- Email: [email protected]
Precautions When Using Enflurane
INHALATIONAL ANESTHETICS
327
Precautions When Using Enflurane Sandee M. Hartsfield, DVM, MS From the Department of Veterinary Small Animal Medicine and Surgery, Texas Veterinary Medical Center, Texas A & M University College of Veterinary Medicine, College Station, Texas
The disadvantages of enflurane compared with halothane, isoflurane, and methoxyflurane relate to the function of various organ systems during general anesthesia and to its cost. Economically enflurane is expensive for induction and maintenance of general anesthesia in small animals. The cost of a milliliter of liquid enflurane is much more than a comparable amount of halothane and similar to the cost for methoxyflurane. The induction and maintenance concentrations for enflurane are higher in both dogs and cats than for halothane, isoflurane, and methoxyflurane; therefore, the total cost of using enflurane for both induction and maintenance of anesthesia exceeds the costs for the other agents. Central nervous system function during enflurane anesthesia is characterized by high voltage activity, and seizures can occur. Seizures can be induced in animals by auditory stimulation, and hypocapnia decreases enflurane' s seizure threshold. Cerebral metabolic _oxygen consumption may increase by as much as 50% in enflurane-anesthetized animals. Enflurane also increases cerebrospinal fluid production in laboratory animals by about 30% to 50%. Enflurane should not be selected for patients scheduled for intracranial surgery or for a patient with a history of seizures, cranial trauma, or cerebrovascular disease. Dogs and cats are easily induced with enflurane because of its rapid uptake. During maintenance, many of the dogs and cats receiving only enflurane demonstrated spontaneous motor movements or "jerking," which could be interpreted as light anesthesia by an individual unfamiliar with enflurane anesthesia. These movements are not dependent on anesthetic depth. Convulsive activity and motor movements may be eliminated by certain premedicants (opioids) and induction agents (thiopental). Enflurane is a dose-dependent cardiovascular depressant. In equipotent concentrations, enflurane produces greater depression of myocardial function and cardiac output than halothane. During hemorrhage, greater reductions in cardiac output and arterial pressure have been demonstrated for enflurane than for the other halogenated inhalational anesthetics. Although enflurane does not sensitize the heart to development of catecholamine-induced arrhythmias as much as halothane, it does lower the threshold for epinephrine-induced arrhythmias. Respiratory function is significantly affected by enflurane anesthesia. This author found that both dogs and cats anesthetized and maintained with enflurane at moderate depths of anesthesia became
328
!NHALATJONAL ANESTHETICS
apneic for extended periods of time (up to 10 minutes) if no surgical stimulation occurred. Surgical stimulation usually resulted in spontaneous ventilation. Enflurane reduces tidal volume and depresses normal respiratory reflexes. Respiratory function is similarly depressed, however, by all of the halogenated anesthetics. Biotransformation of enflurane (2.4%) is less than for methoxyflurane (50%) or halothane (20%) but greater than for isoflurane (0.17% ). Enflurane has the potential for metabolism to free fluoride ion, which is toxic to renal tubules, but the risk appears to be minimal. Some drugs have been shown to enhance enflurane metabolism significantly and increase production of free fluoride ion. Enflurane reduces hepatic blood flow and cardiac output more than isoflurane; thus enflurane appears to be less desirable for patients with renal or hepatic disease. SUMMARY
Enflurane offers few advantages over halothane, and it is more expensive than halothane. It causes greater cardiopulmonary depression and induces seizure activity. When economy and systemic effects are considered, enflurane offers no real benefits for veterinary anesthesia.
