1978). This view is also reflected in the definition of pain (Mersky, 1979) adopted in 1986 by the International Association for the Study of Pain: ''an unpleasant sensory or emotional experience associated with actual or potential tissue damage."

Acceptable levels of pain range from the threshold at which it is first detected to the upper limit of tolerance. The pain threshold is the point at which pain is first perceived during noxious stimulation. The pain threshold, of course, represents only minimal pain and is not commonly associated with stress or distress (Wolff, 1978). It has been shown to be essentially the same in humans and in warm-blooded vertebrate animals (Hardy et al., 1952; Vierck, 1976). (As discussed further in Chapters 4 and 5, the committee feels that the equivalence of the pain threshold across species is conceptually important for the recognition and alleviation of pain in diverse species. However, some feel that the equivalence is based on unproven assumptions about the measurement of stimuli and that similarity among species cannot be known with certainty.)

Pain tolerance is essentially the upper limit or highest intensity of pain that will be accepted voluntarily. Pain tolerance varies in a given organism under different circumstances and within and between species, depending on a number of factors, such as the experimental situation, motivation, previous painful experience, and the level of anxiety or fear. The pain sensitivity range is the difference between the pain threshold and pain tolerance (Wolff, 1978).

Although animals lack the ability to communicate their pain verbally, they can exhibit behaviors and physiologic responses similar to those of humans in response to noxious or tissue-damaging stimuli. The behaviors include simple withdrawal reflexes, vocalization, and learned responses ranging from guarding an injured limb to attempting to escape, avoid, or terminate a painful stimulus. The physiologic responses include those associated with an acute stress reaction, such as changes in blood pressure and heart rate and the activation of the pituitary-adrenal axis (see Chapter 4).

Succeeding chapters will review the stress and potential distress that can be induced by pain and the choice of pharmacologic and nonpharmacologic methods to alleviate pain and pain-induced distress.

Anxiety and Fear are emotional states that are traditionally associated with stress. They can be adaptive, in that they inhibit an organism's actions that could lead to harm or cause it to act in ways that allow it to escape from potentially harmful situations; they can also potentiate the affective quality of pain.

Anxiety and fear are not sharply differentiated behaviorally or physiologically. The causes of anxiety are usually assumed to be less specific than the causes of fear. For example, an animal in a new environment or experiencing a novel but benign procedure might be described as anxious. Fear is more often used to describe an emotional state that results from an experienced or known danger in the immediate environment. For instance, a dog that has gone through a painful experience in a particular setting might vocalize or try to escape when placed in that setting again. Thus, fear usually refers to a focused response to a known object or

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