I posted a poll on Twitter on 12/31/24 asking whether microcytes are associated with lower or higher red cell deformability. It seems a bit counterintuitive (after all smaller cells should have an easier time squeezing through narrow capillaries), but the correct answer is that microcytosis is associated with lower red blood cell (RBC) deformability. 64% answered correctly! The graphic below shows results from two studies (among several) that support this conclusion.
How does this relate to oxygen delivery?
- O2 delivery = cardiac output x O2 content of blood
- Cardiac output can be expressed as:
- Heart rate x stroke volume
- Mean arterial pressure/total peripheral resistance (TPR)
- TPR, in, turn, is inversely proportional to blood viscosity
- Blood viscosity is determined primarily by:
- Cellular viscosity, especially RBCs as determined by:
- Hematocrit (Hct)
- RBC aggregation
- RBC deformability, which depends on:
- Internal viscosity of the RBC (determined primarily by the Hb concentration)
- Surface area to volume ratio of the RBC
- Viscoelastic properties of the RBC (flexibility of the membrane)
- And to a far lesser extent by plasma viscosity
- Cellular viscosity, especially RBCs as determined by:
So, the smaller the RBC, the lower its deformability, the greater the blood viscosity and the lower the cardiac output and O2 delivery.
How does the volume/size of the RBC determine deformability? Presumably by altering the surface area to volume ratio of the cell.
Clinical correlate: Patient with polycythemia vera and microcytosis secondary to iron deficiency (a common association owing to platelet dysfunction and secondary GI blood loss along with increased iron demand from brisk erythropoiesis) may in theory have elevated blood viscosity compared to patients with polycythemia vera with the same Hct but who do not have iron deficiency/microcytosis.
