia and allows a lesser concentration of the inhalant to be used. Fentanyl induced bradycardia can be treated with an anticholinergic. Fentanyl is also available in combination with the tranquilizer droperidol. This neuroleptanalgesic drug combination is useful IM and IV for sedation or induction. Other than the potential for a fentanyl induced bradycardia, the cardiovascular depressant effects are minimal.
The alpha-2 agonists include xylazine, detomidine, medetomidine and romifidine. These drugs produce the majority of their pharmacologic effects by stimulating alpha-2 receptors and are used clinically for sedation, muscle relaxation and analgesia. Larger dosages produce a variety of side effects including repiratory depression, bradycadia and bradyarrhythmias (first and second degree heart block), hyperglycemia, decreases gut motility and diuresis. Vomiting is a common side effects following intramuscular administration to dogs and cats. When administered clinically in small dosages these drugs are excellent for preanesthetic medication or as adjuncts to general anesthesia. The most important point being that they be administered in small dosages (approximately 1/10 of the label recommendation). All of the effects of the alpha-2 agonists can be reversed by administration of an alpha-2 antagonist; yohimbine, tolazoline, atipamezole.
The barbiturates, cyclohexamines (ketamine, Telazol®), propofol and etomidate are injectable anesthetics use to produce short-term (min.) anesthesia or for induction to general anesthesia with an inhalant.
The ultrashort acting barbiturates thiopental, thiamylal, and methohexital are pharmacologically similar. These drugs are cleared from the body by metabolism, and/or redistribution. The duration of action of thiamylal and thiopental (thiobarbiturates) are in large part dependent on redistribution from the central nervous system into muscle and fat. This gives them a predictably short duration of action (5-15 minutes) in normal healthy dogs and cats (Table 2). Induction of anesthesia with the thiobarbiturates and methohexital is extremely rapid and struggle free following IV bolus administration providing the patients are properly preanesthetized. The thiobarbiturates are alkaline in solution (pH=10-11), and accidental perivascular injection can result in inflammation and necrosis at the injection site. Methohexital differs from the thiobarbiturates in that it is cleared more rapidly from the plasma. There are also breed differences in pharmacokinetics, with the thiobarbiturates having an unacceptably long duration of action in sight hound breeds. Because these drugs are highly lipid soluble weak acids that are highly protein bound, their pharmacokinetic behavior is altered by changes in acid-base balance, albumin content, and the concurrent administration of other drugs. Acidosis increases the amount of active non-ionized drug, similarly a decrease in plasma protein binding induced by hypoalbuminemia or the presence of other drugs highly bound to albumin increases the amount of active drug. For these reasons the barbiturate dose should be reduced in critically ill patients that are acidotic or hypoalbuminemic. The thiobarbiturates induce a dose dependent CNS depression, resulting in a dramatic decrease in cerebral metabolic oxygen consumption, cerebral blood flow, and intracranial pressure.
Barbiturates cause a dose related depression of respiration. The response to inspired carbon dioxide is depressed, and the respiratory response to hypoxia is diminished. Apnea is common following an IV bolus.
The barbiturates depress cardiac function. Cardiovascular depression varies with dose and rate of administration. Bolus injection to normal dogs results in tachycardia, increased peripheral vascular resistance, a transient increase in systemic arterial pressure, and increased myocardial oxygen consumption. Left ventricular contractility decreases. Large dosages of thiobarbiturates can induce ventricular arrhythmias or bradyarrhythmias. These arrhythmias are transient and normal sinus rhythm usually returns within 10-15 minutes of administration. The thiobarbiturates also enhance the arrhythmogenic effects of epinephrine in halothane anesthetized dogs. The incidence of arrhythmias is decreased by simultaneous administration of lidocaine, or prior administration of acepromazine.
The most commonly used dissociative anesthetic in veterinary medicine is ketamine. Ketamine is used in a variety of species, either alone or in combination with tranquilizers and sedatives. The term dissociative anesthesia implies a pharmacologic uncoupling of sensory input from conscious sites within the brain. The mechanism of ketamine’s CNS and analgesic effects is unclear but may be related to decreases in brain acetylcholine, facilitation of GABA, or stimulation NMDA and opioid receptors. Ketamine produces waxy limb rigidity, wide eyed stare, maintenance of a corneal and laryngeal reflexes, and occasional violent recoveries. The drug is unsuitable for use in dogs unless it is combined with a sedatives or tranquilizers. Most typically ketamine has been combined with xylazine, acepromazine, and diazepam in dogs and cats.
An advantage of ketamine is its relative lack of cardiovascular depression. Ketamine typically increases heart rate, cardiac output, cardiac contractility, and systemic blood pressure in dogs and cats. These effects are due at least in part to a centrally mediated increase in sympathetic tone. Because of the cardiovascular sparing effect ketamine is useful for anesthesia of the critically ill patient. Ketamine, however can induce cardiovascular depression or tachycardia in patients with limited cardiovascular function (heart failure, hemorrhagic shock) and maintenance of cardiovascular function should not be assumed when ketamine is administered to compromised patients.
