Ineffective Orifice Area: Practical Limitations of Accurate EOA Assessment for Low-Gradient Heart Valve Prostheses.

The goal of this study was to demonstrate the range in effective orifice area (EOA) values that may be possible given the ISO 5840 definition of EOA and the practical limits in the accurate measurement of pressure differential across large diameter valves. A 31 mm mechanical valve was tested on a commercially available pulse duplicator configured for mitral valve testing and tuned to nominal conditions. The experimental data was used as a basis for performing Monte Carlo analyses with published specifications for commonly used pressure sensors as well as measurement equipment accuracy requirements described in ISO 5840. The sources of error were modeled as normally distributed random variables and the simulation was iterated 1,000,000 times. Experimentally-derived EOAs ranged from 2.7 to 5.0 cm2, while the Monte Carlo simulation provided results ranging from approximately 0.4 to 6.7 cm2. Many of these results are clearly non-physical with EOAs larger than the valve’s geometric orifice area and exceedingly short positive pressure differential periods, yet they align with other published results for the same valve model. The volatility of the standard EOA formulation at low mean gradients combined with the difficulty in accurately measuring such small differentials with industry-standard fluid pressure transducers results in a performance metric which is very sensitive to test execution, particularly for low-gradient prostheses.