Pharmacology Every Technician Anesthetist Must Know

Introduction

Of all the clinical responsibilities a veterinary technician may carry, anesthesia monitoring is among the most consequential and pharmacologically demanding. A patient under general anesthesia cannot communicate discomfort or alert the team when something feels wrong. Changes in blood pressure, heart rate, respiratory pattern, or anesthetic depth must be recognized and interpreted by the technician monitoring the case.

That responsibility requires more than knowing how to read a monitor. Understanding how anesthetic drugs affect the patient helps technicians determine why values may be changing and what interventions may be appropriate when problems arise.

The 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats note that lack of monitoring increases the odds of anesthetic death by a factor of 5 to 35, and that the dedicated anesthetist is one of the most critical variables in reducing anesthetic-related complications.¹ In most general practices, that anesthetist is a veterinary technician. A strong foundation in pharmacology is therefore an essential part of safe anesthesia monitoring.

Understanding the Opioid Landscape

Opioids are commonly included in anesthetic protocols because of their role in managing perioperative pain. However, the level and duration of analgesia they provide can vary considerably between drugs. Understanding how different opioids interact with their receptors helps technicians recognize why one may be selected over another based on the patient, procedure, and expected level of pain.

Receptors in the central nervous system act as locks, and opioids are the keys that fit them, binding to mu, kappa, and delta receptors to alter the transmission and perception of pain.²,³ Full agonists fit and activate the receptor completely, partial agonists bind but only partially activate it, and agonist-antagonists stimulate one receptor type while blocking another.²,⁴ Full mu-agonists, such as morphine, hydromorphone, fentanyl, and methadone, produce the most potent analgesia and are the appropriate choice for patients undergoing moderate to severely painful procedures.¹

Buprenorphine is a partial mu-agonist where it binds tightly to the mu receptor but produces a reduced intensity response. ⁴ This makes it an excellent choice for mild to moderate pain and for long-duration management; however, it introduces a critical clinical consideration. Because buprenorphine has a high receptor relationship, if a full mu-agonist is needed later (for example, if a procedure turns out to be more painful than anticipated), its blocking effect may interfere with the efficacy of that rescue drug. ⁴ The AAHA guidelines recommend not using buprenorphine within eight hours of surgery if a full mu-agonist will be part of the anesthetic plan. ¹

Butorphanol works differently because it is an agonist-antagonist that triggers kappa receptors instead of mu receptors like a true agonist.² The AAHA guidelines list an effective analgesic window of roughly 20 to 60 minutes in dogs and about 90 minutes in cats, though dose and route can shift that window, making it an unreliable sole analgesic for a genuinely painful procedure.¹,²,⁴ It also provides effective mild analgesia and strong sedation, making it highly valuable in multi-modal protocols when combined with other medication classes.¹ A technician who understands this can recognize why butorphanol might appear in a premedication protocol for a patient undergoing radiographs but not for one heading into orthopedic surgery.

Fentanyl, by contrast, is the workhorse of intraoperative analgesia in higher-pain surgical cases. Its onset is within one to two minutes intravenously, with a relatively short duration of effect, and it is most delivered as a constant rate infusion (CRI) to maintain steady plasma levels without the peaks and troughs that intermittent bolus dosing produces. ¹, ² CRIs allow the anesthetist to titrate to effect, increasing or decreasing the infusion rate in response to the patient’s physiologic indicators of pain or depth of anesthesia. Technicians managing a fentanyl CRI need to understand that this drug decreases the minimum alveolar concentration (MAC) of the inhalant (meaning lower vaporizer settings are required to maintain the same anesthetic depth) and can contribute to bradycardia through Vago mimetic effects. ¹

Methadone is a full mu-opioid agonist that also blocks NMDA receptors. NMDA receptors are found in the central nervous system and help process nerve signals, including those involved in pain. ² With repeated or intense painful stimulation, these receptors can contribute to wind-up, where the nervous system becomes increasingly sensitive to pain. By blocking NMDA receptors, methadone can help reduce this pain amplification, making it useful for patients with significant existing pain or those undergoing painful procedures. ⁵ It also causes less histamine release than morphine, which can be beneficial for patients prone to hypotension. ⁵

Alpha-2 Agonists: Powerful, Reversible, and Misunderstood

Dexmedetomidine and medetomidine produce sedation, muscle relaxation, and analgesia by activating alpha-2 adrenergic receptors in the central nervous system. Activation of these receptors decreases the release of norepinephrine, reducing sympathetic nervous system activity and producing a calmer, more sedated patient. ¹, ² Their analgesic effects are also synergistic with opioids, allowing the combination to provide stronger pain control than either drug alone.¹,²

Alpha-2 agonists have a predictable, two-phase effect on the cardiovascular system. Initially, they cause vasoconstriction, which increases blood pressure. In response to this rise in blood pressure, the body slows the heart rate, resulting in reflex bradycardia. As the drug’s effects within the central nervous system become more prominent, sympathetic activity decreases. Blood pressure may then fall, while bradycardia and decreased cardiac output can persist. ²

A technician who does not understand this mechanism may interpret the bradycardia as a problem requiring anticholinergic treatment when, in fact, in an otherwise stable patient with normal blood pressure, this may represent a physiologically expected response that does not require intervention. That distinction, bradycardia occurring on its own versus alongside hypotension, is what drives the treatment decision.

