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

The research investigates the effects of intravenously administered histamine on various lung functions in ponies. The results reveal a decrease in arterial oxygen tension and functional residual capacity of the ponies without altering arterial carbon dioxide, pH, respiratory frequency, tidal volume, or the mean arterial blood pressure.

Methodology and Findings

• This experiment was carried out on standing, sedated ponies wherein different doses of histamine were intravenously infused. Lung compliance (Cdyn) and pulmonary resistance (RL) were measured in each pony, and the experiment was repeated thrice with a week’s gap between each test.
• In the study, the histamine doses were calculated in such a way they brought about a 50% and 75% decrease in Cdyn and a 300% and 600% increase in RL. These specific histamine doses were identified as ED50Cdyn, ED75Cdyn, ED300RL, and ED600RL.
• The diverse parameters monitored before and after the infusion of histamine included respiratory frequency, tidal volume, minute ventilation, functional residual capacity, arterial oxygen tension, arterial carbon dioxide tension, and arterial blood pressure.
• Findings from this experiment suggested that the infusion of histamine lowered the arterial oxygen tension and the functional residual capacity. However, it was found not to have a noticeable impact on arterial carbon dioxide tension, pH, respiratory frequency, tidal volume, minute ventilation, and mean arterial blood pressure.

Reproducibility and Variability

• There was significant variation in the response to the histamine doses across the individual ponies; this was seen as at least a 6-fold variability in ED50Cdyn, ED75Cdyn, ED300RL, and ED600RL.
• Despite this variation in switching the subjects, the histamine responses were highly reproducible within individual subjects, showing susceptibility to histamine is consistent within an individual over time.
• Age also seemed to play a role in the histamine responses, with ED50Cdyn, ED75Cdyn, ED300RL, and ED600RL values all having tended to decrease with the advance in age.