Time-based V-slope for determination of anaerobic threshold

The standard method for determining the noninvasive anaerobic threshold (AT) is the V-slope (VS) method. It is often supplemented by the ventilatory equivalent (VEQ) and end-tidal (ET) methods. However, a comparison issue arises: VS plots VCO₂ against VO₂ (x-axis), whereas VEQ and ET conventionally plot each variable against elapsed exercise time. This study aimed to develop a time-based VS graph to align all three methods on the same x-axis.

V-slope is drawn as a square with equal sides, with the diagonal line representing the respiratory exchange ratio (R) = 1. V-slope generally follows the R = 1 line. This provides a framework in which the vertical difference (d) between a VCO₂ data point and the corresponding point on the R = 1 line may be used and plotted against time; that is, d equals the measured VCO2 minus VCO2 on the R = 1 line. Since VCO2 equals VO2 on the R = 1 line, d equals VCO₂-VO₂. This new measure, d (mL/min, VCO2), combines the relation between VCO2 and VO2 into a single value, which now can be plotted against time. In this graph the diagonal of the conventional V-slope (R = 1) becomes the horizontal x-axis (time). The plot using this modification of VS identifies AT as the first upward inflection point against a flat baseline. Data from 127 subjects (age: 51 ± 21; 66 healthy, 20 treated for cardiovascular risk factors, 41 with cardiac disease) who underwent maximal symptom-limited exercise were analyzed. Each of the four AT methods (d, VS, VEQ, ET) was independently assessed in a blinded manner. Reliability was evaluated using Altman's coefficient of repeatability (CR). Statistical comparison of variability of each detection method was performed after the squared difference was log transformed because the data was not normally distributed.

The mean AT (mL/min) and detection rates for d, VS, VEQ, and ET were 857 ± 414 (93%), 939 ± 407 (96%), 946 ± 176 (82%), and 862 ± 379 (85%), respectively. The d method identified AT significantly earlier than the conventional VS did (p < 0.001). The CR of d, VS, VEQ, and ET was 117, 115, 152, and 175 mL/min, respectively. The mean squared difference of d method was significantly smaller than those of VS, VEQ or ET (p < 0.001), indicating improved reliability of AT detection by d. The collective reviewing of all three time-aligned graphs (d, VEQ, ET) revealed that delayed or suppressed ventilatory responses to CO₂ (d vs VEQ) in 48 subjects (38%).

The time-based V-slope was created by plotting changes in VCO2 from the R = 1 diagonal line against time; it aligned all three AT detecting methods on the same time axis, allowing precise detection of differences between methods. It also made AT detection more reliable, probably because the upward inflection breakpoint arises from the flat baseline and does not have the overlapping data points seen in V-slope, making it easier to identify a breakpoint. The d graph also identified AT significantly earlier than V-slope did.