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Advantages and Guidelines for Using Methoxyflurane
316
INHALATIONAL ANESTHETICS
The use of nitrous oxide requires additional equipment, which adds to the expense and complexity of the anesthetic protocol. Nitrous oxide is supplied under high pressure, which increases its potential for high pressure leaks. Both high and low pressure leaks cause environmental pollution. The minimal contribution provided by the use of nitrous oxide comes with a considerable number of potential complications and disadvantages. Its use should be carefully considered. References 1. Eger EI, Saidman LJ: Hazards of nitrous oxide anesthesia in bowel obstruction and
pneumothorax. Anesthesiology 26:61, 1965 2. Haskins SC, Knapp RG: Effect of low carrier gas flows (50% oxygen/50% nitrous oxide) on inspired oxygen tension in anesthetized dogs. JAm Vet Med Assoc 180:735738, 1982
Advantages and Guidelines for Using Methoxyflurane Peter H. Cribb, BSc, MRCVS From the Department of Veterinary Anesthesiology, Radiology, and Surgery, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
Methoxyflurane has a low volatility and a low saturated vapor pressure. Although an end-tidal concentration of only 0.3% to 0.6% methoxyflurane in oxygen is required for surgical anesthesia, a concentration of about 3% is required to achieve a timely induction, and this is the maximum concentration of methoxyflurane that can be achieved. This makes methoxyflurane suitable for use with in circle vaporizers, which are considerably cheaper than precision vaporizers. Methoxyflurane may be administered using a circle system with either a vaporizer in circuit or a vaporizer out of circuit. Nonrebreathing systems such as a Bain's or an Ayre's T-piece may also be used. Methoxyflurane is highly fat-soluble and is the most potent of the clinically used inhalational anesthetics, It is a good analgesic, even in subanesthetic concentrations. Induction and recovery are slow, and it takes many hours for all absorbed methoxyflurane. to be excreted from the body. Postoperative analgesia is good. As with any volatile anes-
INHALATIONAL ANESTHETICS
317
thetic, all waste gases should be scavenged and vented out of the operating room. Palpebral and pedal reflexes disappear early in the induction period, and their disappearance should not be relied on as indicators of a surgical plane of anesthesia. The eye assumes a central position at a light plane of surgical anesthesia, and slight jaw tone may be present. As depth of anesthesia increases, heart rate and respiratory rate decrease. These cardiopulmonary changes, together with a lack of response to surgical stimuli, are good indicators of anesthetic depth. Care must be taken with controlled ventilation because at deeper levels of anesthesia (> 2.0 minimum alveolar concentration [MAC]) cardiac output decreases significantly and hypotension may occur. The slow induction that occurs with methoxyflurane means that it is generally not suitable for mask induction, although depressed or well-sedated patients can be mask induced, particularly in conjunction with nitrous oxide. Nitrous oxide decreases the concentration of methoxyflurane required because it is a good analgesic, and it allows faster induction and recovery owing to its rapid uptake and the second gas effect. Nitrous oxide should be used at between 50% and 66%. Induction is usually by means of an injectable anesthetic or a neuroleptanalgesic combination. Although the use of opioids decreases the concentration of methoxyflurane required, they do cause respiratory depression, and methoxyflurane itself is a marked respiratory depressant. If excessive respiratory depression occurs, assisted or cqntrolled ventilation may be required. Hypotension and hypoxia must be avoided to maintain good hepatic oxygenation because methoxyflurane does reduce hepatic blood flow. Cases of hepatic failure following prolonged anesthesia with hypotension have been reported. The properties of methoxyflurane make it a suitable agent for stable anesthesia at a light plane of anesthesia, when rapid changes in depth are not required and when a slow recovery will not be a problem. These properties make it a good agent for anesthesia for orthopedic surgery. At a surgical plane of anesthesia, the eyeball is usually centrally placed with methoxyflurane, and this can be of assistance for eye surgery, obviating the use of muscle relaxants or stay sutures. If nonsteroidal anti-inflammatory drugs have been used for the control of inflammation and for analgesia, it is important to avoid the use of methoxyflurane owing to additive renal effects. Methoxyflurane and nitrous oxide combination gives very stable anesthesia in birds. Methoxyflurane has been used safely in virtually every species of laboratory animal and in most exotic species including reptiles and amphibians. Because vaporization is low and induction slow, methoxyflurane has proved suitable for open drop induction in small laboratory animals for simple procedures such as blood sampling. Care should be taken to minimize exposure of personnel working in these areas. Methoxyflurane does cause an increase in antidiuretic hormone (ADH) release, and urine production may be reduced during anesthesia.
