Initiating inspiration outside the medulla does produce eupneic breathing

Abstract

ARTERIAL PCO2 IS DICTATED by the ratio between the level of pulmonary gas exchange and alveolar ventilation (VA), not minute ventilation. If so, how could isocapnia be maintained if only the magnitude (tidal volume, VT) and duration (TI and TTOT) of the neural output produced by the central pattern generator of breathing (CPG) can be altered? In other words, what is the strategy used by the respiratory control system to match its output to the level of pulmonary gas exchange to keep arterial blood gas homeostasis if this neural output con- tributes to dead space ventilation? In this Viewpoint, we will show that when the spontaneous breathing is replaced by a cortically triggered breathing pattern at various breathing frequencies (we will name this new rhythm an "ecto-rhythm"), the relationship between TTOT and VT has a positive intercept that averages the pulmonary dead space. As a result, the coupling between ventilation and the pulmonary gas exchange becomes possible (17). We will present the idea that whenever isocapnia is maintained during spontaneous or automatic breathing, the magnitude and dura- tion of breath cycles must display the same relationship as during an ecto-rhythm (15). Conversely, we will discuss the implications of the observation that breathing can display the same essential regulatory properties whether generated outside or within the pontomedullary CPG area (7). Ecto-rhythm, breathing pattern, and gas exchange: the re-