Longitudinal and radial distensibility of the porcine aortic root

The aortic root has been shown to be a highly distensible structure. The function of the aortic valve is intimately related to the expansion of the aortic root, and current nonexpansible stent designs may affect its performance. We therefore measured the radial and longitudinal expansion of the porcine aortic root as a function of pressure in both a static pressurization model and in an isolated working heart model. The radial and longitudinal expansion of the aortic root was measured using a custom-built digital sonomicrometer. Multiple ultrasonic crystals were sutured exterior to the commissures and along the length of the aortic root, and their separation was tracked at varying aortic pressures. In static testing, we found that commissural separation at zero pressure was 26% +/- 7% (mean +/- standard deviation) less than at 120 mm Hg, whereas the longitudinal distance between the base of the valve and the commissures decreased by 11% +/- 9%. Approximately one quarter of the total dimensional change occurred over the physiologic range of 80 to 120 mm Hg. In the isolated porcine heart model, we measured a greater distensibility than in the static tests. For example, at aortic pressures of 120/80 mm Hg (systolic/diastolic), the diameter of the aortic root would be 22% +/- 6% less at 80 mm Hg than at 120 mm Hg. The longitudinal dimensions would be 15% +/- 8% less at 80 mm Hg than at 120 mm Hg. We conclude that the aortic root contracts significantly when depressurized, as during valve replacement surgery, and that the in vivo distensibility of the aortic root is much greater that what is generally measured in vitro. These results suggest that dimensional changes in the implanted prosthetic valve and the recipient aortic root must be considered to achieve both optimal valve orifice and, in the case of distensible valves such as allografts, a proper valve cusp geometry.