Acoustic reflectometry esophageal profiles minimally affected by massive gas ventilation

Acoustic reflectometry can be used to distinguish between breathing tube placement in an esophagus vs the trachea via characteristic area-distance profiles for both cavities. In the cardiopulmonary resuscitation setting, capnography may be useless because the patient has little or no pulmonary circulation. With the breathing tube in the esophagus, can massive ventilation with a manual resuscitation bag, as might occur in the cardiopulmonary resuscitation setting, markedly alter the form of the obtained esophageal reflectometry profile? Nine hounds were induced, endotracheally intubated, mechanically ventilated, and anesthetized. Area-distance profiles were obtained with a 2-microphone acoustic reflectometer customized to measure areas up to 50 cm. Acoustic reflectometer profiles were obtained in intubated esophagi as follows: (1) baseline nonventilated state, (2) after aggressive 2-handed manual ventilation with high inspiratory pressures, rapid respiratory rates, and large tidal volumes for periods of 0.5, 1, and 1.5 minutes, upon detachment of the resuscitation bag, and (3) after esophagogastric decompression. We hypothesized that massive gas ventilation has no effect on the esophageal peak areas (null hypothesis), and used a paired t test for statistical significance (P < .05). For times of 0.5, 1.0, and 1.5 minutes, the ventilation volumes (mean +/- SD) were 25 +/- 7, 49 +/- 8, and 70 +/- 18 L. Massive gas ventilation caused minimal broadening and slight distal spread of the basal "hump". The mean peak area change was 0.18 +/- 0.35 cm2. For a paired t test (n = 9, df = 8), the corresponding t value was 1.54, with a P value of .16, which was incompatible with the null hypothesis. The experimental observations indicate a minimal effect of massive gas ventilation on the acoustic reflectometry esophageal profile. Hence, operator recognition of the altered canine acoustic reflectometer profile as that of an esophageal cavity is maintained, indicating that acoustic reflectometry may be useful in correctly identifying the site of breathing tube placement in out-of-hospital cardiac arrest situations despite massive esophageal ventilation.