INVITED EDITORIAL
Assessment of respiratory mechanics in small animals: the simpler the better?

¹ Department of Medical Informatics and Engineering, University of Szeged, H-6720 Szeged, Hungary; and ² Department of Internal Medicine, University of Genoa, 16132 Genoa, Italy

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THE RECENT INTEREST IN SMALL animal models, which are particularly suitable for genetic and translational studies on the pathophysiology of bronchial asthma, has prompted the search for measurements of lung function that are easy to implement and, if possible, noninvasive. In 1997, Hamelmann et al. (6) proposed a plethysmographic method to assess airway responsiveness in mice, which eventually gained wide popularity. The experimental setup of this method is indeed extremely attractive: the animal is awake, free to move, and restrained only in the sense that it is placed in the plethysmograph. Only box pressure (P_b) is measured, and a dimensionless parameter, called "enhanced pause" (Penh), is derived from the shape of the P_b decay during expiration and the ratio of the inspiratory and expiratory maxima of P_b. It was found that Penh increased during bronchoconstriction and, in anesthetized animals, correlated with pulmonary resistance, while it was apparently independent of breathing frequency and pattern (6). These observations led to the consideration of Penh as a valid surrogate for pulmonary resistance.

The enthusiasm about this "unrestrained plethysmography" (UP), however, was paralleled by some skepticism. This was because it was unclear how a mechanical property could be estimated on the basis of a single quantity not coupled to the respiratory system in any unique way or, in terms of systems analysis, assessed without any measured or standardized driving to which the response belongs. Mitzner and Tankersley (10) questioned the key assumptions of UP and also raised serious concerns about the appropriateness of its experimental validation. In a review article on mouse models of airway hyperresponsiveness, Drazen et al. (2) stressed the necessity to validate Penh with measurements of airway caliber based on known physical principles. Peták et al. (12) compared UP with the low-frequency oscillation technique in mice exposed to 100% O₂ and found a sharp increase in Penh, whereas the airway resistance (Raw) decreased and the tissue parameters remained unchanged. This implies that Penh may be completely unrelated to the mechanical properties of the lung but exclusively determined by the breathing pattern, which may differ widely between different species and under different experimental conditions.

The lack of a theoretical basis and the questionable specificity of UP have apparently been recognized by some of the authors of the original publication (6), who included standard (and invasive) measurements of respiratory mechanics in subsequent studies (e.g., Ref. 1). It should also be noted and appreciated that one of the authors of the original article is involved in a critical reevaluation of UP also published in this issue of the Journal of Applied Physiology (9), a paper representing a laudable mission intended to avoid further confusion about UP and to prevent its indiscriminate use in respiratory research.

The key issue about UP is the identification of the sources of P_b, i.e., the physical processes occurring when an animal is breathing unconditioned air and no signal but P_b is available. There is substantial agreement (6, 10) that P_b depends on 1) alveolar gas compression and expansion to generate flow through the airways, and 2) tidal volume (VT) plus any difference in temperature and humidity between the inspired and alveolar gases. The chief object of disagreement is the relative importance of these pressure sources, a question that badly needed to be addressed quantitatively. Lundblad and colleagues (9) point out that, when mice breathe spontaneously in a box where air is at room temperature and humidity, approximately two-thirds of P_b originate from gas conditioning and approximately one-third from gas compression and expansion. They also show that, under BTPS conditions, the time integral of P_b over inspiration is accurately predicted by a term containing Raw, lung volume, and VT, which recalls the theory of plethysmographic measurement of Raw based on energy dissipation (8). The conclusion is that, unless lung volume and VT are measured or controlled, Penh will not be suitable to characterize airway mechanics (6). It is left to the reader to realize that this condition can hardly be fulfilled with an unrestrained animal in a box and to return to the reality of classic plethysmography as described by DuBois et al. in 1956 (3).

The need for noninvasive and repeatable measurements of airway mechanics in experimental animals cannot be met by technically demanding and sophisticated, yet specific and sensitive methods such as low-frequency oscillation technique (12). Meaningful mechanical variables can be obtained in restrained awake animals by using a double-chamber plethysmograph and forced oscillations (7, 11) or a head-out plethysmograph to measure respiratory flow (4). The article by Glaab et al. (5) in this issue of the Journal of Applied Physiology describes the use of head-out plethysmography in the assessment of bronchoconstrictor responses in conscious rats. The method is based on the measurement of midexpiratory flow (EF₅₀) during tidal breathing and has been validated by establishing the relationships between EF₅₀ in conscious and anesthetized animals and between EF₅₀ and pulmonary conductance (GL) in anesthetized animals. Under a variety of experimental conditions, EF₅₀ and GL exhibited a fairly close relationship, which justifies the authors' conclusion that EF₅₀ is an appropriate index of bronchoconstriction in a rat model of asthma, although, as noted by Mitzner and Tankersley (10), "almost all respiratory mechanics variables show qualitative correlations." Despite some differences between EF₅₀ and GL, probably reflecting different sensitivities to airway and tissue components, this method has the advantages of being relatively simple and based on meaningful physical quantities.

The present commentary is not intended to create a vacuum in methodology by underlying the inadequacy of a technique and then to propose another method with which to fill the vacuum. There is nowadays a broad choice of experimental possibilities, including the few we mentioned above, which yield rather similar respiratory quantities; these techniques differ considerably as regarding the dimensions of sophistication, confirmed validity, and the ease of instrumentation.

	FOOTNOTES

Address for reprint requests and other correspondence: Z. Hantos, Dept. of Medical Informatics and Engineering, Univ. of Szeged, Korányi fasor 9, H-6720 Szeged, Hungary (E-mail: hantos{at}dmi.u-szeged.hu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

10.1152/japplphysiol.00526.2002

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