Does haemorheology explain the paradox of hypoxemia during exercise in elite athletes or thoroughbred horses?

This research article investigates if haemorheology (the flow properties of blood) can explain the phenomenon of arterial oxygen levels significantly dropping during exercise in elite athletes and thoroughbred horses, a condition known as exercise-induced arterial hypoxemia (EIAH).

Introduction and Background

• Exercise-induced arterial hypoxemia (EIAH) is a condition characterized by a substantial reduction in the partial pressure of oxygen in the arteries, seen commonly in aerobically trained athletes and athletic animal species.
• The potential causal mechanisms previously identified include ventilation/perfusion inequality and limitations in pulmonary diffusing capacity.
• In thoroughbred horses, EIAH is often accompanied by exercise-induced pulmonary haemorrhage (EIPH), which leads to the hypothesis that stress failure of pulmonary capillaries causing diffusion limitations may be at play.

Role of Hematocrit and Red Blood Cells (RBC) deformability

• Hematocrit (Hct) level, the proportion of blood given by red blood cells, has been observed to affect blood flow distribution and increase shear stress in pulmonary capillaries during intense exercise.
• Any alteration in the deformability of red blood cells can lead to aggregation at low shear rate, particularly in post capillary veinules.
• The researchers highlight that there is contrasting data on the effects of blood rheology on EIPH in horses, yet they also posit that a substantial increase in hematocrit during exercise could cause vessel wall stress.

Impact of Hematocrit Increase in Humans

• In humans, a steep ascent in hematocrit may contribute to EIAH, along with red blood cell deformability.
• Pulmonary capillaries may not withstand the stress caused by the high-speed blood flow during intense activities, consequently limiting the oxygen-carrying capacity of the blood.

Conclusion and Further Research

• Despite these observations, there’s no consensus among scientists on the effect of blood rheology on exercise-induced hypoxemia.
• The study suggests that further research into the study of blood rheology as it pertains to athletic performance, particularly in the context of extreme physical exertion, is of keen interest.