Q.2 What is the pathophysiology of breathlessness?

Dyspnoea is a signal that there is inadequate ventilation. The brain compares the amount of respiration it expects or requires with the amount that is actually occurring. When these are mismatched, the person experiences dyspnoea.

Need for ventilation > physical breathing that is occurring = DYSPNOEA!

Respiration may be increased in response to signals from:

Source of Signal
Example of Cause of Dyspnoea
Carotid bodies and medulla
Blood O2, CO2, H+
Juxtacapillary (J) receptors (in lungs)
pulmonary interstitial oedema
Left heart failure
(on exertion)
- left ventricular output fails to increase
- left ventricular end-diastolic pressure increases
- pulmonary venous pressure increases
- interstitial fluid leaks
Stretch receptors (in lungs)
Muscle spindles (chest wall, diaphragm)
stretch and tension of the respiratory muscles
Muscle fatigue

N.B. There is a psychological component of dyspnoea as well. Increased ventilation normally occurs during exercise but isn't generally sensed as abnormal.