Resuscitation Fluids
In addition to blood, commonly used fluids for intraoperative use are divided into three categories: conventional crystalloids, colloids, and hypertonic solutions. Some less commonly used fluids include blood substitutes. Crystalloids are balanced salt solutions that freely cross the capillary endothelium and rapidly equilibrate with extravascular fluids. Colloid contain larger molecules (i.e. proteins or starches) that exert the oncotic activity that, in normal situations, keeps water within the intravascular space. In the sick patient, who may have abnormal endothelial permeability, the colloid may not work as predicted by normal physiology. In this section, the choice of initial fluids and the question of glucose utilization are addressed.
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CRYSTALLOIDS
Conventional crystalloids are fluids that contain a combination of water and electrolytes. They are divided into "bal\-anced" salt solutions (e.g., Ringer's lactate solution, Plasma-Lyte, Normosol) and hypotonic solutions. E ither their electrolyte composition approximates that of plasma, or they have a total calculated osmolality that is similar to that of plasma.
Normal saline (0.9%) is actually hypertonic with respect to sodium and especially to chloride, if the osmolality is calculated. However, when normal saline is subjected to a freezing point depression test in an osmometer, its osmolality is approximately 285 mOsm/kg. The calculated value is derived by simple addition of its ionic constituents, whereas the measured value is affected by ionic association or dissociation. Sodium chloride has a relative osmolality of 1 compared with that of sodium and chloride, the value of which is 2. Other balanced electrolyte solutions are slightly hypotonic in vitro (265 mOsm/kg) in comparison with their calculated values and normal plasma. Solutions that contain less than the concentration of electrolytes found in Ringer's lactate solution are not used often intraoperatively.
When an electrolyte-free solution such as D5W is administered, less than 10% stays intravascular. Approximately two thirds is distributed to the intracellular space. Intravascular resuscitation is minimal, and cellular swelling occurs. The administered free water causes a decrease in the serum and interstitial electrolyte concentrations (dilutional effect) and may lead to symptomatic hyponatremia.
When an electrolyte-free solution such as D5W is administered, less than 10% stays intravascular. Approximately 2/3 are distributed to the intracellu lar space. Imtravascular resuscitation is minimal and cellular swelling occurs. The adjinistration of free water causes a decrease in serum and interstitial electrolytes (dilutional effect) and may lead to symptomatic hyponatremia.
When solutions such as 0.2% or 0.45% saline are administered, similar, although slightly less pronounced, redistribu\- tion occurs. Therefore, a balanced salt solution with a sodium concentration of 130 mMoI/L or more is normally chosen when major operative procedures are performed and when excessive blood loss is anticipated. More hypotonic solutions and D5W should be restricted to minor procedures and for some pediatric operations.
Normal saline (0.9% saline solution):
Ringer's lactate
There is no volume expanding advantage of any of these over the others. \par }{\f1 \par \par }{\b\f1
Hypertonic Saline Solutions
Hypertonic saline solutions include 1.8%, 3%, 5%, 7.5%, and 10% sodium chloride solutions. Other anions such as lactate and acetate may be incorporated. They are some\- times mixed with colloids such as dextran. Because the osmolality of hypertonic solutions exceed that of intracellular water and because sodium and chloride ions can not freely cross cell membranes, the ECF becomes slightly hyperosmolar. A gradient f or water to pass from the cells into the extravascular compartment is established, and the extracellular volume is expanded by approximately 2.5 L after administering 1 liter of 3% saline. Because electrolytes freely cross capillary membranes, the fluid is divided between the intravascular and extravascu lar compartments according to their relative volumes. Al\-though hypertonic saline solutions increase the intravascular volume more than would the same volume of a balanced salt solution, they do so at the expense of a decreased intra\cellular volume. If large volumes of previously administered balanced electrolyte solutions have already increased intra\-cellular volume (remember that most are, in effect, slightly hypotonic), hypertonic saline is therapeuti c. If not, cellular dehydration can result.
Potential Complications
The use of hypertonic saline solution has recently increased, due to increased use in intraoperative administration and trauma resuscita tion.
A major concern hypernatremia. However, hypernatremic complications have not been reported in the clinical trials. It is usually transient, especially when these solutions are used with halai iecxl electrolyte or colloid solutions. Comprehensive reviews of many of the aspects of hypertonic saline have been published. Hyperchloremic acidosis may occur owing to the large chloride load. However, substitutimi of hypertonic sodium acetate, although transiently improving acid-base parameters, has not been found to improve outcome and, curiously, increases lactemia.
