Nerve Stimulator and Ultrasound Guidance for Regional Anesthesia in Small Animals

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Complementary Modalities: Peripheral nerve stimulation (PNS) and ultrasound (US) guidance are complementary techniques for regional anesthesia in small animals, with PNS identifying nerve proximity via motor twitch and US providing real-time visualization of nerve, needle, and injectate spread.
• Ultrasound Superiority for Visualization: Ultrasound offers direct visualization of local anesthetic distribution, confirming correct tissue plane placement and reducing risks of intramuscular, intravascular, or intraneural injection, whereas PNS provides indirect confirmation of spread.
• Nerve Stimulator for Deep/Difficult Blocks: Nerve stimulation is particularly valuable for deep blocks, obese patients, or when ultrasound visualization is compromised, offering a functional confirmation of proximity to motor nerves.
• Combined Approach Enhances Safety: Combining PNS with US, often using an echogenic needle, provides a dual confirmation of needle placement and injectate spread, with PNS acting as a safety check for intraneural placement at low currents (<0.2 mA).
• Local Anesthetic Systemic Toxicity (LAST) Mitigation: Ultrasound guidance significantly reduces the risk of LAST by enabling direct visualization to avoid intravascular injection, but requires careful dose calculation, incremental injection, and aspiration, especially in geriatric patients with reduced clearance.
• Needle Visibility is Critical: Echogenic needles and maintaining a shallow insertion angle relative to the ultrasound beam are crucial for optimal needle tip visualization, reducing the risk of losing the needle tip during advancement and improving block success.

Regional anesthesia in dogs and cats has moved from a niche skill to a core component of perioperative analgesic planning. The two dominant techniques for locating nerves and confirming injectate placement are peripheral nerve stimulation and ultrasound guidance. This article compares these modalities for the practicing small animal veterinarian, covering their underlying principles, practical execution, limitations, and the evidence supporting each approach. It addresses the clinical question of when to choose one technique over the other, how to combine them, and how to recognize and avoid the common failure modes that compromise block quality or patient safety.

The intended reader is a veterinarian already comfortable with basic anesthetic monitoring who wishes to incorporate or refine regional anesthetic techniques. The article assumes familiarity with local anesthetic pharmacology and standard aseptic preparation. It does not provide drug doses, as current formulary and label references must be consulted for each agent and patient.

At a Glance

Physiology and Technique Principles

How Nerve Stimulation Locates Nerves

Peripheral nerve stimulation relies on the principle that an electrical current delivered through an insulated needle tip depolarizes motor axons when the tip is sufficiently close to the nerve. The evoked motor twitch indicates proximity. As the current is reduced, the stimulating field shrinks, and a twitch that persists at lower currents (typically below 0.5 mA) suggests the needle tip lies within or immediately adjacent to the nerve sheath. The relationship between current amplitude and distance is not linear, and the optimal threshold current for safe injection remains debated in the veterinary literature. The technique provides no information about the distribution of injected local anesthetic, and abolition of the twitch during injection is an indirect and unreliable indicator of successful spread.

Motor responses are specific to the nerve or plexus being targeted. For example, the radial nerve produces extension of the elbow and carpus, while the sciatic nerve produces flexion of the stifle and extension of the tarsus. The clinician must know the expected motor response for each target and adjust needle position until the appropriate twitch is obtained. A response at currents above 0.5 mA may indicate the needle is too far from the nerve to guarantee block success, while a response only at currents below 0.2 mA raises concern for intraneural placement.

How Ultrasound Visualizes Nerves and Injectate

Ultrasound guidance uses real-time imaging to identify the target nerve, surrounding vessels, fascia, and muscle planes. Nerves appear as hypoechoic rounded or oval structures with internal hyperechoic dots, the fascicular pattern, though this appearance varies with the nerve, the transducer frequency, and the angle of insonation. High-frequency linear transducers, typically 10 to 18 MHz in small animals, provide the resolution needed for superficial nerves. Deeper structures such as the brachial plexus roots or the sciatic nerve at the level of the greater trochanter may require lower frequencies with a consequent loss of resolution.

