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Department of Anesthesiology; Brigham and Women’s Hospital; Harvard Medical School; Boston, MA; bhavani{at}capnography.com
To the Editor:
The study by Krauss et al. (1) raises some important concerns that must be considered in the interpretation of the results and conclusion.
First, the underlying physiological principles of time capnography (carbon dioxide versus time) as employed by Krauss et al. are old and have inherent fallacies that could lead to erroneous results (2–4). There is no way the authors could determine inspiratory time and expiratory time accurately from time capnograms without simultaneously measuring and superimposing respiratory flow rates over the capnograms (2). The side-stream technology of carbon dioxide (CO2) measurement, as described by the authors, is limited in this regard. The authors’ assumption that the onset of the up-stroke and the onset of down-stroke denote the beginning of expiration and inspiration, respectively, is not always true. For the same reason, the use of the old terminology of "ABCDE" is not physiologically correct and has been replaced by phase 0 (inspiratory phase) and the three phases of expiration: Phase I (dead space gases), Phase II (mixing of dead space gases with alveolar gases), and Phase III (alveolar gases) (2–7). The beginning of phase I (expiration) and the beginning of phase 0 (inspiration) cannot be delineated in a time capnogram without superimposing respiratory flow rates over time capnograms. Phase I actually begins before the up-stroke, as CO2 concentration in dead space gases is zero, assuming that there is no rebreathing. Similarly, expiration may end before the actual down-stroke on the capnogram, the remainder being expiratory pause. Therefore, incorporation of the inspiratory and expiratory times as determined by the authors into their algorithm for analysis could result in methodological errors.
Second, volume capnography is a better technique for studying the hypothesis in question because the slope of phase III (alveolar plateau) in the volume capnogram is a better reflection of the ventilation perfusion status of the lung than the corresponding slope in a time capnogram (7). For example, the incidence of positive slope phase III, as seen in volume capnograms, is 100%, which is not reflected in a time capnogram (the phase III appears flat). This is a result of exponentially decreasing expiratory flow that is completed in the very proximal part of alveolar plateau (phase III). A smaller volume of expired gases (approximately the final 15%) occupies half of the time available for expiration, so that a similar change in Pco2 is distributed over a greater length of time in the time capnogram than in the volume capnogram (7). Moreover, the expiration may be complete well before the down-stroke, and the reminder of phase III could be expiratory pause (depending on the respiratory rate) until the onset of the next inspiration.
Finally, demarcation to locate angle b may not be possible in time capnograms obtained in some patients with chronic obstructive lung disease. These time capnograms are quite distinct from those shown by the authors. There is prolongation of phase II that merges almost in a linear fashion with up-sloping phase III. Under these circumstances, however, a volume capnogram will have a distinct demarcation between phases II and III.
References
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