The Dependent Variable
Rebuilding physiology from first principles
Medicine is full of familiar equations, gradients, curves and concepts. We learn them early, use them at the bedside, and eventually stop asking what they actually mean.
But when you trace some of those ideas back to the underlying physics, something uncomfortable happens.
Some turn out to be useful abstractions that have gradually been mistaken for mechanisms. Some are shorthand that works until it is pushed too far. And some, I think, are simply wrong.
The Dependent Variable is an attempt to work out which is which.
I take familiar ideas in cardiovascular and critical-care physiology and deconstruct them from first principles: what the physical system is actually doing, which variables are genuinely independent, and which pressures, flows or gradients simply emerge from the state of the system.
That means not accepting an equation as an explanation merely because it describes a relationship. It means going back to the original experiments, asking what was actually manipulated and measured, and testing the interpretation against the underlying physics.
I am interested in the version of physiology that survives contact with physics.
The subjects will range widely: haemodynamics, the microcirculation, fluid therapy, albumin, capillary exchange, the endothelial glycocalyx, acid–base physiology, shock, cardiovascular ultrasound and other areas where mechanism matters clinically.
Some pieces will be detailed and technical. Others will start with a very practical bedside problem.
The common purpose is the same:
deconstruct the model, test it against first principles, and rebuild only what survives.
Making difficult physiology understandable
Some of the most useful physiology in medicine is also some of the least accessible.
I think complicated ideas should be made simpler without making them less true.
So even when an article goes deep, my aim is to make the reasoning explicit: define the terms, explain what the equations actually say, separate description from mechanism, and build the argument one step at a time.
You should not need to be a mathematical physiologist to understand the physiology that guides your clinical decisions.
Why does it matter?
Physiology is not merely an academic description of what happens. It is the model we use to interpret what we see at the bedside.
If the model is wrong — or if an abstraction is mistaken for a mechanism — then the conclusions we draw from pressure, flow, volume, ultrasound or laboratory measurements can be wrong too.
Get the mechanism right, and clinical reasoning becomes much more reliable.
What you’ll find here
Some articles are deep dives: following an argument through the original physiology, experiments and competing interpretations until we understand what the system is actually doing.
Others are more clinically focused: starting with a problem we encounter in anaesthesia or intensive care and asking what physiology actually helps us make sense of it.
The Venous Return Wars series is one example of the first approach.
I usually publish around once a week, although not to a rigid deadline.
About me
I’m Ashley Miller, a consultant in Anaesthesia and Intensive Care Medicine in the UK and the National Lead for FUSIC Haemodynamics.
Much of my clinical, teaching and academic work has centred on haemodynamics, cardiovascular ultrasound, shock, fluid therapy and applied cardiovascular physiology.
This publication grew out of two interests:
getting the mechanism right, and explaining it clearly.
Why subscribe?
Free subscribers receive regular essays exploring cardiovascular and critical-care physiology, from practical clinical questions to new ways of thinking about familiar mechanisms.
Paid subscribers also receive the major deep dives: the pieces where I go back to the original physiology and evidence, take an argument apart from first principles, and build it back into a more complete synthesis.
These are the articles that require the most research and thought, and the ones I hope will become the defining pieces of The Dependent Variable.
Paid subscriptions also support the time required to do this work properly and help keep the publication independent.
If you enjoy questioning received wisdom and understanding why physiological systems behave as they do rather than simply memorising what happens, you will probably feel at home here.
