Permissive hypotension

Permissive hypotension or hypotensive resuscitation is the use of restrictive fluid therapy, specifically in the trauma patient, that increases systemic blood pressure without reaching normotension (normal blood pressures). The goal blood pressure for these patients is a mean arterial pressure of 40-50 mmHg or systolic blood pressure of less than or equal to 80. This goes along with certain clinical criteria. Following traumatic injury, some patients experience hypotension (low blood pressure) that is usually due to blood loss (hemorrhage) but can be due to other causes as well (for example, blood leaking around an abdominal aortic aneurysms). In the past, physicians were very aggressive with fluid resuscitation (giving fluids such as normal saline or lactated Ringer's through the vein) to try to bring the blood pressure to normal values. Recent studies have found that there is some benefit to allowing specific patients to experience some degree of hypotension in certain settings. This concept does not exclude therapy by means of i.v. fluid, inotropes or vasopressors, the only restriction is to avoid completely normalizing blood pressure in a context where blood loss may be enhanced. When a person starts to bleed (big or small) the body starts a natural coagulation process that eventually stops the bleed. Issues with fluid resuscitation without control of bleeding are thought to be secondary to dislodgement of the thrombus (blood clot) that is helping to control further bleeding. Thrombus dislodgement was found to occur at a systolic pressure greater than 80mm Hg. In addition, fluid resuscitation will dilute coagulation factors that help form and stabilize a clot, hence making it harder for the body to use its natural mechanisms to stop the bleeding. These factors are aggravated by hypothermia (if fluids are administered without being warmed first it will cause body temperature to drop).

It is becoming common in hemorrhaging patients without traumatic brain injury. Due to the lack of controlled clinical trials in this field, the growing evidence that hypotensive resuscitation results in improved long-term survival mainly stems from experimental studies in animals. Numerous animal models of uncontrolled hemorrhagic shock have demonstrated improved outcomes when a lower than normal blood pressure (mean arterial pressure of 60 to 70 mmHg) is taken as the target for fluid administration during active hemorrhage. The first published study in humans, in people with penetrating torso trauma, has demonstrated a significant reduction in mortality when fluid resuscitation was restricted in the prehospital period. However, it is important to note that the objective of that study was the comparison between standard prehospital and trauma center fluid resuscitation versus delayed onset of fluid resuscitation (fluid not administered until patients reached the operating room). A more recent study (2011) performed by the Baylor Group on patients who required emergency surgery secondary to hemorrhagic shock was randomized to a mean arterial pressure (MAP) of 50mmHg versus 65mm Hg. The lower MAP group was found to need less total IV fluids, used fewer blood products, had lower early mortality (within the first 24 hours - which accounts for a large portion of mortality in trauma patients) and trended towards lower 30-day mortality and less postoperative coagulation, concluding that permissive hypotension is safe. Two large human trials of this technique have been conducted, which demonstrated the safety of this approach relative to the conventional target (greater than 100 mmHg), and suggested various benefits, including shorter duration of hemorrhage and reduced mortality. Johns Hopkins group performed a retrospective cohort review from National Trauma Data Bank that found a statistically significant difference in mortality for patients treated with pre-hospital intravenous fluids. Clinical data from well-controlled, prospective trials applying the concept of permissive hypotension in trauma patients are still missing.