A 22-year-old female with a history of persistent post-measles cough and wheezing since childhood. Recent years show increased frequency and severity of attacks. Hospitalized due to unresolved asthma exacerbation after a cold 1 week ago. Examination: apathy, dyspnea, orthopneic position, no significant cyanosis with oxygen. Bilateral wheezes, heart rate 126/min with regular rhythm and pulsus paradoxus. Which test should be immediately performed for objective assessment?