How is the anaerobic threshold determined noninvasively in a cardiopulmonary stress test?

Updated: May 14, 2020
  • Author: Kevin McCarthy, RPFT; Chief Editor: Nader Kamangar, MD, FACP, FCCP, FCCM  more...
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Identifying the anaerobic threshold noninvasively: The noninvasive determination of the anaerobic threshold can be accomplished by analyzing time averaged (20- to 30-s intervals) plots of parameters measured or calculated during the CPX test. Two methods can be used, the V-slope method and the ventilatory equivalent method. Both methods allow determination of the same physiologic event, the increased production of carbon dioxide by isocapnic buffering of lactic acid produced by anaerobic metabolism and yield comparable estimations.

V-slope method: The V-slope method of determining the anaerobic threshold makes use of the fact that carbon dioxide production (VCO2) plotted against oxygen consumption (VO2) shows a slope of slightly less than 1 for work below the anaerobic threshold. A line of best fit for points obtained from the start of exercise is drawn through this plot to obtain the initial slope (S1). When this slope changes to a steeper slope (S2), it indicates an increase in carbon dioxide production from the isocapnic buffering of lactic acid. The intersection of S1 and S2 mark the anaerobic threshold, typically reported as either the absolute value of the oxygen uptake (VO2, mL/min) at that point or as the percentage of the predicted peak VO2.

Ventilatory equivalent method: The ventilatory equivalent method of determining the anaerobic threshold makes use of the derived values known as the ventilatory equivalents for oxygen and carbon dioxide. Carbon dioxide production (VCO2) and oxygen consumption (VO2) divided into the minute ventilation (VE, L/min) are known as the ventilatory equivalents for carbon dioxide (VE/VCO2) and oxygen (VE/VO2). When time averaged (20- to 30-s intervals) plots of VE/VO2 and VE/VCO2 are plotted against time, the point at which the VE/VO2 is seen to increase without a simultaneous increase in the VE/VCO2 marks the anaerobic threshold.

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