Prior to each test, the zero-load setting on the ergometer was checked, and a calibration was performed with a 4-kg weight. Ventilatory and pulmonary gas exchange variables were measured breath-by-breath in all tests using a portable system (K4b 2 , COSMED) which has been previously validated at HA ; the accuracy of the telemetric system has been previously established [18, 19]. The system comprised a face mask, analyzer unit (containing O₂ and CO₂ gas analyzers), heart rate monitor, and battery. The analyzer unit with battery pack, face mask, and tubing (weight 0.8 kg) was attached to the subject with a harness and connected to a personal computer by an Ethernet cable connection. The face mask contained a turbine for measurement of volume and flow; calibration was performed with a 3-l syringe (Hans Rudolph, Kansas City, MO, USA) over a range of different flow profiles. Respired gas, sampled continuously from a port within the turbine via a Nafion polymer capillary (PermaPure©, Toms River, NJ, USA), was analyzed at 100 Hz using rapid-response O₂ (polarographic) and CO₂ (infrared) analyzers (mean response time 120 ms) which were automatically thermostated and compensated for ambient variations in barometric pressure, humidity, and environmental temperature. Analyzers‘ calibration was performed using two precision-analyzed gas mixtures spanning the respired range. The volume and gas concentration signals were sampled and digitized every 10 ms, and time-aligned, i.e., correcting for the transport delay between the turbine and gas analyzers and for the analyzer rise time . HR was measured from a chest strip and recorded every breath. SpO₂ was monitored non-invasively by finger pulse oximetry (Masimo Rad-5, Masimo Corporation, Irvine, CA, USA).

Analytical analyses

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Differences among measured responses were determined by a Student’s paired t test. Pearson’s product–moment correlation coefficient (R 2 ) was used to identify correlations between criterion variables. The level of statistical significance was set at P < 0.05. Group data are presented as mean ± SD. The limits of agreement between the V ? E / V ? CO 2 and the ? V ? E / ?HR methods for VCP estimation were evaluated by the Bland-Altman analysis , where the individual differences are plotted against their respective means. We proceeded with such a type of analysis if a significant linear correlation between methods was previously observed. The same statistical approach was also performed to compare the correspondence between ?_L and the breakpoint between S₁” and S₁‚ in the ? V ? E / ?HR relationship, when the latter was detectable.

Results

The main results of the incremental tests are illustrated in Table 2. At HA, compared to SL, a significant reduction in V ? O 2 heights , WR_peak, HR_peak, SpO_2peak, and ?_L was observed; on the contrary, V ? Epeak at HA was appreciably higher.

Figures 1 and 2 show ? V ? E / ? O 2 , V ? E / V ? CO 2 , P_EtO₂, P_EtCO₂ vs. V ? CO 2 , and ? V ? E / ?HR relationship in two representative subjects at SL (upper panels) and HA (lower panels). As shown in Figure 3, in all subjects, a breakpoint in ? V ? E / ?HR , which occurred at the VCP estimated by the V ? E / V ? CO 2 method, was clearly discernible both at SL and HA. No significant differences were found in V ? O 2 measured at VCP ( V ? CO 2 -VCP; Table 3) between methods utilized to identify the threshold (VCP- ? V ? E / ?HR vs. VCP- V ? E / V ? CO 2 ).