4 Fluid, Electrolyte, and Acid–Base Disorders in the Surgery Patient Stephen F. Lowry Objectives 1. To understand the normal electrolyte composition of body fluids and how they are modified by injury and surgical disease. 2. To understand the importance of evaluating fluid status. 3. To recognize the clinical manifestation of common electrolyte abnormalities and methods for their correction. 4. To understand the common manifestation of acid–base abnormalities. Cases Case 1 A 72-year-old man undergoes subtotal colectomy for massive lower GI bleeding. He receives five units of blood during and following opera- tion and is NPO for 6 days while receiving dextrose 5% in water (D5/W) at a rate of 125mL/hour. Urine output remains normal with specific gravity of 1.012. On the sixth postoperative day, he is disori- ented and combative. Among the results of workup are serum sodium = 119mEq/L, potassium = 3.6mEq/L, chloride = 85mEq/L, glucose = 120mg/dL, blood urea nitrogen (BUN) = 24. Case 2 A 40-year-old woman presents with a 1 week history of persistent upper abdominal pain in association with nausea and vomiting. She tolerates only small amounts of clear fluids by mouth. No diarrhea is present. Physical examination is unrevealing except for loss of skin turgor and reduced breath sounds over the right chest. Lab re- sults include sodium = 138mEq/L, potassium = 2.6mEq/L, HCO3 = 62 4. Fluid, Electrolyte, and Acid–Base Disorders in the Surgery Patient 63 43mEq/L. A blood gas is obtained, revealing pH = 7.57, Pao2 = 98mm Hg, Paco2 = 52mmHg, base excess = 10. Case 3 A 58-year-old woman presents with a 1-week history of confusion, lethargy, and persistent nausea. She has new complaints of back and hip pain. Past history includes a mastectomy for breast cancer 5 years previously. Laboratory values obtained during evaluation include hematocrit (Hct) = 41, white blood count (WBC) = 9000, platelets = 110,000, sodium = 137mEq/L, potassium = 3.8mEq/L, BUN = 25mg/dL, albumin = 3.4g/dL, bilirubin = 1.5g/dL, alkaline phos- phatase = 350IU/L, calcium = 14.2mg/dL. Introduction An understanding of changes in fluid, electrolyte, and acid–base con- cepts is fundamental to the care of surgical patients. These changes can range from mild, readily correctable deviations to life-threatening abnormalities that demand immediate attention. This chapter outlines some of the physiologic mechanisms that initiate such imbalances and methods to systematically evaluate the diverse clinical and biochemi- cal data that lead to decisions regarding therapy. The information and data presented below are intended for application in adult patients, although the principles espoused also are germane to pediatric patients. Basic Concepts The Stress Response The normal physiologic response to injury or operation produces a neuroendocrine response that preserves cellular function and pro- motes maintenance of circulating volume. This is readily demon- strable in terms of retention of water and sodium and the excretion of potassium. Many stimuli can produce this response, including many associated with trauma or operation. Activation of several endocrine response pathways increases the levels of antidiuretic hormone (ADH), aldosterone, angiotensin II, cortisol, and catecholamines. Hyperosmo- larity and hypovolemia are the principal stimulants for ADH release, which increases renal water resorption from the collecting ducts and raises urine osmolarity. Aldosterone, the principal stimulus for renal potassium excretion, also is increased by angiotensin II, which can increase both renal sodium and water retention. Aldosterone also is increased by elevated levels of potassium, a common consequence of tissue injury. Hydrocortisone and catecholamine release also contribute to the excretion of potassium. 64 S.F. Lowry Body Fluid Compartments Total body water (TBW) approximates 60% of body weight (BW) and is divided among the intracellular volume (ICV) as 40% of BW and an extracellular volume (ECV) representing 20% of BW. The ECV is divided further into an interstitial fluid volume (IFV) pool, which is roughly 15% of BW, and the intravascular or plasma volume (PV), which approximates 5% of BW. The TBW is the solvent for most of the solutes in the body, and it is assumed that water moves freely between the ECV and ICV in an effort to equalize the concentration of solutes within each space. However, the solute and colloid concentrations of the ICV and ECV differ markedly. The ECV contains most of the body sodium, while the predominant ICV cation is potassium. Albumin rep- resents the dominant osmotically active colloid within the ECV and virtually is excluded from the ICV. The exogenous administration of electrolytes results in the distribution of that ion to the usual fluid com- partment of highest preferential concentration. Electrolytes When an electrolyte dissolves in water, it releases positive and nega- tive ions. Although, as noted above, their concentrations vary between