Gianluca Bini, DVM MRCVS DACVAA

Maintaining anesthetic stability during veterinary procedures presents unique challenges due to the dynamic nature of anesthesia and individual patient variability. This lecture explores anesthetic agents, MAC sparing techniques, and CRI implementation strategies for enhanced intraoperative stability.

1. Anesthetic Agents Overview

Inhalation Agents

Isoflurane:

MAC: 1.3%
Blood:Gas Coefficient: 1.46 (slower induction/recovery)

Sevoflurane:

MAC: 2.3%
Blood:Gas Coefficient: 0.68 (faster induction/recovery)

Desflurane:

Newer, more expensive option.

Mechanism: Act on GABA receptors with no analgesia.
Monitoring: Essential due to significant cardiopulmonary depression.

Minimum Alveolar Concentration (MAC)

Definition: The minimum alveolar concentration required to prevent movement in response to a supramaximal stimulus in 50% of patients.
Implications: Lower MAC indicates higher potency.
Blood:Gas Coefficient: Lower coefficient = faster changes in anesthetic depth.

2. Adjusting Anesthetic Depth

Fresh Gas Flow Rate: Adjusting the flowrate on the flowmeter and the vaporizer settings influences the rate at which the desired concentration of anesthetic gas fills the circuit.
Typical Volumes:

Small Animal Circuit: 10L
Large Animal Circuit: 50L

Time Constants:

Higher flow rates = faster changes.

3. Pain and Anesthetic Depth

Indicators of Pain: Increased heart rate, blood pressure, movement.

Intervention:

Locoregional techniques (e.g., nerve blocks, epidurals)
Systemic analgesics (opioids, alpha-2 agonists).

4. MAC Sparing Techniques

Locoregional Analgesia

Advantages:

Superior intraoperative stability.
Improved postoperative pain scores.
Reduced opioid consumption.
Greater food intake
Potentially cheaper in larger dogs

Examples:

Dental: Maxillary nerve block, inferior alveolar nerve block.
Hind Limb: Femoral/Sciatic.
Front Limb: RUMM, Brachial Plexus.
Limb Blocks: Femoral/sciatic nerve block, RUMM.
Thorax: intercostals block.
Abdomen: TAP block, quadratus lumborum block.

Epidural Administration

Injection Site: L7-S1 most common.
Techniques:

Hanging drop technique:

The hanging drop technique involves filling the hub of the epidural needle with sterile saline. As the needle passes through the ligamentum flavum into the epidural space, the negative pressure within the epidural space draws the saline inward, creating a visible drop movement. This method is simple, non-invasive, and helps confirm correct needle placement.

Loss of resistance method:

The loss of resistance technique uses a syringe filled with saline or air attached to the epidural needle. As the needle is advanced through the ligamentum flavum, resistance is felt. Once the needle enters the epidural space, there is a sudden loss of resistance, indicating proper placement. This technique provides a tactile confirmation and is commonly used in both human and veterinary anesthesia.

Considerations: Patient position, body condition, and anatomy.

5. Drug Selection for Anesthetic Stability

Alpha-2 Adrenergic Agonists

Examples: Medetomidine, Dexmedetomidine.
Pros:

Sedation, analgesia, muscle relaxation.
Reversible with atipamezole.
Decreases intracranial pressure.

Cons:

Cardiovascular effects (biphasic vasoconstriction, bradycardia).
Increased urine production, hyperglycemia.

Dosage:

Medetomidine: 2-4 mcg/kg IV as a bolus; 2-4 mcg/kg/min IV as a CRI (no loading dose needed).
Dexmedetomidine: 1-2 mcg/kg IV as a bolus; 1-2 mcg/kg/min IV as a CRI.

MAC ISO Sparing Effects:

Dogs: ~50%
Cats: ~20%

Side Effects:

Biphasic vasoconstriction/vasodilation and reflex bradycardia.
Bradypnea with increased tidal volume.
Suppressed insulin production and increased urine output.

Midazolam (Benzodiazepine)

Mechanism: Enhances GABA effect on GABA-A receptor.
Pros:

Minimal respiratory impact.
Useful for sedation in compromised patients or very young (less than 1 week old patients).
Can be reversed with flumazenil (0.01 mg/kg IV).