The needle is advanced under direct vision, either in-plane, where the entire needle shaft and tip are visualized alongside the ultrasound beam, or out-of-plane, where only a cross-section of the needle is seen. In-plane approaches are generally preferred because they allow continuous visualization of the needle tip, which is the critical structure for avoiding vascular or neural injury. The spread of local anesthetic is then observed as a hypoechoic fluid pocket expanding around the nerve. Direct visualization of spread is the principal advantage of ultrasound over nerve stimulation, as it confirms that the injectate is in the correct tissue plane instead of intramuscular, intravascular, or intraneural.

Needle Selection and Visibility

Needle visibility under ultrasound is a limiting factor for block success. A prospective study of twelve commercially available nerve block needles found that visibility varied significantly with needle design, the angle of insertion relative to the ultrasound beam, and the surrounding medium Ultrasound characteriztics of needles for regional anesthesia. Visibility was best when the needle was parallel to the transducer face and deteriorated as the angle increased. Echogenic needles, which have textured or dimpled surfaces that reflect more ultrasound energy, improve tip identification. A comparison study in Thiel cadavers found that one echogenic needle design was significantly more visible than a standard needle, particularly for out-of-plane insertions Echogenic regional anesthesia needles: a comparison study in Thiel cadavers. For veterinary practice, selecting an echogenic needle and keeping the insertion angle as shallow as tissue depth permits will reduce the risk of losing the tip during advancement.

Combining Nerve Stimulation and Ultrasound

The two techniques are complementary instead of mutually exclusive. Many veterinary anesthesiologists use a nerve stimulator connected to an echogenic needle while imaging with ultrasound. The stimulator provides a functional confirmation that the needle is near the target nerve, while ultrasound confirms the needle tip location and the spread of injectate. This dual approach is particularly useful for deep blocks where ultrasound resolution is poor, or in patients where the nerve is difficult to identify sonographically due to obesity or prior trauma. The combination also provides a safety check: if the stimulator produces a twitch at very low current while ultrasound shows the needle tip within the nerve fascicle, the needle should be withdrawn before injection.

The veterinary literature supports the increasing use of objective nerve location methods, with nerve stimulation and ultrasound both contributing to the precision of regional techniques Regional anesthetic techniques for the thoracic limb and thorax in small animals. However, outcome comparisons between techniques remain limited, and large-scale clinical studies are still needed to determine whether one modality confers superior analgesia or fewer complications.

Safety Considerations

Local Anesthetic Systemic Toxicity

Ultrasound guidance reduces, but does not eliminate, the risk of local anesthetic systemic toxicity. Direct visualization of the needle tip and injectate spread helps prevent intravascular injection, which is a leading cause of toxicity. The risk is not uniform across patient populations. Geriatric patients are at disproportionately increased risk due to comorbidities and reduced muscle mass, and inadvertent overdosing has been identified in case reports involving older human patients Local Anesthetic Toxicity in the Geriatric Population. The same principle applies to older dogs and cats, where reduced hepatic metabolism and decreased cardiac reserve amplify the consequences of an overdose. Ultrasound guidance should be combined with careful dose calculation, incremental injection, and aspiration before each aliquot.

Ultrasound Bioeffects

Diagnostic ultrasound is generally safe, but the energy delivered to tissue is not zero. Thermal and mechanical effects have been demonstrated in animal studies, and regulatory agencies have set output limits based on the thermal index and mechanical index displayed on the machine Potential adverse ultrasound-related biological effects: a critical review. For regional anesthesia procedures, which are typically brief, the risk is low. The clinician should still use the lowest output settings that provide adequate image quality and should avoid prolonged imaging of a single site, particularly over sensitive structures such as the eye or developing bone.

Anatomical Considerations in Dogs and Cats

The sonographic appearance of nerves differs between species and between individual patients. In dogs, the brachial plexus is located deep to the scalenus muscles and can be imaged from a lateral approach caudal to the scapula. The sciatic nerve is best imaged at the level of the greater trochanter, where it lies deep to the biceps femoris muscle. In cats, the smaller body size allows higher frequency transducers and correspondingly better resolution, but the reduced tissue depth means that the needle tip can pass through the target plane quickly. The transversus abdominis plane block, which targets the thoracolumbar spinal nerve branches within the abdominal wall, has been described in dogs using ultrasound guidance, with cadaveric studies showing reliable staining of the T11 through L3 segmental branches Ultrasound-guided transversus abdominis plane block in the dog: an anatomical evaluation. This block illustrates the value of ultrasound for fascial plane techniques, where the target is a tissue plane instead of a discrete nerve and nerve stimulation is not applicable.

