Permissive Hypotension with Tim Rainer

I caught up with Prof Tim Rainer who will be presenting a keynote at the Rwanda Emergency Care Association’s conference this May.  Tim is Professor of Emergency Medicine at the Cardiff University School of Medicine.  I asked Prof Rainer for a sneak peek into his keynote topic on permissive hypotension in the context of major trauma
 
What is Permissive Hypotension and why is it important?
Permissive hypotension, in its simplest form, is the use of restrictive fluid therapy that increases systemic blood pressure without reaching normal pressures.  The aim is to achieve a mean arterial pressure of 40 to 50 mmHg, which is generally equivalent to a systolic blood pressure of ≤ 80mmHg, or the presence of a just palpable radial pulse.
There is a more complex definition, which is the use of restrictive fluid therapy and/or inotropes and/or vasopressors as appropriate to increase systemic blood pressure without reaching normal pressures.  This definition is important practically as hypotension is not always due to hypovolaemia (or reduced preload) but may also be due to altered cardiac inotropy and/or peripheral vasodilatation.
The subject is important because there is considerable uncertainty regarding whether permissive hypotension is more or less appropriate than normotensive resuscitation, and whether this might impact on patient survival.

Is there any evidence based guidance that supports the practice?
The National Institutes for Health and Care Excellence is a British organization that evaluates levels of evidence and makes expert recommendations on best practice for a variety of clinical conditions.  In the NICE Guidelines 39, which focuses on major trauma management in the United Kingdom, many important questions were raised.  One was, ‘What are the most clinically and cost effective fluid resuscitation strategies in the major trauma patient (hypotensive versus normotensive)?’ The NG39 also stated, ‘studies have indicated that limiting the amount of fluids administered using permissive hypotension during the initial resuscitation period may improve trauma outcomes. However, the evidence for the practice remains limited and practice may differ depending on type of injury (penetrating or blunt). Moreover, much of the evidence was made before the use of haemostatic resuscitation and clear guidance on resuscitation strategy is still required.’
 
Does permissive hypotension apply differently in head injury?
A particularly interesting area is head injury, which some clinicians believe should be evaluated differently from major trauma.  However, an entry into pubmed of the MeSH terms hypotension and craniocerebral trauma revealed no clinical trials!  Hypoxia and hypotension are considered the greatest threat to functional outcome in brain injury, but there is no level 1 clinical evidence to support or refute this statement.
 
The evidence for permissive hypotension in major trauma doesn’t sound too robust? 
No, it isn’t.  There is limited level 1 research in penetrating trauma to justify permissive hypotension early in trauma care, and limited feasibility studies from blunt trauma.  But there are no definitive studies.  In 2005, the Cochrane Collaboration reported that there was ‘No evidence from trials to support or not to support the use of early or larger intravenous fluid in uncontrolled bleeding’ and ‘1/3rd deaths due to shock from blood loss . . . there is uncertainty about the best time to give fluid and what volume of fluid should be given. . . .increasing fluid will maintain BP, it may also worsen bleeding by diluting clotting factors.’
 
So basically we’re just doing some educated guesswork?  I understand that there are some military recommendations for permissive hypotension.  Do they know something we don’t?
The US, UK and Israeli military support permissive hypotension but this is against a background of limited evidence.  There is considerable uncertainty regarding such recommendations for civilian care.
 
What can we look forward to in your keynote? 
There are many unknowns when it comes to permissive hypotension which I intend to unpack at the conference.  My presentation will discuss the importance and difficulties in setting up a clinical trial to answer these questions.  
 
You can catch prof Rainer in Kigali on 12 May 2017 at the Rwanda Emergency Care Association's conference