http://evolve.elsevier.com/Edmunds/LPN/

There are many nerve paths that carry the sensation of pain from an injured part of the body to the brain. This means that there are several different places to block the feeling of pain. Some feelings of pain may be treated by exercise, heat, ice, or other methods. Some pain is so severe that it requires drug treatment. There are now a wide range of drugs used in controlling pain. The most common drugs for mild pain relief include over-the-counter analgesics such as aspirin and acetaminophen. (These drugs are described separately in Chapter 18 because they also have actions other than pain reduction.) Many of the drugs used for treating severe pain are opioids. (An opioid is any substance that produces stupor associated with analgesia, and are used to treat severe pain.) Natural opioids come from opium, which comes from unripe seed capsules of the poppy plant. Opium contains many chemicals, including morphine and codeine. (Heroin is diacetylmorphine, which chemically breaks down into morphine.) Opioid medications interact with specific receptors in the brain and spinal cord to reduce pain. There are also nonopioid analgesics that produce moderate pain relief for conditions not severe enough to require an opioid.

In addition to natural opioids, artificial or synthetic opioids have been developed. It was hoped that many of these new drugs would not be as addictive as morphine, but this was not so. However, these new drugs are useful for pain management and to reverse the effects of opioids. Many of these new drugs are made by changing morphine chemically. Morphine is the basic chemical from which the synthetic opioid analgesics hydrocodone, hydromorphone, and oxycodone have developed. Other classes of synthetic opioids are made of different chemicals but have actions similar to morphine.

Analgesics used in pain management are classified according to their mechanism of action. Opioids are classified as agonist, partial agonist, or agonist-antagonist medications. The term agonist means “to do”; the term antagonist means “to block.” An agonist drug binds with the receptors to activate and produce the maximum response of the individual receptor. A partial agonist produces a partial response of this type. An opioid agonist-antagonist drug produces mixed effects, acting as an agonist at one type of receptor and as a competitive antagonist at another type of receptor. This information is key to understanding the rest of the chapter. Refer back here if needed.

The mechanism of action for opioids is determined by where they bind to specific opioid receptors inside and outside the central nervous system (CNS). There are six types of opioid receptors (delta, epsilon, kappa, mu [types one and two], and sigma). These receptors are responsive to the opiates (have opiate receptors) in each of these areas that interact with autonomic nervous system nerves that carry pain messages (including the release of neurotransmitters), and this interaction produces changes in the person’s reaction to pain. Some opioids block a particular receptor; others stimulate a particular receptor. Although analgesia can occur with the stimulation of delta, epsilon, and kappa receptors, most action occurs at the mu and kappa receptors. Mu receptors produce analgesia, respiratory depression, sedation, euphoria, decreased gastrointestinal (GI) motility, and constricted pupils (miosis). These mu receptors may also lead to physical dependence. Kappa receptors produce analgesia, sedation, and are also associated with decreased GI motility and miosis. Sigma receptors seem to produce mostly unwanted effects.

Morphine is the main opioid agonist drug with which all other pain management drugs are compared (Table 17-1). It is used a great deal in acute care and also in hospice settings for dying patients who have severe pain. Codeine, hydrocodone (Hydromet), and oxycodone (OxyContin) are often used in combination with acetaminophen in the outpatient setting. Hydromorphone (Dilaudid) is very potent (with regard to the number of milligrams that are equivalent to morphine) and is only used for treating severe pain not relieved by morphine. It comes as a powder for compounding as well as a tablet.

Table 17-1

Equivalent Doses of Opioid Analgesics Compared with Morphine 10 mg PO and IM

ANALGESIC	ORAL DOSE (mg)	PARENTERAL DOSE (IM) (mg)
morphine	60 single dose;	10
	30 repeated doses
morphine (MS Contin)	60
hydromorphone	7.5	1.5
fentanyl	NA	0.1
codeine	200	130
hydrocodone	30	NA
levorphanol	4	1-2
oxycodone	15-20	NA
meperidine	300	75-100
methadone	10-20	5-10

IM, Intramuscular; IV, intravenous; NA, not applicable.
Indicates “Must-Know Drugs,” or the 35 drugs most prescribers use.

Modified from Brunton L, Lazo L, Parker K, editors: Goodman and Gilman’s pharmacological basis of therapeutics, ed 11, New York, 2005, McGraw-Hill.

Opioid agonist-antagonist drugs may be preferred over opioid agonists for use in patients in the community because their risk for abuse is less. A common drug, pentazocine (Talwin) which is estimated to be about 1/6 as potent as morphine has limited use because of its CNS toxicity. In efforts to limit the abuse of these drugs, the federal government created many regulations that describe who may prescribe or administer opioids. Nurses learn and follow these rules. The nurse may have responsibility for keeping opioids in a safe place, typically a locked cabinet, and account for their use in the hospital or nursing home setting.

