Luis H. Tello, MV, MS, DVM
The choice of the perfect fluid among colloids and crystalloids for resuscitation in ER or critical patients in small animal practice has long been a subject of debate among practitioners, primarily because there are data to support arguments for both or even mixed forms of therapy.
The British Medical Journal published recently a meta-analysis on the use of albumin in the critically ill human patients, reviewing many controlled trials (RCTs) involving more than 1,400 patients. After further analysis, the relevant conclusion was that albumin may actually increase mortality.
This review had an impact on practice in 1998 when it was published, influencing clinicians to use fewer albumin solutions in their therapies, but it was later criticized as being flawed when subsequent reviews did not support the authors' conclusion.
Recently, the completion of the Saline v/s Albumin Fluid Evaluation (SAFE) study has shed new light on this issue. This double-blind RCT enrolled 7,000 human patients from 16 ICUs in Australia and New Zealand over an 18-month period. Patients were randomized to receive either 4% human albumin or normal saline from time of admission to the ICU until death or discharge.
Mean arterial blood pressure, central venous pressure, heart rate, and incidence of new organ failure were similar in both groups; however, in a subgroup analysis, differences between trauma and sepsis patients were observed. The relative risk (RR) of death in patients with severe sepsis who received albumin vs. saline was 0.87. The RR of death in albumin-treated patients without severe sepsis was 1.05 (P = 0.059).
The results were the opposite in trauma patients. The overall mortality rate in trauma patients was higher when albumin vs. saline was used for volume resuscitation (13.5% v/s 10%). When patients with traumatic brain injury (TBI) were studied separately, the mortality rate was 24.6% in patients who were treated with albumin compared with 15% in patients who were treated with saline (RR 1.62, 95% confidence interval, -1.12 to 2.34, P = 0.009). Based on these results, the administration of albumin appears to be safe for up to 28 days in a heterogeneous population of critically ill patients, and it may be beneficial in patients with severe sepsis.
However, the safety of albumin administration has not been established in patients with traumatic injury, including traumatic brain injury (TBI).
Although the differences in mortality rates in trauma and TBI patients were observed in a subgroup analysis and consequently have limited validity, this is a strong signal, especially in TBI patients. A new study designed to evaluate patients with head trauma has been set up to examine these differences.
However, multiple new published studies and trial reports revealed contradictory information about the best formula to combine different type of fluids, rates or rhythms of infusion, remarking the fact that individual, tailored, looking for end-goals and monitored therapies are the best option based on the current available information.
The medical management of the critical patient requires a comprehensive approach and the usage of multiple monitoring and therapeutics. These patients may respond initially to a regular symptomatic therapy, but when the case evolves into more complicated conditions, it is imperative for the clinicians to understand the physiopathology and the progression of the signs to interpret their clinical data and adjust the treatment accordingly.
The estimations are that approximately 60% of the entire body is made up of water (more in dogs, less in cats) and that is called total body water (TBW). The TBW is distributed into 2 compartments: intra- and extracellular. The intracellular water comprises the majority of TBW, accounting for approximately 2/3 (66%), but it is water not readily available or easily exchangeable.
Extracellular water represents the remaining 33%. The extracellular fluid is divided into two components: the interstitial fluid, which makes 75% and plays the role of "water bank" in the body; and the intravascular component (plasma), which makes 25% of the extracellular fluid.
There are also some small amounts of water in other compartments such as the gastrointestinal system, bladder, cerebrum-spinal and ocular fluid, which typically accounts for < 1% of the TBW, so is not included in the classical formulas. These are sometimes referred to as transcellular fluid or third-space fluid.
Clinically, water deficits can be divided into perfusion deficits or hypovolemic deficit (intravascular space) and dehydration (interstitium). A tissue perfusion deficit is associated with decreased oxygen delivery for the cells, less production of energy, and therefore is considered a life-threatening condition.
Currently 4 stages are described for a fluid resuscitation plan: 1) assessment of the severity and type of shock; 2) picking up the combination of fluids; 3) planning your goals (endpoint resuscitation) and the protocol; and 4) monitoring and evaluating the patient for your goals and re-planning.