Ketamine produces does dependent respiratory depression characterized by decreased tidal volume, increased frequency, and apneustic breathing. The depression is transient and of no consequence in normal healthy patients, however patients with pre-existing cardiopulmonary depression may experience severe respiratory insufficiency following ketamine. Ketamine is a potent bronchodilator, and has been used for anesthesia in asthma patients
The combination of ketamine and diazepam or midazolam is useful for short-term chemical restraint or anesthetic induction in critically ill patients. The combination is associated with minimal cardiopulmonary depression, and is administered as a 1:1 mixture by volume, 1 ml of the combination per 9 kg body weight. Analgesics (butorphanol, oxymorphone, alpha-2 agonists) should be added to this combination to enhance analgesia.
Etomidate is a nonbarbiturate, non-narcotic, ultrashort acting intravenous anesthetic. Anesthesia is rapidly induced following IV administration, and its hypnotic duration of action is approximately 5-10 minutes. Etomidate is rapidly metabolized in the liver and plasma by non-specific esterases. This rapid metabolism makes it suitable for use in patients with liver dysfunction. Etomidate causes minimal changes in cardiac contractility, heart rate, cardiac output, and systemic blood pressure are minimally effected. Etomidate produces mild respiratory depression. Side effects from etomidate infusion include pain at the injection site, myoclonus, and gagging and retching. These effects are lessened by premedication with a tranquilizer or sedative. Etomidate suppresses adrenocortical function for 2-6 hours following a single bolus dose and may be contraindicated in debilitated or immunosuppressed patients. Etomidate is useful for anesthetic induction in patients with cardiovascular instability.
Propofol is a nonbarbiturate ultrashort acting intravenous anesthetic that can be administered as a bolus or by infusion due to rapid clearance. It is similar to the thiobarbiturates but not as cumulative. Unlike the barbiturates it does not sensitize the myocardium to catecholamine induced arrhythmias and is less likely to produce respiratory and cardiovascular depression at clinically recommended dosages.
Inhalation anesthetics are the cornerstones of long-term (greater than 15 minutes) anesthesia of the critically ill patient. Their duration of action is not appreciably dependent on metabolism and anesthetic depth can be rapidly adjusted compared to injectable anesthesia. Extremely debilitated patients can be rapidly and safely “masked down” with relatively low concentrations of the inhalants. Patients that are alert should be induced to anesthesia using an injectable technique and then receive an inhalant for anesthetic maintenance. The inhalation drugs in common use include nitrous oxide and the volatile anesthetics methoxyflurane, halothane, isoflurane and sevoflurane. Enflurane is rarely uses in veterinary practice and will not be discussed.
Nitrous oxide is a gas that can not be used by itself to sustain anesthesia. It is used concurrently with the volatile inhalation anesthetics to provide analgesia and sedation with relatively minor cardiopulmonary depression. Nitrous oxide is very insoluble in blood and other tissues which is responsible for its extremely rapid onset and termination of action. Nitrous oxide hastens uptake of concurrently used volatile anesthetics (second gas effect). This is advantageous during induction with a soluble drug like methoxyflurane. This advantage is minimal with the relatively insoluble halothane or isoflurane. Nitrous oxide is approximately 30 times more soluble in the body than nitrogen. Since nitrogen is a major component of gas spaces within the body, nitrous oxide moves into these gas filled spaces faster than nitrogen moves out. This results in an increase in volume or pressure of the space. For this reason nitrous oxide is contraindicated for use in patients with pneumothorax or bowel obstruction. Nitrous oxide leaves the body rapidly following termination of anesthesia. If the patient is hypoventilating and breathing room air, hypoxia may ensue. For this reason the patient should be maintained on 100% oxygen in the immediate post operative period if nitrous oxide was a component of the anesthetic regimen.
Halothane, isoflurane and sevoflurane are similar in their effects on the different body systems. They all produce a dose dependent cardiovascular depression. However, at equally potent analgesic concentrations isoflurane and sevoflurane produce the least depression followed by halothane. Isoflurane and sevoflurane induces peripheral vasodilatation and cardiac output is maintained by an increase in heart rate. A disadvantage of halothane is that it sensitizes the heart to the arrhythmogenic effects of catecholamines. Because of this halothane is contraindicated for use in patients prone to development of cardiac arrhythmias.
Halothane, isoflurane and sevoflurane produce dose related ventilatory depression, with halothane producing the least depression. Respiratory depression is associated with an increased arterial carbon dioxide concentration and respiratory acidosis. For this reason, patients with a metabolic acidosis, concurrent respiratory dysfunction, increased intracranial pressure, or any other condition in which respiratory depression in undesirable should be ventilated during inhalation anesthesia.
Table 1. Anesthetic drugs commonly used for premedication in critically ill patients
Table 2. Parenteral anesthetic drugs
Table 3 Volatile Inhalation Anesthetics *
*Assumes fresh gas flow rate of 20-40 ml/kg/min oxygen. If nitrous oxide is added to the fresh gas, flow rate then decrease vaporizer setting by approximately 0.25-0.5.
These settings are for a precision vaporizer located outside the anesthetic circle. Vaporizer settings for in-the-circle, on precision vaporizers are not listed.