Management of refractory hypotension during adrenalectomy in a dog treated with phenoxybenzamine

Pheochromocytoma is a functional tumor of the adrenal medullary chromaffin cells that releases excessive secreted catecholamines, often malignant and invasive of blood vessels. Preoperative treatment with an alpha-1 adrenergic receptor antagonist, such as phenoxybenzamine, has been suggested to reverse chronic vasoconstriction A 12-year-old, 8 kg, female spayed Shih Tzu dog was presented for left adrenalectomy. Clinical signs included polyuria and polydipsia, and lethargy. Abdominal ultrasound revealed chronic renal changes and an adrenal mass with phrenicoabdominal vein and vena cava invasion. Tests for hyperadrenocorticism were negative. No urine metanephrine test was performed. Phenoxybenzamine (5 mg orally twice daily) was started 1 month before surgery. Anesthetic premedication consisted of methadone (0.3 mg kg^-1) intravenously (IV) and atropine (0.02 mg kg^-1) intramuscularly. General anesthesia was induced with alfaxalone (1.5 mg kg^-1 IV) and midazolam (0.3 mg kg^-1 IV) and maintained with isoflurane in oxygen delivered via a circle breathing system. Systemic arterial blood pressure was measured invasively. During surgery, arterial hypotension was present for 195 minutes and treated with lactated Ringer’s solution and hydroxyethyl starch boluses, dopamine (5–15 μg kg^-1 min^-1 IV) and phenylephrine (0.1–0.5 μg kg^-1 min^-1 IV) for 195 minutes and 100 minutes, respectively, aiming to maintain mean arterial blood pressure (MAP) ≥ 60 mmHg. Arterial blood gas analyses showed hypoxemia intraoperatively and in recovery. Despite supplemental oxygen, hypoxemia did not improve. The owners decided to euthanize the dog. This report describes the use of an irreversible alpha-1 adrenergic receptor antagonist to treat severe preoperative hypertension (MAP 45 mmHg) associated with a malignant adrenal gland tumor and the subsequent challenges in managing arterial blood pressure intraoperatively.