Definitions
How does the International Association for the Study of Pain (ISAP) define pain?
IASP defines pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.
What physiologic and endocrine markers form the basis of acute pain scoring systems?
Physiologic parameters: heart rate, respiratory rate, blood pressure, pupil dilation
Endocrine markers: epinephrine, norepinephrine, cortisol
What are nociceptors and what function do they serve?
Nociceptors are specialized free nerve endings of primary afferent fibers located in cutaneous tissues, muscle, and viscera.
They function to encode the localization, intensity, and duration of noxious stimuli.
Most nociceptors are nonselective ion channels gated by temperature, chemical ligands, or mechanical shearing forces. Once activated, the channels permit Na+ and Ca2+ ion influx, producing an inward depolarizing current, which if sufficient activates voltage gated Na+ channels causing further depolarization and initiation of action potentials.
What is the principal excitatory neurotransmitter in the spinal cord and brain? What are two types of receptors exist for this neurotransmitter?
Glutamate is the principal excitatory synaptic neurotransmitter.
Two types of receptors:
- Metabotropic receptors: G-protein coupled receptors
- Ionotropic receptors: ligand-gated ion channels
Ex. NMDA receptors
What is the mechanism of action of local anesthetic medications?
- Interrupt the generation and propagation of action potentials in neural tissue resulting in transient loss of sensory, motor, and autonomic function
- Occurs by blocking Na channels in the nerve membrane
What is nociception?
Nociception refers to the unconscious activity induced by a noxious stimulus in specialized nerve receptors, peripheral nerves, and the central nervous system.
In contrast, pain is a conscious experience.
What dimension of pain is measured with a unidimensional scoring system? What is a disadvantage of this type of system?
These systems measure intensity.
They are highly subjective and not linear.
Examples -
Simple description scale
Visual analog scale
Numeric rating scale
What are the five major components in the pathway of nociception?
- Transduction: Nociceptors transduce high-threshold stimuli into electrical activity.
- Transmission: Transmission of the signal to the spinal cord.
- Modulation: Modulation of the signal by inhibition or amplification.
- Projection: Projection of the signal from the spinal cord to the CNS.
- Perception: Central integration producing a pain experience unique to the individual - conscious perception of pain.
Describe the three-order neuron pathway of nociception.
- A noxious stimulus in the periphery activates the primary afferent fiber that transmits the information to the dorsal horn of the spinal cord.
- Cell bodies of both types of afferent nociceptive nerve fibers are contained in the dorsal root ganglia and extend axons to synapse with dorsal horn neurons within the gray matter of the spinal cord.
- Projection neurons receive input from primary afferent neurons and project to neurons in the medulla, pons, midbrain, thalamus, and hypothalamus.
- Supraspinal neurons integrate signals from spinal neurons and project to subcortical and cortical areas where pain is perceived.
What are indications and contraindications for epidural anesthesia?
Indications - anesthesia and analgesia for procedures caudal to the patient's umbilicus
Contraindications - local infection, bleeding disorder, uncorrected hypotension or hypovolemia, congenital or traumatic anatomic abnormalities, sepsis or bacteremia
What is the difference between allodynia and hyperalgesia?
Allodynia is pain that is caused by a stimulus that usually does not result in pain.
Hyperalgesia is an increased response to a stimulus that is normally painful.
What are examples of behaviors observed in dogs and cats experiencing pain?
Posture - hunched back, base-wide stance, prayer position, head/tail tucked, frequent position changes, reluctance to lie down
Gait - stiff gait, lameness, reluctance to move
Abnormal movements - shaking, trembling
Interaction - reduced willingness to interact
Demeanor - aggression, submissive behavior
Attention to painful area - staring at area, licking, guarding
Palpation of the painful area triggers a response
Vocalization - deviation from normal pattern
Appetite - hyporexia or anorexia
Grooming - appropriate grooming in cats tends to be decreased when in pain
Describe the structure and function of AB fibers.
- Large, myelinated sensory fibers
- Activated by low-intensity stimuli
- Normally conduct non-noxious information (touch, vibration, pressure, and rapid movement)
Name the four locations where modulation of pain occurs.
- Spinal cord dorsal horn
- Rostroventral medulla (RVM) of the brainstem
- Periaqueductal grey matter (PAG) in the midbrain
- Thalamocortex
Describe how to perform an epidural at the lumbosacral space. Name three techniques to determine correct needle placement.
