Question 20 from the first paper of 2011 asked about all 3 major aspects of damage control resuscitation:
The basic principle of permissive hypotension is the pursuit of haemostasis rather than blood pressure. With this in mind, one aims to perfuse the organs with some sort of minimum blood pressure (an SBP of 70-80mmHg or a MAP of 50mmHg appears to be safe), thereby limiting the volume of resuscitation fluid required. A smaller volume of resuscitation fluid means less haemodilution; a lower blood pressure means the sources of arterial blood loss will be slower to lose the blood, and more easily controlled by the pressure of internal self-tamponade.
The LITFL entry on this topic is an excellent starting point, and it is also where the time-poor exam candidate should end their reading. If however you've got a few hours to kill, trauma.org has an excellent bibliography of all the current research. This sort of reading should start with Ken Mattox's brilliant editorial article. " I have lived long enough to recall a long list of the techniques "de jour" to elevate the blood pressure" he starts, presumably in a Texan drawl. He also earns massive kudos by quoting Machiavelli at the end. Excellent stuff from one of the original authors of the 1994 Bickell paper, but perhaps best left for after the exam.
Argument from biology:
MAP of 50 appears to be associated with decreased transfusion requirements but not increased adverse events. This is consistent with the idea that MAP only vaguely correlates with organ perfusion, and that in pathological states aiming for normal organ perfusion may in any case be counterproductive. Evolution has guided the survival of those primates who are able to tolerate haemorrhagic shock long enough to recover and breed; this has been going on for some time in the absence of effective prehospital retrieval services. Specific defensive strategies developed by the mammalian autonomic nervous system have largely involved the intentional decrease in the perfusion of nonvital organs. Ergo, it is probably inappropriate to aim for a normal healthy perfusion of those systems.
Argument from analogy:
The tolerance (indeed, the pursuit) of systolic pressures around 50-100mmHg has already become accepted practice in the management of rupture abdominal aortic aneurysm (van der Vliet, 2007), though anything below 70mmHg may still be too low (Powell et al, 2014). The intra-abdominal exsanguination which occurs there is functionally similar to that experienced by blunt torso trauma patients. It seems unusual to use a seperate set of standards for two sorts of analogous intra-abdominal haemorrhage. What's good for the ruptured AAA should also be good for the ruptured spleen.
Argument from zoology:
Animal models of permissive hypotension uniformly support this practice. Mapstone et al (2003) reviewed the then-existing sum of animal trials and came to the conclusion that within the animal model setting, permissive hypotension is beneficial.
Argument from unaided logic
The systolic blood pressure is the variable which is least related to organ system perfusion. It makes no sense at all to have a resuscitation strategy in shock (defined as inadequate tissue oxygenation) guided by the systolic blood pressure. It would perhaps be wisest to ignore it completely.
Argument from technicality
If we decide that the systolic blood pressure is a worthless parameter and decide we are going to ignore it, which variables do we use to guide resuscitation? Without endpoints, there is nothing to guide management. MAP is well validated as a therapeutic target and is known to correlate with organ bloodflow (if not oxygen delivery and utilisation per se). It is hard to abandon traditional variables in the absence of agreement on which better variable to measure (or, indeed, how low one can go with the traditional ones).
Argument from sloth
Permissive hypotension may be interpreted as the permission to ignore hypotension of all causes; the result may be an intolerable increase in the rate of missed cardiac tamponade and tension pneumothorax.
Argument from competing interests
If the patient has a traumatic brain injury, the practice of permissive hypotension conflicts with the imperative of maintaining cerebral perfusion. This is explored to a satisfactory depth in the discussion of a recent case report (Kohli et al, 2014) as well as the reader's response to it (Senthilkumaran et al, 2015). I will summarise by saying that the easy answer is to always protect the brain above all other priorities. Permissive hypotension may become possible later, if cerebral oximetry is available (i.e. one decides to target something other than ICP/CPP)
Argument from gerontology
If the patient is elderly, theoretically they may require a higher MAP target, and the pursuit of a "young" MAP might result in tragic underperfusion of elderly organs. However, this theory has not been supported by a recent audit of trauma database (Bridges et al, 2015) which could not find any association between mortality and the combination of old age and hypotension.
Bickell et al (1994): prospective trial; 598 adults with penetrating torso injuries. Some received standard pre-hospital fluid therapy; in others the fluids were delayed until they got to the operating theatre. There was a small mortality benefit in favour of delayed resuscitation (30% vs 38%), though it must be pointed out that both the groups had similar blood pressure on arrival to the operating theatre.
Turner et al (1999): randomised controlled trial, 1309 trauma patients. In the treatment group, resuscitation with fluid was delayed until they reached the operating theatre, much like the Bickell study. No difference in mortality and morbidity was found. The trial was crippled by protocol violations (of those who were supposed to get fluids only 31% actually got fluids; of those not supposed to get fluids, 20% received them in violatio of protocol). Overall, it was difficult to draw any conclusions from this.
Dutton et al (2002): randomised controlled trial, 110 patients with haemorrhagic shock. The systolic pressures between groups were 100 mmHg and 114 mmHg. Survival was identical: 92.7%, with four deaths in each group. The proponents of permissive hypotension were gladdened by the absence of any increased mortality with the lower blood pressure. Opponents of the practice pointed at the lack of benefit, and the relatively normal blood pressure even among the "hypotensive" group.
Scheriber et al (2015): multicentre RCT; 192 patients randomised. The hypotensive group was really hypotensive (SBP >70). Surprisingly, they did very well: the 24 hour mortality was 5%, vs 15% in the normotensive group. The difference was even greater on sub-group analysis of the blunt injury patients (3% vs 18%)
The HypoResus Trial has completed, and its publication is awaited; the groups received either a 2000ml or a 250ml bolus of saline. According to the (already available) study results, 24 hour mortality was better among the small-volume group (5% vs 15%).