Aswariya was worried about her 9 year old girl, known case of asthma, she brought her child with a 1 day history of shortness of breath that is increasing in severity. She had a previous upper respiratory tract infection 1 week prior which had resolved. Chest x-ray reveals bilateral hyperinflation. On arrival, she was given oxygen, nebulized beta-2 agonist, and oral prednisolone. She is now drowsy, respiratory rate is 30 and her SpO2 is 90%. Which of the following is the SINGLE most appropriate investigation?