318
INHALATIONAL ANESTHETICS
As with any anesthetic, urine production should be monitored in patients in which renal compromise may exist or when anesthesia may extend over several hours. Postoperatively in dogs and cats there is a decreased urine concentrating ability for about 3 days, and urine output, fluid intake, and the state of hydration should be monitored in compromised patients. High output renal failure, which occurs in humans following methoxyflurane anesthesia and initially reported in association with drugs such as tetracyclines, has not been shown to occur clinically in dogs. It would be wise, however, to avoid the use of methoxyflurane in animals with renal disease. Methoxyflurane has been used to control seizure activity in some cases of poisoning, such as with strychnine. SUMMARY
Methoxyflurane anesthesia is particularly useful for orthopedic surgery (muscle relaxation, analgesia, and slow recovery), ophthalmic surgery (central eye position), and small laboratory animals (high therapeutic index and slow induction).
Precautions When Using Methoxyflurane Doris Dyson, DVM, DVSc From the Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada
The main disadvantages related to the use of methoxyflurane in small animals stem from the high blood/gas partition coefficient and the low vapor pressure. Induction of anesthesia, transfer from injectable to inhalant, and changes in the anesthetic plane occur slowly, often requiring at least twice the time that is noted with the use of less soluble inhalants such as halothane. Attempts to speed induction with high anesthetic concentration is limited by the maximum concentration achievable with methoxyflurane (approximately 3%). For these reasons, I believe that mask induction with methoxyflurane is impractical. Smooth transfer from an induction with injectable anesthetics to methoxyflurane maintenance requires early high vaporizer control knob settings to minimize the possibility of awakening occurring from the injectable before the transfer is complete. The anesthetist may need to
INHALATIONAL ANESTHETICS
The use of nitrous oxide requires additional equipment, which adds to the expense and complexity of the anesthetic protocol. Nitrous oxide is supplied under high pressure, which increases its potential for high pressure leaks. Both high and low pressure leaks cause environmental pollution. The minimal contribution provided by the use of nitrous oxide comes with a considerable number of potential complications and disadvantages. Its use should be carefully considered. References 1. Eger EI, Saidman LJ: Hazards of nitrous oxide anesthesia in bowel obstruction and
pneumothorax. Anesthesiology 26:61, 1965 2. Haskins SC, Knapp RG: Effect of low carrier gas flows (50% oxygen/50% nitrous oxide) on inspired oxygen tension in anesthetized dogs. JAm Vet Med Assoc 180:735738, 1982
Advantages and Guidelines for Using Methoxyflurane Peter H. Cribb, BSc, MRCVS From the Department of Veterinary Anesthesiology, Radiology, and Surgery, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
Methoxyflurane has a low volatility and a low saturated vapor pressure. Although an end-tidal concentration of only 0.3% to 0.6% methoxyflurane in oxygen is required for surgical anesthesia, a concentration of about 3% is required to achieve a timely induction, and this is the maximum concentration of methoxyflurane that can be achieved. This makes methoxyflurane suitable for use with in circle vaporizers, which are considerably cheaper than precision vaporizers. Methoxyflurane may be administered using a circle system with either a vaporizer in circuit or a vaporizer out of circuit. Nonrebreathing systems such as a Bain's or an Ayre's T-piece may also be used. Methoxyflurane is highly fat-soluble and is the most potent of the clinically used inhalational anesthetics, It is a good analgesic, even in subanesthetic concentrations. Induction and recovery are slow, and it takes many hours for all absorbed methoxyflurane. to be excreted from the body. Postoperative analgesia is good. As with any volatile anes-
INHALATIONAL ANESTHETICS
317
