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Figure 1 The digital volume pulse (DVP) and first derivative (dV/dt, lower trace) recorded before and after systemic administration of GTN (500 µg sublingually). The notch or point of inflection at height, b, is identified by the local maximum in the first derivative. The height of the inflection point (IP_DVP) is expressed as percent DVP amplitude, a. The IP_DVP falls after GTN. The time between the first and second peak of the DVP (T_DVP) was measured in some experiments.