Determinants and detection of anaerobic threshold and consequences of exercise above it.

During exercise, the level of oxygen consumption (VO2) above which aerobic energy production is supplemented by anaerobic mechanisms causing a sustained increase in lactate and metabolic acidosis is termed the anaerobic threshold. The VO2 at which the anaerobic threshold occurs is influenced by the factors that affect oxygen delivery to the tissues, being increased when oxygen flow is enhanced and decreased when oxygen flow is diminished. The anaerobic threshold is an important functional demarcation since the physiologic responses to exercise are different above the anaerobic threshold as compared with below the anaerobic threshold. Above the anaerobic threshold, in addition to the development of metabolic acidosis, exercise endurance is reduced, VO2 kinetics are slowed so that a steady state is delayed, and minute ventilation increases disproportionately to the metabolic requirement and a progressive tachypnea develops. The anaerobic threshold can be measured directly from lactate concentration with good threshold detection from a log-log transformation of lactate and VO2. This threshold defines the VO2 at which the lactate/pyruvate ratio increases. As bicarbonate changes reciprocally with lactate, its measurement can also be used to estimate the lactate threshold. But most conveniently, changes in gas exchange caused by the physical-chemical event of buffering of lactic acid by bicarbonate can be used to detect the anaerobic threshold during exercise.