Intra-abdominal Pressure

Intra-abdominal pressure has different values in different parts of the abdominal cavity at any given moment. The abdominal cavity is a hermetically sealed sac filled with fluid and organs of a semi-fluid consistency, which partly contain gases. This content exerts hydrostatic pressure on the bottom and walls of the abdominal cavity. Therefore, in the normal vertical position, the pressure is highest at the bottom, in the hypogastric region: according to the latest measurements by Nakasone, in rabbits it is +4.9 cm of water column. The pressure decreases in the upward direction; slightly above the navel it becomes equal to 0, i.e., atmospheric pressure; even higher, in the epigastric region, it becomes negative (-0.6 cm). If the animal is placed in a vertical position head down, the relationships are reversed: the region with the highest pressure becomes the epigastric region, and the lowest becomes the hypogastric region. In humans, intra-abdominal pressure cannot be measured directly; instead, one must measure the pressure in the rectum, bladder, or stomach, where a special probe connected to a manometer is inserted for this purpose. However, the pressure in these organs does not correspond to the intra-abdominal pressure, because their walls possess their own tension, which alters the pressure. Hormann found the pressure in the rectum of standing humans to be from 16 to 34 cm of water; in the knee-elbow position, the pressure in the rectum sometimes becomes negative, down to -12 cm of water. Factors that change intra-abdominal pressure in the sense of increasing it are 1) an increase in the contents of the abdominal cavity and 2) a decrease in its volume. In the first sense, accumulations of fluid in ascites and gases in meteorism act, and in the second, the movements of the diaphragm and the tension of the abdominal muscles. During diaphragmatic breathing, the diaphragm pushes into the abdominal cavity with each inhalation; although the anterior abdominal wall moves forward, since its passive tension increases in the process, the result is an increase in intra-abdominal pressure. During quiet breathing, intra-abdominal pressure has respiratory fluctuations within the range of 2-3 cm of water column. The tension of the abdominal muscles has a much greater influence on intra-abdominal pressure. When straining, one can obtain a pressure in the rectum of up to 200-300 cm of water column. Such an increase in intra-abdominal pressure is observed during difficult defecation, during childbirth, during "straining," when blood is squeezed out of the veins of the abdominal cavity, and also during the lifting of heavy weights, which can serve as a cause for the formation of hernias, and in women, for displacements and prolapse of the uterus.