thetic, all waste gases should be scavenged and vented out of the operating room. Palpebral and pedal reflexes disappear early in the induction period, and their disappearance should not be relied on as indicators of a surgical plane of anesthesia. The eye assumes a central position at a light plane of surgical anesthesia, and slight jaw tone may be present. As depth of anesthesia increases, heart rate and respiratory rate decrease. These cardiopulmonary changes, together with a lack of response to surgical stimuli, are good indicators of anesthetic depth. Care must be taken with controlled ventilation because at deeper levels of anesthesia (> 2.0 minimum alveolar concentration [MAC]) cardiac output decreases significantly and hypotension may occur. The slow induction that occurs with methoxyflurane means that it is generally not suitable for mask induction, although depressed or well-sedated patients can be mask induced, particularly in conjunction with nitrous oxide. Nitrous oxide decreases the concentration of methoxyflurane required because it is a good analgesic, and it allows faster induction and recovery owing to its rapid uptake and the second gas effect. Nitrous oxide should be used at between 50% and 66%. Induction is usually by means of an injectable anesthetic or a neuroleptanalgesic combination. Although the use of opioids decreases the concentration of methoxyflurane required, they do cause respiratory depression, and methoxyflurane itself is a marked respiratory depressant. If excessive respiratory depression occurs, assisted or cqntrolled ventilation may be required. Hypotension and hypoxia must be avoided to maintain good hepatic oxygenation because methoxyflurane does reduce hepatic blood flow. Cases of hepatic failure following prolonged anesthesia with hypotension have been reported. The properties of methoxyflurane make it a suitable agent for stable anesthesia at a light plane of anesthesia, when rapid changes in depth are not required and when a slow recovery will not be a problem. These properties make it a good agent for anesthesia for orthopedic surgery. At a surgical plane of anesthesia, the eyeball is usually centrally placed with methoxyflurane, and this can be of assistance for eye surgery, obviating the use of muscle relaxants or stay sutures. If nonsteroidal anti-inflammatory drugs have been used for the control of inflammation and for analgesia, it is important to avoid the use of methoxyflurane owing to additive renal effects. Methoxyflurane and nitrous oxide combination gives very stable anesthesia in birds. Methoxyflurane has been used safely in virtually every species of laboratory animal and in most exotic species including reptiles and amphibians. Because vaporization is low and induction slow, methoxyflurane has proved suitable for open drop induction in small laboratory animals for simple procedures such as blood sampling. Care should be taken to minimize exposure of personnel working in these areas. Methoxyflurane does cause an increase in antidiuretic hormone (ADH) release, and urine production may be reduced during anesthesia.
318
INHALATIONAL ANESTHETICS
As with any anesthetic, urine production should be monitored in patients in which renal compromise may exist or when anesthesia may extend over several hours. Postoperatively in dogs and cats there is a decreased urine concentrating ability for about 3 days, and urine output, fluid intake, and the state of hydration should be monitored in compromised patients. High output renal failure, which occurs in humans following methoxyflurane anesthesia and initially reported in association with drugs such as tetracyclines, has not been shown to occur clinically in dogs. It would be wise, however, to avoid the use of methoxyflurane in animals with renal disease. Methoxyflurane has been used to control seizure activity in some cases of poisoning, such as with strychnine. SUMMARY
Methoxyflurane anesthesia is particularly useful for orthopedic surgery (muscle relaxation, analgesia, and slow recovery), ophthalmic surgery (central eye position), and small laboratory animals (high therapeutic index and slow induction).
Precautions When Using Methoxyflurane Doris Dyson, DVM, DVSc From the Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada
The main disadvantages related to the use of methoxyflurane in small animals stem from the high blood/gas partition coefficient and the low vapor pressure. Induction of anesthesia, transfer from injectable to inhalant, and changes in the anesthetic plane occur slowly, often requiring at least twice the time that is noted with the use of less soluble inhalants such as halothane. Attempts to speed induction with high anesthetic concentration is limited by the maximum concentration achievable with methoxyflurane (approximately 3%). For these reasons, I believe that mask induction with methoxyflurane is impractical. Smooth transfer from an induction with injectable anesthetics to methoxyflurane maintenance requires early high vaporizer control knob settings to minimize the possibility of awakening occurring from the injectable before the transfer is complete. The anesthetist may need to