Low-frequency vs. high-frequency respiratory mechanics after methacholine challenge in artificially ventilated rabbits

Specificity and sensitivity of respiratory impedance in assessing reversibility of airway obstruction in children

Flow in the upper airway wall induces significant error in estimating respiratory impedance by the standard forced oscillation technique in subjects with airway obstruction and may be minimized by oscillating pressure around the subject's head (head generator technique). The aim of this study was to determine whether the latter improves the power of forced oscillations in detecting airway response to bronchodilators in children. Seventy-five children with airway obstruction were studied (ages 5.5 to 15 years old). Fifty-three had asthma and 22, cystic fibrosis. A bronchodilator was administered, and the percent changes in respiratory resistance at 10 Hz (Rrs10), 20 Hz (Rrs20), respiratory compliance (Crs), and resonant frequency (fn) with standard and head generator were compared with the corresponding change in FEV1. The response was positive in 38 (delta % FEV1 > or = 15%) and negative in 37 patients. Data on Rrs10, Crs, and fn could not be obtained in 7, 8, and 4 subjects, respectively, for technical reasons. The delta % Rrs20 was not different between head and standard generator in nonresponders (mean +/- SEM: -19.0 +/- 4.5, vs -11.8 +/- 3.1%), but significantly larger with head than standard generator in responders (-54.1 +/- 3.0 vs -26.5 +/- 2.4%; p < 0.001). The optimal decision level determined by Receiver Operation Characteristic analysis showed that, compared with the standard method, the head generator improved the specificity of Rrs20 (78 vs 65%) with no change in sensitivity (76% for both). Resonant frequency had larger sensitivity with standard than with head generator (91 vs 53%) but slightly lower specificity (70 vs 78%). Finally, delta %Crs was more specific (72 vs 67%) and more sensitive (68 vs 52%) with standard than with head generator. The overall incidence of false results was lower with the head generator than with the standard generator for resistance and lower with the standard generator than with the head generator for fn and compliance. Thus, the head generator improves the diagnostic power of the forced oscillation resistance in establishing the reversibility of airway obstruction, but parameters derived from the reactance may have better diagnostic value with the standard method.

Persistent increased lung response to methacholine after normobaric hyperoxia in rabbits

This study was performed to determine the occurrence and time course of airway hyperreactivity following exposure to normobaric hyperoxia. Twenty-six rabbits were studied. Twelve served as control (group 1), and 14 were exposed to normobaric hyperoxia (FiO2 > or = 95%) for 48 h: 9 rabbits (group 2) were studied after 1 day of recovery in room air and 5 (group 3) after 7 days. The rabbits were anesthetized, curarized and artificially ventilated. Respiratory resistance (Rrs) and elastance (Ers) and their respective changes induced by cumulative doses of aerosolized methacholine were assessed by the multiple linear regression analysis of airway pressure, tidal flow and volume. Weight-specific Rrs and Ers were significantly higher in group 2 (respectively, 87.7 +/- 6.5 cmH2O.L-1.sec.kg and 1100.2 +/- 87.1 cmH2O.L-1.kg, mean +/- SEM) than in group 1 (respectively, 65.2 +/- 3.2 cmH2O.L-1.sec.kg and 904.4 +/- 49.7 cmH2O.L-1.kg (P < 0.05)), but were not different from group 3 (79.4 +/- 7.9 cmH2O.L-1.sec.kg and 952.3 +/- 125.0 cmH2O.L-1.kg, respectively). The dose of methacholine required to increase Rrs by 50% (PDRrs50) was significantly lower in both treated groups: 0.37 +/- 0.11 mg in group 2 and 0.51 +/- 0.19 mg in group 3 vs 2.07 +/- 0.51 mg in group 1 (P < 0.05)). PDErs50 was significantly lower in group 2 (0.45 +/- 0.15 mg) and 3 (0.75 +/- 0.43 mg) compared with controls (1.11 +/- 0.26 mg (P < 0.05)). These results show that hyperoxia induces an increase in Rrs and Ers, and airway hyperreactivity in the rabbit. The latter is prolonged beyond the immediate post-exposure period.