Heart Monitor

FIG. 1 is a front-perspective view of a heart monitor in accordance with the design; FIG. 2 is a front-elevation view thereof; FIG. 3 is a rear-elevation view thereof; FIG. 4 is a right-side view thereof; FIG. 5 is a left-side view thereof; FIG. 6 is a top view thereof; and, FIG. 7 is a bottom view thereof. The broken lines present in the drawings illustrate portions of the heart monitor that form no part of the claimed design.