Valve resistance.

W e recently proposed that hemodynamic resistance is a useful index of valvular stenosis,' and there have since been additional studies investigating this index.2.3 The proven usefulness of calculated valve area in describing stenosis raises the important question of why such a proposal was made. The question is all the more pertinent because resistance is calculated from the same data as area. This editorial is an attempt to answer the question directly. In our view, the benefits of this index derive as much from the new ideas that emerge from the concept of resistance as from its greater accuracy in describing stenosis' and predicting outcome.2 We offer four reasons, described separately below, for proposing that resistance be used, at least as an adjunctive index, in describing stenosis: (1) the theoretical basis for the area calculation is incorrect; (2) resistance appears to remain more constant than calculated area when flow varies; (3) further hemodynamic calculations follow naturally from resistance but not from calculated area; and (4) resistance suggests further useful studies of stenotic valves, whereas calculated area does not. The first two of these reasons suggest that resistance might be a more satisfactory or a more accurate index of stenosis, and a recent clinical study showed that resistance gives a better measure of the severity of stenosis when cardiac output is low.2 We regard these as the less important reasons for our proposal. We believe that, despite its limitations, calculated valve area is a useful clinical index and should be retained. To us, the more important reasons for our proposal are the last two: in essence, that the use of resistance suggests future directions for hemodynamic investigations. A major point to be made is that resistance can be used to predict the relation between pressure gradient and flow across the stenotic valve. Whether resistance remains constant or not, a knowledge of the relation between pressure and flow can be very helpful in understanding the hemodynamic effects of stenosis.