Pulmonary effects of intravenous histamine in the conscious pony: dose-response relationships and reproducibility.

In standing sedated ponies, we measured dynamic lung compliance (Cdyn) and pulmonary resistance (RL) during IV infusion of various doses of histamine; this was done in a given pony 3 times at 1-week intervals between experiments. Histamine doses causing a decrease in Cdyn of 50% and 75% and an increase in RL of 300% and 600% (ED50Cdyn, ED75Cdyn, ED300RL and ED600RL, respectively) were interpolated from the histamine dose-response curves. We also measured respiratory frequency, tidal volume, minute ventilation, functional residual capacity, arterial oxygen tension, arterial carbon dioxide tension, arterial oxygen tension, arterial blood pressure before and after infusion of histamine sufficient to cause a decrease in Cdyn of at least 75% and at least a 600% increase in RL. Histamine decreased arterial oxygen tension and functional residual capacity, but did not alter arterial carbon dioxide tension, pH, respiratory frequency, tidal volume, minute ventilation, and mean arterial blood pressure. There was at least a 6-fold variability in ED50Cdyn, ED75Cdyn, ED300RL, and ED600RL between ponies, but these values were very reproducible for each individual. The ED50Cdyn, ED75Cdyn, ED300RL, and ED600RL all decreased with increasing age.