oes not compromise neural responsiveness in the areas under study (NIH, 1991). That is critical because reducing the level of anesthesia after NMBDs have been administered is problematic. Obtaining a performance based assessment of the adequacy of the new anesthesia level may require that NMBDs be withdrawn to assess skeletal muscle response; however, this can entail difficulties because a long period may be needed to restore muscle responses (Hildebrand, 1997). Noninvasive assessment of neuromuscular function with examination of evoked responses of skeletal muscle to peripheral motor nerve stimulation can facilitate monitoring both the induction of paralysis and the recovery from NMBDs (Hildebrand, 1997). Anesthesia and general physiologic state should be monitored for each animal during each procedure (Mason and Brown, 1997; NRC, 1996). Notations
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should include the time, date (if appropriate), drugs or solutions administered, and the name or initials of the person making the entry. A number of physiologic measures are helpful in monitoring animals on NMBDs, including heart rate, electroencephalogram, arterial blood pressure, blood oxygen saturation, urine production and pH, end-tidal CO2 and/or blood gas concentration, rectal temperature, and general autonomic signs of arousal, such as salivation, pupil size, and lacrimation (Hildebrand, 1997; NIH, 1991). Physiologic measures should be documented periodically throughout an experiment. The periodicity of monitoring and documentation should be more frequent during the initial stages of experimentation (such as every 15 minutes) and gradually extended once the animal has been stabilized on NMBDs. The details of the specific physiologic measures to be monitored and the frequency and means of documentation should be described in the research protocol and approved by the IACUC. The use of automated multifunction measuring devices—which maintain a running record of such metabolic measures as blood pressure, blood oxygenation, expiratory CO2, and pulse rate—can greatly facilitate the monitoring of anesthesia and general physiologic state (Vogler, 1997). However, not all devices maintain a historical record, and regular measurements should be recorded under these circumstances. The use of automated devices cannot substitute for direct monitoring of the animal by a human observer (see also Mason and Brown, 1997), and a human observer should be present at all times during a prolonged nonsurvival procedure, as the clinical status of the animal can change quickly and require intervention. Monitoring data should be filed by experiment and animal and kept for at least the duration of the overall project (NIH, 1986). The issues involved in maintaining an experimental animal in good physiologic condition during a prolonged recording experiment are similar to those involved in other situations that require the long-term maintenance of animals in clinical situations (NIH, 1991). Animals that are paralyzed must be ventilated artificially, and standard veterinary practices should be followed when selecting the gas mixture and anesthetic or analgesic used (Vogler, 1997). In some cases, gaseous anesthetics or analgesics are included in the gas mixture (such as isoflurane and N2O), while in other cases, room air with or without added oxygen is used. The use of a mixture of 50% O2 and 50% N2O may be helpful, in that N2O potentiates intravenous anesthetics (NIH, 1991). However, N2O increases cerebral blood flow, and this side effect may be of special concern to researchers performing intracranial procedures (Drummond et al., 1987). Animals exposed to artificial ventilation for long periods should be hyperinflated (sighed) at regular intervals to help to avoid collapse of the pulmonary alveoli (atelectasis) (Vogler, 1997). Hydration of the inspired gases is also helpful in preventing desiccation of lung tissues. That can be accomplished by passing the air through fluid in a flow-through system or by using a closed-loop system that obviates additional hydration; passive humidifica