Limitations and Failure Modes

Nerve stimulation fails when the nerve is not motor, when the patient is under deep neuromuscular blockade, or when the nerve has been damaged by trauma or disease. It also provides no information about the spread of injectate, so a block can fail despite a satisfactory twitch response. Ultrasound fails when the target cannot be identified, when the needle tip is lost from the imaging plane, or when the injectate spreads in an unexpected direction due to fascial barriers. The clinician should have a predefined rescue strategy for each block, including the option to switch modalities or abandon the block in favor of systemic analgesia.

Equipment Selection and Setup

The choice between nerve stimulator, ultrasound, or a combined approach depends on available equipment, patient size, and the specific block performed. Ultrasound guidance alone is now considered the standard for most peripheral nerve blocks in small animals, with nerve stimulation reserved for specific indications. When both modalities are available, combined use can improve accuracy, particularly for deep blocks such as the lumbar plexus or sacral plexus where ultrasound visualization is challenging.

Nerve Stimulator Configuration

A peripheral nerve stimulator delivers a short-duration electrical impulse through an insulated needle. The stimulating needle must be insulated along its shaft with only the bevel exposed to ensure current density remains high at the tip. Set the initial current at 0.5 to 1.0 mA with a pulse duration of 0.1 ms for motor nerve location. Reduce the current incrementally as the needle approaches the nerve. A motor response persisting at 0.3 to 0.5 mA indicates close proximity to the nerve without intraneural placement. Responses below 0.2 mA suggest intraneural needle position and mandate needle withdrawal before injection.

The type of motor response observed provides information about which nerve component is stimulated. For mixed nerves, a response at lower current suggests closer proximity to the nerve trunk. Twitches of the appropriate muscle group confirm correct needle placement. For sensory nerves without a motor component, such as the saphenous nerve, nerve stimulation is unreliable and ultrasound guidance is preferred.

Ultrasound Machine Settings

A high-frequency linear transducer (10 to 18 MHz) is appropriate for most superficial blocks in dogs and cats. A microconvex transducer (8 to 10 MHz) improves access for deep blocks such as the brachial plexus or lumbar plexus in larger dogs. Set the depth so the target nerve and surrounding landmarks occupy the central third of the image. Adjust gain to optimize nerve echogenicity while minimizing artifacts from adjacent fascia.

Needle visibility depends on the angle between the needle and the ultrasound beam. Needles are best visualized when inserted at angles of 30 to 45 degrees to the transducer face. Steeper angles reduce visibility because the ultrasound beam reflects away from the transducer. Echogenic needles, which have textured surfaces or etched patterns, improve visibility at steeper angles and in out-of-plane approaches. In-plane approaches allow continuous visualization of the needle shaft and tip, while out-of-plane approaches show only a cross-section of the needle, making tip localization more difficult.

Step-by-Step Approach for Common Blocks

Brachial Plexus Block

Position the patient in dorsal recumbency with the thoracic limb abducted 45 to 90 degrees. Place the transducer in a transverse plane over the axillary region, just caudal to the scapulohumeral joint. Identify the axillary artery and vein as anechoic circular structures. The brachial plexus nerves appear as hypoechoic oval or round structures clustered around the vessels.

Insert the needle in-plane from the cranial or caudal aspect of the transducer. Advance the needle tip to the perineural space adjacent to the nerve bundle. Inject a small test volume of local anesthetic and observe for hydrodissection of the fascial plane. Continue injection while monitoring for appropriate spread around all nerve components. A single injection that surrounds the entire plexus is acceptable, but separate injections around individual nerve branches may improve block completeness.

Transversus Abdominis Plane Block

The transversus abdominis plane block provides analgesia to the abdominal wall. Place the transducer in a transverse plane over the lateral abdominal wall, caudal to the last rib. Identify the three abdominal muscle layers: external abdominal oblique, internal abdominal oblique, and transversus abdominis. The target plane lies between the internal abdominal oblique and transversus abdominis muscles.

