Principles of Medicine for Engineers

Part 1: A Review of Systems

Studying the systems of medicine in finer focus

I just finished studying for STEP2 and recently found out I passed. While I decided not to apply for residency towards the beginning of COVID, after seeing what can only be described as a clown-car, I did love going through medical training as an engineer.

Source: https://tenor.com/view/steampunk-mechanical-heart-gif-3579322

It was hard, but I stuck with my gut: a small set of basic principles should be enough to explain most of what I was told to memorize.

So this series is an effort to put my fresh-out-of-board-exams mind down to paper.

The goal of this article series

This article series has one goal and one goal only: freeze my current post-USMLE brain state onto paper so I can organize them better for the eventual textbook (lol sure). Or at least for reminiscing in diagrams.

Source: https://www.popsci.com/science/article/2010-09/q-can-brain-freeze-cause-long-term-brain-damage/

I won’t worry about polish, I’ll try my best to organize on concepts, but I’m trying to avoid any bits of perfectionism that may slow down my fleeting medical prowess.

So let’s begin…

Fuzzy Models

Engineers build up understanding in stages, not in pieces. This means we try to see how all parts dance together, and then steadily make the choreography more and more detailed. But, at all stages, we’re looking at the full-picture, and not parts.

Engineers look at the full picture (or word) and then steadily bring it into focus. The blurry version can be considered a “first-order” understanding, while the crisp word is a higher-order. Image from: https://giphy.com/gifs/work-focus-clear-l4FGHqrRZNPkfoLCw

I wanted to learn first-order principles that accounted for as much of the material as possible. If something required second-order principles or more complicated, I laughed and moved on with my life.

But what does it mean to be first-order and not second-order? How do these basic principles make sense of systems-level medicine? Do these principles actually hold or are they just artifacts of test-taking?

What is a system?

First, it’s important to actually understand what a system actually is — they may not actually teach this to you in medical school, even while calling their curriculum “systems-based medicine”.

A system is a box around something you care about, which transforms a set of inputs into a set of outputs. We can do this with things like the heart, the liver, even the brain.

A system is simple: it’s a box drawn around something you think you care about, it’s an input into that box, and it’s an output out of that box.

What is a first-order principle?

The system transforms an input into an output. So we can say that the box transfers energy or information from the input into the output.

This transfer function can take on one of many shapes:

Three different systems, each transforming an input I into an output O in a different way.

First-order principles are straightforward — literally — in that they’re lines. Let’s look at the three shapes above and fit a “first-order understanding” to each of them.

First order (green) understandings of each system. Notice how the line has a little bit where it does a really good job. But when the system itself isn’t a line, it starts to wander away a little, then a lot.

In these three systems we see that a first-order understanding can be added. But, unless the system itself is a line, the first-order understanding can start to be very, very wrong.

Many times, this is ok. But sometimes it’s not ok…

What is a second-order principle?

A second order principle is anything higher than a first-order principle — anything more complicated than a simple line.

I like to keep things in the space of polynomials in this metaphor: my second-order principles are quadratic functions.

Second-order principles (blue) have an extra term in them that lets them bend around more. This means that they can be straight lines by setting the bend = 0. Or they can bend to better capture our more curvy systems. The keen observer will note that the blue lines are not parabolas — my drawing software has its limitations…

Second order principles have an extra part to them: bendiness. This bendiness can be used to curve into data and better track with it. Importantly, second order principles contain all first order principles. I can make a second order principle look like a first order principle.

This may seem trivial, but it’s actually a very important part of the problem solving process. We’ll see later.

Summary

In this first article I outline what a system is and how first- and second-order principles can be used to understand systems.

We haven’t gotten to the medicine part yet, so you may be very, very confused what any of this has to do with medicine, or even STEP2.

Next we’ll start building the basic principles of medicine: (patho)physiology.

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