Fixed vs nonfixed coronary stenosis: the response to a fall in coronary pressure in a canine model.

The resistance of a coronary stenosis capable of passive vasomotion may change in response to alterations in intraluminal pressure. We studied the effects of a fall in perfusion pressure on the hemodynamics of two types of coronary stenosis in 12 open chest dogs: 1) a stenosis produced by placing a soft wire snare around the artery, and 2) a stenosis that was fixed in severity, produced by introducing and tying a short section of hollow plastic tubing into the coronary artery. Aortic pressure, coronary pressure proximal and distal to the stenosis, and coronary flow were measured. Pressure in the stenotic artery was lowered by tightening a snare placed very proximally on the artery. In arteries with a snare stenosis, lowering proximal coronary pressure by a mean of 31 mm Hg caused a rise in stenosis resistance from 0.86 +/- 0.06 to 1.38 +/- 0.14 units (p less than 0.001). However, in arteries with a fixed plastic stenosis, lowering the proximal coronary pressure by a mean of 30 mm Hg caused a fall in stenosis resistance from 0.36 +/- 0.04 to 0.21 +/- 0.03 units ( p less than 0.001). The increase in resistance of the snare stenoses as pressure was lowered may be attributed to passive narrowing of the stenotic segment, suggesting that these stenoses were not fixed in severity. However, the fixed plastic stenoses did not show an increase in resistance as pressure fell because the tubing was not capable of passively narrowing. Therefore, the hemodynamics of a coronary stenosis depend importantly on the type of stenosis that is present and whether it is capable of vasomotion.