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Human ventilation during a progressive exercise to maximal attainable levels, expressed as a function of oxygen consumption (VO2), shows two successive inflection points or ventilation thresholds (VT). Employing the terminology of Skinner-McLellan-Kinderman, the lower ventilation threshold (VT1) is suggested to coincide with the onset of blood lactate accumulation and the upper ventilation threshold (VT2) is possibly of a neurogenic origin. At close to 70-80% of the maximal attainable work-rate, core temperature thresholds for ventilation were also demonstrated (White and Cabanac 1996). However, it is not known if the work rates at these core temperature thresholds for ventilation are the same or different than the work rates at VT1 and VT2. The purpose of the present study was to compare the VO2 at the core temperature thresholds for ventilation to both VT1 and VT2. The goal was to address if these three separate ventilation thresholds are similar or different physiological or metabolic responses. Six fit male, college-aged subjects pedaled a cycle ergometer on 2 occasions in progressive exercise tests until the point of exhaustion. In one exercise session work rate was increased by 20W/2 min (slow ramp) and in the other by 40W/2 min (fast ramp). Subjects were instrumented for esophageal temperature (Tes), skin temperatures and their expired gases were collected to assess oxygen consumption (VO2), carbon dioxide production (VCO2) and minute ventilation (VE). In both exercise sessions, ventilatory equivalents for oxygen consumption (VE/VO2) and carbon dioxide production (VE/VCO2) were plotted as a function of both Tes and VO2. These plots allowed the determination of VO2 at each of the Tes threshold for ventilation, VT1 and VT2. In the slow ramp session the VO2 of 2.85± 0.23 l/min at the Tes threshold for VE/VO2 (p< 0.01) and the VO2 of 2.44± 0.19 l/min at the Tes threshold for VE/VCO2 (p< 0.01) were significantly greater than the VO2 of 1.81± 0.11 l/min at VT1. In the fast ramp session the VO2 of 2.87±0.16 l/min at the Tes threshold for VE/VO2 (p< 0.001) and the VO2 of 2.68±0.15 l/min at the Tes threshold for VE/VCO2 (p< 0.01) were both significantly greater than the VO2 of 1.15±0.23 l/min at VT1. Comparisons of VO2 at the Tes threshold for ventilation to VO2 at VT2 gave different results. In the slow ramp session the VO2 at the Tes threshold for VE/VO2 was not significantly different than the VO2 of 3.01 ± 0.17 l/min at VT2. The VO2 at the Tes threshold for VE/VCO2 in the same slow ramp session was significantly lower (p <0.01) than VT2. In the fast ramp session the VO2 at the Tes thresholds for VE/VO2 and VE/VCO2 were both not significantly different than the VO2 of 2.71 ± 0.18 /min at VT2. In conclusion, the VO2 at the Tes thresholds for ventilation is significantly greater than the oxygen consumption at VT1 and appears not to be different than oxygen consumption at VT2. The results appear to support that VT1 is a ventilatory response occurring at significantly lower VO2 than either VT2 or the Tes threshold for ventilation.
White, M.D. & Cabanac, M. (1996) Exercise Hyperpnea and hyperthermia in human. J. Appl. Physiol. 81, 1249-1254.