Tear propagation in isolated, pressurized porcine thoracic aortas

To determine the pressure required for the initial medial tear, and for propagtion of dissections, within the media of isolated pressurized porcine thoracic aorta, and to determine whether these vary with tear depth.

India ink-stained saline was injected into the media of 48 fresh porcine descending thoracic aorta that were distended with 130 mmHg pressure. The fluid was infused into the media through a 25 gauge needle connected to a constant infusion pump, and the pressure at the entrance to the needle was monitored with a Cobe pressure transducer. The two lumens were not connected. Blebs were made at different depths and measured at the end of the experiment with a Starrett gauge. Seven casts were made in pressurized aortas and nine in nonpressurized ones to determine the shape of the blebs by injecting different volumes of mercox casting material into the media by hand.

Mean tearing pressure, expressed as the transmural pressure between the bleb and the true lumen (which was at 130 mmHg) was 547 mmHg (range 208 to 995). Mean propagation pressure was 54 mmHg (range 25 to 93). The ratio of the two pressures was 10.1 (range 5.2 to 21.7). None of these pressures was correlated with tear depth. Casts showed that the leading edge was sharp in all directions. Small blebs were roughly spherical, and large ones were cylindrical with roughly hemispherical ends between the cylinder and the sharp leading edge.

Dissections can propagate at pressures that could be reached under physiological and certainly pathological conditions. The initial tear requires pressures that are too high to create biologically except with trauma. The leading edge of the dissection appears to be very sharp and likely explains why the dissection propagates at relatively low pressures. Tear depth does not affect the results.