Sepsis and Infectious Disease

1. What sepsis actually is

Start with the infection. Somewhere in the body, bacteria or another organism is growing where it should not be — the lungs, the urinary tract, a wound, the blood itself.

Normally your immune system fights that locally. In sepsis, the response stops staying local. It goes systemic, and it turns on the body's own vessels.

That's sepsis: a dysregulated response to infection. The chemicals your immune system releases make blood vessels dilate and leak.

Picture a Chinese finger trap. Pull on it and it tightens down on your fingers — that's a vessel constricting, holding pressure in. Sepsis does the opposite. The vessels widen out and the walls get leaky, so fluid and pressure escape the container instead of being held in it.

Two things happen at once. The container gets bigger, because the vessels dilate. And the container gets holes in it, because the vessels get permeable. Your circulating volume is suddenly too small for the space it has to fill.

That's why this is called distributive shock. The blood is not lost. The heart is not failing. The volume you have is mal-distributed — pushed into a vascular space that got too big and too leaky to hold it.

So sepsis is going to give you a patient whose tissues cannot get enough oxygen, even when the numbers on your monitor still look okay. Early on, the skin can be warm and flushed and the blood pressure can still be normal, because the body is still compensating. Late sepsis is mottled and cool, because it is not compensating anymore.