Permissive hypotension or hypotensive resuscitation[1] is a term used to describe the use of restrictive fluid therapy, specifically in the trauma patient, that increases systemic pressure without reaching normotension (normal blood pressures). Following traumatic injury some patients experience hypotension that is usually due to blood loss (hemorrhage) but can be due to other causes as well. In the past physicians were very aggressive with fluid resuscitation 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 is termed "permissive hypotension". 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. [2]
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.[3] To date, the only published study in humans, in victims of penetrating torso trauma, has demonstrated a significant reduction in mortality when fluid resuscitation was restricted in the prehospital period. [4] 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). 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.[5][6] Clinical data from well controlled, prospective trials applying the concept of permissive hypotension in trauma patients are still missing.
Following injury the otherwise healthy individual has a natural ability to clot off bleeding. The higher the pressure in your vessels, the harder it is for the bleeding to stop, since the fluid essentially "pushes" the clot out and consequently the bleeding resumes. In more technical terms: hypotension facilitates in vivo coagulation. This is especially true in patients who still have active bleeding. Attempts to normalise blood pressure in case of uncontrolled bleeding as in victims with penetrating trauma, may result in increased blood loss and worse outcomes. In this context, restriction of fluid resuscitation may actually improve outcome. This concept has been supported by animal studies that have demonstrated aggressive fluid resuscitation increase the volume of hemorrhaging fluid to a significant level as well as decreasing mortality.[7][8]
Another issue with aggressive fluid resuscitation is the potential for hypothermia if fluids that are stored at room temperature are used. If these fluids are not warmed prior to infusion (which sometimes time does not permit for), this can result in a significant drop in core body temperature. Hypothermia is associated with many problems including bleeding disorder, organ failure and hypotension itself is one of the three components in the "Triad of Death" that is feared by all trauma specialists.
Crystalloid fluid is used in initial resuscitive efforts. These fluids do not contain any clotting factors or erythrocytes (red blood cells). This can result in a dilution of the clotting factors, leading to poorer control of bleeding, and dilution of erythrocytes, which if becomes severe, can impair the delivery of oxygen to tissues causes further ischemic damage. In addition, crystalloids have an acidic pH. Administration of large quantities of isotonic or slightly hypertonic crystalloid solutions such as 0.9% normal saline or lactated ringer's can aggravate or cause a metabolic acidosis. Acidosis is another component of the "Triad of Death" that is related with poor myocardial (heart muscle) function.
Patients with preexisting hypertension are at higher risk of death and morbidity during permissive hypotension. This is due to the shift in the autoregulatory curve to the right for hypertensive patients.
Permissive hypotension relies on the heart's ability to pump fluid through the body efficiently. Less intravascular fluid results in a less fluid filling the heart (lower end diastolic volume) which results in a lower amount of volume pumped out of the heart (stroke volume). This is based on the frank-starling law of the heart. Healthy patients are able to compensate for lower volumes Patients with preexisting cardiovascular disease that limits myocardial function such as angina pectoris and preexistence of cardiovascular disease limits the use of this concept. This can result in limited coronary perfusion and resulting ischemic damage to the heart and potentially a myocardial infarction (heart attack).[9]
This concept may also be an issue in cerebrovascular disease and carotid artery stenosis; compromised renal (kidney) function in certain conditions, where low blood pressure may induce sludge (thickening of blood) and lead to occlusion of the vessel lumen.
In a high percentage of polytraumatized patients the brain is also affected. The results from the Traumatic Coma Data Bank show the influence of the presence or absence of hypotension (defined as one or more recordings of a systolic blood pressure ≤90 mm Hg) or hypoxia (PaO2 <60 mm Hg) at the time of admission on the outcome of patients who suffer traumatic brain injury and hypotension at admission to the hospital showed twice the mortality and a significant increase in morbidity when compared with patients who were normotensive. The concomitant presence of hypoxia and hypotension upon admission resulted in a 75% mortality.[10] Evidence strongly suggests that the avoidance or minimization of hypotension during the acute and postinjury period following traumatic brain injury had the highest likelihood of improving outcomes of any one single therapeutic maneuver.[11]