Randomization The study subjects were randomized into two arms, using computer-generated sequence of binary code: interventional arm with dexamethasone administration and control group with a placebo - physiological solution. Anaesthesiology nurse, who was not a part of the research team, randomly selected a vial for each enrolled patient, which was later administered to the patient. The number of the ampule corresponded with the patient's number.

Standardized anaesthesiology protocol General anaesthesia was administered in every paediatric patient according to a standardized anaesthesiology protocol. Each subject received premedication with midazolam in the dose of 0.5 mg/kg of body weight, per orally, 45 minutes before the surgical procedure. Inhalation induction of general anaesthesia was initiated with Sevoflurane, the vaporizer was set to 8%, using the carrying mixture of gases O2/air in the 1:1 ratio. After induction of anaesthesia, peripheral venous line was secured, with subsequent administration of an opioid - Sufentanil in the dose of 0.2 uq/kg of body weight, together with the research drug in the dose of 0.15mg/kg of body weight, with the maximum dose of 4 mg administered intravenously. The airways were secured in accordance with standard procedures used at our centre - armed laryngeal mask. In case of any leakages, air leaks, or any other problems with the laryngeal mask, the airways were secured with orotracheal intubation and introduction of tamponade into hypopharynx. General anaesthesia was further maintained using Sevoflurane, towards the values of MAC 1.2-1.5. Fluid therapy was guided by the 4/2/1 ml/kg of body weight /hr. rule - using Ringer's solution. Each subject was given standard analgetization for the early post-operative period: Paracetamol in the dose of 15mg/kg of body weight i.v. in the course of 10 minutes of the surgical procedure. Standard monitoring was provided for every patient: ECG, measurement of blood pressure, saturation, capnometry, inspiration and expiration Sevoflurane concentrations. After the surgery, every patient was woken up from anaesthesia, the laryngeal mask was removed (extubation) with spontaneous ventilation, the patient was further transferred to PACU.

Data collection Anaesthesiology team (anaesthetist + nurse), who was responsible for administration of general anaesthesia according to the standardized protocol, and who was trained to perform the study according to the study protocol, recorded the data concerning Dexamethasone/Placebo administration into a special Case Report Form, containing identification and number of the study drug. The form was transferred together with the patient to PACU, where the occurrence of PONV was monitored and recorded, together with the first value of the BARF score, by a PACU nurse and the child's parent. The form concerning the incidence PONV at inpatient department was provided for the parent who was present at the bedside of the patient at all times, up to the child's discharge from the hospital - the parent noted the number of vomiting episodes and BARF score 8 hours after the surgery (late BARF score); the parent was further instructed to ask the child ever 2 hours about possible sickness. In case of vomiting, the time and number of vomiting episodes were recorded.