USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES


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Official USMLE Practice Exams

Stored in the cells of the atria, ANP is released when atrial pressure increases. The hormone counteracts the effects of the renin-angiotensin-aldosterone system by decreasing blood pressure and reducing intravascular blood volume. See How atrial natriuretic peptide works.

Stretch that atrium The amount of ANP that the atria release rises in response to a number of conditions; for example, chronic renal failure and heart failure. Anything that causes atrial stretching can also lead to increases in the amount of ANP released, including orthostatic changes, atrial tachycardia, high sodium intake, sodium chloride infusions, and use of drugs that cause vasoconstriction. Thirst Perhaps the simplest mechanism for maintaining fluid balance is the thirst mechanism.

Thirst occurs as a result of even small losses of fluid. Losing body fluids or eating highly salty foods leads to an increase in ECF osmolality. This increase leads to drying of the mucous membranes in the mouth, which in turn stimulates the thirst center in the hypothalamus. In an elderly person, the thirst mechanism is less effective than it is in a younger person, leaving the older person more prone to dehydration. See Dehydration in elderly people. Ages and stages Dehydration in elderly people The signs and symptoms of dehydration may be different in older adults.

Quench that thirst Normally, when a person is thirsty, he drinks fluid. The ingested fluid is absorbed from the intestine into the bloodstream, where it moves freely between fluid compartments. This movement leads to an increase in the amount of fluid in the body and a decrease in the concentration of solutes, thus balancing fluid levels throughout the body.

Answer: A. Because your body would probably be dehydrated, it would try to retain as much fluid as possible. To retain fluid, ADH secretion increases. Fluid would move out of the hypotonic container into the other container to equalize the concentration of fluid within the two containers. Osmosis occurs when fluid moves from an area with more fluid to an area with less fluid. Hydrostatic pressure, which pushes fluid out of the capillaries, is opposed by colloid osmotic pressure, which involves: A.

Answer: C. Albumin in capillaries draws water toward it, a process called reabsorption. Juxtaglomerular cells in the kidneys secrete renin in response to low blood flow or a low sodium level. The eventual effect of renin secretion is an increase in blood pressure. Giving a hypertonic I. Because the concentration of solutes in the I.

Scoring If you answered all five questions correctly, congratulations! If you answered fewer than four correctly, pour yourself a glass of sports drink and enjoy an invigorating burst of fluid refreshment! References Ambalavanan, N. Fluid, electrolyte, and nutrition management of the newborn. Organic and biochemistry for today 7th ed. Wait, R. Fluids, electrolytes, and acid—base balance. Mulholland et al. A look at electrolytes Electrolytes work with fluids to maintain health and well-being.

Electrolytes are crucial for nearly all cellular reactions and functions. Ions Electrolytes are substances that, when in solution, separate or dissociate into electrically charged particles called ions. Some ions are positively charged and others are negatively charged. Several pairs of oppositely charged ions are so closely linked that a problem with one ion causes a problem with the other.

Sodium and chloride are linked that way, as are calcium and phosphorus. A variety of diseases can disrupt the normal balance of electrolytes in the body. Understanding electrolytes and recognizing imbalances can make your patient assessment more accurate. Anions and cations Anions are electrolytes that generate a negative charge; cations are electrolytes that produce a positive charge. An electrical charge makes cells function normally. See Looking on the plus and minus sides.

Looking on the plus and minus sides Electrolytes can be either anions or cations. The anion gap is discussed in chapter 3, Balancing acids and bases. Individual electrolytes differ in concentration, but electrolyte totals balance to achieve a neutral electrical charge positives and negatives balance each other. This balance is called electroneutrality. Hooking up with hydrogen Most electrolytes interact with hydrogen ions to maintain acid-base balance. The major electrolytes have specialized functions that contribute to metabolism and fluid and electrolyte balance.

Major electrolytes outside the cell Sodium and chloride, the major electrolytes in extracellular fluid, exert most of their influence outside the cell. Sodium concentration affects serum osmolality solute concentration in 1 L of water and extracellular fluid volume. Sodium also helps nerve and muscle cells interact. Chloride helps maintain osmotic pressure water-pulling pressure. Gastric mucosal cells need chloride to produce hydrochloric acid, which breaks down food into absorbable components.

More outsiders Calcium and bicarbonate are two other electrolytes found in extracellular fluid. Calcium is the major cation involved in the structure and function of bones and teeth.


