Pain Assessment and Multimodal Analgesia

Why Pain Assessment Matters in Veterinary Practice

Animals cannot describe their pain in words, so the veterinary team must detect and quantify it through careful observation. Unrecognized or undertreated pain slows healing, increases stress hormones, suppresses the immune system, and reduces patient welfare. As a veterinary assistant or technician, you are often the person watching the patient most closely—your accurate pain assessments directly affect the care the supervising veterinarian can provide.

Remember: assessing pain is within your scope; deciding on analgesic treatment and administering prescription drugs is the veterinarian's responsibility. Always report your findings promptly and document them accurately.

The Physiology of Acute Pain

Acute pain begins with nociception—the process by which tissue damage activates specialized receptors called nociceptors. The signal travels through peripheral nerves to the spinal cord and then to the brain, where it is interpreted as pain. Four steps summarize the pathway:

1. Transduction – tissue injury converts a mechanical, thermal, or chemical stimulus into an electrical signal at the nociceptor.

2. Transmission – the signal travels along A-delta and C fibers toward the spinal cord.

3. Modulation – the signal is amplified or dampened in the dorsal horn of the spinal cord. Repeated stimulation can cause wind-up, a state of spinal cord sensitization that makes subsequent pain harder to control.

4. Perception – the brain registers and responds to the signal.

Wind-up is a key reason that preemptive analgesia—giving pain medication before the painful stimulus occurs—is standard in modern veterinary anesthesia. It is much easier to prevent pain from becoming established than to reverse it once wind-up has begun.

Validated Pain Scales

Subjective guessing is unreliable. Validated pain scales give you a structured, repeatable method to score pain and track it over time. Two scales you will use in this course are:

• Glasgow Composite Measure Pain Scale – Short Form (CMPS-SF): Designed for dogs. Evaluates six behavioral categories (vocalization, attention to wound, mobility, response to touch, demeanor, posture/activity) with weighted scores. A score at or above the threshold value indicates intervention is needed; that threshold is noted on the scale itself.
• Colorado State University (CSU) Pain Scale: Available for dogs and cats separately. Uses a 0–4 numeric scale combined with behavioral descriptors and body-tension assessment. Visual and descriptive cues help even newer staff apply it consistently.

When using either scale, observe the animal before approaching, then assess response to gentle palpation near the surgical site. Document the score, the time, and your initials. Reassess at regular intervals as directed by the veterinarian or hospital protocol.

The Concept of Multimodal Analgesia

No single drug perfectly blocks every step of the nociceptive pathway. Multimodal analgesia combines agents from different drug classes, each targeting a different mechanism, so that lower doses of each drug can be used. Lower doses reduce the risk of adverse effects while providing superior pain control compared with a single high-dose drug. A typical plan includes agents given before surgery (preemptive), during surgery (intraoperative), and after surgery (postoperative).

Classes of Analgesic Agents

| Class | Primary Mechanism | Common Example |

|---|---|---|

| Opioids | Bind mu (and other) opioid receptors in the CNS and periphery to block pain transmission | Hydromorphone, morphine, buprenorphine |

| NSAIDs | Inhibit cyclooxygenase (COX) enzymes, reducing prostaglandin synthesis at the site of injury | Meloxicam, carprofen |

| Local anesthetics | Block voltage-gated sodium channels, preventing nerve conduction at the site of administration | Lidocaine, bupivacaine |

| Alpha-2 agonists | Activate alpha-2 adrenergic receptors in the CNS and spinal cord, producing sedation and analgesia | Dexmedetomidine |

| NMDA antagonists | Block N-methyl-D-aspartate receptors in the spinal cord, reducing wind-up and central sensitization | Ketamine (at sub-anesthetic doses) |

Each class has specific contraindications and side-effect profiles. NSAIDs, for example, require caution in patients with renal disease or hypotension because they reduce renal prostaglandins that support blood flow. Opioids can cause respiratory depression at higher doses. Always consult Plumb's Veterinary Drug Handbook and confirm with the supervising veterinarian before any drug is drawn up or administered.

Building a Multimodal Plan: Your Role

As a veterinary assistant or technician student, you may be asked to draft a proposed analgesic plan for the veterinarian's review. A simple framework for a routine spay might look like this:

• Pre-operative: An opioid (for preemptive analgesia) combined with an alpha-2 agonist (for sedation and analgesia).
• Intraoperative: A local anesthetic line or splash block at the incision site; low-dose ketamine added to the maintenance protocol to limit wind-up.
• Postoperative: An NSAID administered once the patient is stable and well-hydrated; continued opioid dosing as needed based on pain scores.

This plan is a communication tool for the veterinarian, not an independent order. Document your proposed rationale, present it clearly, and implement only what the veterinarian approves and directs.

Safety Points

• Never administer or adjust analgesic drugs without a direct veterinarian order.
• Reassess pain scores at the intervals prescribed; a rising score may signal a complication such as internal bleeding or wound dehiscence.
• Observe for signs of opioid over-sedation: extreme obtundation, slow or irregular breathing, pinpoint pupils (in cats). Report immediately.
• Record every pain score, time, and any behavioral change accurately in the medical record.