DISORDERS OF FLUID AND ELECTROLYTE BALANCE
The Basics
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Fluid and electrolyte disorders are volume related, compositional, or both. Diagnosis and therapy focuses on measurements such as blood pressure, pulse, central venous pressure, serum electrolyte values, arterial bl ood gas partial pressures, and pH. These are, however, gross indicators of what is really important: normal cellular function and satisfactory, if not optimal, metabolic status.
Normal compensatory responses to fluid and electrolyte abnormalities preserve volume and composition. In the extreme, however, composition (e.g., electrolyte content) is sacrificed to ensure adequate volume. The "volumes" of importance are blood (plasma), interstitial fluid (functional extracellular fluid [FECV]), and intracellula r fluid (ICF). Thus, mechanisms that initially act to maintain oxygen delivery at the cellular level ultimately can result in hyperosmolar or hypoosmolar states that may be life-threatening.
The proper analysis of fluid/electrolyte problems is a three-dimensional approach. This approach involves assessing: (1) the total mass or total body stores of each electrolyte, which is the product of its concentration and the volume of its distribution; (2) the rate of electrolyte movement in and out of the body, i.e., balance; and (3) the movement of each electrolyte in and out of each compartment, i.e., changes in body composition, in health and various pathophysiologic states.
Electrolytes are essential for normal cellular function. Alterations in circulating electrolyte concentrations are common in critically ill patients and occur in most patients admitted to the intensive care unit during their hospital stay. Abnormal electrolyte concentrations reflect altered metabolic status. The most commo n form of monitoring composition of the body fluids is measuring electrolyte concentrations in fluids, particularly serum. However, this is not by any means the best way to do it, as this data must be combined with other clinical and paraclinical informati on as outlined below. The meaning and treatment of disorders of the primary extracellular and intracellular volumes and ions are described in this chapter.
Volume
1. Assess skin turgor, mucous membranes, changes in body weight, urine output, blood pressure, and pulse.
2. Invasive measurements (e.g., central venous pressure, pulmonary arterial occlusion pressure) are less frequently necessary than is commonly thought. Better used in situations where a significant abnormality such as renal failure or congestive heart failure complicate diagnosis and treatment. The patient's mental status (alert, oriented, somnolent, confused) may be the most important feature regardless of "the numbers."
Composition
1. Measure serum electrolytes and glucose, and serum and urine osmolality before diuretics are given. Determine serum glucose.
2. Assessment of osmolality is most important to define the basic problems. Specific electrolyte and glucose abnormalities guide replacement therapy.
Crystalloids are balanced salt solutions that freely cross the capillary endothelium and rapidly equilibrate with extravascular fluids. Colloids contain larger molecules that exert oncotic activity that normally keeps water within the intravascular space. In the sick patient, who may have abnormal endothelial permeability, the colloid may not work this way.
There is no volume expansion advantage of one crystalloid over the other. There is controversy about the advantage of colloids over crystalloids (see below). The choice of the fluid for resuscitation depends on the clinical situation and logistics. Colloid therapy is less favored for early volume restoration. It may play a role in bum therapy after the first 24 hours or in severely hypoalbuminemic states (e.g., albumin level >1.5 to 2.0 g/dL) but their beneficial effect is questionable.
ELECTROLYTE CONCENTRATIONS AND BODY FLUIDS COMPARTMENTALIZATION
Total Body Water = 0.6 x (kg body weight) - for males
Total Body Water= 0.5 x (kg body weight) - for females
Total Body Water
Extracellular water
Osmosis is the way water moves between these spaces.Solutes create the concentration gradients that promote osmosis. Sodium is the major extracellular cation. Magnesium and potassium}{\f1\fs24 are the major intracellular cations. The Na+/K+ ATPase, an energy-consuming pump, maintains the concentrations across the cellular membrane. Vascular membranes and pumps are relatively permeable to water and to these cations.
Albumin (60-70 k-daltons) and larger proteins maintain the colloid-oncotic pressure that keeps the water in the extracellular space.
The changes in normal electrolyte concentrations, masses, and distributions after trauma, stress, and critical illness have been studied. The average healthy, 70-kg male has about 3,400 mEq of potassium ion (48 mEq/kg body weight) and about 2,800 mEq of sodium ion (40 mEq/kg body weight) in about 40 liters of bo dy water (6). Below the age of 60, the body water averages 54% in males and 49% in females. In general, newborns have the highest percentage of water (75%); older persons and females have relatively less water.
Body water is divided into intracellular (ICW), extra-cellular (ECW), and transcellular (third-space) compart\- ments. ICW and ECW move back and forth through the cell membrane. The ICW in adult males averages 57% of the total body water (TBW), or 31% of body weight; in adult females, these values average 53% and 26%, re\-spectively.
