Tear propagation in isolated, pressurized porcine thoracic aortas

OBJECTIVE: To determine the pressure required for the initial medial tear, and for propagation of dissections, within the media of isolated pressurize d porcine thoracic aorta, and to determine whether these vary with tear dep th. DESIGN: India ink-stained saline was injected into the media of 48 fresh po rcine descending thoracic aorta that were distended with 130 mmHg pressure. The fluid was infused into the media through a 25 gauge needle connected t o 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 conn ected. Blebs were made at different depths and measured at the end of the e xperiment with a Starrett gauge. Seven casts were made in pressurized aorta s and nine in nonpressurized ones to determine the shape of the blebs by in jecting different volumes of mercer casting material into the media by hand . RESULTS: Mean tearing pressure, expressed as the transmural pressure betwee n the bleb and the true lumen (which was at 130 mmHg) was 547 mmHg (range 2 08 to 995). Mean propagation pressure was 54 mmHg (range 25 to 93). The rat io of the two pressures was 10.1 (range 5.2 to 21.7). None of these pressur es was correlated with tear depth. Casts showed that the leading edge was s harp in all directions. Small blebs were roughly spherical, and large ones were cylindrical with roughly hemispherical ends between the cylinder and t he sharp leading edge. CONCLUSIONS: Dissections can propagate at pressures that could be reached u nder physiological and certainly pathological conditions. The initial tear requires pressures that are too high to create biologically except with tra uma. The leading edge of the dissection appears to be very sharp and likely explains why the dissection propagates at relatively low pressures. Tear d epth does not affect the results.