Pathophysiology of fluid imbalance

Fluid imbalance can arise due to hypovolemia, normovolemia with maldistribution of fluid, and hypervolemia. Trauma is among the most frequent causes of hypovolemia, with its often profuse attendant blood loss. Another common cause is dehydration, which primarily entails loss of plasma rather than whole blood. The consequences of hypovolemia include reduction in circulating blood volume, lower venous return and, in profound cases, arterial hypotension. Myocardial failure may result from increased myocardial oxygen demand in conjunction with reduced tissue perfusion. Finally, anaerobic metabolism due to reduced perfusion may produce acidosis and, together with myocardial dysfunction, precipitate multi-organ failure. The splanchnic organs are particularly susceptible to the deleterious effects of hypotension and hypovolemic shock, and these effects, depending upon their duration and severity, may be irreversible despite restoration of normovolemia by fluid administration. Patient monitoring in the intensive care unit typically relies upon central venous pressure devices, whereas the primary focus in the operating theater is blood volume deficit estimated from suction devices. However, estimates of intraoperative blood loss can be inaccurate, potentially leading to inappropriate fluid management. Normovolemia with maldistribution of fluid can be encountered in shock-specific microcirculatory disorders secondary to hypovolemia, as well as pain and stress. Consequent vasoconstriction and reduced tissue driving pressure, as well as leukocyte and platelet adhesion, and liberation of humoral and cellular mediators, may impair or abolish blood flow in certain areas. The localized perfusion deficit may contribute to multi-organ failure. Choice of resuscitation fluid may be important in this context, since some evidence suggests that at least certain colloids might be helpful in diminishing post-ischemic microvascular leukocyte adherence. Excessive volume administration may lead to fluid overload and associated impairment of pulmonary function. However, entry of fluid into the lungs may also be facilitated by increased vascular permeability in certain pathologic conditions, especially sepsis and endotoxemia, even in the absence of substantially rising hydrostatic pressure. Another condition associated with elevated vascular permeability is systemic capillary leak syndrome. The chief goal of fluid management, based upon current understanding of the pathophysiology of fluid imbalance, should be to ensure adequate oxygen delivery by optimizing blood oxygenation, perfusion pressure, and circulating volume.     

Albumin and artificial colloids in fluid management: where does the clinical evidence of their utility stand?

Key questions remain unresolved regarding the advantages and limitations of colloids for fluid resuscitation despite extensive investigation. Elucidation of these questions has been slowed, in part, by uncertainty as to the optimal endpoints that should be monitored in assessing patient response to administered fluid. Colloids and crystalloids do not appear to differ notably in their effects on preload recruitable stroke volume or oxygen delivery. Limited evidence nevertheless suggests that colloids might promote greater oxygen consumption than crystalloids. It remains unclear, in any case, to what extent such physiological parameters might be related to clinically relevant outcomes such as morbidity and mortality. Recent randomized controlled trial results indicate that, at least in certain forms of fluid imbalance, albumin is effective in significantly reducing morbidity and mortality. Much further investigation is needed, however, to determine the effects of colloid administration on clinically relevant outcomes in a broad range of critically ill patients. The ability of administered colloids to increase colloid osmotic pressure (COP) constitutes one mechanism by which colloids might reduce interstitial oedema and promote favourable patient outcomes. However, the applicability of this mechanism may be limited, due to the operation of compensatory mechanisms such as increased lymphatic drainage. Attempts to increase COP might also be less useful in states of increased vascular permeability such as acute respiratory distress syndrome, although this issue has by no means been settled by empirical data. Colloids are clearly more efficient than crystalloids in attaining resuscitation endpoints as judged by the need for administration of far smaller fluid volumes. Among the colloids, albumin offers several advantages compared with artificial colloids, including less restrictive dose limitations, lower risk of impaired haemostasis, absence of tissue deposition leading to severe prolonged pruritus, reduced incidence of anaphylactoid reactions, and ease of monitoring to prevent fluid overload. The cost of albumin, nevertheless, limits its usage. Crystalloids currently serve as the first-line fluids in hypovolaemic patients. Colloids can be considered in patients with severe or acute shock or hypovolaemia resulting from sudden plasma loss. Colloids may be combined with crystalloids to obviate administration of large crystalloid volumes. Further clinical trials are needed to define the optimal role for colloids in critically ill patients.     

