Thread by @karimbrohi

Have been asked a few times where I got the 60mmHg figure from in this tweet. Should answer this in a long, well-reasoned paper, not Twitter. But here goes...

First and most importantly, the number 60 is largely irrelevant, the ethos is far more important than the number.

https://twitter.com/karimbrohi/status/1083036811717500929

...OK, some background points first:

There is a difference between imminent exsanguination and later effects of systemic malperfusion.

Being shocked is not the same as bleeding to death.

Maintaining a patient at 60mmHg will lead to some degree of later organ dysfunction. This is not the same as dying in the next few minutes/hours from exsanguination.

In trauma, early deaths from exsanguination >>> deaths from organ dysfunction.

The lower the BP, the longer the shock, the more severe the organ dysfunction.

Any discussion of permissive hypotension is really talking about minutes, maybe an hour or so.

The shorter the timeframe, the lower the transient tolerable BP.

There are two main reasons for running BP low in bleeding trauma patients: A. Dilutional coagulopathy. B. Worsening bleeding ("don't pop the clot")

(B) is important but (A) is really important.

Conceptually, If you pour 1L crystalloid into 4L (remaining) blood volume, you've diluted that patient's clotting factors by 20%. If they bleed another litre and you add another litre of crystalloid, then they're running close to 50% of starting clotting factors already.

Red blood cell transfusions will also dilute of course, but have oncotic and physical factors allowing less volume in their favour.

This is why providing a consistent, balanced transfusion of PRBC & FFP is so important.

So if in terms of dilutional coagulopathy:

Crystalloid >>> RBCs > RBCs/FFP ?> Whole blood

((Note as physiological solutions none of these will CORRECT an established coagulopathy))

Crystalloid is also horrible because it directly damages the endothelium and itself activates inflammation.

And worse, it leads to interstitial oedema which dramatically reduces cellular oxygenation and can directly cause death due to cerebral oedema.

So I would tolerate a pretty low BP to avoid crystalloid. This would be a different BP than if I had RBCs, and that in turn would be a different BP than if I had FFP as well.

So why 60mmHg? Because you obviously need some basal perfusion. And for me the most important organ here is the heart. The brain is secondary (Sorry brain). If you don't keep the heart alive you can forget about everything else.

So my lowest threshold for not giving fluids is a palpable central pulse / MAP 50 / Systolic 60 (because no one ever watches the MAP in resus).

If I had RBCs I'd be transfusing a bit, if I had RBC+FFP I'd be transfusing more.

While a patient is bleeding I am UNcomfortable if the SBP is ABOVE 90 and we are still transfusing.

So 60mmHg is not a hard number. It is *my* lower threshold, tolerated for a short time, in actively bleeding patients, rather than give crystalloid,based on my assessment of what we know about bleeding physiology, coagulopathy and trauma outcomes.

/end. (Colloids are much worse than crystalloid - don't get me started).