What is the benefit of reducing hematocrit in a patient with polycythemia vera (PV), a condition characterized by elevated hematocrit, in terms of its effect on blood viscosity?

From the Guidelines

The reduction in hematocrit through phlebotomy in polycythemia vera is beneficial primarily because it reduces blood viscosity. In polycythemia vera, the excessive production of red blood cells increases the blood's thickness, making it more viscous. This high viscosity impairs blood flow through small vessels and increases the risk of thrombosis, as seen in the patient's symptoms of pruritis and nose bleeds 1. By removing blood through phlebotomy, the hematocrit decreases, which directly lowers blood viscosity. Lower viscosity improves blood flow dynamics, reduces the workload on the heart, and decreases the risk of clot formation. The CYTO-PV trial, a high-quality study published in 2022, randomized 365 adults with PV to either more intensive treatment (hematocrit <45%) or less intensive treatment (hematocrit 45%–50%) and found that targeting a hematocrit level <45% significantly reduced the composite primary endpoint of time until death from cardiovascular causes or major thrombotic events 1. While other parameters like cardiac output may improve secondarily as a result of reduced viscosity, the primary benefit is the direct reduction in blood thickness. This improvement in blood rheology helps alleviate symptoms and reduce complications associated with polycythemia vera, including the patient's nose bleeds and risk of thrombotic events. Key points to consider in the management of polycythemia vera include:

• The use of low-dose aspirin to prevent thrombotic complications, as established by the ECLAP study 1
• The importance of maintaining a hematocrit level <45% to reduce the risk of cardiovascular death and major thrombotic events, as demonstrated by the CYTO-PV study 1
• The role of phlebotomy in reducing blood viscosity and alleviating symptoms associated with polycythemia vera.

From the Research

Reduction in Hematocrit Benefits

The reduction in hematocrit is beneficial because it:

• Reduces blood viscosity 2, 3, 4

Effects of Reduced Hematocrit

Reducing hematocrit:

• Decreases the risk of thrombosis 2, 3, 4
• Is a key component of managing polycythemia vera, with a target hematocrit of less than 45% 2, 3, 4
• Can be achieved through phlebotomy, which is often used in combination with other therapies such as hydroxyurea 2, 3, 4

Relationship Between Hematocrit and Blood Viscosity

There is a direct relationship between hematocrit and blood viscosity, with higher hematocrit levels leading to increased blood viscosity 2, 3, 4

• Increased blood viscosity can lead to a higher risk of thrombosis and other cardiovascular complications 5, 6