Patient description: The patient is a 3-day-old female infant with jaundice that started one day ago. She was born at 34w of gestation and kept in an incubator due to her gestational age. Vital signs were reported as: axillary temperature: 36.3°C, heart rate: 154 beats/min, respiratory rate: 37 breaths/min, and blood pressure: 65/33 mm Hg. Her weight is 2.1 kg, length is 45 cm, and head circumference 32 cm. She presents with yellow sclera and icteric body. Her liver and spleen are normal to palpation. Laboratory results are as follows: Serum total bilirubin: 21.02 mg/dL Direct bilirubin of 2.04 mg/dL AST: 37 U/L ALT: 20 U/L GGT: 745 U/L Alkaline phosphatase: 531 U/L Creatinine: 0.3 mg/dL Urea: 29 mg/dL Na: 147 mEq/L K: 4.5 mEq/L CRP: 3 mg/L Complete blood cell count within the normal range. She is diagnosed with neonatal jaundice that may require phototherapy.

Clinical trial eligibility criteria: eligible ages (years): 0.0-999.0,  Hypoxic Ischaemic Encephalopathy Infants will be eligible for for the trial if the St Michael's hospital standard for cooling and additional for xenon administration are met. St Michael's hospital standard for standard hypothermia treatment of 72 hrs: A: Neonates born at greater than 36 weeks gestation (estimated or clinical assessment) with at least ONE of the following: 1. Apgar score of ≤5 at ten minutes after birth 2. Continued need for resuscitation, including tracheal or mask ventilation, at ten minutes after birth 3. Acidosis, defined as either umbilical cord pH or any arterial, venous or capillary pH within 60 minutes of birth less < 7.00 4. Base deficit ≥16 mmol/L in umbilical cord blood sample or any blood sample within 60 minutes of birth (arterial or venous blood). If the infant meets criterion A then assess for neurological abnormality using criterion B and C (by trained personnel): B: Moderate or Severe encephalopathy as evidenced by any of the following: 1. Altered state of consciousness (reduced or absent responses or pathological irritability and hyper responsive and at least ONE or more of the following: 2. Hypotonia 3. Abnormal reflexes including oculomotor or pupillary abnormalities 4. Absent or weak suck 5. Clinical seizures, as recorded by trained personnel And C: At least 30 minutes duration of amplitude-integrated electroencephalography (aEEG) recording that shows abnormal background aEEG activity. The decision to cool is based on the worst 30 min section of the aEEG, not the best [35] or seizures (clinical or electrical) thus meeting ONE of the following: 1. Normal background with some (> 5 min) electrical seizure activity 2. Moderately abnormal activity (upper margin of trace >10μV and lower margin <5μV) 3. Suppressed activity (upper margin of trace <10μV and lower margin of trace <5μV) 4. Definite seizure activity Additional for xenon: Before being considered for additional inhaled xenon therapy via the breathing gas mixture, the infant would need to meet further additional entry (all must be met): 1. Intubated, ventilated, sedated, being cooled 2. ≤ 5 hours old 3. Any seizures under control 4. Weight > 2nd centile for gestational age 5. Stable cardiovascular parameters; Mean arterial pressure >40mmHg. 6. Oxygen requirement via mechanical ventilator ≤ 40%. 7. Positive End Expiratory Pressure (PEEP) requirement ≤ 8cm H2O 8. Arterial (preferable)/capillary/venous pCO2 within acceptable range (<7kPa) 9. Postnatal age ≤ 5 hours 10. Absence of major congenital abnormalities, imperforate anus and in particular any bowel obstruction, congenital abnormalities suggestive of chromosomal anomaly or other syndromes that brain dysgenesis. Congenital syndromes affecting the brain should be excluded when diagnosed  for cooling in the CoolXenon3 study: 1. Infants expected to be greater than 3 hours of age at the time of starting cooling treatment. 2. Futility. Where prognosis is considered to be hopeless e.g. no cardiac output for 20 minutes