Increase in blood viscosity in the sprinting horse: can it account for the high pulmonary arterial pressure?

This research article investigates the viscosity changes in horse blood after racing, and whether this viscosity alteration can explain the high pulmonary arterial pressure observed in sprinting horses. The study also assesses the effects of a drug called frusemide on these factors.

Overview of the Research

• The study was conducted on 49 Thoroughbred horses and involved blood samples taken before and after racing. The researchers aimed to explore changes in “apparent viscosity” and “relative viscosity”, two measures of the thickness and ‘stickiness’ of the blood.
• Frusemide, a drug commonly used to treat high blood pressure, was also investigated to see if it influenced these measures.
• Shear rate and packed cell volume (PCV) – aspects indicating the density of red blood cells and the speed at which blood can move under certain pressures – were also studied in relation to exercise.

Frusemide’s Effects on Blood Viscosity

• Results showed small increases in apparent viscosity in horses that were given frusemide, however, no differences were observed in the relative viscosity.
• Independent of receiving frusemide or not, horses did not show any frusemide’s influence on red blood cell density or other factors, such as shear rate, which might affect the blood’s apparent viscosity.

Changes in Blood Viscosity During Racing

• Blood viscosity was found to be slightly (though significantly) higher after racing than before, at any particular PCV and shear rate.
• However, the relative viscosity was lower in blood samples taken after racing than those taken before.
• The researchers identified PCV increase during racing as the most significant contributing factor to blood viscosity increase, as the blood becomes less sensitive to shear rate changes when the horses are engaging in physical exercise.

Implications for Pulmonary Arterial Pressure

• With an increase in PCV from 40 to 65% at shear rates above 225/s, the apparent blood viscosity approximately doubled, but this increase alone could not account for the observed elevated pulmonary vascular pressure in racing horses.
• The researchers concluded that additional factors, especially those causing the high left atrial pressure, must be taken into account. The elevated pressure may have a multifactorial cause, implying the need for further research.