Permissive hypotension
Encyclopedia
Permissive hypotension or hypotensive resuscitation 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.
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. 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. 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. Clinical data from well controlled, prospective trials applying the concept of permissive hypotension in trauma patients are still missing.
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.
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.
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).
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. 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.
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
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
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
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
Inotrope
An inotrope is an agent that alters the force or energy of muscular contractions. Negatively inotropic agents weaken the force of muscular contractions...
or vasopressors, the only restriction is to avoid completely normalizing blood pressure in a context where blood loss may be enhanced.
It is becoming common in hemorrhaging patients without traumatic brain injury
Traumatic brain injury
Traumatic brain injury , also known as intracranial injury, occurs when an external force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...
. 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
Mean arterial pressure
The mean arterial pressure is a term used in medicine to describe an average blood pressure in an individual. It is defined as the average arterial pressure during a single cardiac cycle.-Calculation:...
of 60 to 70 mmHg) is taken as the target for fluid administration during active hemorrhage. To date, the only published study in humans, in victims of penetrating torso trauma, has demonstrated a significant reduction in mortality
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
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
Trauma center
A trauma center is a hospital equipped to provide comprehensive emergency medical services to patients suffering traumatic injuries. Trauma centers grew into existence out of the realization that traumatic injury is a disease process unto itself requiring specialized and experienced...
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. Clinical data from well controlled, prospective trials applying the concept of permissive hypotension in trauma patients are still missing.
Pathophysiology
Following injury the otherwise healthy individual has a natural ability to clot offCoagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
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
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
coagulation
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
. 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.
Another issue with aggressive fluid resuscitation is the potential for hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
if fluids that are stored at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
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
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
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
Dilution
Dilution may refer to:* Reducing the concentration of a chemical* Serial dilution, a common way of going about this reduction of concentration* Homeopathic dilution* Dilution , an equation to calculate the rate a gas dilutes...
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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. Administration of large quantities of isotonic
Isotonic
The term isotonic may refer to;*Isotonic for the term associated with muscle contraction*An isotone in nuclear physics*Sports drinks are sometimes designed in an isotonic way to assist athletes rehydrate while balancing electrolytes...
or slightly hypertonic crystalloid solutions such as 0.9% normal saline or lactated ringer's
Lactated Ringer's solution
Lactated Ringer's solution is a solution that is isotonic with blood and intended for intravenous administration. It may also be given subcutaneously....
can aggravate or cause a metabolic acidosis
Metabolic acidosis
In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the...
. Acidosis is another component of the "Triad of Death" that is related with poor myocardial (heart muscle) function.
When Permissive Hypotension Should be Avoided
Patients with preexisting hypertensionHypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
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
Stroke volume
In cardiovascular physiology, stroke volume is the volume of blood pumped from one ventricle of the heart with each beat. SV is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood in the ventricle at the end of a beat from the 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
Coronary perfusion pressure
Coronary perfusion pressure refers to the pressure gradient that drives coronary blood pressure, meaning the difference between the aortic diastolic pressure and the right atrial pressure. It is a term used mainly in research concerning cardiac arrest...
and resulting ischemic damage to the heart and potentially a myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
(heart attack).
This concept may also be an issue in cerebrovascular disease
Cerebrovascular disease
Cerebrovascular disease is a group of brain dysfunctions related to disease of the blood vessels supplying the brain. Hypertension is the most important cause; it damages the blood vessel lining, endothelium, exposing the underlying collagen where platelets aggregate to initiate a repairing process...
and carotid artery stenosis
Carotid artery stenosis
Carotid stenosis is a narrowing or constriction of the inner surface of the carotid artery, usually caused by atherosclerosis .-Signs and symptoms:...
; 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
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
(PaO2 <60 mm Hg) at the time of admission on the outcome of patients who suffer traumatic brain injury
Traumatic brain injury
Traumatic brain injury , also known as intracranial injury, occurs when an external force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...
and hypotension
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
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. 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.