Abdominal distension alters regional pleural pressures and chest wall mechanics in pigs in vivo TAKASHI MUTOH, WAYNE J. E. LAMM, LISA J. EMBREE, JACOB HILDEBRANDT, AND RICHARD K. ALBERT Medical Service, Veterans Administration Medical Center, and Department of Medicine, University of Washington, Seattle, Washington 98195 MUTOH,TAKASHI,WAYNE J.E. LAMM,LISA J. EMBREE, JA- COBHILDEBRANDT,ANDRICHARD K. ALBERT. Abdominaldis- tension alters regional pleural pressures and chest wall mechanics in pigs in Go. J. Appl. Physiol. 70(6): 2611-2618, 1991.-Ab- dominal distension (AD) occurs in pregnancy and is also com- monly seen in patients with ascites from various causes. Be- cause the abdomen forms part of the “chest wall,” the purpose of this study was to clarify the effects of AD on ventilatory mechanics. Airway pressure, four (vertical) regional pleural pressures, and abdominal pressure were measured in five anes- thetized, paralyzed, and ventilated upright pigs. The effects of AD on the lung and chest wall were studied by inflating a liq- uid-filled balloon placed in the abdominal cavity. Respiratory system, chest wall, and lung pressure-volume (PV) relation- ships were measured on deflation from total lung capacity to residual volume, as well as in the tidal breathing range, before and 15 min after abdominal pressure was raised. Increasing abdominal pressure from 3 to 15 cmH,O decreased total lung capacity and functional residual capacity by -440% and shifted the respiratory system and chest wall PV curves downward and to the right. Much smaller downward shifts in lung deflation curves were seen, with no change in the transdiaphragmatic PV relationship. All regional pleural pressures increased (became less negative) and, in the dependent region, approached 0 cmH,O at functional residual capacity. Tidal compliances of the respiratory system, chest wall, and lung were decreased 43, 42, and 48%, respectively. AD markedly alters respiratory sys- tem mechanics primarily by “stiffening” the diaphragm/abdo- men part of the chest wall and secondarily by restricting lung expansion, thus shifting the lung PV curve as seen after chest strapping. The less negative pleural pressures in the dependent lung regions suggest that nonuniformities of ventilation could also be accentuated and gas exchange impaired by AD. abdomen; diaphragm; regional ventilation; pregnancy; ascites; lung mechanics; respiratory mechanics; chest restriction REDUCTIONSINTOTALLUNGCAPACITY (TLC)andfunc- tional residual capacity (FRC) occur in pregnant women as well as in patients with ascites (1,16). With pregnancy the alveolar-to-arterial 0, difference is increased, as is lung resistance, whereas dynamic compliance is report- edly unchanged (5, 9, 14). Although it is obvious that abdominal distension (AD) should affect respiration, the mechanisms by which this occurs have not been fully explained. In the upright posture pleural pressure increases (be- comes less negative) from the upper (nondependent) to the lower (dependent) lung regions (2, 12, 20). Regional lung volume (VL) and ventilation are determined in large part by the regional transpulmonary pressure (Ptp,r) (19), by the changes in Ptp,r that occur with ventilation, and by regional lung compliance (CL). Conditions predis- posing to reductions in FRC or local transpulmonary pressure also predispose to nonuniform ventilation and, consequently, to widened alveolar-to-arterial 0, differ- ence. The respiratory system is commonly divided into two components: the chest wall and the lung. Although the diaphragm forms the caudal boundary of the chest cav- ity, the diaphragm is mechanically coupled to the abdomi- nal wall and contents. Any change in AD should, there- fore, alter chest wall characteristics. Increased abdomi- nal volume (Vab) will change abdominal pressure (Pab) and diaphragmatic pressure (Pdi) and could indirectly affect regional pleural pressures (Ppl,r). As a result, re- gional FRC and CL in the tidal volume range could also be altered by AD. We hypothesized that AD would primarily reduce chest wall compliance (Ccw) but also, by lowering VL, would reduce Ptp,r at FRC and thus CL as well. METHODS Animal prepara tion 13.3-15.4 kg were use . (Fg . Id f:r 1). Five piglets weighing the experiments. Xylazine (Rompun, 18 mg/kg) and ketamine HCl (2 mg/ml) were injected intramuscularly followed by pentobarbital so- dium (20 mg/kg) intravenously. Subsequent doses of pentobarbital were given as needed and before adminis- tration of pancuronium bromide (3 mg). Pancuronium was given 15 min before recording the first pressure- volume (PV) curve and again (2 mg) 15 min before record- ing the second. After tracheotomy and endotracheal tube insertion all pigs were ventilated with 100% 0, (Harvard Apparatus, model 618, tidal volume = 150 ml, frequency = 24-28/ min). A catheter was inserted through the right jugular vein into the superior vena cava to administer medications. A second catheter was inserted through the right femoral artery into the aorta to monitor systemic blood pressure and heart rate. Gastric balloon. A gastric pressure-monitoring balloon (6 cm long) made from condom rubber was inserted through the lumen of a 16F catheter and positioned at 2611