Study to Evaluate the Optimal Dose of Remifentanil During MRI of the Heart Under General Anaesthesia

Study to Evaluate the Optimal Dose of Remifentanil Required to Ensure Apnoea During Magnetic Resonance Imaging of the Heart Under General Anaesthesia

What is the optimal maintenance dose of remifentanil to ensure apnoea, during breath hold episodes in children having cardiac MR imaging with general anaesthesia?

The management of congenital heart disease depends upon accurate imaging of the heart to define anatomical abnormalities. The use of magnetic resonance imaging has grown in response to this need. Young children, however, are unable to cooperate sufficiently to allow the scan to occur; this necessitates the use of general anaesthesia.

The optimal anaesthetic management of children in this situation is complex. Imaging of an acceptable quality depends on a low (normal) heart rate which is relatively constant during the scan and frequent breath holds to avoid respiratory artefacts. Children undergoing such scans commonly have significant cardiac and non-cardiac disease. A technique, currently used at Alder Hey Hospital, is to use intravenous agents to provide anaesthesia: commonly a mixture of the intravenous anaesthetic propofol and the potent opiate remifentanil. The patient's own ventilation is suppressed by the use of remifentanil which also reduces heart rate and heart rate variability. Controlled ventilation is provided via a ventilator and endotracheal tube. Adequate doses of remifentanil are required to ensure suppression of the patient's ventilation whilst excess doses are avoided to reduce the risk of adverse effects (primarily arterial hypotension) and ensure rapid recovery.

Remifentanil dose has been studied during surgery(3-6), however this is a unique indication and optimal dosage has not been formally studied. In particular the absence of any stimulation from surgery is likely to mean that substantially lower doses of remifentanil will be required.

Inclusion Criteria:

• Children of one year (of age) or older and younger than 7 years of age.
• Scheduled for cardiac MR imaging under general anaesthesia at Alder Hey Children's Hospital.
• Parental Consent

Exclusion Criteria:

• Hypersensitivity to any study drug
• Known abnormal response to opiate analgesics or co-morbidity associated with abnormal central control of breathing
• Families unable to understand or complete consent
• Any other contraindication to proposed anaesthetic technique: at the discretion of the responsible anaesthetist.
• Documented significant renal or hepatic dysfunction