What is the timing of the movement at the mid-to-lower left sternal edge? When we understand the timing, we can also define the significance. Let's again use the cotton swab and note there is not only a systolic impulse at the lower left sternal edge that is timed with the systolic impulse at the upper left sternal edge, there is also a presystolic impulse at the lower left sternal edge. Let us look together. [Cut-away]

And, what was the significance of the precordial impulse we felt at the mid-to-lower left sternal edge? Well, to begin with, impulses in that area reflect pathology of the right ventricle. We felt a systolic sustained impulse. That impulse says that the right ventricle is hypertrophied and/or dilated. But, in addition, we felt an impulse prior to systole and the presystolic impulse tells us that there is a reduced compliance of the right ventricle and, as the atrium contracts, as blood is ejected into the right ventricle, it meets that resistance, vibrations are set up and appreciated on palpation as a presystolic impulse.