The ornamental design for a sphygmomanometer, as shown and described.
FIG. 1 is a perspective view of a sphygmomanometer showing our new design;
FIG. 2 is a top view thereof, showing it as viewed from the front;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top view thereof, showing it as viewed from the top;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a front elevational view thereof.
The alternating dot dash lines immediately adjacent the shaded areas represent the boundaries of the claim, and all other broken lines depict environmental subject matter. None of the broken lines form a part of the claimed design.