- Patient in sternal or lateral recumbency
- Clip the fur around the LS space
- Perform a sterile surgical preparation of the skin
- After washing hands and while wearing sterile gloves, palpate the wings of the ilium and the seventh lumbar vertebrae spinous process - slide the index finger caudally into the divot of the LS space
- Insert a 20- or 22- gauge, 1.5 to 3 inch spinal needle perpendicular to the skin ensuring the needle is on midline
- Needle is advanced through the interspinous ligament/ligamentum flavum
- Hanging drop, loss of resistance syringe, or electrostimulation can be used to confirm proper placement
What is the difference between physiologic pain and pathologic pain?
Physiologic or nociceptive pain arises from noxious stimuli associated with the risk of tissue damage. It is typically protective and results in initiating withdrawal reflexes and avoidance responses. It is characterized by a high stimulus threshold, localized and transient, and a stimulus-response relationship.
Pathologic pain implies that tissue damage has already occurred. It is characterized by a low stimulus threshold and an exaggerated response to noxious stimuli (hyperalgesia). It is felt at sites of the injury (primary hyperalgesia) and in surrounding areas (secondary hyperalgesia).
What are action units (AUs)? Give an example of an AU described in cats.
Action units (AUs) are unit changes in facial expression produced by facial muscle activity. Pain AUs are involuntary in nature and cannot be properly suppressed, amplified, or stimulated. In people, they are more sensitive to pain intensity than verbal self-report.
Pain AUs described in cats include bases of the pinnae moving away from each other; dorsal movement of the nose, mouth, and cheek areas; and eyes narrowing.
Describe the structure and function of Aδ fibers.
- One of the principal nociceptive afferents along with C fibers
- Small, myelinated, rapidly conducting
- Conduction velocity 5-30 ms
- Responsible for sensation of physiologic pain, fast pain, or "first pain" (sharp, transient, localized) - lasts only as long as the stimulus is activating the receptor
- Small receptive fields
- Specific, high-threshold ion channels activated by noxious thermal or mechanical input
What is sensitization?
Nociceptors are unique among sensory receptors in that repeated activation may lower their threshold and result in an enhanced response to subsequent stimuli.
What are the indications and potential complications of performing an intercostal nerve block?
- Indications: analgesia for lateral thoracic wall (rib fractures, flail chest), possible improvement in postoperative ventilation through analgesia in thoracotomy patients
- Complications: inadvertent vascular puncture, inadvertent advancement into thoracic cavity causing pneumothorax
What is the difference between adaptive and maladaptive pain?
Adaptive pain includes nociceptive and inflammatory pain. It is a normal response to tissue damage and allows for a protective response. Inadequate management of adaptive pain may alter brain and spinal cord function leading to maladaptive pain.
Maladaptive pain does not have protective properties. The longer the duration of pain, the more likely it is to switch to maladaptive pain. Pathologic pain that stops serving a protective function becomes maladaptive.
What are negative consequences of untreated pain?
- Prolonged mechanical ventilation
- Hemodynamic instability
- Delirium
- Depressed immunity and infections
Describe the structure and function of C fibers.
- Small, unmyelinated
- Conduction speed 0.5-2 ms
- Large receptive field compared to Aδ fibers
- Responsible for pathologic pain, slow pain, or "second pain" (poorly localized, dull/aching/burning sensation that persists despite termination of the noxious stimulus)
- Considered polymodal - can be activated by mechanical, thermal or chemical stimuli
What are the systemic responses to pain?
Pain increases sympathetic tone and causes catecholamine release leading to tachycardia, vasoconstriction, decreased gastrointestinal blood flow, decreased bladder tone, and increased skeletal muscle tone. Pain induced tachycardia causes an increase in myocardial work and oxygen demand.
Pain activates RAAS and induces secretion of cortisol, glucagon, ADH, growth hormone, IL-1, and decreased insulin secretion. A catabolic state characterized by hyperglycemia, proteolysis, lipolysis, Na and water retention, and decreased GFR is created.
Catabolism causes decreased wound healing and immune response.
Describe how to perform an intercostal nerve block.
- Intercostal nerve lies immediately caudal to the rib in close proximity to the intercostal vessels
- Most often achieved by blocking 4 or 5 consecutive nerves
- Patient in lateral recumbency
- Clip fur and aseptically prepare skin
- Palpate the rib as far dorsally as possible to allow the nerve to be blocked proximally
- 1.5 inch spinal needle is advanced through the skin on the lateral aspect of the rib
- Needle is walked caudally off the rib just until it can be advanced medially just caudal to the rib
- Caution should be used not to direct the needle beyond the depth of the rib and enter the pleural space
- Stylet is removed and the needle hub is inspected for blood
- Aspirate the syringe to ensure the needle is not placed intravascularly
- Slowly inject the desired volume - ensure there is no resistance to injection
- Withdraw the needle and syringe together