Nominal size in six bileaflet mechanical aortic valves: A comparison of orifice size and biologic equivalence

Objectives: Nominal size remains the standard by which valves are compared, but its relationship with orifice area and the patient tissue annulus diameter may differ according to valve design. The aims of this study were to measure the orifice size and compare biologic equivalence in six bileaflet mechanical heart valve designs. Methods: The inflow aspect of each of 29 valves was photographed then digitized, and the maximum internal diameter and orifice area were calculated. Biologic equivalence was assessed with a series of machined polypropylene blocks. Results: The orifice area ranged between 159 and 222 mm(2) for the six size 19 valves. The internal diameter ranged from 1.6 to 4.6 mm less than the manufacturer's nominal size. Biologic equivalence assessed from an estimate of tissue annulus diameter with machined blocks ranged from 1.0 and 3.5 mm larger than nominal size for the intra-annular valves. This diameter ranged from 3.5 mm smaller to 1.5 mm larger than nominal size for the supra-annular valves. Conclusion: There are major differences between nominal size and biologic equivalence. This may lead to confusion when attempting to make comparisons between different valve designs with the same nominal size. A clearer sizing nomenclature is required and could be based on in vitro assessment of tissue annulus diameter or an alphanumeric code.