Intravenous histamine administration in ponies with recurrent airway obstruction (heaves).

The research investigated how intravenous administration of histamine affects the pulmonary function and airway reactivity of ponies with recurrent airway obstruction (RAO), commonly known as ‘heaves’. The study found that during disease flare-ups, these ponies react more sensitively to histamine. However, this heightened response was not seen when the disease was in remission.

Objectives and Methodology

• The main aim of this study was to measure the effects of intravenous histamine on the pulmonary function and airway reactivity of ponies diagnosed with recurrent airway obstruction (RAO).
• The researchers employed a regimen that included monitoring the ponies during a period of clinical remission (period A), after exposure to a barn environment (period B), and during two recovery phases (periods C and D).
• The measured parameters included PaO2 (partial pressure of oxygen), dynamic compliance (Cdyn), pulmonary resistance, tidal volume, respiratory frequency, and the histamine dose required to reduce Cdyn to 65% of base-line value (log ED65Cdyn).
• The study involved control group of ponies that were age-, weight-, and gender-matched with the main group of ponies suffering from RAO.

Results and Findings

• During periods A, C, and D, no significant differences were observed between the affected and control ponies in terms of the measured parameters.
• Exposure to the barn environment (period B) resulted in decreased Cdyn, PaO2, and ED65Cdyn and an increase in pulmonary resistance in ponies with RAO but not in control ponies.
• The slope of the histamine dose-Cdyn response curves was similar between the two groups and was not affected by either barn exposure or return to pasture.
• The study revealed a poor correlation between ED65Cdyn and indices of airway caliber.
• During periods of acute airway obstruction, ponies with a history of heaves demonstrated increased reactivity to intravenous histamine. However, this was not the case during disease remission.

Conclusion

• This study concluded that during acute attacks, ponies with a history of heaves have a hyper-reactivity to intravenous histamine.
• However, during disease remission, their airway response to histamine is similar to that of control ponies.
• The hyperreactivity observed in affected ponies following exposure to a barn environment could not be purely explained by alterations in the baseline airway caliber.