Pulse pressure variation, stroke volume variation and dynamic arterial elastance

We read with interest the recent article by Monge Garcia and colleagues [1]. We have two comments regarding this interesting physiological study.

First, although we support their observation [2], their explanations may lead to some confusion. Indeed, the Edwards Vigileo™ system-FloTrac™ sensor calculates stroke volume (SV) using the equation:

where σAP is the standard deviation of the arterial pressure (AP) curve and Khi a constant quantifying arterial elastance and vascular resistance [3]. As stroke volume variation (SVV):

Which means that SVV (%) ≈ pulse pressure variation (PPV; %) for FloTrac™.

With the present mathematical equation, it can be demonstrated that the SVV calculated by FloTrac™ (SVV_FT) is not influenced by Khi, which means that SVV_FT calculation does not include effective arterial elastance (Ea). The originality of Garcia and colleagues' finding is that they realized that when measuring PPV conventionally [4], the ratio PPV/SVV_FT becomes a mirror of a dynamic Ea, as the conventional PPV selected integrates Khi (Figure 1). However, when the present innovative method is used, PPV and SVV should be sampled during the same period of time.

Figure 1

Model of two different arterial elastances (Ea and Ea) on the pulse pressure variation (PVV) relationship curve, with the same stroke volume variation (SVV) value.

In conclusion, we believe that dynamic Ea predicting arterial pressure response to volume loading in preload-dependent patients is an interesting physiological concept. However, this demonstration cannot be achieved without taking into consideration the impact of PPV/SVV sampling.