Blood loss is a common condition in severely critical patients; therefore, it is not well tolerated in the animal. The dangers of hemorrhage are related to a cardiovascular system that operates with a relatively small volume and responds to the Frank-Starling curve in the heart with the possible purpose to limit cardiac work and conserve energy.
Crystalloid fluids are mixtures of sodium chloride and other physiologically active solutes. They are generally isotonic with plasma and have sodium as their major osmotically active particle. The distribution of sodium determines the distribution of infused crystalloid fluids. Sodium is the major solute in the extravascular space, and 75% of the extracellular space is extravascular. Therefore, infused sodium will reside primarily outside the vascular compartment.
Aggressive fluid resuscitation with crystalloid solutions such as Normosol-R, LRS, or normal saline with an end goal of 3 volumes of crystalloid for each volume of blood lost has been widely accepted as the standard management of hemorrhagic shock secondary to trauma. However, some questions arise about the safety in such an approach due to the risk of overload, elevated CVP, reduction of the myocardium perfusion, pulmonary and peripheral edema, and dilution coagulopathy.
In feline patients, there are more doubts and red flags, because the intravascular blood volume is much smaller than in dogs, and the total shock bolus of crystalloid is suggested around 45–60 ml/kg. One among many protocols accepted in cats is initially infusing 10–30 ml/kg rapidly over 10–20 minutes, while the animal is carefully observed for a response or for evidence of complications.
This dose can then be repeated if necessary. If the hemodynamic values (MM, CRT, BP, HR and mentation) in the patient begin to improve, administration may be decreased down before the total bolus has been given. Clinical values such as packed cell volume (PCV), total solids (TS) or total protein (TP), electrolytes, and blood glucose should be monitored before, during, and after such therapy.
Some research has suggested that the use of small volumes of hypertonic crystalloid solutions is recommended because the reduced volumes of fluid required decreases the risks of fluid overload, therefore reducing the chance for developing pulmonary edema.
Hypertonic saline (1.7%, 3%, 5%, 7.5%) is used in hypovolemic and traumatic shock with or without hyperoncotic substances. Experimental studies show hypertonic saline solutions will improve microcirculatory flow, possibly by reducing shock-induced endothelial swelling. With endotoxic shock models, hypertonic saline is more effective than isotonic crystalloids in proving cardiac output and oxygen transport, but only very transiently. In another study, no benefit was seen. No controlled trials exist in veterinary medicine.
Colloids are large molecular-weight substances that do not readily pass across capillary walls. The particles retained in the vascular space will exert an osmotic force that keeps fluid in the blood vessels. Colloids are more effective than crystalloids for increasing vascular volume, because they remain within the vasculature and provide more consistent plasma volume expansion. Based on this effect, it has been widely accepted that small volumes of colloid need to be used compared to crystalloid, for a similar cardiovascular effect.
The main colloid substance in the blood is a natural protein: albumin. Albumin sustains the oncotic pressure, but has shown to induce a sustained increase in the glutathione in lung epithelial cells and to inhibit some deleterious cytokines. Thus, it appears that albumin may have multiple physiologic effects, including boosting the antioxidant potential and modulation of redox balance, hence attenuating inflammation.
In human medicine, it was accepted that there would be a relationship between the albumin level and disease severity in critically ill patients such as multiple trauma. However, no clear correlation has been established by the evidence.
The choice of colloids v/s crystalloids for volume resuscitation in critical patients has long been a subject of debate among veterinary and human practitioners.
Although mortality is but one factor in assessing colloids versus crystalloids, the following recommendations have been made in septic patients: prompt and adequate fluid therapy is the mainstay of treatment of septic shock. Colloid and crystalloid fluids lower hemoglobin concentration, oxygen-carrying capacity, and whole blood viscosity. The choice of fluid should take into account its effect on plasma oncotic pressure (COP). Severe decreases in COP should be avoided. Volume treatment should be individualized and titrated to individual needs.
References are available upon request.