Continuing Education Activity

Permissive hypotension is the act of maintaining a blood pressure lower than physiologic levels in a patient that has suffered from hemorrhagic blood loss. The practice is employed in order to maintain adequate vasoconstriction, organ perfusion, and prevent an undesired coagulopathy during initial fluid resuscitation. This activity outlines the evaluation and management of permissive hypotension and highlights its role in the improvement of clinical outcomes of critically ill patients.

Objectives:

• Explain the pathophysiology of permissive hypotension.
• Describe the typical management of permissive hypotension.
• Outline management considerations for patients requiring permissive hypotension resuscitation.
• Identify the importance of collaboration and communication amongst the interprofessional team to improve outcomes for patients receiving permissive hypotension resuscitation.

Introduction

Permissive hypotension, also commonly known as hypotensive resuscitation, is the method of non-aggressive fluid resuscitation in which volume repletion is limited in order to sustain a goal systolic or mean arterial pressure (MAP) below normal physiological conditions in the setting of volume loss. Such a technique is commonly employed in the trauma setting for patients experiencing acute hemorrhagic volume depletion. Various sources have outlined a decrease in mortality when blood pressures are adjusted below the normal physiologic range. Different sources have stated a quicker recovery and a decrease in postoperative recovery time when permissive hypotension is put into practice. This is, in contrast, to immediately normalizing an individual's blood pressure.[1][2] Results, however, have not delineated a standard algorithmic approach to pressure stabilization in patients with different confounding factors such as age, pre-existing health conditions, and different methods of injury. Given such different methods of achieving permissive hypotension, the challenge of hypotensive resuscitation lies in how low one should set the limiting threshold for permissible hypotension with respect to a patient's age, demographics, and condition causing the acute volume depletion.

Anatomy and Physiology

It is believed that introducing too much fluid at once could cause an increase in the cardiac preload leading to a rise in the cardiac output. This rise in cardiac output, in effect, raises the mean arterial pressure. The result is a decrease in peripheral vasoconstriction resulting in further blood loss.[3] Permissive hypotension may allow for minimal vasodilatory effects of the peripheral vasculature while allowing for adequate organ perfusion to prevent multi-systemic organ failure. It has also been proposed that such practice of diminished fluid resuscitation may prevent hydrostatic damage to already injured vessels.[4] Lastly, it is believed that aggressive fluid repletion may potentially alter the normal physiologic coagulation response.[4][5] With overhydration, coagulopathy in the form of dilutional coagulopathy may result. This, in turn, may necessitate the addition of blood products to better manage homeostatic coagulopathy.[4][6]

Indications

Hypotensive resuscitation can be applied in treating numerous cases of hypovolemic shock. However, this form of volume resuscitation is mainly used in the setting of a traumatic hemorrhage to rapidly stabilize the patient before more definitive procedures can be performed to address the source of bleeding. This strategy should not be utilized after bleeding is controlled in the operating room. Special consideration must be made in evaluating the underlying etiology and background of the patient's presentation to the clinical setting.

The practice of hypotensive resuscitation has mainly been studied and indicated for hemodynamically unstable patients. This is especially true for those who present with severe hemorrhage from either penetrating or blunt trauma. Although its role in blunt trauma is less clear. Patients should be evaluated for any prior history that may put them at risk during such a procedure. Patients who do not suffer from chronic hypertension are appropriate candidates for this practice.[7] Patients without long-term cardiovascular abnormalities or those with endocrine abnormalities (e.g., syndrome of inappropriate antidiuretic hormone (SIADH), diabetes insipidus, etc.) may also exhibit an appropriate physiologic response to the procedure.

Contraindications

In patients with blunt trauma, this strategy may lead to poor outcomes due to tissue hypoperfusion. The data are insufficient to suggest its prehospital use in cases involving blunt trauma. Significant mortality rates have been reported among those blunt trauma patients, who have been subjected to a permissive hypotensive treatment strategy. The permissive hypotensive strategy is contraindicated in patients presenting with traumatic brain injury. In such situations, a mean arterial pressure (MAP) of greater than 80 mmHg (a cerebral perfusion pressure of approximately 60 mmHg) is required in order to maintain cerebral perfusion pressure.

