Pain management start to finish (Proceedings)

Understanding pain

Terminology

• Nociception: the process by which signals generated by the stimulation of nociceptors are transmitted from the site of injury to the central nervous system and ultimately results in the perception of pain by the brain.

• Nociceptors: a receptor for noxious stimuli

• Analgesia-the absence of pain without loss of consciousness

• Allodynia-a painful response to a normally non-painful stimulus

• Antagonistic-blocking of the receptor

• Agonistic-enhancing of the receptor

The pain pathway-a 3 neuron-chain

• 1st order-at the site of injury or pain

• 2nd order-ascending the spinal cord

• 3rd order-projecting into the cerebral cortex

2 distinct nociceptors

• A-delta nerve fibers-conduct pain quickly, known as “first pain”, this is the sharp pain of the initial injury

• C-fibers transmit pain slowly, known as “second pain”, this is the dull, throbbing pain following the painful stimulus

• These nociceptors exist throughout the body including deep and superficial tissues.

The nociceptive process

This is the process of pain recognition within the cerebral cortex and what happens to the noxious stimuli as it travels along the pain pathway-

We start with transduction which takes place at the site of injury-the physical energy is picked up by the nociceptors and translated into an electrical impulse. These nociceptors exist as a-delta also known as fast fibers and responsible for that first sharp pain and c fibers also known as slow fibers responsible for that secondary dull throbbing pain.

Transmission carries the impulse from the site to the spinal cord where modulation takes place.

Here, within the spinal cord there are endogenous systems to inhibit pain. we have the ability to ease our own pain to some degree thru opioid, serotonergic, and noradrenergic systems

That's why when we are injured that first initial excruciating pain seems to dull and lessen in a short period of time

From the spinal cord, the impulse then reaches the cerebral cortex and the pain is recognized by the brain.

Knowing this nociceptive process is the key to unlocking how we can prevent and control the pain response.

Recognizing pain

Physiologic signs of pain-increased blood pressure, heart rate, peripheral vasoconstriction (pale mucous membranes), and increased respiration.

Behavioral signs of pain-vocalization, aggression, hiding, hunched posture, guarding the site of injury, licking, chewing or biting at the site of injury.

Important point

The pain response can be detrimental to any patient, but especially a problem for older patients with marginally compensated organ function.  The stress of pain can result in decompensation during the procedure, during recovery or in the days immediately after the procedure._

Pain management strategies

• Preemptive Analgesia-the use of analgesics before the patient is exposed to pain.
• This includes the use of preanesthetic medication such as opioids and nsaids.

The multi-modal approach

• Defined as two or more analgesic drug classes used simultaneously to manage pain.

• The pain impulse transmission can be interrupted at each point of the pain pathway.

• At the site of Transduction (peripheral site of injury)-

Local anesthetics

• Opioids

• NSAIDS

• At the site of Transmission (to inhibit pain impulse conduction)

• Local anesthetics

• alpha 2 agonists

• At the site of Modulation (inhibit central pain)

• Local anesthetics

• Opioids

• alpha 2 agonists

• NSAIDS

• To inhibit Perception:

• Inhalant anesthetics (does not inhibit pain impulse, only the perception of pain)

• Opioids

• Alpha 2 agonists

A list of commonly used drugs by class

Opioids.  These mimic our own endogenous system responsible for modulating nociception, in simple terms easing the pain..  They are considered the big guns in pain relief.  There are subclasses of opioids that bind with specific receptors found within the synapse.  The mu receptors are responsible for the most profound analgesia.  Pure mu agonists-remember agonists enhance the receptor, provide even more analgesia at the site of those receptors. Within the pure mu agonist class are drugs like morphine, hydromorphone and fentanyl.

Opioid antagonists block those receptors, Naloxone blocks the receptor and prevents opioids from binding with it.  This is the reversal action of this drug. 

Bupenorphine is classified as a partial opioid agonist and bind at the mu receptors but only partially activates them, still very effective and has a relatively long duration lasting anywhere from 4-8 hours.

Mixed agonist-antagonists like butorphanol are less effective on severe pain. They enhance some receptors while blocking others.  Butorphanol also has a much shorter duration of efficacy.  We typically don't use this drug for pain, but do use it if we need more sedation for those special patients.

For our regional nerve blocks we use bupivicaine and lidocaine-these are the local anesthetics most used in dentistry.  We'll discuss the use of these drugs more in depth in the next lecture.

Alpha 2 agonists like dexmedetomidine bind with the alpha 2 receptors and when used in combination with an opioid, have a synergistic effect and provide tremendous pain relief and sedation.  Use caution when using these combinations because of cardiopulmonary side effects.  I have used this combination in general practice for routine surgeries young spays and neuters with great success especially for those puppies that come in that are very energetic and getting a catheter placed can be challenging.  It also makes for an incredibly smooth and quiet recovery for these patients.    But for our dental patients who are usually senior patients and have underlying heart issues we typically don't use this combination.

NSAIDS like meloxicam and carprofen were used primarily for chronic pain, but their use as an analgesic in peri and postoperative pain is on the rise and becoming commonplace usage. When used with an opiate there is a synergistic effect on pain.  These of course are contraindicated in those patients with liver or kidney issues.

Adjunctive analgesics are drugs that are primarily used in a  capacity other than pain management, but may be helpful in certain painful situations.  Antidepressants, anticonvulsants and corticosteroids are just some of the drugs in this class.  The primary type of drug we will be discussing is the NMDA antagonist.  Ketamine and amantodine are examples of an NMDA antagonist.  I'll go into the use of this drug more in depth later in the lecture.

The wind-up phenomenon

Also known as central sensitization.  A condition in which spinal neurons become hypersensitized after receiving repeated and prolonged pain impulses.  Consequently, the pain threshold is altered, becoming significantly lower.  Pain impulses that prior to this condition did not trigger the physiological pain response, now cause the pain response._  This is also known as allodynia.  Preemptive and Multimodal pain management will prevent this phenomenon from occurring.

Ketamine is the most commonly used drug to prevent and/or reverse the wind-up phenomenon. Ketamine is an NMDA receptor antagonist .  NMDA (N-methyl-D-aspartate) receptors are present within the spinal cord and the brain and become hypersensitized after prolonged painful stimulus.  By blocking these receptors, reduces the excessively painful responses.   The use of ketamine alone does not provide an adequate level of  analgesia, however when used in conjunction with an opioid in a constant rate infusion provides an optimum level of analgesia._

Constant rate infusion

This is a useful technique that can have profound effects on the management of severe and chronic pain states.  Although it is an extremely effective tool it is not used as readily as it could be in the general practice.  Calculating correct dosing and dilution can be intimidating and time consuming.  The Veterinary Anesthesia Support Group (vasg.org) website created by Dr. Bob Stein becomes an invaluable aide in this task.  This website not only contains extensive information regarding the physiology and management of pain, but also spreadsheets that are interactive with the user in calculating drug dosages for syringe infusion and constant rate infusion of analgesic agents. 

In providing the correct combination of analgesics, as well as, the right timing of these drugs, pain management becomes exponentially more effective.  Ultimately, the benefits to the patient are numerous especially with regards to recovery.   Quality patient care must include an effective and comprehensive pain management protocol.

References

1 Holmstrom, S.E., Veterinary Dentistry for the Technician and Office Staff

April 1999, 73-74_2 Paddleford, R.  Manual of Small Animal Anesthesia 1999, 227-229_3 Stein, B., Thompson S.  Analgesic Constant Rate Infusions  October 2005_