The effects of positive expiratory pressure on peritoneovenous shunt flow

Cirrhotic patients with peritoneovenous shunts may require mechanical ventilation. Despite the importance of flow to shunt patency and the relevance of intrathoracic pressure to that flow, the relationship between shunt flow and positive airway pressure has not been documented. To study the effects of positive expiratory pressure (PEEP) on shunt flow, models of ascites (n = 8) were created in adult male mongrel dogs. Each animal was anesthetized, intubated, and mechanically ventilated. Peritoneovenous shunts with in-line electromagnetic flow meters were surgically placed. Shunt flow, central venous pressure (CVP), and intraabdominal pressure (IAP) were monitored. Initial intraabdominal pressures were adjusted by infusion of warmed saline and positive expiratory airway pressures were added in increments. Changes in pressures (IAP, CVP) and shunt flow were tabulated and analyzed with linear and polynomial regression. Intraabdominal and central venous pressures increased linearly with PEEP at different rates such that IAP-CVP varied inversely with PEEP. Shunt flow varied inversely as a polynomial function of PEEP. Analyses of these relationships allowed creation of a nomogram which can be interpolated to indicate required intraabdominal pressure needed to maintain shunt flow throughout the clinically useful range of positive airway pressure.