COMMENTS
This article compared EF50 values that were measured by two different procedures.
As published by Lomax (1), there are two types of whole-body plethysmographs (WBP).
I will use the terminology that he used.
Pressure plethysmograph (PWBP): a sealed chamber
Flow plethysmograph (FWBP): a chamber with a pneumotachograph in its wall
The authors of this article used an FWBP to measure the EF50, as well as other respiratory parameters.
Under the Results section, the authors discussed the EF50 as follows:
“The second respiratory metric that which we focused on is the mid-tidal expiratory flow (EF50). EF50 values represent the flow rate at which 50% of tidal volume of an individual breath has been expelled, measured in milliliters per second [22] [their reference #22, for Glaab et al.]. While EF50 bears some similarities with forced expiratory flow 50% (FEF50), a standard measure in human spirometry [23][their reference #23, for Weinberger et al.], differences in the wave form and magnitude between natural and forced breaths make comparisons difficult. Despite these differences both measures can provide insights into the early portion of the breathing curve.
Typically, in vivo asthma studies examining respiratory function have observed reduced level of the EF50, reporting percent reduction relative to baseline [22] [24] [their reference #24, for Hoyman]. The resulting breath curve shifts to the right, as the reduced flow rate requires more time to exhale 50% of the volume (Fig 3A). However, EF50 values increased robustly during SARS-CoV infection compared to mock maximum differences occurring between 2 and 4 days post infection (DPI) (Fig 3B). These data indicate that during SARS-CoV infection, the breath curves shifts to the left, more rapidly exhaling breath to 50% volume (Fig 3A).
Together the results represent a striking contrast with prior studies examining airway function in the context of allergic responses [their reference #24].”
I will now try to explain why this striking contrast has occurred.
1. As given above, the authors used an FWBP. This unit is designed to measure the airflow resistance component during a breath and, as much as possible, to avoid the conditioning component using an PWBP (1).
2. An easy way to measure the EF50 directly is to use a pneumotachograph attached to a head-out body plethysmograph (HoBP) as shown in (2, 3). This will directly measure tidal airflow, and integration with time will yield tidal volume. It is then easy to determine the point at which 0.5 expiratory volume occurs and the corresponding value for airflow. This was abbreviated VD50, but then the abbreviation was changed to EF50 as given in [their references #22 and #24]. This is easily done with mice or guinea-pigs (2, 3, 4, 5); the restraint needed is minimal.
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When the EF50 is measured in this manner, we can also get flow-volume curves (or loops) like those obtained in humans. Plenty of similarities with human data are available, as given in (4, 5). We can also use the same procedure with guinea pigs, challenging them with bronchoconstrictors (or a variety of other toxicants) while under tidal volume augmentation using CO2. The effects are striking (4, 5). We tried CO2 in 8 or 10 different strains of mice, but there was little or no augmentation of tidal volume in this species.
As I prepared this comment, I discovered another very recent article that caught my attention (6). These authors, also expected a lowered EF50 using an FWBP, but instead they found an increase. I also found another article with very large increases in the EF50, also using the FWBP (7). Striking pathology was presented in this article. Therefore, a surprising increase in the EF50 has been found, not only in this article, but by these other authors as well (6, 7).
The reason that this happened was not due to bronchoconstriction (or airways/airflow limitations). Rather, it was due to “rapid shallow breathing” that occurred as a result of lung restriction. See the last sentence on page 497 of (3) for the explanation and then look at your respiratory frequency data. I think you have “rapid shallow breathing” given the extensive pathology and there must have been a long pause introduced before each inspiration as shown in Figure 1 in (3).
Would I object to the use of this FWBP system?
Yes and no. I understand this procedure is easy to perform and provides investigators with respiratory data to show when something is developing with/recovering from infectious agents. It is a good screening system. However, the interpretation can be a problem unless you plot all variables being measured. If possible, you can obtain flow-volume curves as given in (2, 3) at crucial time points. There is really minimum body restriction with mice or guinea pigs to obtain such measurements. For example, head-out spirometry (HoS), the equivalent of HoBP, as far as measurements made, produced excellent results for monitoring the course of lung infection in mice (8). Figure 1 in this reference provides all the necessary information for all variables measured.
References:
1. Lomask, M (2006). Further exploration of the Penh parameter. Exp and Toxicol Pathol 57: 13-20.
2. Vijayaraghavan R, Schaper M, Thompson R, Stock MG, Alarie Y (1993). Characteristic modifications of the breathing pattern of mice to evaluate the effects of airborne chemicals on the respiratory tract. Arch Toxicol 67: 478-490.
3. Vijayaraghavan R, Schaper M, Thompson R, Stock MG, Boylstein LA, Luo JE, Alarie Y (1994). Computer assisted recognition and quantitation of the effects of airborne chemicals acting at different areas of the respiratory tract in mice. Arch Toxicol 68: 490-499.
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4. Alarie Y, Schaper M. (1988). Pulmonary performance in laboratory animals exposed to toxic agents and correlation with lung disease in humans. Chapter 3 in: Loke, J. Ed. Pathophysiology and treatment of inhalation injuries. Marcel Dekker Inc. NY. (you can Google search: Yves Alarie Jacob Loke and you can read Chapter 3).
5. Alarie Y, Iwasaki M, Schaper M (1990). Whole-body plethysmography in sedentary or exercises conditions to determine pulmonary toxicity, including hypersensitivity induced by airborne toxicants. Journal American College Toxicology 9: 407-439.
6. Ramirez-Ramirez E, Torres-Ramirez A, Alquicira-Mireles J, Canavera-Constatin A, Segura-Medina P, Montano-Ramirez M, Ramos-Abraham C, Vargas MH, Arreola-Ramirez JL (2017). Characteristic plethysmographic findings in a guinea pig model of COPD. Exp Lung Research. http://www.tandfonline.co...
7. Cockrell AS, Yount BL, Scobey T, Jensen K, Douglas M, Beall A, Tang X-C, Marasco WA, Heise MT, (2016). A mouse model foe MERS coronavirus-induced acute respiratory distress syndrome. Nature Microbiology 2: 1626 1-11
DOI: 10.1038/nmicrobiol.2016.226
8. Wolbelling F, Munder A, Stanke, F, Tummler, B. Bauman U, (2010). Head-out spirometry accurately monitors the course of pseudomonas aeruginosa lung infection in mice. Respiration 80: 340-346. DOI: 10.1159/000319442
Yves Alarie, PhD
Professor Emeritus
University of Pittsburgh
April 5, 2017
Note: A copy of this analysis will be e-mailed to the following authors cited above: Cockrell AS, and Arreola-Ramirez JL, authors of references (6 and 7) concerning EF50 findings and to Ferris MT, of this article in PLoS ONE concerning EF50 findings.