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Some pain receptors are selective in their response to stimuli, but most are sensitive to more than one of the following types of excitation: (1) mechanical stress of trauma; (2) extremes of heat and cold; and (3) chemical substances, such as histamine, potassium ions, acids, prostaglandins, bradykinin, and acetylcholine. Pain receptors, unlike other sensory receptors in the body, do not adapt or become less sensitive to repeated stimulation. Under certain conditions the receptors become more sensitive over a period of time. This accounts for the fact that as long as a traumatic stimulus persists the person will continue to be aware that damage to the tissues is occurring.
The body is able to recognize tissue damage because when cells are destroyed they release the chemical substances previously mentioned. These substances can stimulate pain receptors or cause direct damage to the nerve endings themselves. A lack of oxygen supply to the tissues can also produce pain by causing the release of chemicals from ischemic tissue. Muscle spasm is another cause of pain, probably because it has the indirect effect of causing ischemia and stimulation of chemosensitive pain receptors.
The perception of pain by an individual is highly complex and individualized, and is subject to a variety of external and internal influences. The cerebral cortex is concerned with the appreciation of pain and its quality, location, type, and intensity; thus, an intact sensory cortex is essential to the perception of pain. In addition to neural influences that transmit and modulate sensory input, the perception of pain is affected by psychological and cultural responses to pain-related stimuli. A person can be unaware of pain at the time of an acute injury or other very stressful situation, when in a state of depression, or when experiencing an emotional crisis. Cultural influences also precondition the perception of and response to painful stimuli. The reaction to similar circumstances can range from complete stoicism to histrionic behavior.
Since this theory was first proposed, researchers have shown that the neuronal circuitry it hypothesizes is not precisely correct. Nevertheless, there are internal systems that are now known to occur naturally in the body for controlling and mediating pain. One such system, the opioid system, involves the production of morphinelike substances called enkephalins and endorphins. Both are naturally occurring analgesics found in various parts of the brain and spinal cord that are concerned with pain perception and the transmission of pain signals. Signals arising from stimulation of neurons in the gray matter of the brain stem travel downward to the dorsal horns of the spinal cord where incoming pain impulses from the periphery terminate. The descending signals block or significantly reduce the transmission of pain signals upward along the spinal cord to the brain where pain is perceived by releasing these substances.
In addition to the brain's opioid system for controlling the transmission of pain impulses along the spinal cord, there is another mechanism for the control of pain. The stimulation of large sensory fibers extending from the tactile receptors in the skin can suppress the transmission of pain signals from thinner nerve fibers. It is as if the nerve pathways to the brain can accommodate only one type of signal at a time, and when two kinds of impulses simultaneously arrive at the dorsal horns, the tactile sensation takes precedence over the sensation of pain.
The discovery of endorphins and the inhibition of pain transmission by tactile signals has provided a scientific explanation for the effectiveness of such techniques as relaxation, massage, application of liniments, and acupuncture in the control of pain and discomfort.
Objective signs of pain can help verify what a patient says about pain, but such data are not used to prove or disprove whether it is present. Physiologic signs of moderate and superficial pain are responses of the sympathetic nervous system. They include rapid, shallow, or guarded respiratory movements, pallor, diaphoresis, increased pulse rate, elevated blood pressure, dilated pupils, and tenseness of the skeletal muscles. Pain that is severe or located deep in body cavities acts as a stimulant to parasympathetic neurons and is evidenced by a drop in blood pressure, slowing of pulse, pallor, nausea and vomiting, weakness, and sometimes a loss of consciousness.
Behavioral signs of pain include crying, moaning, tossing about in bed, pacing the floor, lying quietly but tensely in one position, drawing the knees upward toward the abdomen, rubbing the painful part, and a pinched facial expression or grimacing. The person in pain also may have difficulty concentrating and remembering and may be totally self-centered and preoccupied with the pain.
Psychosocial aspects of tolerance for pain and reactions to it are less easily identifiable and more complex than physiologic responses. An individual's reaction to pain is subject to a variety of psychologic and cultural influences. These include previous experience with pain, training in regard to how one should respond to pain and discomfort, state of health, and the presence of fatigue or physical weakness. One's degree of attention to and distraction from painful stimuli can also affect one's perception of the intensity of pain. A thorough assessment of pain takes into consideration all of these psychosocial factors.
When analgesics are not appropriate or sufficient or when there is a real danger of addiction, there are noninvasive techniques that can be used as alternatives or adjuncts to analgesic therapy. The selection of a particular technique for the management of pain depends on the cause of the pain, its intensity and duration, whether it is acute or chronic, and whether the patient perceives the technique as effective.
Distraction techniques provide a kind of sensory shielding to make the person less aware of discomfort. Distraction can be effective in the relief of brief periods of acute pain, such as that associated with minor surgical procedures under local anesthesia, wound débridement, and venipuncture.
Massage and gentle pressure activate the thick-fiber impulses and produce a preponderance of tactile signals to compete with pain signals. It is interesting that stimulation of the large sensory fibers leading from superficial sensory receptors in the skin can relieve pain at a site distant from the area being rubbed or otherwise stimulated. Since ischemia and muscle spasm can both produce discomfort, massage to improve circulation and frequent repositioning of the body and limbs to avoid circulatory stasis and promote muscle relaxation can be effective in the prevention and management of pain. Transcutaneous electrical nerve stimulation (TENS) units enhance the production of endorphins and enkephalins and can also relieve pain.
Specific relaxation techniques can help relieve physical and mental tension and stress and reduce pain. They have been especially effective in mitigating discomfort during labor and delivery but can be used in a variety of situations. Learning proper relaxation techniques is not easy for some people, but once these techniques have been mastered they can be of great benefit in the management of chronic ongoing pain. The intensity of pain also can be reduced by stimulating the skin through applications of either heat or cold, menthol ointments, and liniments. Contralateral stimulation involves stimulating the skin in an area on the side opposite a painful region. Stimulation can be done by rubbing, massaging, or applying heat or cold.
Since pain is a symptom and therefore of value in diagnosis, it is important to keep accurate records of the observations of the patient having pain. These observations should include the following: the nature of the pain, that is, whether it is described by the patient as being sharp, dull, burning, aching, etc.; the location of the pain, if the patient is able to determine this; the time of onset and the duration, and whether or not certain nursing measures and drugs are successful in obtaining relief; and the relation to other circumstances, such as the position of the patient, occurrence before or after eating, and stimuli in the environment such as heat or cold that may trigger the onset of pain.
acute painA normal physiologic and usually time-limited response to an adverse (noxious) chemical, thermal or mechanical stimulus that is associated with surgery, trauma, and acute illness, and is historically responsive to opioid therapy.
acute painPain management A normal physiologic and usually time-limited response to an adverse (noxious) chemical, thermal or mechanical stimulus, associated with surgery, trauma, and acute illness and historically responsive to opioid therapy. See Pain; Cf Chronic pain.
Patient discussion about acute pain
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