Aerobic power and the main determinants of blood rheology: is there a relationship?

This cross-sectional study aimed to explore the relationship between aerobic power and the main determinants of blood rheology namely plasma viscosity, plasma fibrinogen concentration and haematocrit. Ninety-three normal healthy individuals (VO2max 48.3 ml/kg per min), who were familiarized with the laboratory environment and testing procedures, participated in the study. Aerobic power as assessed by maximal oxygen consumption (VO2max) was determined by using an incremental exercise protocol on either a treadmill or a stationary bike. Oxygen consumption was measured online using a computer-based metabolic system. In a standardized resting condition, venous blood samples were removed from a prominent vein of the nondominant arm. Aliquots of whole blood were measured for haematocrit (in triplicate), whereas plasma was assayed for fibrinogen concentration and viscosity (in duplicate) using semiautomatic coagulometer and capillary viscometer; respectively. The mean values for haematocrit (41.9 +/- 2.5%), plasma viscosity (1.56 +/- 0.27 mPa s) and plasma fibrinogen (272.1 +/- 86.9 mg/dl) were within the normal range for normal participants. Pearson correlation coefficients and regression analysis were used for statistical evaluations. In this population, VO2max negatively correlated with plasma viscosity (P < 0.01) and plasma fibrinogen concentration (P < 0.01). Although VO2max positively correlated with haematocrit, this correlation was not as strong. Thus, high aerobic power as assessed by maximal oxygen consumption appears to be associated with lower plasma viscosity and lower plasma fibrinogen. The significant negative relationships between VO2max and plasma viscosity and plasma fibrinogen might suggest that blood is more dilute in individuals with high aerobic power. This could probably be due to an expansion of plasma volume, which is commonly seen in those who are physically active and exhibit a higher level of cardiorespiratory fitness.