Cons:

No analgesia.
Risk of dysphoria (disinhibition) in small animal species.
Respiratory depressant when combined with other drugs.

Dose: 0.1-0.3 mg/kg IV bolus or CRI.
MAC ISO Sparing Effect: ~15% in dogs.

Ketamine

Mechanism: NMDA receptor antagonist.
Pros:

Minimal respiratory depression.
Increase HR.
Good analgesia (especially for chronic pain)

Cons:

NO muscle relaxation.
Can cause dysphoria in recovery if used as a CRI (stop 30 min before)

Dose:

Bolus: 0.5-3 mg/kg IV.
CRI: 10-30 mcg/kg/min IV.

MAC ISO Sparing Effect:

Dogs: ~30%
Cats: ~60%

Lidocaine (Dogs Only)

Mechanism: Sodium channel blockade.
Pros:

Minimal respiratory changes
Mild analgesia (especially for acute pain)
Anti-arrhythmic
Prokinetic (in vitro)
Neuroprotective (in vitro)

Cons:

Low threshold for toxicity in cats
Potential vasodilation

Dose:

Bolus: 2 mg/kg IV.
CRI: 1-3 mg/kg/h IV.

MAC ISO Sparing Effect:

Dogs: ~30%
Cats: ~25% at “acceptable” doses

Side Effects:

Potential cardiovascular depression at higher doses.

Opioids

Mechanism: Bind to opioid receptors for analgesia.
Types:

Pure Mu Agonists: Hydromorphone, Methadone, Morphine, Fentanyl.
Partial Agonists: Buprenorphine.
Kappa Agonists/Mu Antagonists: Butorphanol.

Effects:

Pure Mu Agonists: Strong analgesia, sedation.
Partial Agonists: Moderate analgesia, sedation.
Kappa Agonists/Mu Antagonists: sedation.
Can be reversed with Naloxone
Side Effects: Bradycardia, respiratory depression, nausea, vomiting, hyperthermia (cats).
Side effects are worse if the patient DOES NOT need them.Conclusion

While monitoring equipment is invaluable in anesthesia, it is not infallible. Understanding the limitations of each device and integrating clinical assessment with technology ensures the safest anesthetic management for veterinary patients.

MAC ISO Sparing Effect:

Fentanyl: ~75% (dogs).
Hydromorphone, Methadone, Morphine: ~30% (dogs).
Buprenorphine: ~30% (dogs).
Morphine, Buprenorphine: no significant sparing effect in cats.

Doses:

Hydromorphone: 0.1 mg/kg IV/IM.
Methadone: 0.1-0.3 mg/kg IV/IM.
Morphine: 0.1-0.3 mg/kg IV/IM.
Fentanyl: 3-5 mcg/kg IV/IM.
Buprenorphine: 20-30 mcg/kg IV/IM.
Butorphanol: 0.1-0.5 mg/kg IV/IM.

6. Combining CRIs for MAC Reduction

Morphine-Lidocaine-Ketamine (MLK)

Protocol:

Morphine 0.2 mg/kg/h + Lidocaine 2 mg/kg/h + Ketamine 0.6 mg/kg/h.
Administer at 2 mL/kg/h.

MAC ISO Sparing Effect: ~55%

Dexmedetomidine-Morphine-Lidocaine-Ketamine (DMLK)

Protocol:

Dexmedetomidine 1 mcg/kg/h + Morphine 0.2 mg/kg/h + Lidocaine 2 mg/kg/h + Ketamine 0.6 mg/kg/h.
Administer at 2 mL/kg/h.

MAC ISO Sparing Effect: ~90%

7. Practical Considerations

Equipment: Syringe pumps are recommended for accuracy and consistency.
Alternatives: Manual administration is possible via LRS bags if pumps are unavailable.

Monitoring: Vigilance is required during CRI administration to avoid complications.

Conclusion

Anesthetic stability requires a multifaceted approach that incorporates appropriate drug selection, monitoring, and the strategic use of MAC-sparing techniques like nerve blocks and CRIs. By understanding the mechanisms and practical applications of various anesthetic agents, clinicians can optimize patient outcomes and improve procedural safety.

References

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