Flat capnogram found in A/E –

All the four options given in the question can cause flat Capnography. But, bronchospasm is the single best answer here as only severe bronchospasm (causing complete respiratory obstruction) can cause a flat capnograph. However, bronchoconstriction is usually partial which causes a steep plateau in capnograph (not flat capnograph). A flat capnograph is seen in accidental extubation, equipment failure, disconnection (dissociation) of a tube, total occlusion, mechanical ventilation failure, cardiac arrest, severe bronchospasm completely obstructing the airway and complete occlusion of pulmonary circulation.