Permissive Hypotension

In my opinion the theory makes a couple of assumptions:^1-2

1.) Clots are actually forming. Coagulopathy can be problematic quite early. As hemorrhage and resuscitation continue in tandem, dilutional and consumptive coagulopathy grow in their influence. The longer the patient is in shock, then the more influence the lethal triad (or diamond) is exerting on the situation making it harder and harder for clots to form. 

2.) That all patients will tolerate hemorrhage and low MAPs the same way. 

3.) The bleeding is venous in nature, in which case PH will support clot formation. High pressure arterial bleeds are going to be difficult to form clots with PH alone due to the hydrostatic pressure of the arterial system and the size of the bleed. But is PH with venous bleeding a good idea?

4.)That the patient presents early enough in the shock continuum to respond to small volume resuscitation. 

A New Way to Think About It

Over time the MAP goal has been extrapolated to one single blanket strategy of "get the MAP to 65 and then stop." There are actually different strategies for the MAP based on the patent's mechanism of injury and/or the presumed source of bleeding:³

 • Penetrating Trauma and/or Assumed Arterial Involvement --> SBP 60-70 and MAP 60-65. We are merely attempting to keep up with the rate of hemorrhage and keep the brain and heart perfused at the same time. The strategy is to give small volume boluses (250cc) at a given time interval (q5-10. min typically) to keep enough fluid in the system to keep the MAP 60-65. The risk is that if the MAP is pushed too high, too soon (before source control), then there is the real risk of making things worse by increasing the rate of bleeding.

 • Blunt Trauma and/or Assumed Venous Involvement --> SBP 80-90 and MAP 65. In the setting of blunt trauma in the prehospital environment (this context is important), there is an imperative to be more deliberate with the resuscitation. If the bleeding is venous in nature and non-compressible, using a hypotensive strategy may well precipitate hemodynamic collapse by reducing venous return to the heart and inducing arrest from hypovolemia. MAP targets should be higher in this population to ensure that there is enough fluid and flow in the venous system to get volume back to the heart and keep the patient perfused. PH in this population is known to be fatal.

 • TBI / Pre-existing Hypertension / Pregnancy --> PH is not a viable solution for these patients.

Trauma management is about the time you have left to restore perfusion before the shock is irreversible. It is very much a surgical emergency and rapid delivery to the OR is the best thing we can do. Beyond that, keeping them alive without making their physiology worse is the next best thing we can do. 

We may think that it is helpful to keep the MAP at 65, but we neglect to think about how well or poorly the heart and brain are being perfused. The heart is likely taking a hit as well as the brain^4-5, and nothing helps shock more than a hypo-perfused heart right?

But PH has limitations. 

While it does slow the rate of bleeding and may protect clots, it does not (on its own) protect the patient from or reverse the untoward effects of the lethal diamond. It also makes an assumption that the patient is effectively forming clots. 20-30% of trauma patients are coagulopathic almost immediately following their injury. 

We have to restore perfusion and aerobic metabolism to overcome the effects of the Lethal Diamond.

 It also puts the patient at risk for anoxic brain injury from hypotension. We generally accept that a MAP of 65 is safe, based on the studies, but it may be less so when applied to different patient populations. For instance, some of the data used to support PH strategies was based on young adults (avg. age of 31) with penetrating torso trauma with very few co-morbidities.6 Chronic diseases and advanced age make the concept of PH less black and white, making its applicability somewhat questionable. 

Close Out

The question is: is the benefit of permissive hypotension strategies really slower bleeding, or is it the avoidance of using large volumes of crystalloids and all of the untoward badness that can accompany it that are making the difference? 

We also have to stop and ask, at what point does this permissive hypotension strategy become deleterious for the patient? When does permissive hypotension become profoundly dangerous hypotension?

Perhaps it is not a blanket strategy that is appropriate for all patients. Keeping all of your trauma patients hypotensive can be ill advised if one were neglect to understand the clinical contexts in which it can be safely applied.

References:

1.) Brohi K, Cohen MJ, Davenport RA. Acute coagulopathy of trauma: mechanism, identification and effect. Curr Opin Crit Care. 2007 Dec;13(6):680-5. doi: 10.1097/MCC.0b013e3282f1e78f. PMID: 17975390.

2.) MacLeod JB, Lynn M, McKenney MG, Cohn SM, Murtha M. Early coagulopathy predicts mortality in trauma. J Trauma. 2003 Jul;55(1):39-44. doi: 10.1097/01.TA.0000075338.21177.EF. PMID: 12855879.

3.) Bonanno FG. The Need for a Physiological Classification of Hemorrhagic Shock. J Emerg Trauma Shock. 2020 Jul-Sep;13(3):177-182. doi: 10.4103/JETS.JETS_153_19. Epub 2020 Sep 18. PMID: 33304066; PMCID: PMC7717460.

4.) Wiles, M.D. (2017), Blood pressure in trauma resuscitation: ‘pop the clot’ vs. ‘drain the brain’?. Anaesthesia, 72: 1448-1455. https://doi.org/10.1111/anae.14042

5.) Spaite DW, Hu C, Bobrow BJ, Chikani V, Sherrill D, Barnhart B, Gaither JB, Denninghoff KR, Viscusi C, Mullins T, Adelson PD. Mortality and Prehospital Blood Pressure in Patients With Major Traumatic Brain Injury: Implications for the Hypotension Threshold. JAMA Surg. 2017 Apr1;152(4):360-368. doi: 10.1001/jamasurg.2016.4686. PMID: 27926759; PMCID: PMC5637731.

6.) Bickell WH, Wall MJ Jr, Pepe PE, Martin RR, Ginger VF, Allen MK, Mattox KL. Immediate versus delayed fluid resuscitation for hypotensive patients with penetrating torso injuries. N Engl J Med. 1994 Oct 27;331(17):1105-9. doi: 10.1056/NEJM199410273311701. PMID: 7935634.