Fluid and Electrolyte Management of the Surgical Patient PDF

3 Fluid and Electrolyte Management of the Surgical Patient chapter Matthew D. Neal Introduction 83 Concentra...

3 Fluid and Electrolyte Management of the Surgical Patient chapter Matthew D. Neal Introduction 83 Concentration Changes / 86 Postoperative Fluid Therapy / 97 Body Fluids 83 Composition Changes: Etiology and Fluid Management in Enhanced Recovery Total Body Water / 83 Diagnosis / 88 After Surgery (ERAS) Pathways / 97 Fluid Compartments / 83 Acid-Base Balance / 91 Special Considerations for the Metabolic Derangements / 91 Postoperative Patient / 98 Composition of Fluid Compartments / 83 Osmotic Pressure / 84 Fluid and Electrolyte Therapy 93 Electrolyte Abnormalities in Specific Parenteral Solutions / 93 Surgical Patients 98 Body Fluid Changes 85 Alternative Resuscitative Fluids / 94 Neurologic Patients / 98 Normal Exchange of Fluid and Correction of Life-Threatening Malnourished Patients: Refeeding Electrolytes / 85 Electrolyte Abnormalities / 94 Syndrome / 98 Classification of Body Fluid Changes / 85 Preoperative Fluid Therapy / 96 Acute Renal Failure Patients / 99 Disturbances in Fluid Balance / 85 Intraoperative Fluid Therapy / 97 Cancer Patients / 99 Volume Control / 86 INTRODUCTION Fluid Compartments Fluid and electrolyte management is paramount to the care of TBW is divided into three functional fluid compartments: the surgical patient. Changes in both fluid volume and electro- plasma, extravascular interstitial fluid, and intracellular fluid lyte composition occur preoperatively, intraoperatively, and (Fig. 3-1). The extracellular fluids (ECF), plasma, and intersti- postoperatively, as well as in response to trauma and sepsis. The tial fluid together compose about one-third of the TBW, and the sections that follow review the normal anatomy of body fluids, intracellular compartment composes the remaining two thirds. electrolyte composition and concentration abnormalities and The extracellular water composes 20% of the total body weight treatments, common metabolic derangements, and alternative and is divided between plasma (5% of body weight) and inter- resuscitative fluids. These concepts are then discussed in stitial fluid (15% of body weight). Intracellular water makes up 1 relationship to management of specific surgical patients and approximately 40% of an individual’s total body weight, with their commonly encountered fluid and electrolyte abnormalities. the largest proportion in the skeletal muscle mass. Composition of Fluid Compartments BODY FLUIDS The normal chemical composition of the body fluid compart- ments is shown in Fig. 3-2. The ECF compartment is bal- Total Body Water 2 anced between sodium, the principal cation, and chloride Water constitutes approximately 50% to 60% of total body and bicarbonate, the principal anions. The intracellular fluid weight. The relationship between total body weight and total compartment is composed primarily of the cations potassium body water (TBW) is relatively constant for an individual and is and magnesium, the anions phosphate and sulfate, and proteins. primarily a reflection of body fat. Lean tissues such as muscle and The concentration gradient between compartments is maintained solid organs have higher water content than fat and bone. As a by adenosine triphosphate–driven sodium-potassium pumps result, young, lean males have a higher proportion of body weight located within the cell membranes. The composition of the as water than elderly or obese individuals. In an average young plasma and interstitial fluid differs only slightly in ionic compo- adult male, TBW accounts for 60% of total body weight, whereas sition. The slightly higher protein content (organic anions) in in an average young adult female, it is 50%.1 The lower percent- plasma results in a higher plasma cation composition relative to age of TBW in females correlates with a higher percentage of the interstitial fluid, as explained by the Gibbs-Donnan equilib- adipose tissue and lower percentage of muscle mass in most. rium equation. Proteins add to the osmolality of the plasma and Estimates of percentage of TBW should be adjusted downward contribute to the balance of forces that determine fluid balance approximately 10% to 20% for obese individuals and upward by across the capillary endothelium. Although the movement of 10% for malnourished individuals. The highest percentage of ions and proteins between the various