FIG. 13:  One of the mechanisms by which opioids are thought to provoke central apneas is through an altered loop gain.  Loop gain is an engineering term used to describe the overall gain of any system controlled by feedback loops.  As shown in this Figure, it is generally described by the equation that calculates the ratio of the response to a given disturbance to the disturbance itself.  So a high loop gain, or a high-gain, system responds quickly and vigorously to any disturbance, whereas a low-gain system responds more slowly and weakly. 

The two primary factors influencing loop gain are the controller gain and the plant gain, and both are important in ventilatory stability.^[10]  For example, chemoresponsiveness to hypoxemia and hypercapnia is part of the controller gain.  When a person has a high controller gain, it means that their hypercapnia responsiveness is brisk and tends to overshoot. 

Plant gain reflects the effectiveness of any given change in ventilation to eliminate CO₂ and increase stores of O₂.  So a high plant gain would occur, for example, if a small change in ventilation produced large changes in PCO₂, and a condition that would be described as a high plant gain would be, e.g., having low dead space.