Airway and tissue constrictions are greater in closed than in open-chest conditions

We measured lung impedance (Z(L)) before and after four doses of methacholine (Mch) infusion in five intact chest (with esophageal balloon) and six open-chest dogs from 0.2 to 8 Hz with an optimal ventilator waveform. From ZL, we estimated airway resistance (R-aw) and inertance (I-aw) and tissue viscance (Gr) and elastance (HL). Two-way analysis of variance revealed that: (I) Mch had a strong influence on all parameters (p < 0.001), but small effect on hysteresivity, eta L = GL/HL; (2) closed-chest GL and HL were significantly higher and I-aw lower than their open-chest values (p < 0.002, p < 0.05 and p < 0.0001); and (3) at the highest Mch dose, the relative increase in R-aw was six times higher in the closed-chest condition. The reduced impact of Mch on open-chest mechanics may be due to constrictions superimposed on grossly different lung configurations and/or some humoral effects initiated by the thoracotomy. We conclude that Mch doses that elicit mild constriction in open-chest condition can cause a severe constriction in intact animals. (C) 1997 Elsevier Science B.V.