3. contraindications to Nitrous Oxide: 1-patients who require pure oxygen ventilation 2- intracranial hypertension 3- disorders of consciousness that impede the cooperation of the patient, 4-pneumothorax; 5-emphysematous bulla, 6-pulmonary embolism; 7-immersion accident, 8-bloating abdominal trauma 9- facial trauma that affects the area of application of the mask,10- patients who received the type gases SF6, C3F8, C2F6 (used in eye surgery), at least in the previous three months.