Minimal volume,hypotense resuscitation

Large volume fluid resuscitation attempting to normalise physiological parameters in hypovolaemic shock has become the accepted management practice during the last 30 years. This doctrine, based on research in the 1950s, teaches that shock increases mortality, aggressive resuscitation improves outcome and normalisation of vital signs protects against multiple organ dysfunction. The wide acceptance of this doctrine is demonstrated by the central role it plays in the American College of Surgeons Advanced Trauma Life Support (ATLS) course and its Australian equivalent the Early Management of Severe Trauma (EMST) course. During the late 1980s, a number of animal research papers demonstrated severe limitations to the earlier work performed in the 1950s and proposed an alternative approach using hypotense or minimal fluid resuscitation. Controlled haemorrhagic shock is hypovolaemic shock in which the source of the bleeding is easily controlled without operation and hence aggressive fluid resuscitation can be pursued with minimum risk. Uncontrolled haemorrhagic shock is hypovolaemic shock due to bleeding which cannot be controlled without surgery. The restoration of blood pressure towards normal levels may lead to dislodgement of thrombus and loss of vascular spasm in damaged vessels, with a subsequent increase in blood loss. It is in this situation that hypotense resuscitation is thought to be of most value. Hypotense resuscitation is defined as the use of fluid resuscitation to maintain blood pressure at lower than normal levels which are sufficient to maintain life, but minimise the risk of exacerbating internal bleeding. Prompted by animal research a number of human studies have been undertaken to clarify the role of fluid resuscitation in uncontrolled haemorrhage. At present, there is wide acceptance of the use of hypotense or minimal volume resuscitation for ruptured abdominal aortic aneurysm and a recent demonstration that morbidity and mortality are decreased by the use of hypotense resuscitation in penetrating truncal trauma. There are however many other clinical situations that may produce uncontrolled haemorrhagic shock about which we have little clinical data to predict appropriate levels of fluid resuscitation. These include ectopic pregnancy, gastro-intestinal haemorrhage and blunt multi-system trauma. This paper will analyse the animal studies that demonstrate the physiological effects of the various fluid resuscitation regimes and discuss all the clinical papers on the subject of hypotense resuscitation. An attempt will then be made to integrate this data into current Australian practice and give broad guidelines on the modern management of uncontrolled haemorrhagic shock, based on minimal volume or hypotense resuscitation.     

Clinical skills: assessing and treating shock: a nursing perspective

This article outlines the pathophysiology associated with hypovolaemic, cardiogenic and distributive shock, and discusses how each of these might present clinically in the patient. Nursing assessment of a patient in shock is explored, and the use of tools such as the pulse oximeter is examined. The evidence base for a variety of interprofessional interventions is analysed, including fluid therapies such as blood transfusion, the use of crystalloids and colloids, and drug therapies such as the use of inotropic and vasoactive agents. The nursing role in managing the patient in shock is considered throughout. The importance of recognizing the clinical presentation of shock is highlighted, with an emphasis on understanding the pathophysiology and potential systemic effects. Treatment is discussed and covers: providing optimal oxygen therapy, appropriate patient monitoring and location of care, using effective communication skills, assisting with activities of living, psychological support, and working collaboratively to maximize the overall quality of patient care delivered.     

Cardiopulmonary interactions and volume status assessment

Assessment of the hemodynamics and volume status is an important daily task for physicians caring for critically ill patients. There is growing consensus in the critical care community that the "traditional" methods-e.g., central venous pressure or pulmonary artery occlusion pressure-used to assess volume status and fluid responsiveness are not well supported by evidence and can be misleading. Our purpose is to provide here an overview of the knowledge needed by ICU physicians to take advantage of mechanical cardiopulmonary interactions to assess volume responsiveness. Although not perfect, such dynamic assessment of fluid responsiveness can be helpful particularly in the passively ventilated patients. We discuss the impact of phasic changes in lung volume and intrathoracic pressure on the pulmonary and systemic circulation and on the heart function. We review how respirophasic changes on the venous side (great veins geometry) and arterial side (e.g., stroke volume/systolic blood pressure and surrogate signals) can be used to detect fluid responsiveness or hemodynamic alterations commonly encountered in the ICU. We review the physiological limitations of this approach.     

