Commentary on “Restrained whole body plethysmography for measure of strain-specific and allergen-induced airway responsiveness in conscious mice”

Abstract

To the Editor: We read the manuscript by Lofgren et al. (7) with great interest and would like to compliment them for an excellent comparison and validation of plethysmographic methods to assess airway function in mice. There are two errors that we would like to bring to the authors' notice. First, the units of specific airway resistance (sRaw) should be cmH2O·s not cmH2O/s {sRaw = Raw × TGV = (pressure/flow) × volume, i.e., units are [cmH2O/(ml/s)] × ml = cmH2O × s, where Raw is airway resistance and TGV is thoracic gas volume}. Second, the authors' contention that this study demonstrates for the first time the measurement of sRaw using restrained whole body plethysmography (RWBP) in conscious mice is incorrect. Investigators from our laboratory at the Institute of Genomics and Integrative Biology, Delhi, have adapted the guinea pig method described by the late Dr. K. P. Agrawal (4) under his guidance and have already published data using this method (5, 9) in peer reviewed indexed journals. Being a relatively straightforward adaptation, it was not published independently as a technique. We now routinely use the commercial double-chamber plethysmography (DCP) setup by Buxco systems (2). Since we have been using both methods for some time now, we would like to present our slightly different perspective. While we agree with most of the conclusions presented by Lofgren and colleagues, we are surprised that they find sRaw to be lower by RWBP compared with DCP. This is opposite to our experience for mice and guinea pigs. In mice, we find sRaw-DCP to be ∼1.1 cmH2O·s similar to the range reported by Flandre et al. (6) and used to find sRaw-RWBP to be ∼2.1 cmH2O·s. Similarly, the original studies for RWBP (4) and DCP (8) reported higher sRaw values for healthy guinea pigs using RWBP (2.09 cmH2O·s) compared with DCP (1.24 cmH2O·s). This difference can be attributed partly to leakage, which can be minimized but not eliminated. In RWBP, leakage of air around the nares tends to artificially increase measured sRaw, while in DCP leakage at the collar artificially reduces measured sRaw. In this context, it is difficult to understand the significantly lower sRaw-RWBP compared with sRaw-DCP in this study. A possibility that the authors consider is that perhaps the collar was extremely tight during DCP, causing upper airway compression and elevating sRaw-DCP. While possible, it may be worthwhile to also consider possible causes for an artificially low sRaw by the RWBP method. Inadvertent introduction of phase differences between the box signal and the flow signal during RWBP can change the angle of the loop. We used a real-time XY display of the signals on an oscilloscope to minimize this. The authors report a post hoc reconstruction of the XY loop from the primary data. Could there be temporal misalignment during this or possibly alteration of phase during signal processing and/or filtration? While this is of academic interest to us, this should be of limited practical significance to the broader community, since such systematic errors are canceled out during comparisons. We would like to thank the authors on reviving interest in this very useful technique that has also been modified to measure lung volumes in humans (3), which could possibly be scaled down and applied to mice. Another possible improvement in whole body plethysmography could come from nasal occlusion of spontaneously breathing mice, transitioning them to an oral mode of breathing that allows nasal airway changes to be excluded. While this cannot be done during RWBP in the current form, we have applied it to DCP with good results (2). In summary, there is more to noninvasive plethysmography than the much-maligned enhanced pause (1). Noninvasive airway mechanics provide information about airway function comparable to more invasive approaches, while retaining the inherent advantages of being more physiological and suitable for longitudinal follow-up.