A Systematic Method to Detect the Metabolic Threshold from Gas Exchange during Incremental Exercise

Abstract

Incremental exercise consists of three domains of exercise intensity demarcated by two thresholds. The first of these thresholds, derived from gas exchange measurements, is defined as the metabolic threshold (V̇O₂θ) above which lactate accumulates. Correctly and reliably identified, V̇O₂θ is a non-invasive, sub-maximal marker of aerobic function with practical value. This investigation compared variability in selection of V̇O₂θ among interpreters with different levels of experience as well as from auto-detection algorithms employed by a commercially available metabolic cart (MC). Ten healthy young men performed three replicates of incremental cycle exercise during which gas exchange measurements were collected breath-by-breath. Two experienced interpreters (E) and four novice interpreters (N) determined V̇O₂θ from plots of specific response variables. Interpreters noted methods used and confidence in their selections. V̇O₂θ was automatically determined by the MC. Interclass correlations indicated that E agreed with each other (mean difference, 21 mL·min^-1) and with the MC (23 mL·min^-1), but not with N (-664 to 364 mL·min^-1); N did not agree among themselves. Despite good overall agreement between E and MC, differences >500 mL·min^-1 were seen in 50% of individual cases. N expressed unduly higher confidence and used different V̇O₂θ selection strategies compared with E. Experience and use of a systematic approach is essential for correctly identifying V̇O₂θ. Current guidelines for exercise testing and interpretation do not include recommendations for such an approach. Data from this study suggests that this may be a serious shortcoming. Until an alternative schema for V̇O₂θ detection is developed prospectively, strategies based on the present study will give practitioners a systematic and consistent approach to threshold detection.

Keywords: Incremental exercise testing; lactate threshold; metabolic threshold; oxygen uptake.

Figure 1.

The carbon dioxide output (V̇CO₂) versus oxygen uptake (V̇O₂) relationship. The metabolic threshold (V̇O₂θ) is identified at the point at which V̇CO₂ increases out of proportion to the increase in V̇O₂. V̇O₂θ is identified on the x-axis at a value of 1.40 L/min. The horizontal line at V̇CO₂ = 1.75 L/min represents the ventilatory threshold (V̇CO₂θ) as determined in Figure 2. The line of identify intersects the origin of the x- and y-axes.

Figure 2.

The minute ventilation (V̇_E) versus V̇CO₂ relationship. The vertical line at V̇CO₂ = 1.75 L·min^-1 identifies the ventilatory threshhold V̇CO₂θ.

Figure 3.

Panel A. The ventilatory equivalents for oxygen (V̇_E/V̇O₂) and carbon dioxide (V̇_E/V̇CO₂). V̇O₂θ, the first threshold, is identified by the dual criteria of a systematic increase in V̇_E/V̇O₂ without an increase in V̇_E/V̇CO₂. V̇O₂θ, the second threshold, is identified by a systematic increase in V̇_E/V̇CO₂. Panel B. End-tidal gas tensions for oxygen (P_ETO₂) and carbon dioxide (P_ETCO₂). V̇O₂θ, the first threshold, corresponds with the requirement that P_ETO₂ systematically increases while P_ETCO₂ does not decrease. Note that V̇O₂θ is identified at V̇O₂ = 1.40 L/min. V̇O₂θ, the second threshold, corresponds with the requirement that P_ETCO₂ systematically decreases.

Figure 4.

Linear regressions of V̇O₂θ selected by pairs of interpreters. Data points are means of triplicate trials. The diagonal dashed line represents the line of identity (x = y); the solid diagonal line is the regression line; 95% confidence limits are shown by the curved lines. Panel A shows E1 versus E2. Panels B-E show the mean V̇O₂θ selected by N1, N2, N3, and N4, respectively versus the mean of E1 and E2. Panel F is the regression of the mean V̇O₂θ selected by auto-detection by the metabolic measurement system (MC) versus the mean of E1 and E2.

Figure 5.

Bland-Altman plots of the difference between the selections of V̇O₂θ by pairs of interpreters versus the mean of their selections. The solid horizontal line represents the mean bias between the pairs of interpreters; the two dashed lines represent the 95% confidence limits of the bias. Each data point is the average of three trials for the ten subjects. Panel A shows E1 vs E2; Panels B-E show the mean of E1 and E2 selections of V̇O₂θ versus those of N1-N4; Panel F shows the mean of E1 and E2 selections versus the MC automated selection of V̇O₂θ.