Oedema in the rested-immobilised horse: a physiological pathology or a pathological physiology?

This research article investigates the occurrence and factors contributing to oedema in horses during periods of rest or immobilisation, focusing on key aspects of equine haemodynamics.

Research Overview

The article considers oedema – swelling caused by fluid retention – which frequently occurs in immobilised or rested horses. It also observes the high erythrocyte sedimentation rate (ESR) – a common blood test measuring the rate at which red blood cells settle at the bottom of a test tube – and significant variations in the horse’s venous haematocrit (Hv) – the ratio of red blood cells to total blood volume in the veins. The researchers hypothesize that these three phenomena are interconnected and tied to resting-induced oedema in horses.

Horse States and Plasma Values

• Researchers observed horses in different states: sedated, rested, and exercised. Measurements taken during these states showed increases in the venous haematocrit and plasma protein concentration, and decreases in the erythrocyte sedimentation rate.
• Horses were sedated using acetylpromazine, a tranquillizer with depressant effects on the central and autonomic nervous systems. “Rested” refers to horses confined in loose boxes with no physical exertion, while “exercised” refers to horses after a period of trotting or cantering.

Interpretation and Assumptions

• The researchers interpret these data through changes in the plasma volume (PV), i.e., fluid content in the blood. They note if the plasma protein concentration (PPC) shifts, it signals changes in PV.
• They highlight physiological assumptions made during PV tracking using plasma protein-bound markers, including the ideas that plasma protein remains within the vascular compartment, and samples retrieved from a significant blood vessel like the jugular vein represent protein content across the entire blood volume.

Comparison with Human Data

• The paper compares its observations in horses to analogous scenarios in humans, citing research indicating that standing after 30 minutes of lying down can lead to a 300ml decrease (about 10%) in human PV. The cited study attributes these changes to the filtration of protein-less liquid across the capillary bed due to posture and activity-induced changes in blood pressure.
• The authors caution that distinct calculations cannot be used to identify blood volume differences based on Hv variations, as it cannot be concluded that Hv represents the haematocrit of the entire blood volume, given the fact that red blood cell volume is not uniform throughout the vascular system.