Preoperative cardiopulmonary exercise testing
Patients preoperatively performed incremental CPET up to volitional exertion in upright position on an electronically-braked cycle ergometer (Lode Corival, Lode BV, Groningen, the Netherlands). Prior to the test, patients were asked to refrain from vigorous physical activity, caffeine, and tobacco for 24 hours and meals for 2 hours, but to continue medication as usual. Seat height was adjusted to the participant’s leg length. Before commencing CPET, forced vital capacity and forced expiratory volume in one second was obtained from maximal flow-volume curves (Ergostik, Geratherm Respiratory, Bad Kissingen, Germany) according to ATS/ERS standards [During CPET, subjects breathed through a facemask (Hans Rudolph, Kansas City, MO, USA) connected to an ergospirometry system (Ergostik, Geratherm Respiratory, Bad Kissingen, Germany). Before every test, calibration for respiratory gas analysis measurements (ambient air and a gas mixture of 16% oxygen and 5% carbon dioxide) and volume measurements (three-liter syringe) took place. Expired gas passed through a flow meter (triple V volume transducer), an oxygen analyzer, and a carbon dioxide analyzer. The flow meter and gas analyzers were connected to a computer that calculated breath-by-breath minute ventilation, oxygen uptake, carbon dioxide production, and the respiratory exchange ratio. Raw unfiltered breath-by-breath data was retrogradely averaged over five different data display intervals.