Pain and Analgesic Drugs

Pain and Analgesic Drugs

• Analgesic drugs selectively block pain sensations without affecting other functions or consciousness.
• Anesthetics block nerve conduction, causing loss of sensation (local) or loss of sensation and consciousness (general).
• Pain is a personal experience, and individuals vary greatly in their perception.
• Nociception is the detection of noxious stimuli, or stimuli that can damage tissue.
• Pain involves four processes: transduction, transmission, perception, and modulation.
• Pain assessment needs a qualified individual who can evaluate the pain.
• A pain assessment tool, like the OUCH scale, helps quantify pain intensity.
• Acute pain is sudden in onset and usually resolves after treatment.
• Nociceptive stimuli, like stepping on a nail, activate nociceptive pathways that cause acute pain.
• Chronic pain lasts longer than six weeks, is often persistent or recurring, and can be challenging to treat. Types include mild musculoskeletal, deep pain, neuropathic pain, and chronic pain of indeterminate cause.
• Neuropathic pain results from nerve injury or infection. Examples include phantom limb pain, trigeminal neuralgia, shingles, and diabetic neuropathy.
• Forms of pain include chronic pain of indeterminate cause, and pain associated with psychiatric disorders (e.g., depression).
• Somatic pain originates in the skin and other tissues. Examples include superficial and visceral (internal organs), specifically vascular and respiratory. Referred pain appears in areas away from the source of pain.
• Factors that influence pain tolerance include discomfort, insomnia, anxiety, fear, anger, boredom, sadness, depression, introversion, social abandonment, mental isolation.
• Factors that improve pain tolerance include relief of symptoms, sleep, relaxation, explanation and support, understanding empathy, diversions, companionship, mood improvement, and understanding the significance of pain.
• Analgesia is a process that aims to alleviate pain by addressing the cause.
• Opioids, specifically, work in the brain.

Opioids

• Opioids are morphine-like compounds that bind to opioid receptors on nerve endings.
• Opioids include morphine, codeine, methadone, fentanyl, diamorphine (heroin), levorphanol, hydromorphone, and oxycodone.
• Analgesics, including opioids, can be categorized as agonists (e.g., morphine) which directly activate receptors; or partial agonists (less potent).
• Agonist-antagonist opioids (e.g., buprenorphine) have both agonist and antagonist effects at different opioid receptors.
• Opioid interactions exist and can be detrimental causing cumulative effects from CNS depressants like antipsychotics, antihistamines, sedatives (benzodiazepines, barbiturates), and alcohol.

Opioid Receptors

• μ (mu) receptors are linked to analgesia ( pain relief) and located in the brain (cortex, medulla, thalamus, limbic system, amygdala), spinal cord, and other areas.
• κ (kappa) receptors influence analgesia and are found in the brain and spinal cord, associated with dysphoria and hallucinations.

Pharmacological Properties of Opioids

• Opioids produce analgesia (pain relief).
• Opioids can induce sedation.
• Opioids can cause feelings of euphoria and tranquility due to central dopaminergic pathways.
• Opioids can suppress the cough reflex through the cough center in the medulla.
• Opioids can lead to respiratory depression (a serious side effect) by directly affecting the brainstem respiratory center.
• Opioids can result in nausea and vomiting caused by stimulation of the chemoreceptor trigger zone of the medulla.
• Opioids can cause the pupils to constrict (miosis).
• Opioids cause tolerance and dependence.
• Opioids can cause constipation, urinary retention, and hypotension.
• Opioids can cause dilation of cutaneous blood vessels (warm skin).
• Opioids can cause itching and sweating.

Opioid Analgesics - Morphine

• Morphine is an analgesic primarily targeting μ receptors in the brain and spinal cord.
• Morphine is administered through intravenous (IV), intramuscular (IM), subcutaneous (SC), oral (PO), or intrathecal routes.
• Morphine's metabolism occurs through the liver, with a half-life of approximately 2-4 hours.
• Morphine is sometimes used for cancer pain, but is not without risk in pregnancy.
• Morphine's pharmacological properties include analgesia, sedation, euphoria, antitussive effects, and respiratory depression.

Analgesics and Cancer Pain

• Chronic cancer pain often requires around-the-clock (ATC) treatment with opioids.
• Breakthrough pain can arise even when chronic pain is managed. Rescue medications are helpful.
• NSAIDs (non-steroidal anti-inflammatory drugs) and adjuvants are used often as added support.
• Sustained release morphine (MS Contin) is a common treatment.

WHO Pain Management Ladder

• The WHO pain ladder is a treatment approach to managing pain with varying drug intervention, from non-opioids to opioids, depending on the severity of pain.

Cancer Pain - NSAIDs and Adjuvants

• NSAIDs are commonly used for pain management in cancer patients.
• Adjuvant therapies (like antidepressants, anti seizure meds, and corticosteroids) can assist in pain management.

Morphine: Contraindications and Cautions

• Morphine is contraindicated in individuals with severe asthma or respiratory insufficiency due to possible respiratory depression.
• Morphine use is cautioned in those with hepatic (liver) dysfunction, and/or elevated intracranial pressure (ICP).

Opioid Analgesics: Adverse Effects

• Opioids commonly cause respiratory depression, CNS depression, nausea and vomiting, constipation.
• Other adverse effects include hypotension, histamine release, urinary retention, diaphoresis, and miosis.

Opioid Analgesics: Interactions

• Opioids can interact with central nervous system (CNS) depressants such as antipsychotics, antihistamines, sedatives, and alcohol.

Moderate Opioid Analgesics, Examples:

• Codeine (3-methylmorphine): a less strong analgesic than morphine, typically needs combination.
• Oxycodone (Oxycontin, Percodan): often combined with acetaminophen (e.g., Percocet), metabolized to work.
• Buprenorphine: agonist-antagonist;

Opioid Antagonists

• Naloxone (Narcan) and naltrexone are opioid antagonists used to reverse opioid-induced respiratory depression.

Treating Opioid Addiction

• Methadone programs and buprenorphine/naloxone (Suboxone) combinations are utilized.

Opioid Tolerance

• Tolerance is a common physiological result of sustained opioid treatment, reducing opioid effects.

Opioid Physical Dependence

• Physical dependence occurs with chronic opioid use; abrupt discontinuation causes withdrawal.
• Withdrawal symptoms are unpleasant but usually not dangerous and can be managed with medical supervision.

Opioids: Implications for Clinical Care

• Oral opioids should be taken with food to minimize gastric upset.
• Respiratory depression is a critical clinical concern. Vital signs monitoring is essential.
• Constipation is a common adverse effect and needs management.
• Orthostatic hypotension should be avoided with slow position changes.