Dr. Mitzner and I clearly differ in our opinions about the usefulnessof lung impedance (4, 6). However, we do agree on certain keythings, such as the lack of utility of Penh, which, as my worthyopponent rightly points out, has seduced many investigatorssolely on account its ease of use (1). But then he makes theerror of suggesting that this unfortunate state of affairs alsoapplies to impedance because equipment is now available to measureit with relative ease (6). While there is no question that acertain amount of confusion currently reigns over the physiologicalinterpretation of lung impedance, this is not a reflection ofany fundamental shortcoming in the quantity itself. Rather,the shortcoming resides in the community of scientists who useit. Indeed, Dr. Mitzner says as much, for example, when he accusesinvestigators of lack of consistency in the use of the constant-phasemodel parameter G (6). As I stated previously (4), in a homogeneouslung G reflects the dissipative properties of the lung tissues.However, both analytical investigation (2) and simulation studieswith anatomically accurate models of the lung (5, 7) have shownthat changes in G relative to H (tissue elastance) can be usedto infer the development of lung derecruitment versus regionalventilation heterogeneities. These are nontrivial physiologicalinsights that, along with those provided by RN (airway resistance)and H, have been used to advantage in several recent studies(e.g., Refs. 3, 5, 8). Dr. Mitzner also charges that "investigatorsoften just extract airway resistance from the impedance" (6).This is true, but again it simply reflects ignorance, not fundamentalfutility. If investigators choose to discard some of the informationpresent in an experimental measurement, the fault (if thereis any) lies with them. To be fair, lung impedance can onlybe properly interpreted in physiological terms in the contextof a mathematical model of the lung (5, 7). This introducesa level of technical complexity that goes beyond what many biomedicalresearchers are used to, which probably explains how far wehave not come in 30 years (6). The utility of impedance thuscomes down to an issue of intellectual accessibility, whichis consequently its greatest impediment to widespread acceptance.Nevertheless, I firmly believe that an appreciation of the utilityof lung impedance is potentially within the grasp of any trainedscientist and well worth the effort.
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