Dynamics of pulmonary gas exchange

Power outputs that are below the anerobic threshold (Θ_an) may be sustained for prolonged durations, whereas power outputs that are greater than oan result in a significant reduction in the tolerable duration to fatigue. The Θ_an may therefore be considered to demarcate exercise intensity into moderate (below) and heavy (above) domains. O₂ uptake (V̇O₂) responds with linear first-order dynamics for sub-Θ_an power outputs with a time constant of ≅ 25 to 35 sec and a "delay" of 15 to 20 sec. A steady state is therefore normally achieved within 3 min. For supra-Θ_an exercise an additional, slower component of V̇O₂ delays the steady state (if attainable). This slow phase of the V̇O₂ response causes the V̇O₂ to rise to values above the steady-state level attainable by fitter subjects at that work rate. The magnitude of this "excess" V̇O₂ correlates highly with the increased arterial blood lactate [L⁻] and becomes marked when [L⁻] exceeds 4 to 5 meq/liter. The Θ_an may therefore be considered a crucial index for sustainable physical activity that is not – or is modestly – fatiguing.