Drawing Up Bupivacaine: The Field Block Request That Exceeds the Dose Ceiling

The math:

• Maximum dose: 22 kg x 2 mg/kg = 44 mg.
• Concentration: 0.5% = 5 mg/mL.
• Maximum volume: 44 mg / 5 mg/mL = 8.8 mL.
• Requested volume: 15 mL = 75 mg = 3.4 mg/kg, which is about 70% above the ceiling.

The epinephrine does not create meaningful additional headroom for bupivacaine in this patient, and it adds its own effects: tachycardia, hypertension, and ventricular arrhythmia risk in a dog already under isoflurane.

What to do: do not draw up or administer the 15 mL, and do not silently "compromise" by giving a smaller volume of your own choosing. Pause and speak to the veterinarian directly, out of earshot of the client if possible: state the patient's weight, the reference maximum, the calculated volume of 8.8 mL, and what the 15 mL request represents. Offer options rather than only a problem:

• Give up to 8.5 mL of bupivacaine as a true field block and accept a shorter duration.
• Reduce the bupivacaine volume and supplement with lidocaine, remembering that local anesthetic toxicity is additive, so the lidocaine dose must be counted against the same total toxicity budget (lidocaine in dogs is roughly 4 to 6 mg/kg without epinephrine; 2% is 20 mg/mL, so about 4.4 to 6.6 mL at 22 kg).
• Add a systemic analgesic strategy instead: an opioid top-up, or a ketamine or lidocaine infusion as ordered.
• Have a trained provider perform an ultrasound-guided block if one is available.

Then document the conversation, the calculation, and the final order in the record.

Recognize the sequence. Local anesthetic toxicity is dose- and route-dependent, and bupivacaine is more cardiotoxic than lidocaine. Signs often progress from central to cardiovascular:

• Early CNS: twitching, muscle tremors, restlessness, nystagmus, increased responsiveness to stimulation, or a sudden change in heart rate or blood pressure during injection.
• Advanced CNS: generalized seizures.
• Cardiovascular: bradycardia, hypotension, ventricular premature complexes, ventricular tachycardia or fibrillation, and cardiovascular collapse. In an anesthetized patient, an unexpected arrhythmia or unexplained drop in blood pressure during or right after a block should raise suspicion immediately.

Response:

1. Stop the injection immediately and note the volume already given.
2. Call the veterinarian, deliver 100% oxygen, and be ready to intubate and ventilate if respiratory effort is inadequate.
3. Control seizures with a benzodiazepine (for example midazolam) or propofol as ordered.
4. Treat hypotension with crystalloid boluses and vasopressors as directed; treat arrhythmias as directed rather than improvising.
5. Start lipid emulsion therapy if cardiovascular signs develop: a bolus of 20% lipid emulsion at about 1.5 mL/kg, repeated once or twice as needed, followed by a continuous infusion (roughly 0.25 mL/kg/min). Know where the bottle is kept before the block is given, not after.
6. Continue ECG, blood pressure, SpO2, and capnography monitoring into recovery, and document the timeline.

Prevention: aspirate before injecting, inject slowly, use the smallest effective volume, and calculate the dose before drawing the drug up.

Preoperative planning:

• Calculate maximum volumes for every local anesthetic the team might use, at the patient's actual weight, before the patient is induced, and write them on the anesthesia sheet alongside the other drug doses. For Luna, that means 8.8 mL of 0.5% bupivacaine and roughly 4.4 to 6.6 mL of 2% lidocaine, plus a note that these ceilings are additive when drugs are combined.
• Decide in advance which blocks will be performed and who will perform them, so the volume is divided between sites rather than improvised.
• Stock the correct syringe sizes. A 3 mL syringe cannot deliver 8 mL, and repeatedly puncturing a 20 mL vial with a small needle invites contamination and needle-stick injury. Use an appropriate syringe, a filter needle or a drawing-up device, and label the syringe with the drug name, concentration, total milligrams, and time.

Know the site-specific cautions: do not use epinephrine-containing local anesthetics near end-arterial sites such as digits, the pinna, the nasal planum, or the tail tip.

Documentation: the drug, concentration, volume, total milligrams, mg/kg, time given, route, who administered it, the veterinarian's order, the patient's weight, monitoring values before and after, and any conversation in which a dose was questioned or changed. If a dose is reduced from what was requested, that decision and its rationale belong in the record. This protects the patient, the team, and the hospital, and it makes the next calculation faster and safer.