This is a report of a case in which an endobronchial blocker was dislodged, leading to severe air trapping and a brief episode of pulseless electrical activity. Bronchial blockade for lung deflation was successfully instituted during emergency repair of a ruptured descending aortic aneurysm. During a period not involving manipulation of aortic cross-clamps, end-tidal CO2 decreased precipitously to zero and airway pressures increased markedly, followed by equalization of intracardiac pressures. Prompt deflation of the endobronchial blocker balloon reversed the problem. We hypothesize that when surgical manipulation dislodged the bronchial blocker into the tracheal position, leading to profound air trapping as successive, stacked tidal volumes were forced distal to the blocker.