Validity of the esophageal balloon technique at high frequencies

The reliability of the esophageal balloon technique in measuring high-frequency changes in pleural pressure (Ppl) was investigated in six normal subjects by studying the amplitude ratio (A) and phase angle (phi) of esophageal (Pes) and mouth (Pm) pressures during airway occlusion and while pseudorandom pressure variations (2-32 Hz) were applied to the chest. The measurements were made with a common esophageal balloon-catheter system connected to a high-impedance piezoresistive transducer. When the cheeks were firmly supported, A averaged 1.08 +/- 0.063 at 2 Hz and 1.06 +/- 0.11 at 32 Hz. Pes increasingly led Pm with increasing frequency, and phi averaged 20.8 +/- 4.0 degrees at 32 Hz. Washing the airways with 80% He-20% O2 reduced phi by 50%. When the cheeks were not supported, A exhibited a strong positive frequency dependence, averaging 1.71 +/- 0.34 at 32 Hz, whereas phi increased much faster below 20 Hz and tended to decrease afterward. Because the esophageal transfer function Pes/Ppl = (Pes/Pm)/(Ppl/Pm), we could estimate Pes/Ppl by computing for individual subjects the pressure difference between the pleura and the mouth based on the lung and upper airway wall properties that were measured separately. The results suggest that the ratio of Pes and Ppl remains close to unity from 2 to 32 Hz, but Pes lags slightly behind Ppl (phi equals about -7 degrees at 32 Hz).