One more physiological consideration

We often use the hemoglobin (Hb) and hematocrit (Hct) interchangeably when discussing and considering patients.

Are the Hb and Hct really interchangeable?

Not exactly. MCHC = Hb/Hct; therefore the relationship between Hb and Hct depends on the MCHC

Let’s see how the Hb and Hct affect oxygen delivery:

Oxygen delivery = cardiac output x oxygen content of blood

As we discussed earlier in the case, Hct affects blood viscosity, which increases total peripheral resistance and reduces cardiac output.
Hb, by contrast, increases the oxygen content of blood.

As Hb increases, so too does Hct (that is the price paid for packaging Hb inside red cells). Therefore as oxygen content increases, cardiac output decreases. This relationship can be illustrated in the following schematic:

Oxygen delivery as a function of Hb/Hct yields a bell-shaped curve. In early stages, as Hb increases, so too does oxygen delivery. However, there comes a point when the negative effects of the Hct on blood viscosity outweigh the positive effects of Hb on oxygen carrying capacity of blood, and the curve begins to shift downward. The apex of the summit, the top of the hill (the point of equipoise) is called the optimal Hb/Hct, which for most mammals is around 15 g/dL and 45% respectively.

When patients deviate to the left of the optimal Hb/Hct (anemia), the Hb (and oxygen carrying capacity of blood) is limiting, so it makes sense to talk about the hemoglobin.

On the other side of the hill are those rare patients with polycythemia. In this case, Hct/viscosity are limiting and the goal of therapy is to reverse course and pull the patient back to the summit (hill top) using phlebotomy or myelosuppressive agents.

Are the Hb and Hct really interchangeable?

So, from a physiological standpoint, it makes most sense to refer to the Hct when discussing a patient with polycythemia.