Anerobic Threshold: Time at which the patient starts to exhale increasing amounts of carbon dioxide (VEco2) to compensate for a build up of lactic acid and resulting metabolic acidosis.
At AT, lactate starts to accumulate. Since it is a weak base, it readily dissociates releasing H+ ions. These H+ ions are bufferred by bicarbonate system. This generates extra CO2. This results in increased rate of release of CO2 (VEco2) without a concomittant increase in oxygen consumption (Vo2).
At this point, there is hyperventilation relative to Vo2, but not relative to Vco2. This is Anerobic Threshold.
If the exercise continues, pH falls. Lower pH stimulates cardotid bodies leading to hyperventilation