Fluid therapy in the PACU

The goal of fluid therapy in the PACU setting is the restoration of blood volume and tissue perfusion. Choosing the type of fluid infusion depends on the preoperative, intraoperative, and postoperative condition of the patient. An understanding of the functional fluid compartments, the composition of body fluids and commercially available fluids, and the steps to evaluate fluid depletion allow one to determine the fluid needs of the patient. The orderly and expedient evaluation of fluid status of the postoperative patient involves the assessment of volume status, concentration status, composition status, and signs and symptoms of inadequate tissue perfusion. Recovery after surgery is a dynamic process, and fluid reassessment should be conducted periodically. Fluid challenges may be necessary in the hypovolemic patient or in patients with clear signs and symptoms of end-organ hypoperfusion. Weil and Rackow and Shoemaker provide useful approaches to fluid challenge guided by CVP and PAP monitoring. The decision of whether to use crystalloids or colloids for fluid resuscitation is complex, controversial, often determined by personal preference and concern over expense, and may be inconsequential as long as fluids are infused appropriate to the needs of the patient. There are disadvantages and advantages to both crystalloid and colloid fluid administration. As with any therapeutic intervention, there are complications with fluid administration, congestive heart failure and pulmonary edema being of more immediate concern. Finally, blood components are colloid-type solutions that should be reserved for specific patient problems. Red blood cells are indicated to increase oxygen-carrying capacity in patients with anemia. Platelets are used to treat bleeding associated with deficiencies in platelet number or function. Fresh frozen plasma is transfused to increase clotting factor levels in patients with demonstrated deficiency. A good understanding of fluid types available, of a systematic approach to evaluating fluid depletion, and of the indications for blood component therapy will allow one to make appropriate decisions when implementing fluid therapy in the PACU.     

Effect of dextran on blood volume and interactions with volume regulatory systems

The effect of 800 ml 3% dextran Ringer's lactate on blood volume was studied in two groups of young surgical patients who had no significant blood loss during operation. One group was normovolaemic, the other was made hypovolaemic by rapid withdrawal of 600 ml blood. A third group, with neither blood loss nor volume administration, served as the control. The main finding was a restoration of the initial blood volume within 48 h of the dextran infusion in normovolaemic subjects and a maintenance of the resituted volume in hypovolaemic subjects. The plasma dextran concentration showed an identical decay in both situations. Total plasma albumin increased in the hypovolaemic group towards the value before bleeding within 24 h, whereas in the normovolaemic group, it decreased progressively after dextran infusion with significant falls after 24 and 48 h. This suggests that the homeostatic mechanisms are based mainly on the discrepant behaviour of albumin in the two volume situations.For the assessment of volume effect of exogenous colloids, the interaction with blood volume regulatory mechanisms should be taken into account. Furthermore, the actual volume situation and the albumin reserves have to be considered.     

AANA Journal course: update for nurse anesthetists--intraoperative fluid management for the pediatric surgical patient

Intraoperative fluid management for the pediatric surgical patient is a critical element of the anesthetic care plan. In contrast with adult patients, the fluid management is systematized by the use of established protocols that calculate fluid on a per kilogram basis. Children are relatively volume sensitive, and mismanagement of fluid and electrolytes can contribute to morbidity and mortality in infants and young children undergoing even the simplest procedures. Failure to correct volume deficiencies can lead to multisystem failure and death. Inappropriate overhydration can result in pulmonary edema and respiratory problems that can prove fatal. Regardless of the fluid management plan, perioperative fluid management must be flexible and take into account the physiologic development and age of the pediatric patient. The goals of intraoperative fluid management are to restore intravascular volume, maintain cardiac output, and, ultimately, ensure provision of oxygen to the tissues.     

