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Volume-related and volume-independent effects of posture on esophageal and transpulmonary pressures in healthy subjects

¹Department of Anesthesia and Critical Care and ²Division of Pulmonary and Critical Care Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts

Submitted 13 June 2005 ; accepted in final form 16 November 2005

Ventilator management decisions in acute lung injury could be better informed with knowledge of the patient's transpulmonary pressure, which can be estimated using measurements of esophageal pressure. Esophageal manometry is seldom used for this, however, in part because of a presumed postural artifact in the supine position. Here, we characterize the magnitude and variability of postural effects on esophageal pressure in healthy subjects to better assess its significance in patients with acute lung injury. We measured the posture-related changes in relaxation volume and total lung capacity in 10 healthy subjects in four postures: upright, supine, prone, and left lateral decubitus. Then, in the same subjects, we measured static pressure-volume characteristics of the lung over a wide range of lung volumes in each posture by using an esophageal balloon catheter. Transpulmonary pressure during relaxation (PL_rel) averaged 3.7 (SD 2.0) cmH₂O upright and –3.3 (SD 3.2) cmH₂O supine. Approximately 58% of the decrease in PL_rel between the upright and supine postures was due to a corresponding decrease in relaxation volume. The remaining 2.9-cmH₂O difference is consistent with reported values of a presumed postural artifact. Relaxation volumes and pressures in prone and lateral postures were intermediate. To correct estimated transpulmonary pressure for the effect of lying supine, we suggest adding 3 cmH₂O (95% confidence interval: –1 to +7 cmH₂O). We conclude that postural differences in estimated transpulmonary pressure at a given lung volume are small compared with the substantial range of PL_rel in patients with acute lung injury.

respiratory mechanics; chest wall; esophageal balloon; supine posture

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