Ventricular tachycardia unresponsive to lidocaine
9-year-old Labrador that had surgery 24 hours ago for a splenic haematoma. He has vomited several times and is receiving a crystalloid without potassium supplementation. He has sustained monomorphic ventricular tachycardia at 230 bpm, with a systolic pressure of 100 mmHg; he is conscious. Two lidocaine boluses of 2 mg/kg have had no effect.
| Parameter | Result | Reference |
|---|---|---|
| Potassium | 2.8 mmol/L | 3.5–5.1 |
| Ionised magnesium | 0.36 mmol/L | 0.43–0.60 |
| Haematocrit | 31% | 37–55 |
| Lactate | 2.4 mmol/L | < 2.5 |
What best explains the lack of response, and what is the next step?
chevron_right Show answer and explanation
A. Insufficient dose: keep giving lidocaine boluses until conversion occurs
Repeating boluses moves towards toxic doses (tremors, seizures, vomiting) without correcting the cause of refractoriness.
B. Probably supraventricular in origin: slow IV diltiazem bolus
This is monomorphic ventricular tachycardia; a calcium channel blocker given in VT can cause severe hypotension and collapse.
C. Hypokalaemia and hypomagnesaemia: correct them and retry the lidocaine
Correct. Class I antiarrhythmics such as lidocaine lose efficacy with hypokalaemia, and magnesium deficiency promotes arrhythmias and makes potassium harder to correct. Supplement potassium without exceeding 0.5 mEq/kg/h and give IV magnesium, then repeat the lidocaine or move to procainamide.
D. Intrinsic class I resistance: IV esmolol constant-rate infusion
This is not intrinsic resistance but a correctable electrolyte problem. A beta-blocker in a dog that has just had surgery and is anaemic may precipitate hypotension.
lightbulb Key takeaway
Before calling a ventricular arrhythmia 'refractory', check potassium, magnesium, pain, hypoxaemia, anaemia and hypovolaemia. Ventricular arrhythmias are common 12–72 hours after splenectomy; treat them if they are fast and sustained, multiform, show R-on-T or cause haemodynamic compromise.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.