Fluid therapy and the resuscitation of traumatic shock

Fluid management of the traumatized patient begins with assessment of volume status via palpation of pulses; evaluation of mental status; and measurement of urine output, arterial blood pressure, and central pressures. Intravascular line placement and choice of initial resuscitation fluids should be individualized to the clinical situation, although in most situations a crystalloid solution continues to be the initial fluid of choice. Following initial stabilization, the intravenous fluid administered can be tailored to a given situation, chosen only after the deranged fluid balance is sequentially classified according to alterations of volume, concentration, and composition. Parenteral fluids may be divided into two groups: crystalloids and colloids. The indications, complications, and controversies surrounding various resuscitation modalities have been reviewed.     

Postoperative fluid management

Postoperative care units are run by an anesthesiologist or a surgeon, or a team formed of both. Management of postoperative fluid therapy should be done considering both patients' status and intraoperative events. Types of the fluids, amount of the fluid given and timing of the administration are the main topics that determine the fluid management strategy. The main goal of fluid resuscitation is to provide adequate tissue perfusion without harming the patient. The endothelial glycocalyx dysfunction and fluid shift to extracellular compartment should be considered wisely. Fluid management must be done based on patient's body fluid status. Patients who are responsive to fluids can benefit from fluid resuscitation, whereas patients who are not fluid responsive are more likely to suffer complications of overhydration. Therefore, common use of central venous pressure measurement, which is proved to be inefficient to predict fluid responsiveness, should be avoided. Goal directed strategy is the most rational approach to assess the patient and maintain optimum fluid balance. However, accessible and applicable monitoring tools for determining patient's actual fluid need should be further studied and universalized. The debate around colloids and crystalloids should also be considered with goal directed therapies. Advantages and disadvantages of each solution must be evaluated with the patient's specific condition.     

Current concepts in resuscitation

Early recognition and differentiation of shock, as well as goal-directed resuscitation, are fundamental principles in the care of the critically ill or injured patient. Substantial progress has been made over the last decade in the understanding of both shock and resuscitation. Specific areas of advancement, particularly pertaining to hemorrhagic shock, include a heightened appreciation of dynamic measurements of preload responsiveness (e.g., respiratory-induced pulse pressure and venous diameter variability), an improved awareness of the detrimental effects of blood product transfusion, and better recognition of the complications of overzealous volume expansion. However, several areas of controversy remain regarding the optimal resuscitation strategy. These include the optimal targets for perfusion pressure and oxygen delivery, endpoints of resuscitation, resuscitative fluid, and transfusion strategies for packed red blood cells and blood products. This article reviews the diagnosis and differentiation of shock, measurements of tissue perfusion, current evidence regarding various resuscitative techniques, and complications of resuscitation.     

Management of early acute renal failure: focus on post-injury prevention

PURPOSE OF REVIEW: In this review, we describe our current understanding of various aspects of secondary renal injury and its prevention. Secondary renal injury indicates any injury to the kidney, which occurs after an initial event has already triggered injury to the organ. RECENT FINDINGS: Analysis of the literature reveals several important fields of possible intervention. First, blood pressure is considered important and hypotension is associated with renal injury. Avoiding hypotension is an important mechanism of renal protection from secondary injury. Similarly, a low cardiac output state should be promptly treated or prevented. Adequate volume resuscitation is also considered important although strong direct evidence for this intervention is not available. There is insufficient evidence to suggest that any drug can specifically increase renal blood flow in man independent of an effect on blood pressure or cardiac output. Specific kidney protective approaches have not yet been identified. Intensive insulin therapy possibly delivers renal protection and deserves further investigation. Modulation of the stress response appears attractive in experimental models but it has not been shown effective in man. Ischemic preconditioning is a useful strategy for renal protection in the experimental setting. An understanding of the mechanisms involved in ischemic preconditioning might assist in developing novel and effective interventions in man. SUMMARY: The pillars of protection from secondary renal injury are similar to those needed to protect the kidney from primary injury: maintenance of adequate intravascular volume, cardiac output, and arterial blood pressure. Novel protective strategies such as intensive insulin therapy require further investigation.     