The practice of employing permissive hypotension must be considered carefully and on a case-by-case basis. In the setting of a traumatic brain injury (TBI), it has been recommended that the practice of permissive hypotension not be used. The Brain Trauma Foundation has recommended against the maintenance of systolic pressures below 90 mmHg in patients with TBI.[8] Patients with traumatic brain injuries have been reported to suffer an increase in the mortality rate when systolic pressures drop below normal physiologic levels. This may have been due to a decrease in cerebral perfusion pressure.[9][10] 

This is especially true in large cerebral hemorrhagic events causing an increase in intracranial pressure. Incremental fluid infusions in these patients have been recommended in order to increase the mean arterial pressure (MAP) above 80 mmHg (allowing for a cerebral perfusion pressure of 60 mmHg) and avoid a potential ischemic event.[11] For this population of trauma patients, recent reports have advised that the management of systolic pressures be based on patient age. Under a retrospective data review, new thresholds for permissive hypotension with respect to patients were created. Based on Brain Traumatic Foundation guidelines, well above the permissive hypotension thresholds, modulation of systolic blood pressures to 110 mmHg in patients 15 to 49 years of age, 100 mmHg for patients 50 to 69 years of age, and 110 mmHg for patients 70 years and older, have been recommended.[12][13]

Preparation

In general, for all patients requiring resuscitation, fluid replenishment can be divided into discrete phases. Vincent and De Baker[14] divided resuscitative methods into four stages, referring to the purposes of each stage of therapy, as follows;

• Rescue or salvage
• Optimization 
• Stabilization 
• Deescalation

The first is the 'rescue phase' during which the resuscitation is aimed at re-establishing the minimum perfusion necessary to sustain life in the setting of a life-threatening hypovolemic shock. Next, the 'optimization phase' is aimed at preventing decompensation after the patient has been rescued from life-threatening shock. This helps ensure an optimal cardiac output for appropriate tissue perfusion. The 'stabilization phase' aims at maintaining a patient's normal physiologic systolic and mean arterial pressures. Then, 'de-escalation' aims at slowing down and removing fluid administration in order to evaluate a patient's independent stability following supportive therapy and fluids. Permissive hypotension has generally been used only in the initial rescue and optimization phases of recovery. 

Technique or Treatment

Different guidelines can be applied when determining what initial infusion volume and with which rate to begin before resuscitation. There have been animal models attempting to determine the most appropriate method in which hypotensive resuscitation could be standardized in humans.[15][16] Research favors resuscitation at a rate of 60 mL/kg/h to 80 mL/kg/h to maintain systolic pressures of 80 mmHg to 90 mmHg or a MAP of 40 mmHg to 60 mmHg.[17] However, there lacks standardized, conclusive data corroborating a numerical cutoff in pressure or a discrete algorithmic approach to the rate and method of pressure stabilization. Many sources are in agreement that permissive hypotension can be achieved in many patients who have either a MAP of around the target number of 50 mmHg or systolic pressure around 80 mmHg to 90 mmHg.[1][2][18][19][20] Fluid infusions around 100 mL to 200 mL at a time, while evaluating the response of the periodic infusions using the mean arterial pressure or systolic blood pressures can be applied to ensure a patient is in a permissible pressure range.[11] 

The method of injury may also play a role in resuscitative efforts. It has been noted that different forms of injury require different systolic blood pressure control. Penetrating traumas may require a systolic blood pressure ranging from 60 mmHg to 70 mmHg. Patients with brain injury may require resuscitation methods that manage systolic pressures ranging between 80 mmHg to 90 mmHg. Finally, those with both traumatic brain injury blunt force trauma should be managed in non-hypotensive conditions with systolic blood pressures ranging between 100 mmHg to 110 mmHg based on age.[21] The overall management of systolic pressures should ultimately be based on the clinical improvement of the patient pursuant to the initial resuscitation methods employed. 

The use of crystalloid versus colloid solutions in initial resuscitation has been debated. Evidence supporting the initial use of crystalloid may have either a positive or no effect on promoting coagulation during the initial resuscitation of a patient undergoing a hypovolemic hemorrhagic bleed.[22][23][24] Reports show that early use of colloid solutions may have either a positive, neutral, or negative effect on early coagulation.[25][26][27] It can be noted that the use of crystalloid 0.9% saline to allow for permissive hypotension has shown to have some pro-coagulatory effects when used in moderation.[28] Only a few solutions should not be used or may be used with great caution during resuscitation. The use of colloid solutions of dextrans or hydroxyethyl starch could hamper coagulation when 1.5 or more liters are administered during resuscitation efforts.[29] 

