Knowledge Check

Sort the clinical manifestation (top) according to whether it is a general findings in all cases of polycythemia or specific to certain causes:

Splenomegaly

Clubbing of nails

Heart murmur

Engorged retinal veins

Plethora

Low SaO2

Hypertension

Renal artery bruit

General finding

Specific finding

What is the cause of plethora in polycythemia?

Click for Answer

Increased red cell mass
Vasodilation

Poiseuille’s law

Poiseuille’s law states that flow of a fluid (Q), or cardiac output, correlated directly with the pressure gradient and inversely with the total peripheral resistance:

Cardiac output = ∆P/TPR

Where TPR = πr⁴/8ηL

The hematocrit (Hct) is the primary determinant of blood viscosity.
Blood viscosity is a determinant of the total peripheral resistance.
As the Hct increases, so too does blood viscosity and total peripheral resistance.
To maintain cardiac output in the face of an increasing total peripheral resistance:
- ∆P increases, leading to hypertension.
- The diameter (thus the radius) of blood vessels increases, leading to plethora.

Note how you get a big bang for you buck with vasodilation (cardiac output increases to the power of 4)!

What is the cause of plethora in polycythemia?

Poiseuille’s law

Cardiac output = ∆P/TPR

Where TPR = πr4/8ηL

Where TPR = πr⁴/8ηL