Hemodynamic and colloid osmotic pressure alterations in the surgical patient

Colloid osmotic pressure (COP) was measured simultaneously with cardiorespiratory measurements in 103 surgical patients suspected of having circulatory problems. In a small subset of 28 patients, measurements were taken before, during, and after surgical operations. Similarly, data sets were taken before, during, and after infusions of colloids and crystalloids to assess the interactions of these variables during the stress of surgery and the administration of fluid therapy. COP was found to decrease during and shortly after surgical operations despite reasonably well-maintained pressure, volume, and flow variables. Concentrated (25%) albumin and plasma protein fraction (PPF) increased COP, cardiac index (CI), CVP, pulmonary capillary wedge pressure (WP), and blood volume, whereas crystalloids transiently increased CI, CVP, and WP but did not significantly change COP and blood volume. Low COP values were weakly related to survival, and COP-WP differences less than or equal to 3 mm Hg were roughly related to ARDS and pulmonary edema.     

Hemodynamic monitoring in shock and implications for management

Objective

Shock is a severe syndrome resulting in multiple organ dysfunction and a high mortality rate. The goal of this consensus statement is to provide recommendations regarding the monitoring and management of the critically ill patient with shock.

Methods

An international consensus conference was held in April 2006 to develop recommendations for hemodynamic monitoring and implications for management of patients with shock. Evidence-based recommendations were developed, after conferring with experts and reviewing the pertinent literature, by a jury of 11 persons representing five critical care societies.

Data synthesis

A total of 17 recommendations were developed to provide guidance to intensive care physicians monitoring and caring for the patient with shock. Topics addressed were as follows: (1) What are the epidemiologic and pathophysiologic features of shock in the ICU? (2) Should we monitor preload and fluid responsiveness in shock? (3) How and when should we monitor stroke volume or cardiac output in shock? (4) What markers of the regional and micro-circulation can be monitored, and how can cellular function be assessed in shock? (5) What is the evidence for using hemodynamic monitoring to direct therapy in shock? One of the most important recommendations was that hypotension is not required to define shock, and as a result, importance is assigned to the presence of inadequate tissue perfusion on physical examination. Given the current evidence, the only bio-marker recommended for diagnosis or staging of shock is blood lactate. The jury also recommended against the routine use of (1) the pulmonary artery catheter in shock and (2) static preload measurements used alone to predict fluid responsiveness.

Conclusions

This consensus statement provides 17 different recommendations pertaining to the monitoring and caring of patients with shock. There were some important questions that could not be fully addressed using an evidence-based approach, and areas needing further research were identified.     

The effects of mechanical ventilation on the cardiovascular system

Positive-pressure ventilation may improve gas exchange, decrease the work-cost of breathing, and rest respiratory muscles, but it also will alter cardiac output and may modify blood flow distribution. Ventilation may induce these hemodynamic changes by altering systemic venous return to the RV (RV preload), pulmonary arterial pressure (RV afterload), ventricular interdependence (LV preload), or transmural LV ejection pressure (LV afterload). These interactions are magnified when the changes in lung volume and intrathoracic pressure are increased or under conditions associated with a reduced effective circulating blood volume or cardiac contractility. An understanding of these interactions is central to the effective management of the ventilator-dependent patient.     

Review article: Prehospital fluid management in traumatic brain injury

The early management of patients who have sustained traumatic brain injury is aimed at preventing secondary brain injury through avoidance of cerebral hypoxia and hypoperfusion. Especially in hypotensive patients, it has been postulated that hypertonic crystalloids and colloids might support mean arterial pressure more effectively by expanding intravascular volume without causing problematic cerebral oedema. We conducted a systematic review to investigate if hypertonic saline or colloids result in better outcomes than isotonic crystalloid solutions, as well as to determine the safety of minimal volume resuscitation, or delayed versus immediate fluid resuscitation during prehospital care for patients with traumatic brain injury. We identified nine randomized controlled trials and one cohort study examined the effects of hypertonic solutions (with or without colloid added) for prehospital fluid resuscitation. None has reported better survival and functional outcomes over the use of isotonic crystalloids. The only trial of restrictive resuscitation strategies was underpowered to demonstrate its safety compared with aggressive early fluid resuscitation in head injured patients, and maintenance of cerebral perfusion remains the top priority.     

