Blood pressure responses as an indicator of the anesthetic level in ground squirrels

In many experiments in visual physiology which employ halothane anesthesia, it is difficult to judge accurately an animal's anesthetic state, particularly so because of the necessity for paralysis and artificial respiration. For the ground squirrel (Citellus tridecemlineatus and C. mexicanus), we find that measures of arterial pressure response to a noxious stimulus combined with mean arterial pressure allow for an accurate division of halothane anesthetic state into five stages. All five stages are identified and are the same regardless of whether the animal is unparalyzed or is paralyzed and artificially respired. Use of these measures in long-term experiments allows for recognition of and adjustment for small perturbations of anesthetic state that may occur even though inhaled halothane concentration remains constant.