Ashley Miller

I’ve started a Substack: The Dependent Variable.

X is great for short-form discussion, but it’s not ideal for building a coherent archive.

This will be the home for longer-form writing on cardiovascular physiology, haemodynamics, fluids, shock, ultrasound and critical care reasoning. All in one place, easier to follow and return to.

First post: open.substack.com/pub/icmteachin…

A narrow pulse pressure suggests a small stroke volume. A wide pulse pressure suggests a hyperdynamic circulation. A low diastolic pressure suggests vasodilatation.

But how much can we really infer from arterial pressure?

In tomorrow's Complicated Ways to Learn Very Little, I work through the physics of pulse pressure, arterial compliance and runoff, before examining whether the diastolic shock index (DSI) tells us anything useful.

Both PP and DSI contain physiological information. The question is whether they tell us anything we couldn't establish more directly. And whether that information changes what we do.

Pulse Pressure & DSI: Complicated Ways to Learn Very Little, Part 3. Friday at 08.00 UTC.
Link to my Substack in the reply.

Voting for @ICS_updates council members is now open. My very good friend Jennie Stephens is running. I’ve worked with her in various ICS capacities for many years now and I can’t think of anyone who would be a better council member. She will be brilliant if elected. Please consider voting for her if you are an ICS member.

The CO₂ gap seems a useful way to assess blood flow.

Tissues continuously produce carbon dioxide. Blood carries it away. If blood flow falls relative to the amount of CO₂ being produced, the venous–arterial difference increases. This is real physiology and, in some respects, the CO₂ gap is a cleaner signal of flow than ScvO₂.

But then we start asking it to do more. We combine it with ScvO₂ to classify haemodynamic states. When that still leaves uncertainty, we divide the CO₂ signal by an oxygen-content difference and try to identify anaerobic metabolism.

Eventually we have two blood gases, several differences, a ratio and a four-box diagnostic matrix.

But have we actually learnt anything that could not have been established more directly?

Part 2 of Complicated Ways to Learn Very Little is out now. Link in reply

The CO₂ gap is a surprisingly good signal of blood flow.

So why do we combine it with ScvO₂, divide it by oxygen content, and use the result to infer anaerobic metabolism?

Tomorrow on The Dependent Variable, Part 2 of Complicated Ways to Learn Very Little.

Some excellent physiology. Some very questionable clinical utility.