Chapter 117 - Part B: When Flow Stops: Obstruction, Pressure, and Systemic Collapse

In this BoardsCast episode, we continue Tobias Chapter 117 — Urethra by destroying the most dangerous misconception in ER medicine:

A blocked cat doesn’t die because it can’t pee.
It dies because pressure turns a local obstruction into a whole-body collapse.

Here’s the core chain:

Obstruction → back pressure → GFR hits zero → postrenal azotemia → hyperkalemia + acidemia → the heart fails.

We walk through why back pressure shuts down filtration mechanically, why creatinine is the “late receipt,” and why the short-term killer is hyperkalemia—with the ECG acting as a survival timer. 

Then we flip management the way Tobias demands:

Stabilize physiology first. Relieve the obstruction second.

You’ll learn:

•  Why urethral obstruction becomes a cardiovascular emergency in disguise
•  The ECG progression of hyperkalemia (tented T waves → P wave loss → wide QRS → arrest) 
•  Why LRS can be preferred over 0.9% saline (acid-base effect matters more than the tiny K⁺ in the bag) 
•  What calcium gluconate actually does (cardioprotection, not potassium removal) 
•  How insulin + dextrose shifts potassium back into cells 
•  The post-unblock trap: post-obstructive diuresis can dehydrate them to death if you don’t match fluids to urine output 

Key takeaway: Pressure keeps traveling until someone stops it.

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