In telecommunication, the term loop gain has the following meanings:
Note: The maximum usable gain is determined by, and may not exceed, the losses in the closed path.
Loop gain is an engineering term used to quantify the gain of a system controlled by feedback loops. Respiration is in fact such a system. The gain being influenced by controller variables (hypercapnic and hypoxic responsiveness), the ability to eliminate CO2)and circulation time, among others. Simply put, loop gain can be defined as the response of a system divided by the disturbance leading to that response:
Loop gain = response to a disturbance / the disturbance itself
It should be evident from this formula that ventilation will become quite unstable when loop gain is more than 1. If the response is always greater than the disturbance, then ventilation can never stabilize and will wax and wane indefinitely. Thus, the higher the loop gain (the closer it is to 1), the potentially more unstable the respiratory control system will become. In sleep medicine the term loop gain describes the respiratory insetability. Ventilatory insetability is likely to be most important in patients with apnoea with only a mild pharyngeal anatomical deficiency. If these individuals have a high loop gain, then the fluctuating upper airway resistance which occurs at sleep onset may be sufficiently destabilising that ventilation will begin to cycle leading to further loss of pharyngeal muscle activation and frank airway collapse (DP White, 2006; RB Fogel 2004).
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