Advance the needle in-plane from a dorsal approach. The needle tip should rest within the fascial plane between these two muscles. Injection produces anechoic fluid spread that separates the muscle layers. Cadaveric evaluation demonstrates that a single injection at this location stains the segmental nerves from T11 through L3, with the most consistent staining of T12 through L2. The block is therefore most reliable for mid-abdominal procedures, with less consistent coverage of the cranial and caudal abdominal extremes.

Sacral and Lumbosacral Blocks

These blocks are technically demanding and benefit from combined ultrasound and nerve stimulator guidance. Place the transducer in a sagittal or transverse plane over the lumbosacral junction. Identify the sacral promontory and the seventh lumbar vertebra. The sacral nerve roots are difficult to visualize directly, so nerve stimulation provides confirmation of needle placement.

Use the nerve stimulator at 0.5 mA and observe for contraction of the anal sphincter, tail, or pelvic limb musculature. Ultrasound confirms the needle tip position relative to the vertebral canal and vascular structures. This combined approach reduces the risk of unintended epidural or intrathecal injection.

Monitoring Parameters During Block Performance

Continuous monitoring during block placement detects complications before they become life-threatening. The following parameters should be assessed throughout the procedure:

Injection pressure monitoring deserves particular emphasis. High resistance to injection suggests the needle tip is within a nerve fascicle or dense connective tissue. Stop injection immediately and withdraw the needle 1 to 2 mm before attempting further injection. Low resistance with visible fluid spread on ultrasound confirms correct needle placement.

Patient-Specific Considerations

Geriatric patients present increased risk for local anesthetic systemic toxicity due to reduced cardiac reserve, decreased hepatic and renal clearance, and lower lean muscle mass. Reduce total local anesthetic volume and dose in these patients, and consider using lower concentrations of drug. Monitor cardiovascular parameters closely during and after injection.

Body condition affects block performance. Obese patients have increased distance from skin to target nerves, making ultrasound penetration and needle manipulation more difficult. The increased tissue depth may require lower frequency transducers and longer needles. Cachectic patients have reduced echogenicity of perineural fat, which can make nerve identification more challenging.

Brachycephalic breeds have altered anatomical landmarks in the cervical and thoracic regions. The brachial plexus may lie more cranially than in mesaticephalic breeds. Confirm the location of the scapula and first rib before needle insertion. Cats have smaller anatomical structures and require proportionally smaller needles and volumes. The brachial plexus in cats is located more cranially than in dogs, and the nerves are smaller and more closely packed.

Documentation and Record Keeping

Document the block procedure in the anesthetic record with sufficient detail to support clinical decision-making and medicolegal review. Record the following elements: patient identification, block performed, guidance method used, needle type and gauge, local anesthetic drug and volume, number of injection attempts, motor response threshold if nerve stimulation was used, ultrasound findings including nerve appearance and injectate spread, complications encountered, and the time of block performance relative to surgery.

Photographic or video capture of the ultrasound image before and after injection provides objective evidence of correct needle placement and injectate distribution. Store these images with the patient record when the practice's imaging system permits. Document any deviation from the planned approach and the reason for the deviation.

The AAHA anesthesia and monitoring guidelines recommend continuous patient assessment throughout the perianesthetic period, and block documentation forms part of this requirement. The WSAVA Global Pain Council guidelines similarly emphasize the importance of recording analgesic interventions and their effects. Consistent documentation supports outcome assessment and quality improvement within the practice.

Recognized Complications and Early Detection

The most serious complication of regional anesthesia is local anesthetic systemic toxicity (LAST). Early signs in dogs and cats include perioral paresthesia, agitation, muscle twitching, and progressive central nervous system excitation that may precede cardiovascular collapse. Geriatric patients carry disproportionate risk because of reduced lean muscle mass and concurrent cardiac or hepatic disease, and inadvertent overdosing accounts for a substantial share of reported toxicity cases in this population local anesthetic toxicity in the geriatric population. Detection depends on continuous electrocardiography, capnography, and pulse oximetry during injection, with fractionated dosing and frequent aspiration to minimize intravascular delivery. Ultrasound guidance reduces but does not eliminate LAST, so lipid emulsion and resuscitation drugs must be immediately available before any block is performed.