fluid compartments is TBW is found in newborns, with approximately 80% of their total restricted, water is freely diffusible. Water is distributed evenly body weight comprised of water. This decreases to approximately throughout all fluid compartments of the body so that a given 65% by 1 year of age and thereafter remains fairly constant. volume of water increases the volume of any one compartment  Key Points 1 Proper management of fluid and electrolytes facilitates cru- 5 Although active investigation continues, alternative resusci- cial homeostasis that allows cardiovascular perfusion, organ tation fluids have limited clinical utility, other than the cor- system function, and cellular mechanisms to respond to sur- rection of specific electrolyte abnormalities. gical illness. 6 Enhanced recovery after surgery (ERAS) protocols have 2 Knowledge of the compartmentalization of body fluids markedly changed perioperative fluid management and are forms the basis for understanding pathologic shifts in these being used more frequently. ERAS minimizes perioperative fluid spaces in disease states. Although difficult to quantify, fluid administration and focuses on early enteral intake to a deficiency in the functional extracellular fluid compart- reduce morbidity associated with IV fluid administration. ment often requires resuscitation with isotonic fluids in sur- 7 Most acute surgical illnesses are accompanied by some gical and trauma patients. degree of volume loss or redistribution. Consequently, iso- 3 Alterations in the concentration of serum sodium have pro- tonic fluid administration is the most common initial intra- found effects on cellular function due to water shifts between venous fluid strategy, while attention is being given to the intracellular and extracellular spaces. alterations in concentration and composition. 4 Different rates of compensation between respiratory and 8 Some surgical patients with neurologic illness, malnutrition, acute metabolic components of acid-base homeostasis require fre- renal failure, or cancer require special attention to well-defined, quent laboratory reassessment during therapy. disease-specific abnormalities in fluid and electrolyte status. relatively little. Sodium, however, is confined to the ECF com- 2 mEq. The number of milliequivalents of cations must be bal- partment, and because of its osmotic and electrical properties, it anced by the same number of milliequivalents of anions. How- remains associated with water. Therefore, sodium-containing ever, the expression of molar equivalents alone does not allow a fluids are distributed throughout the ECF and add to the volume physiologic comparison of solutes in a solution. of both the intravascular and interstitial spaces. Although the The movement of water across a cell membrane depends administration of sodium-containing fluids expands the intra- primarily on osmosis. To achieve osmotic equilibrium, water vascular volume, it also expands the interstitial space by approx- moves across a semipermeable membrane to equalize the con- imately three times as much as the plasma. centration on both sides. This movement is determined by the concentration of the solutes on each side of the membrane. Osmotic Pressure Osmotic pressure is measured in units of osmoles (osm) or The physiologic activity of electrolytes in solution depends on milliosmoles (mOsm) that refer to the actual number of the number of particles per unit volume (millimoles per liter, or osmotically active particles. For example, 1 mmol of sodium mmol/L), the number of electric charges per unit volume (milli- chloride contributes to 2 mOsm (one from sodium and one equivalents per liter, or mEq/L), and the number of osmotically from chloride). The principal determinants of osmolality are active ions per unit volume (milliosmoles per liter, or mOsm/L). the concentrations of sodium, glucose, and urea (blood urea The concentration of electrolytes usually is expressed in terms nitrogen, or BUN): of the chemical combining activity, or equivalents. An equiva- lent of an ion is its atomic weight expressed in grams divided Calculated serum osmolality = 2 sodium + (glucose/18) by the valence: + (BUN/2.8) Equivalent = atomic weight (g)/valence The osmolality of the intracellular and extracellular fluids For univalent ions such as sodium, 1 mEq is same as is maintained between 290 and 310 mOsm in each compart- 1 mmol. For divalent ions such as magnesium, 1 mmol equals ment. Because cell membranes are permeable to water, any % of Total body weight Volume of TBW Male (70 kg) Female (60 kg) Plasma 5% Extracellular volume 14,000 mL 10,000 mL Interstitial Plasma 3500 mL 2500 mL fluid 15% Interstitial 10,500 mL 7500 mL Intracellular volume 28,000 mL 20,000 mL Intracellular volume 40% 42,000 mL 30,000 mL Figure 3-1. Functional body fluid compartments. TBW = total body 84 water.  