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Figure 1 Illustration of AP and inflection points used in analysis. P₁, P₂ and P_d, are the BP at the first and second shoulders and at end diastole, respectively.

In the current version of the PWV Medical SphygmoCor device, AI as defined here is labeled as AI_P2/P1. If the reported AI is greater than 100, then P₂ > P₁, that is the pressure at the second systolic shoulder, is greater than at the first. In terms of the classification of Murgo et al. (17), this corresponds to a type "A" waveform, whereas if P₂ < P₁, a type "C" waveform results. Kelly et al. (2) calculated AI as AI = (P₁ – P₂)/P, that is, AP divided by pulse pressure. This formula gives both positive and negative values of AI.