Comments on "Quantifying hypoxia-induced chemoreceptor sensitivity in the awake rodent" by Morgan et al

Vincent Joseph
2014 Journal of applied physiology  
TO THE EDITOR: In a recent issue of the Journal of Applied Physiology, Morgan et al. (4) presented a methodological paper for determination of hypoxic ventilatory response in awake rats (4). The method proposes to use the value of the oxygen or carbon dioxide equivalent ratio (V E/V O 2 or V E/V CO 2 ) as a function of SpO 2 under graded levels of hypoxia as an accurate way to measure the hypoxic ventilatory response. This approach takes into account the different components of the response to
more » ... ypoxia, i.e., hyperpnea (increased ventilation) and decreased metabolic rate (V O 2 and V CO 2 ) and uses SpO 2 as a surrogate for PaO 2 -the effective stimulus occurring at the level of peripheral chemoreceptors. This approach is a welcome addition to the field and there is no doubt that these parameters must be taken into considerations for adequate and reliable estimation of hypoxic ventilatory response in rodents. We have noticed, however, that the paper proposes few details on the approach used for measurements and calculation of V O 2 and V CO 2 , it is only noted that "Oxygen consumption and carbon dioxide production were measured by sampling O 2 and CO 2 concentrations in the inspired and expired air." It is well recognized that measurements of V O 2 and V CO 2 with this "flow-through" system suffer from three major sources of error: dilution effects of carbon dioxide and water vapor pressure, oxygen analyzer drift, and variability in the CO 2 (and H 2 O) concentration in the inflowing air due the presence of human(s) in the laboratory or to atmospheric conditions (1-3). Furthermore, for the purpose of measurements under hypoxic conditions, an additional source of error that should be considered is the linearity of the O 2 analyzers on the range of O 2 % used, which is particularly important if two different O 2 analyzers (or a dual-channel O 2 analyzer) are used to measure inflowing and outflowing O 2 %. Considering the above points, we would like to ask the authors of the abovementioned paper (4) how these potential sources of errors have been taken into consideration for the measurements of V O 2 and V CO 2 ? DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author(s). AUTHOR CONTRIBUTIONS Author contributions: V.J. drafted manuscript; V.J. edited and revised manuscript; V.J. approved final version of manuscript. REFERENCES 1. Jensen DR, Gayles EC, Ammon S, Phillips R, Eckel RH. A selfcorrecting indirect calorimeter system for the measurement of energy balance in small animals. J Appl Physiol 90: 912-918, 2001. 2. Lighton JR, Halsey LG. Flow-through respirometry applied to chamber systems: pros and cons, hints and tips. Comp Biochem Physiol A Mol Integr Physiol 158: 265-275, 2011. 3. Melanson EL, Ingebrigtsen JP, Bergouignan A, Ohkawara K, Kohrt WM, Lighton JR. A new approach for flow-through respirometry measurements in humans. Am J Physiol Regul Integr Comp Physiol 298: R1571-R1579, 2010. 4. Morgan BJ, Adrian R, Bates ML, Dopp JM, Dempsey JA. Quantifying hypoxia-induced chemoreceptor sensitivity in the awake rodent.
doi:10.1152/japplphysiol.00887.2014 pmid:25512068 fatcat:33vj64sqwvduzmxli75t4jcx3y