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Preface

This is the fifth edition of NMS Surgery . Surgery is changing, and we are addressing these changes with this edition. All chapters have been updated. We have given special attention to the first chapter, Principles of Surgical Physiology, and have attempted to clarify this difficult topic. There is also the addition of an online comprehensive examination consisting of new questions and answers and explanations. In keeping with the original purpose of the National Medical Series for Independent Study , this book continues to present the core material of the specialty of surgery. The text is not meant to be all -inclusive and does not contain minutiae that we felt would be of little use to the reader. The authors have included not only didactic material but also facts that they find useful in clinical practice. Where controversy exists, we have attempted to present all sides fairly and to indicate factors that are essential in the decision -making process. We have also tried to stress situations in which surgeons and all others involved in patient care must work closely to make the most appropriate decisions regarding treatment.

The study questions and answers and explanations have been carefully updated and provided in both print and electronic format to be used in preparation for the United States Medical Licensing Examination Step 2.

The fifth edition of NMS Surgery is written primarily for students and residents in general surgery, but practicing surgeons as well as physicians in other specialties will no doubt find it a useful reference. We hope that all readers will find that the book represents a declaration of the state of surgical art in the year 2007.

Bruce E. Jarrell

R. Anthony Carabasi III

Chapter 1

Principles of Surgical Physiology

Steven B. Johnson

Matthew Lissauer

I Fluid and Electrolytes

A Normal body composition

Body water accounts for 50%–70% of body weight. There is a higher percentage of water in young people, thin people, and men and a lower percentage of water in older people, obese people, and women.

Compartments

Intracellular. This compartment accounts for 30%–40% of body weight (65% of total body water). Most of the body's intracellular water is contained in skeletal muscle cells (lean body mass); very little water is contained in adipose cells, which accounts for the lower percentage of water in obese and older people.

Extracellular. This compartment accounts for about 20%–30% of body weight (35% of total body water) and includes two compartments.

Interstitial. Accounts for 15%–20% of total body weight (25% of total body water).

Intravascular. Accounts for 5% of total body weight (10% of total body water).

Maintenance of the intravascular compartment is essential to survival and is the primary consideration of fluid resuscitation for maintenance of homeostasis. Many regulatory mechanisms exist to supplement this compartment from the interstitial first, then intracellular compartments if the intravascular volume drops.

Two-thirds rule. Determining the exact size of any one of the three compartments is virtually impossible because of the variation among patients (and within the same patient). The two-thirds rule is a simple method of approximating the value: Total body water comprises approximately two thirds of body weight; of this, two thirds is intracellular, and one third is extracellular (i.e., intravascular and interstitial). Of the extracellular portion, two thirds is interstitial, and one third is intravascular. This rule provides a starting point for assessment of the normal patient but must always be adjusted to the clinical presentation of the patient.

Blood volume. Knowing how to calculate the approximate blood volume for a patient is important. Using the two-thirds rule, approximately 7% of body weight is blood volume. This calculation is based on lean body mass for a 70 -kg man, and it varies depending on the patient's age, gender, and body habitus.

Electrolyte composition. Electrolytes determine the amount of water that exists in any one space at any time. Electrolyte concentrations in the intracellular space differ compared with the extracellular spaces (i.e., intravascular and interstitial). Water follows electrolytes across cell membranes to equilibrate osmolality.

Compartments. (Table 1-1) Due to ion pumps (principally Na +/K + ATPase), intracellular and extracellular compartments have different electrolyte compositions.

Intracellular. Principal osmotic cation is potassium. Has higher concentration of osmotic particles than extracellular compartment thus allowing water to flow into the cell, creating turgidity.

Extracellular. Intersitial and intravascular composition nearly but not quite identical. Principal osmotic cation is sodium.

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TABLE 1-1 Electrolyte Composition

*Total of osmotically active particles.

Maintaining equilibrium. Change in osmotic pressure in one compartment causes water to redistribute from the other compartments until equilibrium is returned.

B Maintenance

Water. The amount of water required by a person depends on the person's weight, age, gender, and illness.

Methods of calculating water requirements. There are numerous methods to calculate normal water needs for maintenance.

The amount of body water excreted can be used as an estimate. The major water loss from the body is through urine production. Generally, 0.5 mL/kg/hour is the minimum needed to excrete the daily solute load. The next highest daily water loss is insensible loss (i.e., sweat, respiration, stool; see I D 2), which is estimated as 600–900 mL/24 hour. In a 70 -kg man, minimal water maintenance would be:

(70 kg × 0.5 mL/kg/hour × 24 hr) + 750 mL/24 hour = 1590 mL/24 hours.

Again, this is the minimum and does not take into account any excess loss such as fever, which will increase the insensible loss.