COLLOIDS
Colloids commonly used in the United States include albu\- min, hydroxyethyl starch (HES; also known as hetastarch (Hespan) and dextran. In Europe, gelatin derivatives are available as well. Colloid molecules are suf ficiently large that they normally do not cross capillary membranes in significant numbers. Most of an administered colloid remains intravascular unless an altered permeability condition is present. Distribution of fluid throughout the body is dependent on the forces represented in the Starling equation:
Jv = Kf (PMV - Pis) - §(COPuv - COPis)
§ represents the rate of filtration of fluid across the capillaries; Kf is the ultrafiltration coefficient (a measure of permeability); PMV is the hydrostatic pressure within the microvasculature (i.e., the capillaries); Pis is the hydrostatic pressure in the interstitial space (the tissues); § is the reflection coefficient and is a relative value expressing the ability of the semipermeable membrane to prevent movement of a given solute (in this case, the colloids of interest); COPMV is the colloid oncotic pressure in the microvasculature; and COPis is the colloid oncotic pressure in the tissue.
For colloids "to work" as desired (i.e., remain in the intravascular compartment), § must be large (approaching 1.0). The value of § varies greatly among tissues; for example, the lungs are moderately permeable (§= 0.6); muscle is moderately impermeable (§ = 0.9); and the brain and glomeruli are essentially impermeable to protein entry (§= 0.99 and 1.0, respectively). The § value for other tissues, such as liver, is low (§= 0).
Changes in Capillary Permeability in Pathologic States
During trauma or sepsis, § values may change significantly. A classic example is the increased capillary permeability to albumin in the lungs in ARDS. In such a case, administered colloid may freely move across what ordinarily would be moderately permeable membranes in much the same fashion as does a balanced electrolyte solution. Increased capillary permeability (capillary leak) also occurs at the site of surgical trauma, and administered colloid moves out of the capillaries into the involved interstitium. In this setting, colloids are less effec\- tive than otherwise would be expected for intravascular expansion. They may increase interstitial edema by exerting a reverse oncotic gradient as they accumulate in the tissues.
Oncotic Pressure Gradients
Once colloid molecules leak into the interstitial space, they must he removed to prevent this reverse oncotic pressure gradient and tissue edema. Rarely does a concentration gra\- dient exist for colloid movement from the interstitial space back into the capillaries. Instead, it must be removed by the lymphatic system. Although many tissues, especially the lungs, have a large capacity for lymphatic drainage, others , including skeletal muscle, do not. Removal of colloid is much slower than that of crystalloid, and persistent edema, even to the point of blood flow interruption, sometimes results. This situation is particularly problematic in major trauma and burns.
Albumin
Johnson and coworkens treated severely injured patients with a standardized resuscitation protocol. Approximately one half of the patients received 150 g/day or additional albumin for 3 to 5 days. The patients given supplementa l albumin required greater volumes of whole blood and fresh frozen plasma to obtain normal clotting studies than did those who were resuscitated with crystalloid solutions. Albumin-treated patients had a significant decrease in fibrinogen concentration an d prolongation of the prothrornhin tune that could not be explained by dilution. In contrast, the prolonged thromboplastin time and decreased platelet counts that also occurred in the albumin group were ascribed to dilution. The amount of albumin administered in this study was much greater than that usually given in clinical settings. Other investigators, using smaller doses of albumin, reported clotting abnormalities that could be explained solely on the basis of dilution. In addition, in vitro studies found that albumin did not adversely influence clotting nor did it affect the structure of fibrin clots.
Overall, albumin may exert some mild effects on hemostasis; however, these effects seem to be primarily dilutional as a result of volume expansion. When large volumes of albumin are infused, the degree of volume expansion exceeds that obtained with a comparable amount of c of crystalloid solutions. Therefore, a more pronounced coagulation defect is likely.
Hetastarch (HES= Hydroxyethyl starch)
Its effects have been studied in two major groups of patients. The first consists of healthy patients undergoing leukopheresis for donation of white blood cells. These patients usually receive small amounts (approximately 500 mL) of HES. In one study, 10 donors who received HES during leukopheresis had slight hut significant prolongation of their prothrombin time and prolonged thromboplastin time (mean increases of 0.6 and 2.5 seconds, respectively). Levels of fibrinogen, factor VIII:C, and factor V were similarly reduced but remained within the normal range. In another report, no defects in platelet function were noticed. The second includes those who receive larger doses of HES for trauma and surgery. In these patients, a prolonged partial thromboplastin time and up to a 50% decrease in factor VIII:C occurs with an infusion of 1 L of HES.