Alpha-2 agonists are fully reversible with atipamezole, a significant clinical advantage for managing prolonged recoveries or for reversing excessive sedation. Atipamezole reverses the full range of the alpha-2 agonist’s clinical effects, both the detrimental ones like bradycardia and the beneficial ones like analgesia, so another analgesic agent should typically be given just before reversal to maintain pain control. ²

Induction Agents: Choosing for the Patient, Not the Protocol

Propofol and alfaxalone are two of the most used induction agents in small animal practice, alongside etomidate and injectable combinations such as ketamine or tiletamine-zolazepam. ¹ Both provide rapid, smooth induction, but their pharmacologic differences and the individual needs of the patient can influence which drug is selected.

Propofol causes dose-dependent cardiovascular depression, including bradycardia, decreased cardiac output, and vasodilation. ² These effects are generally transient and well tolerated in healthy patients but may be more significant in those with cardiovascular compromise or those that have already received premedication. Because propofol is highly protein-bound, it should also be used cautiously in patients with significant hypoalbuminemia. ² Cats metabolize propofol more slowly than dogs, increasing the risk of accumulation and delayed recovery with repeated doses or prolonged infusions. ² Propofol does not provide analgesia, so although the patient may be unconscious, pain is not being treated. Appropriate analgesics must therefore be incorporated into the anesthetic protocol when a painful procedure is anticipated. ¹

Alfaxalone has a cardiovascular profile similar to propofol, causing some vasodilation and negative inotropy, or decreased strength of cardiac contraction. These effects can contribute to lower blood pressure. In cats, the higher doses needed to maintain anesthesia may also increase the likelihood of excitement during recovery. ¹,² It is an excellent choice for fractious patients or those requiring IM induction, and its wider therapeutic index in cats makes it a preferred option in many feline protocols.

Etomidate is often selected for induction in hemodynamically unstable patients because it causes minimal cardiovascular depression compared with drugs such as propofol or alfaxalone. It also preserves the baroreceptor reflex, helping the body respond to changes in blood pressure. However, etomidate can temporarily suppress adrenal hormone production, an important consideration in critically ill patients. ¹,²

Ketamine is a dissociative anesthetic and NMDA receptor antagonist commonly incorporated into multimodal anesthetic protocols. ¹ Its NMDA-blocking activity provides an additional approach to managing central sensitization, making low, subanesthetic doses useful for analgesia alongside other pain medications. When administered as a CRI, ketamine can provide this benefit without relying on its dissociative anesthetic effects. ²

Multimodal Analgesia: The Standard, Not the Exception

Multimodal analgesia uses multiple drugs that act at different points in the pain pathway to provide greater overall pain relief while allowing lower doses of individual medications. ¹ Using medications with different mechanisms of action allows pain to be addressed at multiple points rather than relying on a single drug or drug class.

A multimodal pain protocol combines medications that control pain in different ways. This may include an NSAID or grapiprant, a local anesthetic block, an opioid, and additional drugs such as dexmedetomidine or ketamine. Each targets a different part of the pain pathway: anti-inflammatory drugs reduce inflammation and pain at the tissue level, local anesthetics block pain signals from traveling through the nerves, and opioids or alpha-2 agonists reduce how pain signals are processed in the central nervous system. ¹

For technicians, understanding how these drugs work together is an important part of managing anesthesia. Effective analgesia can decrease the amount of inhalant anesthetic needed for maintenance, which can also reduce dose-dependent adverse effects such as cardiovascular depression. ¹ When technicians understand why each drug is included in the protocol, they are better prepared to recognize when pain control may be inadequate, when a drug is producing an unexpected effect, or when the protocol may need to be adjusted for the individual patient.

Multimodal Analgesia Considerations for the 4 Phases of Anesthesia: ¹

The Monitor Tells You What; Pharmacology Tells You Why

Reading the monitor and recognizing an abnormal value is only part of anesthesia monitoring. A rising ETCO₂ may reflect hypoventilation from excessive anesthetic depth, ² a drop in blood pressure after induction may be related to propofol-induced vasodilation,² and an increasing heart rate during a tooth extraction may indicate inadequate locoregional anesthesia.² Understanding the pharmacology behind these changes allows technicians to recognize what may be happening and respond appropriately. That knowledge is what allows a veterinary technician to move beyond simply monitoring anesthesia and become an active part of patient management.

Kelsey Marriott, BS, AAS (Equine) AAS (Veterinary Technology), CVT, is the clinical director and an instructor at Pima Medical Institute’s East Valley campus. She has more than 12 years of experience in veterinary medicine, with a background spanning equine practice, mixed animal, general practice, and specialty medicine. Her work focuses on externship development, clinical training, and strengthening workforce readiness for veterinary technician students. She is actively involved in professional advocacy and serves as a director-at-large for the Arizona Veterinary Technician Association.

References

1. Grubb T, Sager J, Gaynor JS, et al. 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 2020;56(2):59–82.
2. Thomas JA, Lerche P. Anesthesia and Analgesia for Veterinary Technicians and Nurses. 6th ed. St. Louis, MO: Elsevier; 2024.
3. Merck Veterinary Manual. Analgesics used in animals. https://www.merckvetmanual.com/therapeutics/pain-assessment-and-management/analgesics-used-in-animals. Accessed June 2026.
4. dvm360. Understanding opioids in veterinary medicine. https://www.dvm360.com/view/understanding-opioids-in-veterinary-medicine. Accessed June 2026.
5. Tighe MM, Brown MP, eds. Mosby’s Comprehensive Review for Veterinary Technicians. 4th ed. St. Louis, MO: Elsevier.