Figure 3.

Loop gain provides a framework to understand the pathophysiology of Cheyne-Stokes respiration. (A) Simplified conceptual block diagram of the respiratory control system. A disturbance to this system (i. hypoventilation) temporarily raises alveolar and arterial CO₂ (PCO₂) at the lungs (ii) as determined by the “plant”. After a circulatory delay (iii) the controller perceives the blood gas change and increases its output to oppose the original disturbance (iv). Whether or not this oscillation grows and manifests CSR depends on the loop gain of the system. (B) If loop gain is below 1.0, each response is smaller than the prior disturbance and transient disturbances are damped away. (C) If loop gain exceeds 1.0, each response is greater than the prior disturbance, and oscillations grow until Cheyne-Stokes respiration is established.