Heparin in Vascular Surgery

Why Heparin is double-edged? ⚔️ 

Anaesthetist: ACT is 140 👩‍🔬 
Surgeon: Top-up 2500 Heparin please 🩸 

An hour later, numerous stitch hole bleeding points from a patch, and a sizeable haematoma from the contralateral groin. 🥲 
The patient was 45 kg and had CKD. This happened last week.

This is something that occurs more often than one would think. But why so?

Unfractionated heparin is a well-known anticoagulant, and acts against Factors 2 and 10 by enabling and activating Antithrombin. However, there is a uniqueness to it.

Half-life is the time for any drug to drop its concentration to 50%, and is usually a constant denoted as t 1/2. ⏳ 

Heparin is one of the few drugs in a distinguished club, where the t 1/2 is variable!

At low doses, it binds to the endothelial surface and gets cleared at a dose dependent fashion, and t 1/2 is typically short.

Once the endothelial surface is saturated, heparin is free in the plasma and almost acts in an unpredictable fashion binding with various plasma proteins. The t 1/2 is prolonged, the degree depending on these interactions.

ACT levels reflect the effect of circulating heparin, but is fundamentally and in principle an assay that evaluates whole blood clotting (including platelet action) rather than any of its specific sub-components.
The list of caveats for reading an ACT is as long as its normal value.
However, ACT is an easily available, quick-to-perform, fairly reliable bedside test to measure the degree of anticoagulation.

Always keep in mind the unpredictable pharmacokinetics of heparin as well as the nature of an ACT assay before you ask for that top-up.

Anaesthetist: It has been an hour, shall we give some more heparin? 🕐 
Surgeon: Nah we’re good, seems like we’ve given enough to cover for the next patient. Let’s get some Surgicel! 🧐