Acute pulmonary response to intravenous histamine at fixed lung volume in dogs

We measured tracheal pressure (Ptr), tracheal flow, and two alveolar pressures in five open-chest anesthetized and paralyzed dogs. The lungs were maintained at a fixed volume for 50 s while small amplitude oscillations in flow at 6 Hz were applied at the tracheal opening. The measurements of alveolar pressure showed that the resulting oscillations in Ptr were virtually entirely determined by airway resistance (Raw) and consequently gave accurate estimates of the same. A 20-mg bolus of histamine was given intravenously at the start of this period when Ptr was 0.5 kPa. After approximately 10 s the mean Ptr increased sharply by approximately 40% and plateaued after approximately 25 s. Raw, in contrast, continued to increase throughout the oscillation period. Furthermore, the increases in mean Ptr were virtually identical in all dogs, whereas the increases in Raw were highly variable among the dogs. Our results suggest that the increases in mean Ptr caused by histamine were due to contraction of distal elements in the lung, whereas the changes in Raw were due mainly to constriction of more central airways.