Abstract
Capnography and end tidal CO2 (EtCO2) aids the anaesthesiologist in diagnosing problems during all phases of general anaesthesia. Negative arterial to end-tidal carbon-dioxide gradient during anaesthesia has been reported in various conditions including pregnancy, infants and inadvertent exogenous addition of carbon dioxide (CO2) to the expired gas in case of thoracoscopic procedures with iatrogenic injury to lung parenchyma/bronchial tree. Thus, airway injury or intentional opening of airway as a part of surgical step can be diagnosed using a negative arterial and end tidal CO2 gradient. Higher optimal PEEP can be used as a splint across the bronchial cuff in one-lung ventilation which prevents leak from capnothorax and decrease inadvertent entry of CO2 in to the expired gases which erroneously increase arteriolar to end tidal CO2 gradient.