Gastric Inflation in Relation to Airway Pressure

Ruben H, Knudson EJ, Carugati G Acta Anaesthesiology Scandinavia 1961 Vol 5 Pages 107-114 OF HISTORICAL INTEREST

Gastric inflation is a possible side-effect of intermittent positive-pressure respiration using a bag and mask. The study was undertaken to investigate the possible influence of various air pressures in the pharynx or the esophagus on gastric inflation. Twenty anesthetized patients were investigated prior to surgery, both before and after intubation. Air pressures were brought about by insufflation through a mask or, in the intubated patients, through a wide-bore rubber tube. The minimum pressure which produced gastric inflation was determined at the moment a well-defined sound of a gurgling or blowing nature could be heard through a stethoscope at the upper abdomen. Pressures were raised stepwise to produce static or near-static pressures in order to eliminate the influence of airway resistance. Three insufflation sequences were performed for each value to be determined. Pressures were measured using a high tube opening in the pharynx just below the soft palate, and a deep tube opening into the esophagus just above the cardia. During measurement, the mouth and nostrils were closed manually. The influence of head position on insufflation pressures was also determined with the head of the patient placed in neutral, maximally extended and maximally flexed positions. With the head in the normal position, pressures below 15 cmH20 rarely produced insufflation of the stomach, while pressures exceeding 25 cmH20 in most patients, did so. The most common minimum insufflation pressure lay between 15-20 cmH20 using a face mask, 20-25 using the pharyngeal tube, and between 15-20 cmH20 with the esophageal (cardia) tube. The cardia was found to function as a pressure-sensitive valve which in practically all cases opened at pressures higher than 25 cmH20. Maximum backward tilting of the head did not significantly change the insufflation pressures when a high tube was used. The face mask gave essentially the same results except in two patients where higher pressures were recorded. Maximum flesion raised the pressure required for gastric inflation. This is not surprising as flexion is known to produce obstruction as the tongue level in most anesthetised patients. The data suggests that when performing artificial respiration by bag and mask or by expired air (mouth-to-mouth), relatively small inflation pressures should be used to avoid an unnecessary rise of the pressure in the pharynx.