ABSTRACT
During a standard progressive work test, the maximal exercise effort and maximal oxygen consumption attained are primarily determined by the maximal possible oxygen delivery to and extraction by the exercising muscles. The Fick equation describes the three components determining oxygen delivery: heart rate, stroke volume, and arterial-mixed venous oxygen extraction. Heart rate increases in a nearly linear fashion with increasing effort. Maximal exercise heart rate is best predicted by a subject's age, but there is a substantial scatter of values for a given age even among normal subjects. Stroke volume during upright exercise is dependent both on cardiac function and the pumping action of the exercising muscles on the veins within muscle. Hence, stroke volume increases immediately with the onset of leg exercise, with only minimal further increases as the work test continues. Among normal subjects, the observed differences in maximal oxygen uptake are best explained by inter-individual differences in exercise stroke volume. The arterio-venous oxygen extraction increases linearly during a CPET, from roughly 20% of the arterial content at rest to over 75% with maximal effort. As stroke volume remains constant after the onset of exercise, the oxygen-pulse measurement (oxygen consumption divided by heart rate) increases progressively with increasing exercise, a reflection of the increasing arterio-venous oxygen extraction. During a progressive work test, a progressive reallocation of blood flow from splanchnic and renal circulations to the exercising muscle takes place, such that over 85% of blood flow goes to exercising muscle at maximal effort. The delivery of oxygen to the exercising muscle is facilitated by the properties of the hemoglobin molecule that more readily release oxygen in the hot and acidotic environment of exercising muscle.
