Systemic and digital vascular effects of intravenous histamine in the pony.

This research investigates the impact of a 60-minute intravenous (IV) infusion of histamine on the systemic, pulmonary, and digital vasculature in ponies, noting changes in systemic and pulmonary pressures, cardiac output, vascular resistances, and various other physiological responses. The study concludes that the effects caused by histamine do not mimic those of acute alimentary laminitis on the system and digital vascular functions.

Histamine Infusion and Vascular Effects

The research was designed to examine the effects of an IV histamine infusion in ponies. Histamine, administered at 0.5 mg per minute for a duration of 60 minutes, was shown to cause:

• Transient decrease in systemic pressure lasting less than a minute, followed by a brief period of systemic hypertension, before returning to preinfusion levels for the rest of the infusion period.
• An increase in pulmonary arterial pressure coinciding with systemic hypotension.
• An increase in cardiac output and a decrease in total peripheral and pulmonary vascular resistances.

Specific Effects on Digital Flow and Other Observations

Histamine’s interactions with the digit—a part of the pony susceptible to laminitis—were also examined. The effects include:

• A sustained increase in blood flow to the digit during the period of systemic hypertension, but no change in blood flow otherwise.
• No changes in digital lymph flow or protein concentration as a result of histamine infusion.

Additionally:

• The study noted a significant increase to the hematocrit (the volume percentage of red blood cells in blood) after 60 minutes of histamine infusion, despite plasma and blood volumes remaining steady.
• All ponies showed signs of cyanosis (bluish discoloration due to lack of oxygen), hyperpnea (excessive breathing), and sweating as a result of the histamine infusion.

Comparison with Acute Alimentary Laminitis

The study sought to compare the effects of histamine infusion with those of acute alimentary laminitis, a painful condition affecting the hooves of horses. However, it concluded that IV histamine’s systemic and digital vascular effects do not mimic those of this condition.