The ECW is divided into the interstitial water (ISW) and the plasma water by the blood vessels; normally, the ratio of plasma water to ISW is 1:5. A relatively small part of the FCW is liquid; most is a gel loosely bound to mucopolysaccharide connective tissue. ISW and plasma water move across the capillary mem\-brane according to the three forces described in Starling's law of the capillaries: (1) the hydrostatic pressure of the inflowing blood; (2) the filtration pressure, which is the difference between tissue pressure and the pressure at the venous end of the capillary; and (3) the osmotic pressure difference between plasma and ISW. These Starling forces operate under a wide range of pathologic conditions but break down in the terminal stage of respiratory failure, leading to the capillary leak syndrome. Transcellular water is formed by the active transport of ECW across epithelial cells. The normal concentration of transcellular water in the body is about 15 ml/kg body weight, distributed between the gastrointestinal (GI) lumen, the cerebrospinal fluid, the billiary tract, and the lymphatics in the ratio of 7:3:2:3, respectively. In diseases such as peritonitis and ascites, large volumes of fluid accumulate in the peri toneal and pleural cavities, which normally contain only negligible amounts of fluid. With intestinal obstruction, up to 20 liters or more of fluid may accumulate in the bowel lumen. It is not correct to speak of peripheral edema or other expansions of th e ECW as transcellular or third-space fluid shifts.
Excessive salt intake, such as after ingestion of seawater, administration of hypertonic saline or sodium bicarbonate, initially increases the ECW Na+ concentration and osmolality. As water moves from the ICW to the ECW to equalize the solute concen\- trations, the decrease in ICW volume increases the solute (Na+) concentration in this compartment as well. Uncomplicated solute losses produce changes opposite to those of excess salt intake; the ECW bec omes hypotonic and water migrates into the cells. However, solute loss without concomitant water losses is rarely seen clinically. Isotonic alterations are characterized by fluid movements that maintain equal tonicity throughout the major body compartmen ts and eliminate osmotic gradients. Isotonic expansion (e.g. with 0.9% NaCL or RL) expands ECW without changing plasma Na+ concentration or osmolality, and there are no large water shifts into the ICW. However, these patients may develop peripheral edema indicative of ISW expansion. Isotonic loss of Na+ and water is frequently seen with losses of fluid from the GI tract following intestinal obstruction. Usually, this reduces the ECW volume without significantly changing plasma Na+ concentration.
Blood volume (BV) in the adult male ranges from 6.5% to 8% of body weight (mean 7%). In females, blood volume ranges from 5.5% to 7.5% of body weight (mean 6.5%). Plasma water volume, like body water volume, is greater in young muscular m ales and less in short, obese, elderly females. Often, normal values are based on body surface area (BSA), i.e., 2.74 L/m}{\f1\super 2 }{\f1 for males and 2.37 L/m for females. More precise measurements use regres\- sion analysis to relate BV to weight and surface area: for men, BV = 2937 + 26.87 kg body weight; for women, BV = 1124 + 42.6 kg body weight.
Distribution of Water and Electrolytes in Body Compartments
Cells contain slightly more than half the body water and 98% of the potassium ion (K+); the ECW has onl y about 2% of the body K+, i.e., 50 to 85 mEq. The intracellular K+ concentrations normally are 135 to 155 mEq/L of ICW, or about 360 mEq/kg of intracellular solids. By contrast, ECW contains 3.8 to 5 mEq/l of K+, and the ratio of intracellular to extracellular K+ is therefore about 30:1. There is a similar concentration gradient in the opposite direction for sodium ion (Na+), whose intracellular and extracellular concentrations are 5 to 15 mEq/L and 138 to 142 mEq/L, respectively. Water readily moves across the cell membrane to equalize the total solute concentration (osmolarity) inside and outside the cell.
Body balance is the net gain or loss of specific body constituents calculated from careful measurements of the total oral and parenteral intake and the total loss by urine and by feces, sweat, GI, and other extrarenal route. It does not define electrolyte movements within the body, it does describe the net rate of gain or loss for specific electrolytes in various organs.
In catabolic states, about 0.9 ml of Na+-poor water is liberated by the breakdown of each gram of stored tri\-glyceride, glycogen, or cell protein. Normally, about 100 ml of cellular water is released daily; however, extensive catabolism can increase this severalfold. Although cell breakdown does not ordinarily liberate a large volume of fluids, it may become important in acute renal failure where fluid restriction is necessary.