fluid compartments, the distribution of water across fluid compart- ments seeks to equalize the concentration of total solutes and other osmotically active particles. When considering electrolyte problems, it is useful to use the milliequivalent (mEq) measure of their chemical combining capacity. In some cases, this must be converted from the weight expression milligram (mg) expressed on the laboratory report. Table 4.1 assists in this conversion. A millimole (mM) is the atomic weight of a substance expressed in milligrams. A milliosmole (mOsm) is a measure of the number of osmotically active particles in solution. Since mOsm does not depend on valence, the mM dissolved in solution will be the same as mOsm. The osmolarity of a solution depends on the number of active parti- cles per unit of volume (mOsm/L). The normal osmolarity of serum is 290 ± 10mOsm/L. The effective osmolarity (tonicity) involves the mea- Table 4.1. Data for serum electrolytes. Normal Electrolyte mg/dL mEq/L Sodium 322 140 Potassium 17.5 4.5 Calcium 10 5 Magnesium 2.4 2 Chloride 35.7 102 Phosphorus 3.4 2.0 Source: Reprinted from Pemberton LB, Pemberton DK. Treatment of Water, Electrolyte, and Acid-Base Disorders in the Surgical Patient. New York: McGraw Hill, 1994. With per- mission of The McGraw-Hill Companies. 4. Fluid, Electrolyte, and Acid–Base Disorders in the Surgery Patient 65 surement of two solutes, sodium and glucose, that represent nearly 90% of ECV osmolarity. This can be modified by addition of urea con- centration, especially in conditions of uremia. The formula for calcu- lating approximate osmolarity is: POSM = 2 ¥ plasma [Na+] + [glucose]/20 + [BUN]/3 Because water moves freely between fluid compartments, ECV osmo- larity (or tonicity) is equivalent to that in the ICV. Maintenance Requirements There are several principles that underlie the prescription for replacing fluid and electrolytes in surgical patients. This includes a knowledge of normal maintenance requirements as well as replacement for losses. Water The normal losses of water include sensible (measurable) losses from urine (500–1500mL/day) and feces (100–200mL/day), as well as insensible (unmeasurable) loses from sweat and respiration (8– 12mL/kg/day). Cutaneous insensible losses increase by approxi- mately 10% for each degree C above normal. A method to roughly calculate daily normal water requirements is shown in Figure 4.1. The water of biologic oxidation (catabolism) contributes up to 300mL/day and can be subtracted from these calculations. For healthy adults, an estimated daily maintenance fluid requirement approximates 30 to 35mL/kg/day. Sodium Sodium losses in urine can vary widely but, in general, approximate daily intake. The normal kidney can conserve sodium to a minimum level of 5 to 10mEq/L. A figure of 70 to 100mEq Na/day is a reason- able estimate of maintenance level. Potassium The normal excretion of potassium approximates 40 to 60mEq/day. Since the renal conservation of potassium is not as efficient as for sodium, this is the minimum level of daily replacement in healthy adults (0.5–1.0mEq/kg/day). Summary of Normal Maintenance Fluids for Surgical Patients In the absence of other comorbidities or prolonged injury/operation induced stress, the NPO surgical patient is adequately maintained by infusion of variable combinations of dextrose (D5) and saline (up to 0.5 N) containing solutions, with approximately 15 to 20mEq/L of potassium added. The rate of infusion should be adjusted to achieve water replacement as outlined above. Such parenteral solutions, when 66 S.F. Lowry Total Body Water [60% Body Wt. (42L)] INTRACELLULAR EXTRACELLUAR [40% Body Wt. (28L)] [20% Body Wt. (14L)] CATIONS CATIONS Na+ 12.0 mEq/L Ca2+ 4.0 mEq/L Na+ 14.2 mEq/L Ca2+ 2.5 mEq/L K+ 150 mEq/L Mg2+ 34.0 mEq/L K+ 4.3 mEq/L Mg2+ 1.1 mEq/L ANIONS ANIONS Cl– 4.0 mEq/L Proteins 54 mEq/L Cl– 104.0 mEq/L Proteins 14 mEq/L – – HCO3 12.0 mEq/L Other 90 mEq/L HCO3 24 mEq/L Other 5.9 mEq/L 2– – 2– – HPO4 , H2PO4 40 mEq/L HPO4 , H2PO4 2.0 mEq/L INTERSTITIAL PLASMA (10.5 L) (3.5 L) Figure 4.1. Distribution of body water and electrolytes in a healthy 70-kg male. (Adapted from Narins RG, Krishna GC. Disorders of water balance. In: Stein JH, ed. Internal Medicine, 2nd ed. Philadelphia: Lippincott Williams & Wilkins. Reprinted from Nathens AB, Maier RV. Perioperative Fluids and Elec- trolytes. In: Norton JA, Bollinger RR, Chang AE, et al, eds. Surgery: Basic Science and Clinical Evidence. New York: Springer-Verlag, 2001, with permission.) given at an appropriate rate of infusion, suffice to manage the major- ity of postoperative patients. Perioperative Fluid and Electrolyte Requirements The management of fluid and electrolytes in the stressed surgical patient requires a systematic approach to the changing dynamics and demands of the patient. Consideration of existing maintenance requirements, deficits or excesses, and ongoing losses requires regular monitoring and flexibility in prescribing.