Does blood viscosity affect cerebral blood flow? A study in a population living in a high-altitude city

BackgroundViscosity affects flows by increasing resistance to movement; therefore, variations in blood viscosity (BV) levels could modify the cerebral blood flow. ObjectiveThis study aimed to determine the level of correlation between BV and cerebral blood flow in people acclimated to chronic hypoxia who have high BV levels. MethodsProspective observational study was conducted among clinically healthy young adults living in the city of Cusco (3,399 m above sea level). All participants were examined at low and high shear rates (75 and 300 sec-1) to simulate the dynamic component of BV. A transcranial Doppler study of the middle cerebral artery was performed to measure systolic, diastolic, and mean flow velocities (FVs) and resistance and pulsatility indices (PIs). ResultsA total of 131 participants were included. The median viscosity levels were 5.01cP [interquartile range (IQR): 4.45-5.73cP] at 300 sec-1 and 6.16 cP [IQR: 5.58-7.20 cP] at 75 sec-1, the mean FV was 57 m/s [IQR: 50-65 m/s5], and the PI was 0.91 [0.86-1.02]. BV was negatively correlated with mean FV (r: -0.17, p=0.007), while it showed no correlation with other values of blood flow, resistance, or PI. ConclusionsChanges in BV levels have a minimal impact on the mean FV but not on other parameters. This finding suggests that in young and clinically healthy individuals, there are autoregulation mechanisms that compensate for BV variations, although they are not completely understood.