Vascular puncture is the most common mechanical complication. Ultrasound reveals the needle tip approaching a vessel in real time, whereas nerve stimulation provides no vascular feedback. When blood is aspirated, the needle should be redirected and the syringe changed. Intraneural injection produces a sustained motor response at low current thresholds (below 0.3 mA) or visible fascicular swelling on ultrasound. Early detection requires stopping advancement the moment a motor response appears below 0.5 mA and repositioning before injection. Pneumothorax is a recognized risk for brachial plexus and intercostal approaches, post-block thoracic auscultation and, where available, thoracic ultrasound or radiography are warranted when respiratory effort changes.

Common Errors and Corrective Actions

Less experienced operators frequently mistake fascial planes for nerves. Fascia appears hyperechoic and linear, but nerves are typically more heterogeneous with a honeycomb or stippled internal pattern. The corrective action is to rotate the transducer to trace the structure over several centimeters and confirm that it enters a nerve plexus instead of terminating as a flat sheet. Another frequent error is advancing the needle without visualizing the tip, relying instead on tissue movement or patient response. The needle must be seen continuously, if the tip is lost, the transducer should be rocked or the needle withdrawn slightly until the tip reappears ultrasound characteriztics of needles for regional anesthesia.

Injecting when the needle bevel is against the nerve or within the epineurium produces high injection pressure and risks fascicular damage. Watching for injectate spread that tracks around instead of through the nerve is the discriminating check. If injectate does not surround the nerve, the needle should be repositioned. A third common error is using an out-of-plane approach without sufficient experience, which makes tip localization difficult and increases the risk of unintended puncture. Novice operators should prefer in-plane approaches for deep blocks and reserve out-of-plane techniques for superficial sites where the needle path is short and the anatomy is forgiving.

Troubleshooting Table

Evidence Limitations and Expert Disagreement

The veterinary literature on regional anesthesia relies heavily on cadaveric anatomical studies and small clinical case series. For example, the ultrasound-guided transversus abdominis plane block in dogs shows reliable staining of T12 through L2 in cadavers, but the clinical analgesic effect in live animals has not been established with large prospective trials ultrasound-guided transversus abdominis plane block in the dog. Experts disagree on whether motor response thresholds should guide injection when ultrasound confirms needle tip position, and some clinicians advocate abandoning nerve stimulation entirely for superficial blocks. Others maintain that stimulation provides a useful safety check for deep blocks where needle tip visualization is difficult. The comparative efficacy of different block techniques for postoperative analgesia also remains unresolved, with no consensus on which approach provides superior outcomes for specific surgical procedures regional anesthetic techniques for the thoracic limb and thorax.

Referral, Consultation, and Reporting

Referral to a veterinary anesthesiologist or specialist center is appropriate when a patient has coagulopathy, severe cardiac disease, or anatomical distortion from trauma or prior surgery that makes landmark identification unreliable. Consultation with a specialist is also warranted when a block fails repeatedly despite correct technique, because alternative approaches or imaging modalities may be needed. Laboratory involvement is indicated when LAST is suspected, for measurement of serum electrolytes, lactate, and cardiac biomarkers, and when coagulopathy is identified before a planned block. Regulatory reporting obligations vary by jurisdiction. Adverse events involving drug errors, unanticipated death, or suspected product defects should be reported to the relevant national pharmacovigilance authority and the drug manufacturer. Professional liability insurers and veterinary licensing bodies may also require notification of serious complications, and the American Veterinary Medical Association practice resources provide guidance on incident documentation and disclosure.

Frequently Asked Questions

How do I choose between nerve stimulation and ultrasound when I only have access to one modality?

Ultrasound alone is preferred when available because it provides real-time visualization of the needle, the target nerve, and the injectate spread. Nerve stimulation alone remains a reasonable alternative when ultrasound is unavailable, particularly for deep blocks where surface landmarks are unreliable. When using stimulation alone, maintain the current above 0.5 mA and observe the appropriate muscle twitch before injecting. When using ultrasound alone, rely on hydrodissection and direct visualization of perineural spread instead of motor response. The combined approach is ideal but not mandatory. Ultrasound-guided regional anesthesia techniques in small animals describes both modalities as complementary tools that increase procedural objectivity and precision.

What should I do when the ideal block needle is not available?

