Quantification and physiological significance of the rightward shift of the V-slope during incremental cardiopulmonary exercise testing

Background: Ventilatory anaerobic threshold (VAT) is frequently used as a measure of exercise tolerance, with the V-slope method being the standard; however, this needs to be visually determined. Over the years, we have observed that the V-slope itself often appears to shift rightward before the appearance of the VAT (RtShift: rightward shift of V-slope). This phenomenon has long been known to occur during the first 1-2 min of steady-state exercise and disappears thereafter; it is attributed to CO₂ storage, presumably in active muscle. However, during incremental exercise, we have observed that the RtShift persists; furthermore, it seems to be related to the level of VAT. Therefore, we attempted to objectively quantify the RtShift, and to confirm its relationship to an index of exercise tolerance (VAT).

Methods: This study was based on a retrospective analysis of data from 100 cardiopulmonary ramp exercise tests (submaximal) performed by patients with cardiac disease. VAT was determined with the visual V-slope method. The horizontal distances between the diagonal R = 1 line and each data point on the V-slope plot to the right of R = 1 were measured; the average of these measurements was used as an objectively determined estimate of RtShift.

Results: The predominant portion of RtShift occurred earlier than VAT. The mean RtShift was 33.9 ± 25.0 mL⋅min^-1 VO₂, whereas the mean VAT was 635 ± 220 mL⋅min^-1. RtShift positively correlated with VAT (r = 718, p < 0.001), confirming previous visual observations. It also significantly correlated with ΔVO₂/Δwork rate, a marker of oxygen uptake efficiency (r = 0.531, p < 0.001).

Conclusions: We identified that among patients with cardiac disease, V-slope is shifted rightward to varying degrees. The objectively quantified rightward shift of V-slope is significantly correlated with an index of exercise tolerance (VAT). Furthermore, it appears to occur at even lower work rates. This may offer a new objective means of estimating exercise tolerance; however, its exact biological basis still needs to be elucidated.