A series of experiments was devised to test the resistance to flow through the lungs of blood samples with different screen filtration pressures (SFP). The pulmonary artery and left atrial presures were measured in dogs rendered acutely hypotensive by rapid bleeding, and in others subjected to severe soft tissue trauma. Following acute hypotension the SFP always increased before and more markedly in the pulmonary artery than it did in femoral artery or vein blood. In other dogs similar measurements were made in normotensive dogs perfused with blood having a high screen filtration pressure. In each of these series of experiments the pulmonary artery pressure and difference between it and the left atrial pressure increased when the blood of the animal developed a high SFP (during hypotension and after trauma), or when high SFP blood was infused into the animal. It was suggested that the high SFP was due to adhesive and aggregated blood cells, especially platelets and leukocytes, and that these aggregates had occluded numerous blood conduits of the lungs.
Acute hypotension was produced in heparinized and nonheparinized dogs by hemorrhage and by injections of histamine, and adhesiveness and aggregation of blood elements determined periodically by the screen filtration pressure method. This method measures the pressure required to force blood at a constant rate through a standardized screen with multiple square holes each 20 x 20 micra. In some cases the arterial blood was perfused through a glass wool filter in an extracorporeal system both before and during periods of hypotension. The results of these studies justify the following conclusions:
1. Increased adhesiveness and aggregation of platelets and other blood elements which occur during hypotension produced by hemorrhage or intravenous injections of histamine were indicated by an increase in the screen filtration pressure of blood. These changes were not prevented from developing, or significantly altered by heparinization of the animal.
2. The amount of blood which could be obtained during arterial hemorrhage was significantly increased by heparinization of the animal.
3. The adhesive and aggregated platelets and leucocytes, which formed during hypotension, were removed from the blood when it was perfused through a column of glass wool. This treatment was attended by a decrease in the screen filtration pressure of the blood to or below normal.
4. The increase in screen filtration pressure which occurred during hypotension was much less marked if the animals' blood had first been circulated through pyrex glass wool.
5. Filtration of blood through pyrex glass wool was also attended by a progressive decrease in the mean arterial blood pressure of the animal.
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