Standard hypodermic needles can be used for ultrasound-guided blocks, but visibility is reduced, especially at steep angles. Advance the needle parallel to the transducer face, use a shallow angle of 30 to 45 degrees, and inject small volumes of saline to confirm tip location before depositing local anesthetic. Echogenic needles improve visibility at steeper angles and in out-of-plane approaches, as demonstrated in comparative studies of echogenic regional anesthesia needles. Needle visibility also depends on the surrounding medium and the ultrasound machine used, so test the needle in a gel phantom or water bath before the procedure. Ultrasound characteriztics of needles for regional anesthesia confirms that needle design and insertion angle significantly affect ultrasonic appearance.

How does the approach differ in cats compared with dogs?

Cats have smaller anatomical targets, less perineural fat, and a higher risk of local anesthetic systemic toxicity relative to body mass. Use higher frequency transducers, typically 15 to 18 MHz, and smaller gauge needles. Reduce injectate volumes proportionally and aspirate before each injection. The transversus abdominis plane block has been anatomically validated in dogs, but feline-specific spread data are limited, so verify injectate distribution under ultrasound in real time. Ultrasound-guided transversus abdominis plane block in the dog documents segmental nerve staining patterns that may not transfer directly to cats. Monitor cats closely for signs of systemic toxicity because their smaller body mass amplifies the risk described in local anesthetic toxicity reviews.

What documentation should I record after performing a regional block?

Record the block name, approach, needle type and gauge, guidance modality, local anesthetic drug and volume, nerve stimulator settings if used, number of attempts, and any vascular puncture or paresthesia. Document the patient's heart rate, respiratory rate, and blood pressure before, during, and after the procedure. Note the time of block performance and the time of return of motor and sensory function when assessable. Include a pain score using a validated scale at regular intervals. The AAHA anesthesia and monitoring guidelines emphasize that complete anesthetic records support patient safety and continuity of care. Photographs or video clips of the ultrasound image are useful for teaching and medicolegal purposes.

How do I explain the value of regional anesthesia to a client who is concerned about cost?

Frame regional anesthesia as a component of multimodal analgesia that can reduce the requirement for systemic opioids and inhalant anesthetics. Explain that better intraoperative stability and smoother recovery may shorten hospital stay and reduce the need for rescue analgesia. The WSAVA Global Pain Council guidelines support multimodal analgesic approaches as best practice for pain management in companion animals. Be honest that the block adds procedure time and requires specific equipment and training. Offer a written estimate that separates the block fee from the anesthetic monitoring fee. If the client declines, document the discussion and provide an alternative analgesic plan that still addresses expected pain.

What are the minimum monitoring standards during block performance?

Monitor heart rate, respiratory rate, pulse oximetry, and electrocardiography continuously during the procedure. Measure blood pressure at least every 5 minutes. Observe the patient for signs of local anesthetic systemic toxicity, including arrhythmias, hypotension, muscle twitching, or seizures. The AAHA anesthesia and monitoring guidelines recommend that a dedicated person monitor the patient throughout the anesthetic period. Have lipid emulsion and resuscitation drugs drawn up and immediately available before injecting local anesthetic. Continue monitoring for at least 30 minutes after the final injection, because systemic toxicity can present with delayed onset, particularly in patients with reduced cardiac output or hepatic clearance.

Related Clinical & Scientific Guides

• Anesthetic Machine Leak Testing and Pressure Checks: A Step-by-Step Protocol
• Anesthetic Depth Assessment: Reflexes, Eye Position, and Ventilation
• Anesthesia for Patients with Obesity: Challenges and Solutions

References and Further Reading

• Ultrasound characteriztics of needles for regional anesthesia.. 2007.
• Ultrasound-guided transversus abdominis plane block in the dog: an anatomical evaluation.. 2011.
• Local Anesthetic Toxicity in the Geriatric Population.. 2020.
• Echogenic regional anesthesia needles: a comparison study in Thiel cadavers.. 2012.
• Regional anesthetic techniques for the thoracic limb and thorax in small animals: A review of the literature and technique description.. 2018.
• Potential adverse ultrasound-related biological effects: a critical review.. 2011.
• AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. AAHA.
• WSAVA Global Pain Council Guidelines. WSAVA.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.

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• Anesthesia Machine Safety Features and Backup Systems

This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.