2.3. Instrumentation and data collection

All data were collected using a 16‐channel PowerLab system (Powerlab/16SP ML880; AD Instruments; ADI) and analyzed offline using commercially available software (LabChart Pro software 8). Respiratory flow was measured through the use of a pneumotachometer (HR 800L flow head and spirometer amplifier; ADI ML141). Instantaneous ventilation (V̇_I, L/min) was determined as the product of breath‐by‐breath inspired volume (V _TI; calculated from the integral of the flow signal) and respiratory rate (R _R, min⁻¹; calculated by 60 per period of the flow signal). Breath‐by‐breath O₂ and CO₂ were sampled proximal to the mouth and measured in percent using a combined CO₂ and O₂ gas analyzer (ADI ML206), calibrated daily. The pressure of end‐tidal (P _ET)O₂ and P _ETCO₂ was calculated and corrected for BTPS in Torr using the daily atmospheric pressure (Calgary is ≈1,045 m with P _ATM of ≈665 mmHg). Calculating oxygen saturation (ScO₂; (%)) was performed using the previously described equation (Severinghaus, 1979) as in previous studies in our laboratory (Pfoh et al., 2016, 2017) and measured by a peripheral pulse oximeter (SpO₂, ADI ML320) placed on the index finger of the right hand. ScO₂ and SpO₂ were observed at all times to ensure values never fell below 70% saturation for safety. For cardiovascular variables, participants were instrumented for measuring heart rate (HR; ECG electrodes in lead II configuration using the ADI bioamp ML132) and beat‐by‐beat blood pressure (Finometer Pro, Finapres Medical Systems; calibrated for every subject). Instantaneous HR was calculated from the R–R interval (60 per period). Mean arterial pressure (MAP) was calculated as a mean from the raw finger photoplethysmography arterial pressure tracing. Participants kept their eyes closed throughout the protocol while listening to white noise via ear buds to minimize distraction, except when instructions on breath holding were provided.