Plots of ST-segment depression against HR during both the exercise and first three minutes of postexercise recovery (i.e., ST/HR diagrams) illustrating the computerized determination of the ST/HR hysteresis from a single lead.
In these diagrams, ST depression is plotted in an upward direction on the vertical axis, and the negative values represent ST elevation. Typical ST/HR diagrams are shown for a clinically normal subject (upper left panel) with an unambiguously negative hysteresis in ST depression against HR (i.e., clockwise hysteresis loop in recovery) and for a patient with coronary artery disease (upper right panel) with an unambiguously positive hysteresis in ST depression against HR (i.e., counterclockwise hysteresis loop in recovery). In the lower panels, more complex ST/HR diagrams are shown for a normal patient (lower left panel) and for a patient with coronary artery disease (lower right panel), in which cases the direction of the hysteresis in ST depression against HR was changed during the postexercise recovery phase. In graphical sense, the ST/HR hysteresis was obtained by subtracting the area of negative hysteresis from the area of positive hysteresis and dividing the resulted net area by the HRrec. HRexe = maximum HR; HRrec = the minimum heart rate of recovery; HRrec = HRexe-HRrec; 3v-CAD = three-vessel coronary artery disease.