Hemodynamic, blood volume, and oxygen transport responses to albumin and hydroxyethyl starch infusions in critically ill postoperative patients

Hemodynamic, plasma volume, and oxygen transport effects were measured after administration of 500 ml of 5% albumin or 6% hydroxyethyl starch (HES) in hypovolemic postoperative patients using a prospectively randomized crossover design. Both agents produced marked and significant improvement in plasma volume and flow as well as small transient increases in arterial and venous pressures, urine output, colloidal osmotic pressure (COP), and oxygen transport. The authors conclude that HES is a safe, inexpensive, effective plasma expander that has hemodynamic effects similar to those of other colloids. It was apparent from these and other studies that clinically stable postoperative patients may have appreciable blood volume deficits. Routine vital signs correlated poorly with the preinfusion control hemodynamic values or the changes in blood volume status after volume loading. Normal cardiac output, central venous pressure (CVP), and pulmonary arterial wedge pressure (WP) values are commonly seen in critically ill postoperative patients who, nevertheless, may be hypovolemic. Measurement of changes in these variables after a fluid challenge is a useful way to assess plasma volume status.     

Fluid resuscitation in diabetic emergencies — a reappraisal

The first objective in diabetic ketoacidosis is to restore the circulating volume and improve tissue perfusion. In any form of hypovolaemic shock the most efficient way of restoring circulating volume is to be use colloid solutions rather than crystalloids. At least three times the amount of crystalloid must be used to achieve the same effect. The historical reason for using isotonic saline in diabetic ketoacidosis is related not to its similarity to the fluid lost, but to its supposed efficiency in correcting the circulating volume. Excess crystalloid expands the interstitial space which results in pulmonary oedema, peripheral oedema and possibly cerebral oedema. Although currently difficult to define precisely in their more subtle forms, they all produce adverse pathophysiological effects. The fluid loss in diabetic ketoacidosis is equivalent to "half-normal" saline, a relatively hypotonic solution. As well as causing extensive oedema, resuscitation with isotonic saline can increase serum sodium and osmolarity while not providing free water to replace the intracellular losses.     

Induction of hypothermia in patients with various types of neurologic injury with use of large volumes of ice-cold intravenous fluid

OBJECTIVE: Mounting evidence suggests that mild to moderate hypothermia can mitigate neurologic and myocardial injury. The speed of induction appears to be a key factor in determining its efficacy. However, even when the fastest currently available cooling techniques are used, reaching target temperatures takes at least 2 hrs and usually longer. We hypothesized that infusion of refrigerated fluids could be a safe accessory method to increase cooling speed. DESIGN: Prospective intervention study. SETTING: University teaching hospital. PATIENTS: One hundred thirty-four patients with various types of neurologic injury (postanoxic encephalopathy, subarachnoid hemorrhage, or traumatic brain injury). MEASUREMENTS AND MAIN RESULTS: Hypothermia was induced in 134 patients with various types of neurologic injury, by means ice-water cooling blankets and infusion of refrigerated (4 degrees C) saline (110 patients) or saline and colloids (24 patients). An average volume of 2340 +/- 890 mL of refrigerated fluids was infused in 50 mins. Core temperatures decreased from 36.9 +/- 1.9 degrees C to 34.6 +/- 1.5 degrees C at t = 30 mins and to 32.9 +/- 0.9 degrees C at t = 60 mins (target temperature: 32 degrees C-33 degrees C). Monitoring of blood pressure, heart rhythm, central venous pressure, blood gasses, electrolyte and glucose levels, and platelet and white blood cell count revealed no additional adverse effects. Mean arterial pressure increased by 15 mm Hg, with larger increases in blood pressure occurring in hemodynamically unstable patients. No patient developed pulmonary edema. CONCLUSIONS: Induction of hypothermia by means of cold-fluid infusion combined with ice-water cooling blankets is safe, efficacious, and quick. Because the speed of cooling is important to increase its protective effects, we recommend that cold-fluid infusion be used in all patients treated with induced hypothermia. This should be combined with another method to safely and accurately maintain hypothermia once target temperatures have been reached.