Use Of Graphically Depicting Time-based V-slope To Determine Gas Exchange Threshold

Three methods are commonly in use for the determination of gas exchange threshold (GET): v-slope(VS), ventilatory equivalent(VEQ) and end-tidal(ET). For VS, VCO2 is plotted against VO2 (x-axis), and for VEQ and ET, each variable is plotted against elapsed time(x-axis). When it appears that three is discrepancy between the results obtained by VS vs. VEQ or ET, it is often difficult to pinpoint precisely where discrepancy exists, because data points do not correspond on the same y-axis. PURPOSE: To devise a way to construct a time-based v-slope graph. METHODS: A new index, d, was created by subtracting VO2 from VCO2, and it was plotted against time. In Figure a & b, d = VCO2-VO2 (upper) is plotted against time, in the same manner as VEQ(lower) is. Fig(a) represents a case in which v-slope and VEQ agree (type-A response), while Fig(b) shows decreasing VEQ values despite increasing d, indicating inadequate ventilatory response to CO2 (type-B). The database of 127 subjects (age: 51 ± 21, 93 males, 66 healthy subjects, 20 being treated for CV risk factors, and 41 cardiac patients ) who underwent maximal symptom-limited exercise was used to assess the utility of this approach (d). Each of three GET methods was used independently and blinded to other 2 methods. RESULTS: The mean GET (mL/min) by obtained by VS, VEQ and ET was 882 ± 314, 1003 ± 387, and 869 ± 330, respectively (VEQ > VS or VE, p ≤ 0.05). In 2 cases, GET could not be detected by any method. The most remarkable finding was in 14 cases, in which both VEQ/ET could not detect GET, while VS could. This response was observed in 7 normal subjects and 7 cardiac patients. Type-B was observed in 11 of 14 these discordant cases, but only in 35 of 111 cases in which both VS and VEQ/ET could detect GET (p ≤ 0.001). CONCLUSION: Time-based v-slope graphically depicted a clear discrepancy when v-slope and other methods of detecting GET did not agree. Inappropriate ventilatory response to CO2 was the main cause of the discrepancy.