Pre-Clinical Investigation
Localized Transvalvular Pressure Gradients in Mitral Bileaflet Mechanical Heart Valves and Impact on Gradient Overestimation by Doppler

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Background

It has been reported that localized high velocity may be recorded by continuous-wave Doppler interrogation through the smaller central orifices of bileaflet mechanical heart valves (BMHV) and that this may result in overestimation of the transvalvular pressure gradient (TPG). However, the prevalence and clinical relevance of this phenomenon remain unclear, particularly for BMHVs in the mitral position. The objective of this in vitro study was to assess the presence and magnitude of localized high velocity in mitral BMHVs as well as its impact on TPG overestimation by Doppler.

Methods

Nine BMHVs were tested under nine different flow conditions (volumes and flow waveforms) in a simulator specifically designed to assess mitral valve hemodynamics. Flow velocity was measured at three different locations (leading edge, midleaflets, and trailing edge) within the central and lateral orifices of the BMHVs using pulsed-wave Doppler. TPG was measured by pulsed-wave and continuous-wave Doppler and by catheterization.

Results

The maximum flow velocity occurred within the central orifice of the BMHV in 61% of the 81 tested conditions. This locally higher velocity within the central orifice predominantly occurred at the leading edge of the prosthesis. Doppler overestimated mean TPG by an average of 5% to 10% compared with catheterization. The magnitude of the localized high velocity and ensuing overestimation of TPG by Doppler was more important at higher mitral flow volumes (P < .0001) as well as in BMHVs with smaller internal ring diameters (P < .0001).

Conclusions

This study shows that the flow velocity distribution within the three orifices of mitral BMHVs is not uniform and that higher velocity occurs more frequently, but not always, within the inflow aspect of the central orifice. In most mitral BMHVs and flow conditions, this localized high-velocity phenomenon causes small overestimation of TPGs (<2 mm Hg and <10%) by Doppler and is thus not clinically relevant. However, in small mitral BMHVs exposed to high flow rates, the overestimation of TPG due to localized high velocity could become more important and overlap with the range of gradients found in patients with prosthesis dysfunction or prosthesis-patient mismatch.

Section snippets

In Vitro Model

For the purposes of this study, several models and sizes of BMHVs were tested under various pulsed flow conditions in a pulse duplicator previously developed and validated to mimic physiologic and pathologic mitral and aortic flows.8 Briefly, two anatomically shaped deformable silicon molds of the left atrium and ventricle were enclosed inside two separate Plexiglas boxes (called activation boxes; Figure 1A). The molds were activated (compressed or stretched) by adding or removing a volume of

Location of the Maximum Flow Velocity

Figure 3 shows the distribution of the occurrence of the maximum flow velocity within the three orifices of the BMHV according to BMHV model, BMHV internal ring diameter, mitral diastolic flow volume, and mitral flow E/A ratio. Overall, the maximum flow velocity occurred within the central orifice of the BMHV in 61% of the 81 tested conditions. The highest velocity was observed in the LatR orifice in 28% of the cases and in the LatL orifice in only 11% of the cases. The valve size as documented

Discussion

This study confirms the nonuniformity of the flow velocity distribution within the three orifices of BMHVs and the existence of locally higher velocities and TPGs. The main findings are as follows: (1) the maximum flow velocity occurs more frequently, but not always, within the central orifice of the prosthesis; (2) this locally higher velocity within the central orifice predominantly occurs at the leading edge of the prosthesis; (3) Doppler overestimates TPG by an average of 4% to 11% compared

Conclusions

This study shows that the flow velocity distribution within the three orifices of functional mitral BMHVs is not uniform and that higher velocity occurs more frequently within the inflow aspect of the central orifice. In most mitral BMHVs and flow conditions, this localized high-velocity phenomenon causes small overestimation of TPGs (<2 mm Hg and <11%) by Doppler and thus does not pose diagnostic ambiguity with other potential causes of high gradients.

However, in small mitral BMHVs exposed to

Acknowledgments

We thank Johanna Internicola for her work during her internship. We also thank Mrs. A. Anolli, marketing director of cardiac surgery at St. Jude Medical, for providing the St. Jude valves and Mr. Gilbert Pommepuy, of Force 3 Medical Inc., for providing the On-X valves.

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This work was supported, in part, by research grant MOP 102737 from the Canadian Institutes of Health Research (Ottawa, ON, Canada). P.P. holds the Canada Research Chair in Valvular Heart Diseases supported by the Canadian Institutes of Health Research.

Robert M. Lang, MD, FASE served as Guest Editor on this article.

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