What makes the peritoneal drain work? Pressure in the subhepatic space after biliary surgery.

To determine the nature of the forces responsible for the evacuation of fluid from the peritoneal cavity via a drain we measured the pressure in the subhepatic space after biliary surgery. Pressure in that space increased with inspiration and decreased with expiration. The inspiratory increase was larger in the erect than in the supine position. Evacuation of fluid from the subhepatic space via a drain results from intermittent increase in pressure such as occurs during deep inhalation. Drainage is improved by maximal inspiration in the erect position.