The Fatigue Index Test (FIT) is a single breath measurement unique to any individual that gives a simple higher is a better number. This assesses the power generated by the inspiratory muscles and the amount of air moved into the lungs via the PrO2 device, fully representing respiratory capacity. It measures resistance to fatigue and the likelihood of incapacitating breathlessness, vital to well-being and life.

In greater detail, FIT is based upon the ability to inspire 500 ml through the 2 mm leak of the device against the total resisted inspiratory vital capacity (IVC) measured by the power exerted to move the 500 ml tidal volume (TV) divided by the power (using a watts and orifice flow conversion) exerted to inspire from RV to TLC. This value is then multiplied by the time to move the TV divided by the total inspiratory duration. The 2 mm leak provides constant resistance throughout inspiration, while the power output required varies through the range and over time, depending on the individual SMIP shape and capacity. The fraction resulting from this calculation is then divided into 1 to give an equivalent of the whole number.

This novel and comprehensive measurement of inspiratory fatigue is available in the PrO2 device and its accompanying app. The ability to monitor and record this fundamental indicator of respiratory fatigue may be the most accurate reflection of the respiratory system.