The Effects of Droperidol and Ondansetron on Dispersion of Myocardial Repolarization in Children

1. Purpose: to examine in detail the effect of droperidol and ondansetron on Tp-e (an ECG measure of dispersion of repolarization); to search with maximum statistical power for a difference in this parameter before and after exposure to this widely used anaesthetic. To investigate the nature of any dose-response relationship between propofol and mean QTc and Tp-e intervals.
2. Hypotheses: 1. H0: mean pre-operative Tp-e = mean intra-operative Tp-e within each group vs. H1: mean pre-operative Tp-e ≠ mean intra-operative Tp-e within each group. 2. H0: mean intra-operative Tp-e group 1 = mean intra-operative Tp-e group 2 = mean intra-operative Tp-e group 3 vs. H1: mean intra-operative Tp-e group 1 ≠ mean intra-operative Tp-e group 2 ≠ mean intra-operative Tp-e group 3.
3. Justification: Droperidol and ondansetron are anti-emetics. It has long been thought that prolongation of repolarization, however caused, predisposes to a rare malignant ventricular tachyarrhythmia called torsades de pointes (TdP). The classic model for this hypothesis is a group of hereditary conditions collectively known as long QT syndrome. Although rare, this condition usually presents in childhood or early adulthood, with syncope, aborted cardiac arrest or sudden death, secondary to episodes of TdP. The genetic mutation affects the structure and function of myocardial potassium channels involved in repolarization dynamics. Some anaesthetic agents block some of these potassium channels, thus prolonging repolarization, producing an acquired long QT syndrome.

QT interval prolongation per se is associated with, but is not the cause of, TdP. It has been shown recently that exaggeration of a physiological phenomenon called dispersion of repolarization (TDR) provides the right environment and the trigger for TdP. Normal TDR reflects the way that different layers of the myocardial wall repolarize at different rates - the outside fastest, then the inside & finally the middle. Physiological TDR also determines the morphology of the T wave on the surface ECG. The interval between the peak and the end of the T wave is a measure of TDR.

We therefore now have a new tool for assessing the risk posed by a drug that prolongs the QT interval. Evidence is accumulating that, if TDR is not increased, the risk of TdP is not increased, even if the QT interval is prolonged. Conversely, if TDR is exaggerated, the risk of TdP is raised, even if the absolute QT interval is within normal limits.

Whyte & colleagues showed that propofol does not increase TDR, suggesting that the risk of TdP is not increased with this agent. That study examined only one dose at the extreme lower end of the range for surgical anesthesia & had only 80% power. This study is designed to address those weaknesses and investigate more thoroughly the relationship between propofol and TDR, with the aim of being able to provide evidence-based recommendations, where none currently exist, on its use in patients with or at risk of long QT syndromes. 4. Objectives: a) to determine whether there is a significant difference between pre and post-induction mean QTc interval and mean Tp-e interval for each effect-site target concentration of propofol. b) to determine whether there is a relationship between propofol dose, and mean QTc and Tp-e intervals. The primary outcome of the study will be the presence or absence of differences in Tp-e within and between groups of children allocated by randomization to receive droperidol or ondansetron or a combination of both. For each child, the endpoint of the study will be 5 minutes after induction of anesthesia. 5. Research Method: randomised, double-blinded within- and between groups comparative study in 80 unpremedicated ASA I-II children, aged between 3 and 10 years, undergoing elective, day case strabismus; otoplasty; ear, nose & throat; dental surgery. After obtaining written informed parental consent, and patient assent where appropriate, enrolled patients will be randomized to one of 4 groups, to receive a droperidol, ondansetron, a combination, or saline. Block randomization will be prepared using computer generated random numbers. Allocation will be concealed using sealed sequentially numbered opaque envelopes. Prior to induction of anaesthesia, ECG electrodes will be sited at standardised locations for acquisition of a pre-operative 12 lead ECG. An intra-operative ECG, using the same electrode positions, will be taken 5 min after induction of anaesthesia. The patient's involvement in the study will then be complete and the conduct of anaesthesia continued at the discretion of the supervising anaesthetist. All ECGs will be recorded in duplicate, at a paper speed of 50 mm/sec and with no identifying data or automated analysis on the recorded traces. Each ECG will be given a random number three-figure code, to allow identification of paired pre- and intra-operative traces after analysis. IV access will be obtained immediately before induction. Anaesthesia will be induced and maintained with propofol delivered by a syringe pump. Throughout the study period, all children will receive routine monitoring. In an attempt to minimize sympathetic stimulation, laryngoscopy will not be permitted during the study period, and the airway will be maintained either by facemask or laryngeal mask. All the ECG traces will be analysed independently by two of the authors (SS and SW) in accordance with predetermined criteria. Both will be blinded to the study group and to the status of the ECG recording (pre- or intra-operative). Neither will be involved in recruitment or randomisation of patients, or in the conduct of the anaesthesia or acquisition of ECG recordings, all of which will be performed by one of the other investigators.

Data analysis: the QT and Tp-e intervals will be measured for all complete P-QRS-T cycles in leads II and V5 and averaged to give a mean QT interval and Tp-e interval for that lead. The QT interval will be measured from the start of the QRS complex to the end of the T-wave, defined as the point of return to the T-P baseline. If U waves are present, the end of the T-wave will be taken as the nadir of the curve between the T and U waves. The Tp-e interval will be measured from the peak of the T-wave to the end of the T-wave. Monophasic T wave peaks can be identified visually. For more complex T wave morphologies, the peak will be identified according to the criteria of Emori & Antzelevitch.