The esophageal balloon technique for measuring pleural surface pressure (Ppl) has recently been shown to be valid in recumbent positions. Questions remains regarding its validity at lung volumes higher and lower than normally observed in upright and horizontal postures, respectively. We therefore evaluated it further in 10 normal subjects, seated and supine, by measuring the ratio of esophageal to mouth pressure changes (.DELTA.Pes/.DELTA.Pm) during Mueller, Valsalva, and occlusion tests maneuvers at FRC, 20, 40, 60, and 80% VC with the balloon placed 5, 10, and 15 cm above the cardia. In general, .DELTA.Pes/.DELTA.Pm was highest at the 5-cm level, during Mueller maneuvers and occlusion tests, regardless of posture or lung volume (mean range 1.00-1.08). At 10 and 15, cm, there was a progressive increase in .DELTA.Pes/.DELTA.Pm with volume (from 0.85 to 1.14). During Valsalva maneuvers, .DELTA.Pes/.DELTA.Pm also tended to increase with volume while supine (range 0.91-1.04), but was not volume-dependent while seated. Qualitatively, observed .DELTA.Pes/.DELTA.Pm fit predicted corresponding value (based on lung and upper airway compliances). Quantitatively there were discrepancies probably due to lack of measurement of esophageal elastance and to inhomogeneities in .DELTA.Ppl. At every lung volume in both postures, there was at least one esophageal site where .delta.Pes/.DELTA.Pm was within 10% of unity.
