In-Field Validity and Inter-Unit Variability of Metabolic Carts During Simulated Exercise

The present study had three main objectives: (a) to evaluate the in-field validity of different commercially available cardiopulmonary exercise testing (CPET) systems when used by end-users following typical calibration procedures, (b) to measure the variability in accuracy among identical CPET units, and (c) to explore the relationship between the age of the units, as well as the maintenance practices, and their measurement accuracy. Fifty-seven CPET systems, calibrated and operated by end-users in clinical practice, research, or sports settings, were assessed against a metabolic simulator that simulates breath-by-breath gas exchange. The values measured by each system [minute ventilation (V̇E), oxygen uptake (V̇O₂), carbon dioxide production (V̇CO₂), and respiratory exchange ratio (RER)] were compared to the simulated values to evaluate the accuracy. Absolute percentage errors during the simulations ranged from 1.41% to 24.6% for V̇E, 3.29%-10.6% for V̇O₂, 2.86%-13.3% for V̇CO₂, and 1.90%-10.0% for RER. Inter-unit variability (%) ranged from 1.98% to 12.7% for V̇O₂, 1.49%-8.10% for V̇CO₂, and 1.93%-4.24% for RER. No consistent relationship between system age and accuracy was observed, nor between annual maintenance and accuracy. The validity of metabolic carts for measuring respiratory gas variables varied significantly even between identical systems, despite passing manufacturers' calibration checks. Furthermore, inter-unit variability of most systems exceeded intra-unit test-retest variability, thus necessitating caution when using devices interchangeably, as this may increase measurement noise, even within the same laboratory. Most inaccuracies seemed related to technological errors, although some user errors were also identified, indicating the need for a holistic approach to identify errors.