CASE HISTORY: A 4 year old boy is brought by his mother to emergency department in severe respiratory distress. According to his mother, he was playing with his elder brother eating some nuts when she noted severe cough and difficulty in breathing .On examination, the child is restless, irritable and breathless. His Blood Pressure is 130/90 mm of Hg(normal 110/70), respiratory rate is 42 breaths / minute, heart rate is 120 beats / minute(normal 65 to 110) and temperature is 99oF(normal 98.6 oF) .Examination showed bluish discoloration of lips and tongue. When his chest was examined it was found that the right side of his chest was not moving as much as the left one. The breath sounds were present on left side and absent on right side. The child was immediately given Oxygen through facemask, and his condition improved. Some blood was taken out from his radial artery for arterial blood gases and other routine investigation. The laboratory data showed a 70mm of Hg partial pressure of oxygen (normal 100mm Hg) and a 45mm Hg partial pressure of carbon dioxide (normal 40mm Hg) in his arterial blood.