Incoprea Theory
Volume IV · 2026

Theory 04/Pulmonology & Gas Exchange /Respiratory sensation/3 min

Breathlessness Is a Reserve Signal

Patients are breathless with normal oxygen. The sensation is not measuring what the blood is measuring.

The claim

Exertional dyspnoea reflects the dynamic depletion of ventilatory reserve rather than blood gases, so the sensation should track the ratio of demand to capacity in real time, and interventions that change reserve should relieve it without changing oxygenation.

01The evidence

  • Work on exertional dyspnoea has reframed the problem from respiratory limitation to the dynamic depletion of mechanical-ventilatory reserves, which places the sensation in the reserve rather than in the gas.Journal of Applied Physiology, 2025
  • In pulmonary hypertension, the respiratory system was found to be the main determinant of dyspnoea, which is difficult to reconcile with a gas-exchange account.Europe PMC

02The tension

Dyspnoea is treated as a symptom of inadequate gas exchange, and its severity is expected to track oxygen saturation or carbon dioxide. In practice the correlations are weak, and patients with normal gases are often severely breathless while others with worse gases are not.

03The theory

Breathing has a reserve: the difference between what ventilation is required and what the respiratory muscles can deliver. As demand rises or capacity falls, the reserve shrinks, and the sensation scales with how much of it is gone.

This makes dyspnoea a ratio perceived in real time, which explains why it tracks exercise intensity so closely and blood gases so poorly. It also explains why interventions that reduce demand, such as breathing pattern changes, can relieve the sensation immediately without changing gas exchange.

The prediction is that the sensation should correlate with the reserve ratio better than with any gas measurement, and that manipulating reserve at fixed gases should change the sensation. Conversely, changing gases at fixed reserve should not.

This has a practical consequence for rehabilitation: training respiratory muscles increases reserve and should reduce dyspnoea even in patients whose lung function is unchanged, which is exactly what trials show but the gas model cannot explain.

04What this is not

Gas-exchange models of dyspnoea

Those models tie the sensation to arterial gases. This ties it to a ratio of demand to capacity, which predicts relief from reserve-expanding interventions that leave gases unchanged.

05Predictions

  • P1Dyspnoea severity should correlate with ventilatory reserve better than with oxygen saturation.
  • P2Reducing demand at fixed gases should relieve breathlessness.
  • P3Respiratory muscle training should reduce dyspnoea in proportion to the reserve gained.

06What would refute it

Sensation tracking gases at fixed reserve would restore the gas-exchange account.

07What it would change

It would make reserve the target of breathlessness treatment, and change how rehabilitation is prescribed.

08Evidence base

  1. Dynamic depletion of mechanical-ventilatory reserves in exertional dyspnoea — J Appl Physiolhttps://journals.physiology.org/doi/pdf/10.1152/japplphysiol.01051.2025
  2. Respiratory system as the main determinant of dyspnea in pulmonary hypertensionhttps://europepmc.org/article/PMC/9053013