Abstract
Efforts to calibrate measured transthoracic impedances against pulmonary air volumes have been marginally successful. We devised a new nonhomogeneous analogue of the thorax consisting of two coaxial cylinders with different resistivities and dimensions. Measurements were compared with calculations and found to correlate closely. Then, using the mathematical model and data typical of adult male patients, we calculated the relative contributions of all the factors involved in impedance pneumography. We found that changes in electrode radius, chest-wall resistivity and various thoracic dimensions contribute much more to impedance variations than changes in lung resistivity, and conclude that impedance pneumography only has clinical use as a monitor for respiration rates and apnea.
Sommaire
Les efforts déployés pour calibrer les impédances transthoraciques mesurées par rapport aux volumes d'air pulmonaire n'ont été couronnés due d'un succès relatif. Nous avons mis au point un modèle analogique non homogène du thorax composé de deux cylindres coaxiaux ayant des résistivités et des dimensions différentes. Les mesures obtenues ont été comparées aux calculs entre lesquels il existait une corrélation étroite. Utilisant ensuite un modèle mathématique et des données caractéristiques d'un adulte de sexe masculin, nous avons calculé les contributions relatives de tous les facteurs en jeu dans la pneumographie par impédance. Nous avons observé que les changements dans le rayon des électrodes, dans la résistivité de la paroi thoracique et dans les diverses dimensions de la cage thoracique contribuent beaucoup plus aux variations d'impédance que les changements de résistivité des poumons; nous avons conclu que la pneumographie par impédance n'a une utilisation clinique que dans la mesure où elle peut permettre de surveiller le rythme respiratoire et l'apnée.
Zusammenfassung
Die Bemühungen, gemessene transthorakale Impedanzen auf Grund des Luftvolumens in der Lunge zu eichen, waren nur am Rande erfolgreich. Wir haben ein neues, nicht homogenes Analogmodell des Thorax gefunden, das aus zwei koaxialen Zylindern mit unterschiedlichen spezifischen Widerständen und Abmessungen besteht. Die Messungen wurden mit Berechnungen verglichen und es ergab sich ein enger Zusammenhang. Unter Ver wendung des mathematischen Modells und typischer Werte für männliche Erwachsene berechneten wir die relativen Beiträge aller Faktoren in der Impedanz-Pneumographie. Wir stellten fest daß Änderungen des Elektrodenradius und des spezifischen Widerstands der Brustwand sowie unterschiedliche thorakale Abmessungen mehr zu Impedanzschwankungen beitragen als Änderungen im spezifischen Widerstand der Lunge und schlossen daraus, daß die Impedanz-Pneumographie lediglich klinisch als Monitor für Atmungsgeschwindigkeit und Apnoe eingesetzt werden kann.
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Albisser, A.M., Carmichael, A.B. Factors in impedance pneumography. Med. & biol. Engng. 12, 599–605 (1974). https://doi.org/10.1007/BF02477221
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DOI: https://doi.org/10.1007/BF02477221