ABSTRACT
The normal human respiratory system is able to assure full oxygenation of arterial blood regardless of the level of exercise attained. In addition, the augmented ventilation during heavy exercise reduces the arterial PCO2 and partially compensates for the exercise-associated metabolic acidosis. The mechanical characteristics of the lung-chest wall system determine the breathing patterns that will minimize the work of breathing while maintaining a normal PaO2 during exercise. During a CPET, normal subjects never reach the limit of their maximal possible sustained ventilation. The control of ventilation during exercise is driven by a complex combination of neural inputs from cortical centers, brainstem, the carotid body, and exercising muscles and joints. Among normal individuals, there is a relatively broad range of ventilation sensitivity to exercise, such that a normal exercise VE/VCO2 ratio may range between 22 and 34 liters of ventilation per liter of CO2. Analysis of the exercise ventilation response during a CPET provides an important noninvasive means of identifying the point of onset of the metabolic acidosis of exercise during the test, the ventilatory (or anaerobic) threshold. A graphical analysis of the relation between oxygen consumption and carbon dioxide output throughout the CPET (V-slope method) provides the most accurate marker of the exercise increase in arterial lactate. The time of onset of a progressive increase in end-tidal PO2 or increase in VE/VO2 during a test provide a second estimate of a ventilatory threshold that is usually slightly later in the test than the V-slope estimate. If arterial blood gas measurements are obtained during a CPET, then measurements of gas exchange efficiency can be obtained from the course of the alveolar-arterial O2 difference and physiologic dead space during the exercise test.
