Identification of the breathing cycle forms the basis of any breath-by-breath gas exchange analysis. Classically, the breathing cycle is defined as the time interval between the beginning of two consecutive inspiration phases. Based on this definition, several research groups have developed algorithms designed to estimate the volume and rate of gas transferred across the alveolar membrane ("alveolar gas exchange"); however, most algorithms require measurement of lung volume at the beginning of the ith breath (VLi-1 - i.e., the end-expiratory lung volume of the preceding ith breath). The main limitation of these algorithms is that direct measurement of VLi-1 is challenging and often unavailable. Two solutions avoid the requirement to measure VLi-1 by redefining the breathing cycle. One method defines the breathing cycle as the time period between two equal fractional concentrations of lung expired oxygen (FO2) (or carbon dioxide; FCO2), typically in the alveolar phase, whereas the other uses the time period between equal values of the FO2/FN2 (or FCO2/FN2) ratios. Thus, these methods identify the breathing cycle by analyzing the gas fraction traces rather than the gas flow signal. In this review, we define the traditional approach and two alternative definitions of the human breathing cycle and present the rationale for redefining this term. We also explore the strengths and limitations of the available approaches and provide implications for future studies.
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