Cardiopulmonary exercise testing (CPET, CPEX) offers a fully quantifiable measurement of physiological status and maximal aerobic capacity. CPET can be undertaken by any individual capable of performing the chosen exercise modality, which may be an exercise cycle ergometer, treadmill, or arm crank. Maximal exercise is normally limited by the ability of the cardiovascular system to deliver oxygenated blood to the working muscles, despite the capacity for cardiac output to increase by many multiples of basal resting activity. Under conditions of maximal exercise, cardiac output rises from its resting rate of 5 L/min to a maximum of around 20 L/min. As exercise approaches its highest intensity, the buffering capacity of the blood becomes exhausted, so that acidosis occurs. A reduction in breathing reserve at maximal exercise is the cardinal feature of lung disease. Restrictive disease is usually accompanied by an increased respiratory frequency at peak exercise, in an attempt to achieve adequate ventilation despite pathologically low lung volumes.