Alan M. Klide

Prolonged General Anesthesia in MR Studies of Rats

RATIONALE AND OBJECTIVES Magnetic resonance (MR) imaging of laboratory animals may require general anesthesia to minimize body movements over many hours. The anesthetization technique should allow physiologic parameters to remain as close to normal as possible, permit fast recovery, allow safe, repeated use, and avoid attachment of ferrous metal components to the animal. The purpose of this study was to evaluate an anesthetization technique that was developed to meet each of these qualifications. MATERIALS AND METHODS In 15 rats (280-483-g body weight), general anesthesia was induced (with intramuscular ketamine hydrochloride, xylazine hydrochloride, acepromazine maleate, and atropine), a tail vein catheter was inserted, and preimaging surgical procedures were performed. A face mask was applied, the animal was positioned in a dorsal recumbent position on an acrylic board, and an isothermal heating pad was placed on the ventral aspect of the abdominal wall. The rat, on the board, was then inserted into a trough that contained a custom-built, linearly polarized birdcage head coil and placed in the bore of a 4.7-T horizontal-bore magnet. The face mask was connected to a non-rebreathing gaseous anesthetic system, and anesthesia was maintained with 1.5-2.0 L/min oxygen and 0.25%-1.50% isoflurane. Oxygen saturation, heart rate, and rectal temperature were continuously monitored. RESULTS The duration of intramuscular anesthesia was 110 minutes +/- 12, and the duration of gaseous anesthesia was 106 minutes +/- 43. The monitoring equipment permitted display of vital signs. CONCLUSION The method appeared safe, was easy to perform, maintained a stable physiologic state for the parameters monitored, and could be used for repeated anesthesia in the same animal.

Precautions When Using Alpha-2 Agonists as Anesthetics or Anesthetic Adjuvants

Alpha-2 adrenergic agonists have some unique properties that could theoretically make them useful in the perianesthetic period: they reduce the MAC of inhaled anesthetics, and they are reversible. They also have properties that may limit their usefulness, such as marked decreases in cardiac output. Their clinical utility awaits further studies.

Intracranial Dysfunctions: Stupor and Coma

Coma and stupor are pathologic neural states whose signs arise from either massive cortical or brainstem disease or toxicity. A variety of pathological processes can cause brain disease. Often one has to begin treatment before the diagnosis of the cause of the problem is made. At all stages of treatment, great care must be taken to avoid precipitating an intracranial crisis. The physiological derangements that may aid in the diagnosis include arousability, pupil reactivity, various reflexes, breathing patterns, and cardiovascular function. The treatment needs to be both specific and general; i.e., the condition itself must be treated, if possible, and the intracranial pressure must be lowered if it has been raised by the condition. Treatments for raised intracranial pressure include hyperventilation, diuretics, corticosteroids, avoidance of jugular vein compression, coughing, and any change of the position of the head and neck from normal. When sedating or anesthetizing these patients, one should avoid drugs that increase cerebral blood flow such as ketamine and inhaled anesthetics, using instead drugs that lower cerebral blood flow such as thiopental, lidocaine, and narcotics (with ventilation).

The Case for Low Gas Flows

When nitrous oxide is not used, there are no clinically significant, valid reasons for not using low gas flows. There are many clinically significant, valid reasons for using low flows.

The Case for Out of Circuit Vaporizers

Out of circuit vaporizers have some characteristics that differ from in circuit vaporizers. When these are understood, out of circuit vaporizers offer some advantages.

Fresh Gas Flow

If, when using a circle system, the flow of all inhaled gases exactly equals the uptake of all these gases by the patient, the circle system is called a closed circle system. If the fresh gas flows are higher than the uptake of the gases by the body, the circle system is a semiclosed circle system. The mechanical status of the pop-off valve; i.e., open or closed, has nothing to do with the definition of whether the circle is closed or semiclosed.