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Esophageal intubation following a mask ventilation

Occasionally enough alveolar gas can be forced down the esophagus into the stomach during mask ventilation resulting transiently in few capnograms as shown above. The concentration of carbon dioxide falls dramatically after three breaths and to zero in about six breaths. However, a careful analysis of capnograms should alert to such a possibility of esophageal intubation particularly if the tip of the endotracheal tube was not seen going past the vocal cords.  If enough alveolar gas is forced down the esophagus into the stomach during mask ventilation prior to intubation, the concentration of carbon dioxide in the later may be similar to that in the alveoli. After few breaths, the carbon dioxide in the stomach will then be diluted and the waveform will very quickly become flat.

Reference:
Sum-Ping ST. Anaesth Analg 1987;66:481-3.