In addition, serial laboratory tests should be obtained in order to evaluate the degree of coagulopathy and the degree of anemia. Mild to moderate hypovolemic episodes may not require blood products, whereas patients with severe blood loss and rapid decompensation may require the early administration of such products.[30] A hemoglobin level of below 7 g/dL should warrant the transfusion of packed red blood cells. Generally, a 1:1:1 ratio of units of plasma, packed red blood cells, and platelets are used to prevent early coagulopathy and address anemia during early resuscitation.[30] Prothrombin, partial thromboplastin time, bleeding time, and the international normalized ratio should be reviewed in order to assess the adequacy of the transfusion products administered.

After the initial rescue and optimization of resuscitation, hydration can then be managed with urine output checks to evaluate appropriate maintenance fluids. Physicians should also take note of the physical markers of the patient's hydration status. Physicians should check for capillary refill and peripheral pulses, in conjunction with monitoring both the patient's heart rate and blood pressure. Although such measures are not as accurate as serial re-checks in urine output, quick physical exams offer a quick measure to gauge fluid status. 

Clinical Significance

Timing plays an important role in the initial stabilization of a patient’s vital signs. Prehospital (PH) administration of fluids in those patients with major hemorrhagic shock has been shown to be an effective strategy in stabilizing patients before intra-operative procedures and in helping to reduce mortality.[1][31][32] In general, the prehospital administration of intravenous fluid is associated with increased survival in trauma patients. However, the benefits of prehospital fluid administration may not be applicable to all patients with hemorrhage loss. Prehospital fluid crystalloid administration of more than 500 mL in patients without PH hypotension has been shown to have worse outcomes.[32] A goal-directed resuscitation based on the presence or absence of PH hypotension should be adopted. In those with severe, life-threatening hemorrhage. Such practice allows early venous access to permit the administration of appropriate fluid administration, and any other necessary resuscitation agents that may be acutely administered in the hospital setting.[33][34]

Employing the practice of hypotensive resuscitation prevents a compensatory loss of blood by mitigating a vasodilatory effect and by preventing the over-dilution of the coagulation cascade factors, decreasing the need for administration of other blood products such as fresh frozen plasma to preserve clotting function. With caution, permissive hypotension can be a much more cost-effective method of resuscitation. As this method of fluid resuscitation is more thoroughly elaborated in the future, it is already apparent that such a strategy can positively improve patient recovery and expedite discharge while reducing post-hospitalization complications.

Enhancing Healthcare Team Outcomes

The management of permissive hypotension can be a complex endeavor. The procedure allows the medical team to maintain those body fluid pressures appropriate for organ function in the setting of a hemorrhagic shock, due to trauma. Proper coordination and communication must be carefully choreographed within the medical team, in order to adequately evaluate any variable that may alter the management protocol. It is of great importance that first responders quickly garner information regarding the mechanism of injury, in order to better determine the amount and rate of fluid administration.[1][2] In addition, the severity of the volume loss must be assessed in order not only to evaluate the amount of fluids needed; but also to estimate the need for prehospital fluid administration.[1][31][32][35] A patient's pre-existing conditions must then be assessed in order to determine if there are any contraindications to management before fluids are administered. 

Based upon retrospective data collection studies in the trauma setting, reports have suggested that permissive hypotension can generally be achieved in many patients, if the mean arterial pressure is close to the target number of 50 mmHg, or if the systolic pressures are between 80 mmHg to 90 mmHg.[1][2][18][19][20] [Level 4] The fluid resuscitation strategy should depend on the mechanism of injury. Penetrating traumas may require a systolic blood pressure ranging from 60 mmHg to 70 mmHg. Patients with brain injury may require resuscitation methods with systolic pressures between 80 mmHg to 90 mmHg. Those with both traumatic brain injury and blunt trauma should be managed in non-hypotensive conditions with systolic blood pressures ranging between 100 mmHg to 110 mmHg and adjusted based on age.[21] [Level 4] Evidence-based guidelines however have yet to clearly elucidate adjustment of systolic and mean arterial pressures based on other aspects of the patient's background, such as age, weight, height, and gender. It is, therefore, of great importance that nurses and physicians closely monitor the patient's mentation and vitals once initial fluids are started. Following the initial rescue phase, blood pressure may be gradually raised and urine output may be monitored in determining the patient's establishing homeostasis.