Respiratory restriction and elevated pleural and esophageal pressures in morbid obesity

Behazin N, Jones SB, Cohen RI, Loring SH. Respiratory restriction and elevated pleural and esophageal pressures in morbid obesity. J Appl Physiol 108: 212-218, 2010. First published November 12, 2009; doi:10.1152/japplphysiol.91356.2008.-To explore mechanisms of restrictive respiratory physiology and high pleural pressure (P-Pl) in severe obesity, we studied 51 obese subjects (body mass index = 38-80.7 kg/m(2)) and 10 nonobese subjects, both groups without lung disease, anesthetized, and paralyzed for surgery. We measured esophageal and gastric pressures (P-Es, P-Ga) using a balloon-catheter, airway pressure (P-AO), flow, and volume. We compared P-Es to another estimate of P-Pl based on P-AO and flow. Reasoning that the lungs would not inflate until P-AO exceeded alveolar and pleural pressures (P-AO > P-Alv > P-Pl), we disconnected subjects from the ventilator for 10-15 s to allow them to reach relaxation volume (V-Rel) and then slowly raised P-AO until lung volume increased by 10 ml, indicating the "threshold P-AO" (PAO-Thr) for inflation, which we took to be an estimate of the lowest P-Alv or P-Pl to be found in the chest at V-Rel. PAO-Thr ranged from 0.6 to 14.0 cmH(2)O in obese and 0.2 to 0.9 cmH(2)O in control subjects. P-Es at V-Rel was higher in obese than control subjects (12.5 +/- 3.9 vs. 6.9 +/- 3.1 cmH(2)O, means +/- SD; P = 0.0002) and correlated with PAO-Thr (R-2 = 0.16, P = 0.0015). Respiratory system compliance (C-RS) was lower in obese than control (0.032 +/- 0.008 vs. 0.053 +/- 0.007 l/cmH(2)O) due principally to lower lung compliance (0.043 +/- 0.016 vs. 0.084 +/- 0.029 l/cmH(2)O) rather than chest wall compliance (obese 0.195 +/- 0.109, control 0.223 +/- 0.132 l/cmH(2)O). We conclude that many severely obese supine subjects at relaxation volume have positive P-pl throughout the chest. High P-Es suggests high P-Pl in such individuals. Lung and respiratory system compliances are low because of breathing at abnormally low lung volumes.