The effect of rate of administration of tramadol on incidence of post operative nausea and vomiting after any surgical operation for which significant postoperative pain is expected

INTERVENTION: ‘Fast’ group – will receive 2.5 mg/kg tramadol (made to 10 ml with addition of saline) intravenously over 2 minutes and 10 ml saline via a syringe driver over 30 minutes. ‘Slow’ group – will receive 10 ml saline over 2 minutes and 2.5 mg/kg tramadol (made to 10 ml with addition of saline) intravenously via a syringe driver over 30 minutes. CONDITION: Post operative nausea and vomiting PRIMARY OUTCOME: Comparative peak change from baseline in nausea visual analogue score (VAS) scores between fast, slow and placebo tramadol administration groups SECONDARY OUTCOME: Comparative vomiting frequencies, morphine PCA usage, pain scores, duration spent in PACU and patient satisfaction scores between fast, slow and placebo tramadol administration groups over three hours. INCLUSION CRITERIA: English spoken as first language, or fluent as a second language• Scheduled to have elective non‐abdominal surgery under general anaesthesia (including general, orthopaedic, plastic, ENT, urologic, dental or vascular surgery) • Requiring general anaesthesia• Significant post‐operative pain requiring opioid therapy expected.