OSMOTIC AND COLLOIDAL OSMOTIC PRESSURES
When two aqueous solutions of unequal concentrations are separated by a membrane permeable only to water, the water migrates through the membrane to equalize the concentrations of the two solutions. Osmosis describes the net water flux. Osmotic pressure is the hydrostatic pressure applied to the solution of greater concentration, and prevents water movement across the membrane. Osmotic pressure is determined only by the number of molecules in solution, and not by their molecular weight, electrical charge, or valence number.
The capillary basement membrane dividing the ECW into 15W and plasma water is freely permeable to water, less permeable to electrolytes and low-molecular weight crystalloids such as urea, glucose, amino acids, and lactate, and barely permeable to high-molecular-weight colloids such as plasma proteins. Colloidal osmotic pressure (COP), or oncotic pressure, is the osmotic force exerted by plasma proteins, normally 24 +/- 3 torr. When a protein solution is divided from a buffer solutio n by a rigid semipermeable membrane, COP is measured by the height of the protein solution above the buffer solution. Because of their high molecular weights, plasma proteins have low osmotic effects, even though their concentrations are high. The crystalloidal solutes of plasma exert osmotic pressures of about 5,100 torr against distilled water; however, since crystalloids readily equilibrate across the capillary walls, they do not exert a significant osmotic pressure in the body. Therefore, distribution of water between ECW and plasma is largely determined by the difference in protein concentrations across the capillary membrane; thus, COP values have considerable physiologic significance. Ultimately, the osmotic pressure is probably set by intracellul ar mechanisms associated with the body's overall energy state. Attempts to localize an "osmo-stat" in the brain have not been well accepted. Starvation and depletion produce hypoalbuminemia. Hyperalimentation and overadministration of concentrated glucose (50%) and sodium bicarbonate (8%) solutions may exceed the renal capacity to excrete the osmolar load and lead to hyperosmotic nonketotic coma. On the other hand, late-stage renal disease with high BUN values and diabetic ketoacidosis lead to hyperosmolar states.
Osmolarity and osmolality express the sum of molar concentrations of dissolved solutes. Osmolarity is the number of osmols or moles of solute per liter of solvent plus solute. Osmolality is the solute concentration per kilogram of solvent (water). In plasma, small differences between osmolarity and osmolality are due to plasma protein and fat, which comprise about 6% to 8% of plasma solutes. Osmolality is more useful since it is measured by commonly available freezing-point methods, and is not aff ected by changes in plasma protein or fat. Normally, all body compartments are iso-osmolar, and there are no appreciable osmolar gradients except in the renal medulla. Plasma and serum osmolality is usually 285 to 305 mOsm/L. Tonicity describes the relative osmolality of solutions. For example, physiologic saline containing 155 mEq/L of salt (310 mOsm/L) is isotonic with respect to plasma. Hypertonic fluids contain more dissolved particles per liter, and hypotonic solutions contain fewer dissolved particles per liter than does the reference solution. When fluid is added or removed from the body, osmotic forces redistribute body water to eliminate osmolar gradients and maintain isotonicity (fl). After oral ingestion of water or excessive nonelectrolyte fluid administration, water is rapidly and uniformly redistributed throughout the body, increasing both ECW and ICW volume. This triggers volume receptors that increase the output of urine with a low Na+ concentration and low osrnolality. In contrast, water losses decrease the volumes of both fluid compartments, producing a corresponding increase in ECW Na+ concentration and Osmolality.
Meta-analyses and studies have been done. There is no convincing evidence exists that either one improves outcome more than the other. In general, one may (opinion) not to recommend colloids for patients with abnormal capillary permeability.
Shock is a syndrome of inadequate nutrient and blood flow at the cellular level.The goal of the intensivist is to restore adequate cellular perfusion, and delivery and nutrient delivery. In most shock states a relative or absolute decreased blood volume is present. There is decreased venous return, decreased preload and decreased stroke volume resulting in decreased cardiac output and oxygen delivery. Shock should be treated by treating the cause. However, in the initial stages fluid resuscitation is the most important step to keep the patient alive and well, until specific therapy can be started and takes effect. The choice of fluid for critical resuscitation depends on the cause of the shock state. For example, hemorrhagic shock is ideally treated with blood or, more realisticaly,packed red blood cells. However, while the blood is ready, one needs to expand the intravascular volume with the best possible fluid.
Wild-Schumman Classification of Shock
Cardiogenic Shock:
Extracardiac-Obstructive Shock
Oligemic Shock
Distributive Shock
All of these need vigorous fluid resuscitation aside from the specific therapies.
Physiologic Response to Volume Loss
Fresh Blood
Packed Red Blood Cells
Fresh Frozen Plasma
Platelets
Frozen Red Cells
Many other components exist. Those, however, have no use in fluid resuscitation.
Complications of Massive Transfusions
GOALS OF FLUID RESUSCITATION THERAPY