Neuroprotective Effects of Hypothermia Combined With Inhaled Xenon Following Perinatal Asphyxia

Inclusion Criteria: Infants will be eligible for enrolment into the trial if each of the following criteria is fulfilled: * Infants 36 to 43 weeks gestation with at least one of the following: * Apgar score of \<5 at 10 minutes after birth; * Continued need for resuscitation, including endotracheal or mask ventilation, at 10 minutes after birth; * Acidosis defined as pH \<7.00 and/or base deficit \>15 mmol/L in umbilical cord blood sample or any blood sample within 60 minutes of birth (arterial or venous blood). * Moderate to severe encephalopathy consisting of altered state of consciousness (reduced or absent response to stimulation) and hypotonia, and abnormal primitive reflexes (weak or absent suck or Moro response). Clinical severity of HIE will be assessed by Thompson encephalopathy score, and modified Sarnat score. * At least 30 minutes duration of amplitude integrated EEG (aEEG) recording that shows moderately abnormal or suppressed background aEEG activity or seizures Exclusion Criteria: * If treatment with hypothermia is delayed beyond 6 hours, or infants are expected to be \>12 hours of age at the time of randomisation; Infants with ventilatory oxygen requirement \> 70%; Attending clinician considers infant not suitable to participate because of other serious congenital abnormalities, or the infant's condition appears terminal.