We studied the effects of capsaicin on airway and lung tissue mechanics in anesthetized and tracheostomized guinea pigs that were mechanically ventilated at constant tidal volume (8 ml/kg) and breathing frequency (60 breaths/min). Dose-response curves to intravenously infused capsaicin (1, 10, and 100 micrograms/kg) were performed (five animals for each dose, one dose per animal). We measured airflow, volume (by integration of the flow signal), tracheal pressure, and alveolar pressure with an alveolar capsule. Capsaicin induced a dose-dependent increase in airway resistance, tissue resistance, and dynamic elastance. The relationship between airway pressure and flow was fitted by a quadratic (Rohrer) equation. Airflow became more turbulent after infusion of capsaicin. After infusion of 100 micrograms/kg capsaicin, airway pressure (at flow rates of 10 ml.s-1) increased from 3.03 +/- 0.40 (mean +/- SEM) to 9.58 +/- 1.88 cm H2O, whereas the pressure corresponding to viscoelastic properties of lung tissue increased from 0.92 +/- 0.14 to 8.58 +/- 1.12 cm H2O. We conclude that infusion of capsaicin results in mechanical effects in both airways and pulmonary tissue.

1. Mechanical behavior of the respiratory system and pulmonary histopathology were investigated in young (8 months) and old (18 months) adult Wistar rats. 2. Respiratory mechanics were studied under static conditions and during both relaxed and forced expiration. Morphological studies consisted of descriptive light microscopy analysis of intratracheally-fixed, paraffin-embedded pulmonary tissue. 3. Old animals exhibited morphological emphysema and chronic bronchitis, decreased respiratory system (2.40 vs 3.42 cm H2O/ml) and lung (1.32 vs 2.30 cm H2O/ml) elastances, forced vital capacity (13.00 vs 14.32 ml), forced expiratory mean flows between 50 and 75% (35.67 vs 60.50 ml/s) and 75 and 100% (6.67 vs 17.67 ml/s) of forced vital capacity, and an increased respiratory system time constant (0.114 vs 0.082 s) in relation to young rats. 4. These results indicate that old animals suffer from a chronic obstructive pulmonary disease that resembles human pulmonary emphysema.

1. Volume, airflow, tracheal, esophageal and transpulmonary pressures were measured in 6 mechanically ventilated, anesthetized and paralyzed guinea pigs. 2. The elastic and resistive properties of the respiratory system were partitioned into their lung and chest wall components a) following end-inflation occlusion of the airways subsequent to constant flow inspirations, and b) during relaxed expirations following release of occlusion. The values obtained by both methods were similar. 3. Mean respiratory system, lung and chest wall elastances were 3.518, 2.671, and 0.847 cm H2O/ml, respectively. 4. Mean respiratory system, pulmonary and chest wall resistances (at flows of 4.3 ml/s) were 0.302, 0.184, and 0.118 cm H2O ml-1 s, respectively. 5. Respiratory system, lung and chest wall resistances were partitioned into two components: 1) a homogeneous one whose values corresponded to 0.171, 0.095, and 0.076 cm H2O ml-1 s, for respiratory system, lung and chest wall, respectively, and 2) a component due to Pendelluft and stress relaxation and corresponding to 0.131, 0.089, and 0.042 cm H2O ml-1 s, respectively. 6. Resistive pressure vs flow relationships for the respiratory system, lung and chest wall were also determined during the entire tidal expiration. 7. We conclude that the chest wall participates significantly in respiratory system unevenness.