Does elevated hematocrit increase blood viscosity and reduce cerebral blood flow?

Elevated hematocrit is a major determinant of whole-blood rheology, but the clinical consequences depend on plasma volume, shear conditions, red-cell behavior, and vascular compensation.

Viscosity and red-cell concentration

• Higher hematocrit reliably raises whole-blood viscosity. The relationship is nonlinear and becomes steeper at higher hematocrit; a rapid increase above approximately 50% has been described. The effect is greatest at low shear rates, where red-cell aggregation is important.
• A true increase in red-cell mass commonly raises hematocrit and viscosity, but hematocrit is a concentration measure. Plasma-volume loss can raise hematocrit without increasing total red-cell mass (relative erythrocytosis), while plasma expansion can conceal absolute erythrocytosis.
• Viscosity is also modified independently by plasma viscosity, red-cell aggregation, and particularly red-cell deformability at high shear.

Cerebral perfusion and oxygen delivery

• Increased viscosity can reduce cerebral blood flow. In six people with elevated hematocrit (mean 51.5%), normovolemic hemodilution increased cerebral blood flow by 19–23%. Small studies at hematocrits around 47–53% likewise found lower flow and improvement after phlebotomy.
• In extreme erythrocytosis in mice, cerebral blood flow fell 44%. In older adults, each 1 mmol/L higher hemoglobin was associated with lower measured cerebral perfusion, although this observational finding does not establish viscosity as the cause.
• Reduced flow does not invariably mean reduced oxygen delivery: increased arterial oxygen content and vasodilation may compensate. In the small hemodilution study, calculated cerebral oxygen-delivery capacity did not significantly change; similarly, erythrocytotic mice maintained oxygen delivery comparable with controls despite reduced flow.

Bottom line

• The claim is substantially correct: elevated hematocrit increases viscosity and can impede cerebral perfusion, especially at substantial elevations. Impaired tissue oxygen delivery is physiologically credible but depends on whether compensatory increases in oxygen content and vascular dilation can offset the flow reduction.