200 mEq/L 200 mEq/L 85 CATIONS ANIONS CHAPTER 3 154 mEq/L 154 mEq/L 153 mEq/L 153 mEq/L K+ 150 HPO43– 150 CATIONS ANIONS CATIONS ANIONS SO 42– Na+ 142 CI− 103 Na+ 144 CI− 114 FLUID AND ELECTROLYTE MANAGEMENT OF THE SURGICAL PATIENT HCO3 − 27 SO4 2– HCO3− 30 HCO3− 10 3 PO43– SO42– K+ 4 3 PO43– K+ 4 Mg2+ 40 Protein 40 Organic acids 5 Organic Ca2+ 5 Ca2+ 3 acids 5 Mg2+ 3 Protein 16 Mg2+ 2 Protein 1 Na+ 10 Plasma Interstitial Intracellular Figure 3-2. Chemical composition of fluid fluid body fluid compartments. change in osmotic pressure in one compartment is accompanied Classification of Body Fluid Changes by a redistribution of water until the effective osmotic pressure Disorders in fluid balance may be classified into three general between compartments is equal. For practical clinical purposes, categories: disturbances in (a) volume, (b) concentration, and (c) most significant gains and losses of body fluid are directly from composition. Although each of these may occur simultaneously, the extracellular compartment. each is a separate entity with unique mechanisms demanding individual correction. Isotonic gain or loss of salt solution results in extracellular volume changes, with little impact on BODY FLUID CHANGES intracellular fluid volume. If free water is added or lost from the ECF, water will pass between the ECF and intracellular fluid Normal Exchange of Fluid and Electrolytes until solute concentration or osmolarity is equalized between the The healthy person consumes an average of 2000 mL of water compartments. Unlike with sodium, the concentration of most per day, approximately 75% from oral intake and the rest other ions in the ECF can be altered without significant change extracted from solid foods. Daily water losses include 800 to in the total number of osmotically active particles, producing 1200 mL in urine, 250 mL in stool, and 600 mL in insensible only a compositional change. losses. Insensible losses of water occur through both the skin (75%) and lungs (25%) and can be increased by such factors as Disturbances in Fluid Balance fever, hypermetabolism, and hyperventilation. Sensible water Extracellular volume deficit is the most common fluid disorder losses such as sweating or pathologic loss of gastrointestinal in surgical patients and can be either acute or chronic. Acute (GI) fluids vary widely, but these include the loss of electrolytes volume deficit is associated with cardiovascular and central ner- as well as water (Table 3-1). To clear the products of metabo- vous system signs, whereas chronic deficits display tissue signs, lism, the kidneys must excrete a minimum of 500 to 800 mL of such as a decrease in skin turgor and sunken eyes, in addition urine per day, regardless of the amount of oral intake. to cardiovascular and central nervous system signs (Table 3-2). The typical individual consumes 3 to 5 g of dietary salt per Laboratory examination may reveal an elevated BUN level if day, with the balance maintained by the kidneys. With hypo- the deficit is severe enough to reduce glomerular filtration and natremia or hypovolemia, sodium excretion can be reduced to hemoconcentration. Urine osmolality usually will be higher as little as 1 mEq/d or maximized to as much as 5000 mEq/d than serum osmolality, and urine sodium will be low, typically to achieve balance except in people with salt-wasting kidneys. 20 mEq/L, and urine osmo- central nervous system origin and are related to cellular water larity is >300 mOsm/L. Normovolemic hypernatremia can result intoxication and associated increases in intracranial pressure. from renal causes, including diabetes insipidus, diuretic use, and Oliguric renal failure also can be a rapid complication in the renal disease, or from nonrenal water loss from the GI tract or setting of severe hyponatremia. skin, although the same conditions can result in hypovolemic A systematic review of the etiology of hyponatremia hypernatremia. When hypovolemia is present, the urine sodium should reveal its cause in a given instance. Hyperosmolar concentration is