Does the threshold of transcutaneous partial pressure of carbon dioxide represent the respiratory compensation point or anaerobic threshold?

On reaching the respiratory compensation point (RCP) during rapidly increasing incremental exercise, the ratio of minute ventilation (VE) to CO2 output (VCO2) rises, which coincides with changes of arterial partial pressure of carbon dioxide (PaCO2). Since PaCO2 changes can be monitored by transcutaneous partial pressure of carbon dioxide (PCO2,tc) RCP may be estimated by PCO2,tc measurement. Few available studies, however, have dealt with comparisons between PCO2,tc threshold (TAT) and lactic, ventilatory or gas exchange threshold (VAT), and the results have been conflicting. This study was designed to examine whether this threshold represents RCP rather than VAT. A group of 11 male athletes performed incremental exercise (25 W.min-1) on a cycle ergometer. The PCO2,tc at (44 degrees C) was continuously measured. Gas exchange was computed breath-by-breath and hyperaemized capillary blood for lactate concentration ([la-]b) and PaCO2 measurements was sampled each 2 min. The TAT was determined at the deflection point of PCO2,tc curve where PCO2,tc began to decrease continuously. The VAT and RCP were evaluated with VCO2 compared with oxygen uptake (VO2) and VE compared with the VCO2 method, respectively. The PCO2,tc correlated with PaCO2 and end-tidal PCO2. At TAT, power output [P, 294 (SD 40) W], VO2 [4.18 (SD 0.57) l.min.1] and [la(-)] [4.40 (SD 0.64) mmol.l-1] were significantly higher than those at VAT[P 242 (SD 26) W, VO2 3.56 (SD 0.53) l.min-1 and [la(-)]b 3.52 (SD 0.75), mmol.l-1 respectively], but close to those at RCP [P 289 (SD 37) W; VO2 3.97 (SD 0.43) l.min-1 and [la(-)]b 4.19 (SD 0.62) mmol.l-1, respectively]. Accordingly, linear correlation and regression analyses showed that P, VO2 and [la(-)]b at TAT were closer to those at RCP than at VAT. In conclusion, the TAT reflected the RCP rather than VAT during rapidly increasing incremental exercise.