Body weight can be used to estimate the maintenance fluid requirement. This method is often used for pediatric patients because their body weights vary widely. Estimations are 100 mL/kg/day or 4 mL/kg/hour for the first 10 kg of body weight, 50 mL/kg/day or 2 cc/kg/hr for the second 10 kg of body weight, and 20 mL/kg/day or 1 cc/kg/hour for each additional kilogram of body weight. An easy way to remember this is the first 20 kg of weight = 60 cc/hour (10 kg × 4 cc/kg/hour + 10 kg × 2 cc/kg/hour) and then 1 cc/kg/hour above that, so a 50 -kg person = 90 cc/hour.

A given amount of water per kilogram of body weight can be used to determine water maintenance requirements. The value used for this method is generally 35–40 mL/kg/day, adjusted higher or lower based on age (the elderly often require only 15 mL/kg/24 -hour maintenance).

A given amount of fluid can be used, regardless of body weight. Standard orders often include fluid rates of 100–125 mL/hour as a maintenance rate. Again, this should be adjusted for individual patients.

Evaluating maintenance rates. Different patients not only have different maintenance needs, but replacing water or removing excess water may be of concern (see I C 1). Because water requirements vary, the most important aspect of water administration is evaluating the adequacy for the patient in question. Fever, environmental temperature, and respiratory rate can increase insensible loss and increase maintenance requirements. Simple

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methods in the noncritically ill population to monitor adequacy of fluid administration include:

TABLE 1-2 Electrolyte Composition of Gastrointestinal Secretions

Urine output variations. If urine output is high (i.e., >1 mL/kg/hour), then less water may be required. If urine output is low, more water may be required, or further assessment may be necessary.

Elderly. As people age, muscle mass and the number of glomeruli decrease.

The elderly should not be expected to produce as much urine as younger people.

Elderly patients can be pushed into congestive heart failure (CHF) by continued water administration for a low urine output. If responses to fluid bolus or increased infusion is not seen, further diagnostic workup is needed to assess fluid, renal, and circulatory status.

Tachycardia. Can be a sign of dehydration, low intravascular volume.

Adjusting fluid rates for individual patients. Based on the above clinical parameters, rates can be adjusted up or down. Besides providing maintenance fluids, replacing lost fluids or removing excess fluids should alter the fluid rate and composition given to patients (Table 1-2).

Injury, illness, and surgery. Can result in fluid losses due to blood loss, third spacing, insensible losses from diarrhea, fever, etc. Providing more than calculated maintenance fluid to replace losses (such as 1.5 or 2 times maintenance) is necessary. Adequacy of the rate can be judged from the above criteria.

The natural response in patients stressed by injury, illness, or surgery is to retain sodium and water due to the hormonal response to stress––antidiuretic hormone (ADH) secretion––therefore, urine output may be misleading in these patients.

Total parenteral nutrition and maintenance fluids. Patients receiving total parenteral nutrition (TPN) generally should have their maintenance fluid calculated as part of their nutrition. A patient on TPN who does not have ongoing abnormal water losses rarely requires additional fluids.

Diuresis. Patients who require diuresis already are overloaded with fluid, and intravenous (IV) fluids should be held. However, there may be electrolyte or nutritional aspects of fluid administration that require water as a carrier for other substances during diuresis.

Sodium. Normally, people take 150–200 mEq of sodium daily. Much is excreted in the urine. If the body

needs to conserve sodium, it can reduce renal excretion to less than 1 mEq/day. Daily homeostasis is easily maintained with 1–2 mEq/kg/day.

Potassium. The normal daily intake of potassium is approximately 40–120 mEq/day, with about 10%–15% excreted in urine. An amount of 0.5–1 mEq/kg/day is appropriate to maintain homeostasis.

What is a good maintenance IV? (Table 1-3) Using the previous estimates for a 70 -kg male, the weight formula for IV fluid would equal 110 cc/hour. Minimal sodium maintenance would require 70–140 mEq/day, and minimal potassium requirements would be 35–70 mEq/day. In 0.5% normal saline (NS), there is 77 mEq/L sodium, and if one adds 20 mEq/L of potassium, then using 0.5% NS with 20 mEq/L KCL at 110 cc/hour would equal about 2.6 L of fluid, 200 mEq of sodium, and 52 mEq of potassium … pretty close!

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TABLE 1-3 Electrolyte Concentration in Various Intravenous Fluids

*Plasmalyte also contains 27 mEq/L acetate and 23 mEq/L gluconate.

C Water and electrolyte deficits and excesses

Water

Hypovolemia:

Signs of acute volume loss include tachycardia, hypotension, and decreased urine output.

Signs of gradual volume loss include loss of skin turgor, thirst, alterations in body temperature, and changes in mental status.

Replacing water deficits. Acute deficits should be replaced acutely; chronic deficits should be replaced more slowly, with half of the deficit replaced over the first 8 hours and the rest in 24– 48 hours. In the case of hypernatremia with hypovolemia, do not allow the sodium concentration to drop more than 0.5–1 mEq/hour.

Hypervolemia: Well tolerated in healthy patients––they will just urinate the excess.