This chapter is divided into three sections. The first section deals with opioid agonist analgesics. The second section presents opioid agonist-antagonist analgesics. Nonopioid (centrally acting) combination drug analgesics are presented in the third section.

Researchers believe four things are required for pain to occur:

Anxiety, depression, fatigue, and other chronic diseases may increase the perception of pain. Activities designed to distract the patient, create positive attitudes, or provide support may reduce the perception of pain. There are many nondrug methods for doing this. Some of these activities might involve listening to music, massage therapy, cold or hot packs, hydrotherapy, acupuncture, biofeedback, relaxation therapy, art therapy, hypnosis, therapeutic touch, Qigong or Reiki energy therapies, or use of transcutaneous electrical nerve stimulation (TENS) units. Sometimes pain requires nerve blocks or surgical intervention to relieve pain on nerves or structures.

Drugs called opioid agonist analgesics are thought to prevent painful feelings in the CNS (in the substantia gelatinosa [gray matter] of the spinal cord, brain stem, reticular formation, thalamus, and limbic system). (See Chapter 16 for additional information on receptors and neurotransmitters in the CNS.) Figure 17-1 demonstrates how opioids act on neurons.

Nursing Implications and Patient Teaching

n Assessment

Before a patient can be effectively helped with pain relief, it is important to determine the cause of the pain. Some organizations have called for pain assessment to be the fifth vital sign, but this is not universally accepted. Do not simply administer drugs prescribed for pain without understanding the source of the particular pain. Even in a patient with terminal cancer, evaluate each new pain for a specific cause that may be specifically treated. For example, bone pain can often be managed by radiation. Table 17-2 lists the classifications of pain and their characteristics.

Table 17-2

Classification of Pain

CATEGORY	CHARACTERISTICS	EXAMPLE
Nociceptive	Somatic Well localized Dull, aching, or throbbing	Laceration, fracture, cellulitis, arthritis
Visceral	Poorly localized Continual aching Referred to dermatomal sites that are distant from the source of the pain	Subscapular pain arising from diaphragmatic irritation; right upper quadrant pain arising from stretching of liver capsule
Neuropathic	Shooting or stabbing pain superimposed over a background of aching and burning	Postherpetic neuralgia, postthoracotomy neuralgia, poststroke pain, trigeminal neuralgia, diabetic polyneuropathy

Modified from McKenry LM, Tessier E, Hogan MA: Mosby’s pharmacology in nursing, ed 22, St Louis, 2006, Mosby.

When assessing pain, ask the patient to describe the pain. Learn the history of the pain, including when it started, where it is, what it feels like, how often it occurs, and what makes it worse or relieves it. Accept that patients have pain when they say they have it at the intensity level they say it is. Use a pain scale to make the assessment more objective. In addition to what patients say, the nurse may also sometimes see changes in their breathing, blood pressure, and pulse, as well as tense muscles, sweating, and pupil reaction. Also, they may be restless, crying, or moaning.

Learn as much as possible about other parts of the health history, such as whether the patient has a history of allergic or adverse reaction to morphine or related drugs, past ability to deal with pain, and whether there is any reason to think opioid abuse might become a problem.

The Agency for Health Care Policy and Research Clinical Practice Guidelines include the following principles of pain assessment (A-A-B-C-D-E-E):

Ask about pain regularly. Medication is to be given regularly and is more effective if it is given before the patient is in severe pain and begging for medication. There is acceptance that addiction is generally not a concern, especially for patients with chronic pain or terminal illness.

Assess pain systematically. Use pain intensity scales (Figures 17-2 and 17-3).

Believe the patient and family in their reports of pain and what relieves it.

Choose pain-control options appropriate for the patient, family, and setting. The health care provider who makes this decision should be aware of the wishes of the family and individual.

Deliver interventions in a timely, logical, and coordinated fashion.

Empower patients and their families.

Enable them to control their course to the greatest extent possible.

Lifespan Considerations

Pediatric

Pain Measurement

Pain measurement scales exist to help evaluate how much pain the patient is feeling. Children experience pain in the same way as adults. Sometimes they have more difficulty communicating their pain, so providers need to be alert. A pain scale using smiling or frowning faces may help assess a child’s pain (see Figure 17-2).

n Diagnosis

Are there reasons the patient should not use these medications? Are there risk factors for their use? Is the nurse aware of other things that might pose a problem for a patient taking these medications? Report any problems discovered to the registered nurse or physician.

n Planning

Whenever possible, pain treatment should begin with simple and nonopioid analgesics and supportive pain-relief measures first. These measures are described later.

Remember that pain relief is best if the drug is given before the patient has intense pain.

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