End-Expiratory And Tidal Volumes Measured In Conscious Mice Using Single Projection X-Ray Images

The evaluation of airway resistance (Raw) in conscious mice requires both end-expiratory (V-e) and tidal volumes (V-t) (Lai-Fook SJ and Lai YL. J Appl Physiol 98: 2204-2218, 2005). In anesthetized BALB/c mice we measured lung area (A(L)) from ventral-to-dorsal x-ray images taken at FRC (V-e) and after air inflation with 0.25 and 0.50 ml (Delta V-L). Total lung volume (V-L) described by equation: V-L = Delta V-L + V-FRC = KA(L)(1.5) assumed uniform (isotropic) inflation. Total V-FRC averaged 0.55 ml, consisting of 0.10 ml tissue, 0.21 ml blood and 0.24 ml air. K averaged 1.84. In conscious mice in a sealed box, we measured the peak-to-peak box pressure excursions (Delta P-b) and x-rays during several cycles. K was used to convert measured A(L)(1.5) to V-L values. We calculated V-e and V-t from the plot of V-L vs. cos(alpha-phi). Phase angle alpha was the minimum point of the P-b cycle to the x-ray exposure. Phase difference between the P-b and V-L cycles (phi) was measured from Delta P-b values using both room- and body-temperature humidified box air. A similar analysis was used after aerosol exposures to bronchoconstrictor methacholine (Mch), except that phi depended also on increased Raw. In conscious mice, V-e (0.24 ml) doubled after Mch (50-125 mg/ml) aerosol exposure with constant V-t, frequency (f), Delta P-b, and Raw. In anesthetized mice, in addition to an increased V-e, repeated 100 mg/ml Mch exposures increased both Delta P-b and Raw and decreased f to apnea in 10 min. Thus conscious mice adapted to Mch by limiting Raw, while anesthesia resulted in airway closure followed by diaphragm fatigue and failure.