Refined anaesthesia for implantation of engineered experimental aortic valves in the pulmonary artery using a right heart bypass in sheep

Abstract

The feasibility of an anaesthetic protocol developed for surgery during right heart bypass in sheep is reported. Seven female Suffolk sheep, weighing 25–35 kg, were selected for the study. Premedication consisted of midazolam and methadone (both 0.1 mg kg⁻¹ intravenously). Anaesthesia was induced with propofol (2–4 mg kg⁻¹) and maintained with isoflurane in oxygen and continuous rate infusions of propofol (5–7 mg kg⁻¹ h⁻¹) and fentanyl (5 μg kg⁻¹ bolus, 5 μg kg⁻¹ h⁻¹). Cisatracurium (0.2 mg kg⁻¹) provided muscle relaxation. A standard roller pump was used for the extracorporeal circulation. Drugs administered to maintain blood pressure and heart rate within acceptable levels included phenylephrine (3–4 μg kg⁻¹), ephedrine (0.1–0.2 mg kg⁻¹), nitroglycerine (50–150 μg kg⁻¹ h⁻¹) and metoprolol succinate (30–80 μg kg⁻¹). Electrolytes were infused as needed. Postoperative analgesia was provided by an intercostal block (15 mL 0.5% bupivacaine + epinephrine), carprofen (4 mg kg⁻¹) and an opioid (methadone 0.1 mg kg⁻¹ or buprenorphine 0.01 mg kg⁻¹). One sheep became hypoxic during the bypass (PaO₂ 47.7 mmHg). Irregularities of the electrocardiogram were observed during manipulation of the heart in all animals. During the initial phase of the bypass, blood pressure decreased in all sheep, accompanied by dilatation of the heart and large intrathoracic veins in five sheep. With appropriate treatment, blood pressure was restored and easily maintained until the end of the bypass. Weaning from the bypass, using an infusion of nitrates, was smooth. One sheep required a blood transfusion because of severe blood loss and another sheep died postoperatively from respiratory complications. Minor irregularities of the electrocardiogram observed during manipulation of the heart were not life threatening and required no treatment. Decreases in blood pressure at the beginning of the bypass can be expected and require treatment. Nitrates are useful in avoiding volume overload during weaning. The anaesthetic protocol is acceptable for surgery under right heart bypass in sheep.