In addition to its effect on levels of factor VIII:C, HES seems to cause changes in fibrin clot formation and fibrinogenolysis. This characteristic maybe related to incorporation of the HES molecules into the clot with subsequent prevention of solid clot formation.
Pentastarch
Dextran
Clotting deficits associated with dextran probably related to defects in platelet interaction and an antifibrinolytic effects. The platelet-vascular interaction is believed to be primarily associated with an effect on factor VIII. Dextran also seems to he incorporated into the polymerizing fibrin clot so that it alters clot structure and enhances fibrinogenolysis.
Dextran 40 is used in vascular surgery to prevent thrombosis but is rarely employed as a primary volume expander, alone or in combination with hypertonic saline.
Colloids and Hemostasis
Patients who undergo surgery with significant blood loss often have problems with coagulation. However, not all coagulation deficits seers in surgical patients can be related to the use of blood. Colloid solutions have been reported to be responsible in many settings. These deficits are in addition to those expected purely from the dilution associated with large-volume resuscitation.
Few topics in anesthesia and surgery have generated as much controversy as the relative merits of colloids and crystalloids for intraoperative fluid replacement and resuscitation. Numerous animal and human studies have been undertaken to prove that one or the other is superior. In most cases, the choice is based more on personal opinion and dogma rather than on scientific merit.
A metaanalysis (Valanovich ; Surgery, 1988) looked at mortality in 8 published human trials in patients receiving either crystalloid or colloid for resuscita tion. It showed an overall 5.7% decrease in mortality rate in patients resuscitated with crystalloid rather than colloid solutions. Subgroup analysis showed that trauma/sepsis patients had a 12.3% decrease in mor tality when crystalloids were used. On the other hand, when crystalloids were used in patients undergoing elective surgery, there was a 7.8% increase in mortality. The proposed explanation was that patients with trauma and sepsis have an increase in capillary permeability that allows the administered colloid to leak out of the vasculature, to be less effective as an intravascular volume expander, and to slow resolution o f edema from the affected tissues. In patients undergoing elective procedures, the amount of capillary leak, in contradistinction to that in major trauma, is more discretely limited to the surgical site; thus, the use of colloids may be more efficacious i n increasing intravascular volume. This study does not settle the controversy, but it does provide some insight into specific situations when one or the other may be preferable. Most colloid advocates do not recommend these substances as the sole resuscitative fluid. The usual protocol involves initial infusion of crystalloids, followed by the ad\- ministration of colloids when large volumes are necessary to reduce the amount of crystalloids. In general, crystalloids need to be administered in volumes that are approximately 2-3 times that of isooncotic colloid to obtain the same hemodynamic effect. When more concentrated colloid solutions such as 25% albumin are used, this ratio is no longer valid.
The most comprehensive evaluation of colloid therapy was presented in a 1991 workshop on the assessment of plasma volume expander. All of the pertinent clinical trials involving albumin, dextran, and HES were carefully evaluated in terms of efficacy, cost, indications for use, and compl ications. Little evidence was found for either a short-term or long-term benefit from the use of supplemental colloidal agents in blood loss, burns, cardiopulmonary bypass, pulmonary edema, trauma, and nutrition. No evidence suggested that serum albumin l evels as low as 3.0 g/dL were deleterious, and even values as low as 2.0 g/dL have not been clearly shown to be problematic.
PULMONARY EDEMA. Of particular interest was the discussion relating pulmonary edema and the fact that administration of albumin in hypoalbuminemic patients, by abruptly increasing pulmonary artery perfusion pressure, may produce the very complication it is designed to prevent -- interstitial and alveolar flooding.
RENAL FUNCTION. A rise in tir e colloid oncotic pressure a little above normal significantly impairs renal salt and water excretion. No congenital hyperalbuminemic states are known, and the body reacts to transient elevations of albumin by immediately stopping production and accelerat ing catabolism. The adverse renal effects may be associated with the absence of natun4ly occurring states of excess albumin, whereas those in which albumin level is low are common. However, with one exception, in which albumin supplementation of 900 g occurred over several days, no toxic effect of albumin therapy has been shown. The anticoagulant, antiplatelet and fibrinolytic effects of colloidal products have been mentioned.
COSTS. The cost of colloid solutions is as much as 200 times that for an equal volume of crystalloid solution or 50 to 100 times that for an equipotent volume of crystalloid solution. The cost of various intravenous fluids (1992 values) is as follows:
BLOOD SUBSTITUTES
Fluorinated Hydrocarbons
Flubron
Hemoglobin Solutions
Results of Aliquot Therapy