Advantages and Guidelines for Using Isoflurane John W. Ludders, DVM From the Department of Clinical Sciences, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York
Isoflurane, which is one of the newest inhalational anesthetics, represents a benchmark in the development of general anesthetics because its physical properties more closely approach those of the ideal inhalational anesthetic than do previous inhalational drugs. Isoflurane is nonflammable at anesthetic concentrations, and unlike halothane or methoxyflurane, it is stable and does not require the addition of a preservative. 5 Furthermore, it is not broken down by sunlight or strong alkali. 5 Isoflurane has a faster onset of action and recovery because of its relatively low blood solubility compared with other inhalational anesthetics. Although anesthetic inductions with isoflurane in human patients may be slower than expected because its pungent odor causes
327
Precautions When Using Enflurane Sandee M. Hartsfield, DVM, MS From the Department of Veterinary Small Animal Medicine and Surgery, Texas Veterinary Medical Center, Texas A & M University College of Veterinary Medicine, College Station, Texas
The disadvantages of enflurane compared with halothane, isoflurane, and methoxyflurane relate to the function of various organ systems during general anesthesia and to its cost. Economically enflurane is expensive for induction and maintenance of general anesthesia in small animals. The cost of a milliliter of liquid enflurane is much more than a comparable amount of halothane and similar to the cost for methoxyflurane. The induction and maintenance concentrations for enflurane are higher in both dogs and cats than for halothane, isoflurane, and methoxyflurane; therefore, the total cost of using enflurane for both induction and maintenance of anesthesia exceeds the costs for the other agents. Central nervous system function during enflurane anesthesia is characterized by high voltage activity, and seizures can occur. Seizures can be induced in animals by auditory stimulation, and hypocapnia decreases enflurane' s seizure threshold. Cerebral metabolic _oxygen consumption may increase by as much as 50% in enflurane-anesthetized animals. Enflurane also increases cerebrospinal fluid production in laboratory animals by about 30% to 50%. Enflurane should not be selected for patients scheduled for intracranial surgery or for a patient with a history of seizures, cranial trauma, or cerebrovascular disease. Dogs and cats are easily induced with enflurane because of its rapid uptake. During maintenance, many of the dogs and cats receiving only enflurane demonstrated spontaneous motor movements or "jerking," which could be interpreted as light anesthesia by an individual unfamiliar with enflurane anesthesia. These movements are not dependent on anesthetic depth. Convulsive activity and motor movements may be eliminated by certain premedicants (opioids) and induction agents (thiopental). Enflurane is a dose-dependent cardiovascular depressant. In equipotent concentrations, enflurane produces greater depression of myocardial function and cardiac output than halothane. During hemorrhage, greater reductions in cardiac output and arterial pressure have been demonstrated for enflurane than for the other halogenated inhalational anesthetics. Although enflurane does not sensitize the heart to development of catecholamine-induced arrhythmias as much as halothane, it does lower the threshold for epinephrine-induced arrhythmias. Respiratory function is significantly affected by enflurane anesthesia. This author found that both dogs and cats anesthetized and maintained with enflurane at moderate depths of anesthesia became
328
!NHALATJONAL ANESTHETICS
apneic for extended periods of time (up to 10 minutes) if no surgical stimulation occurred. Surgical stimulation usually resulted in spontaneous ventilation. Enflurane reduces tidal volume and depresses normal respiratory reflexes. Respiratory function is similarly depressed, however, by all of the halogenated anesthetics. Biotransformation of enflurane (2.4%) is less than for methoxyflurane (50%) or halothane (20%) but greater than for isoflurane (0.17% ). Enflurane has the potential for metabolism to free fluoride ion, which is toxic to renal tubules, but the risk appears to be minimal. Some drugs have been shown to enhance enflurane metabolism significantly and increase production of free fluoride ion. Enflurane reduces hepatic blood flow and cardiac output more than isoflurane; thus enflurane appears to be less desirable for patients with renal or hepatic disease. SUMMARY
Enflurane offers few advantages over halothane, and it is more expensive than halothane. It causes greater cardiopulmonary depression and induces seizure activity. When economy and systemic effects are considered, enflurane offers no real benefits for veterinary anesthesia.
Advantages and Guidelines for Using Isoflurane John W. Ludders, DVM From the Department of Clinical Sciences, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York
Isoflurane, which is one of the newest inhalational anesthetics, represents a benchmark in the development of general anesthetics because its physical properties more closely approach those of the ideal inhalational anesthetic than do previous inhalational drugs. Isoflurane is nonflammable at anesthetic concentrations, and unlike halothane or methoxyflurane, it is stable and does not require the addition of a preservative. 5 Furthermore, it is not broken down by sunlight or strong alkali. 5 Isoflurane has a faster onset of action and recovery because of its relatively low blood solubility compared with other inhalational anesthetics. Although anesthetic inductions with isoflurane in human patients may be slower than expected because its pungent odor causes