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Bicarbonate plays a vital role in acid-base balance. Fundamental phosphorus The body contains phosphorus in the form of phosphate salts. Sometimes, the words phosphorus and phosphate are used interchangeably. Phosphate is essential for energy metabolism. Combined with calcium, phosphate plays a key role in bone and tooth mineralization. It also helps maintain acid-base balance. Magnificent magnesium Magnesium acts as a catalyst for enzyme reactions. It regulates neuromuscular contraction, promotes normal functioning of the nervous and cardiovascular systems, and aids in protein synthesis and sodium and potassium ion transportation.

Electrolyte movement When cells die e. Like fluids, they move around trying to maintain balance and electroneutrality. Electrolyte balance Fluid intake and output, acid-base balance, hormone secretion, and normal cell function all influence electrolyte balance. Because electrolytes function both collaboratively, with other electrolytes, and individually, imbalances in one electrolyte can affect balance in others. See Understanding electrolytes. Understanding electrolytes Electrolytes help regulate water distribution, govern acid-base balance, and transmit nerve impulses.

They also contribute to energy generation and blood clotting. Check the illustration below to see how electrolytes are distributed in and around the cell. See Interpreting serum electrolyte test results, page 26, for a look at normal and abnormal electrolyte levels in the blood. See the whole picture When you see an abnormal laboratory test result, consider what you know about the patient. With that said, look at the whole picture before you act, including what you know about the patient, his signs and symptoms, and his electrolyte levels.

See Documenting electrolyte imbalances, page Fluid regulation Many activities and factors are involved in regulating fluid and electrolyte balance. A quick review of some of the basics will help you understand this regulation better. Fluid and solute movement As discussed in chapter 1, active transport moves solutes upstream and requires pumps within the body to move the substances from areas of lower concentration to areas of higher concentration— against a concentration gradient.

Adenosine triphosphate ATP is the energy that moves solutes upstream. With potassium, the reverse happens: A large amount of potassium in intracellular fluid causes an electrical potential at the cell membrane. As ions rapidly shift in and out of the cell, electrical impulses are conducted. These impulses are essential for maintaining life. Organ and gland involvement Most major organs and glands in the body—the lungs, liver, adrenal glands, kidneys, heart, hypothalamus, pituitary gland, skin, gastrointestinal GI tract, and parathyroid and thyroid glands —help to regulate fluid and electrolyte balance.

As part of the renin-angiotensin-aldosterone system, the lungs and liver help regulate sodium and water balance as well as blood pressure. The adrenal glands secrete aldosterone, which influences sodium and potassium balance in the kidneys. These levels are affected because the kidneys excrete potassium, or hydrogen ions, in exchange for retained sodium. The heart says no The heart counteracts the renin-angiotensin-aldosterone system when it secretes atrial natriuretic peptide ANP , causing sodium excretion. The hypothalamus and posterior pituitary gland produce and secrete an antidiuretic hormone that causes the body to retain water which, in turn, affects solute concentration in the blood.

Where electrolytes are lost Sodium, potassium, chloride, and water are lost in sweat and from the GI tract; however, electrolytes are also absorbed from the GI tract.

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Discussions of individual electrolytes in upcoming chapters explain how GI absorption of foods and fluids affects their balance. The glands play on The parathyroid glands also play a role in electrolyte balance, specifically the balance of calcium and phosphorus. The parathyroid glands usually two pairs are located behind and to the side of the thyroid gland.

The thyroid gland is also involved in electrolyte balance by secreting calcitonin. This hormone lowers an elevated calcium level by preventing calcium release from bone. Calcitonin also decreases intestinal absorption and kidney reabsorption of calcium. Kidney involvement Remember filtration? Filtration occurs in the nephron the anatomic and functional unit of the kidneys.

Some fluids and electrolytes are reabsorbed through capillaries at various points along the nephron; others are secreted. Age can play an important role in the way kidneys function—or malfunction. Older adults are also at risk for electrolyte imbalances. Their kidneys have fewer functional nephrons, a decreased glomerular filtration rate, and a diminished ability to concentrate urine.

Normally functioning kidneys maintain the correct fluid level in the body. Sodium and fluid balance are closely related. The kidneys also rid the body of excess potassium. When the kidneys fail, potassium builds up in the body. High levels of potassium in the blood can be fatal. For more information about which areas of the nephron control fluid and electrolyte balance, see How the nephron regulates fluid and electrolyte balance.

How the nephron regulates fluid and electrolyte balance In this illustration, the nephron has been stretched to show where and how fluids and electrolytes are regulated.

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How diuretics affect balance Many patients—whether in a medical facility or at home—take a diuretic to increase urine production. Diuretics are used to treat many disorders, such as hypertension, heart failure, electrolyte imbalances, and kidney disease. Keeping a close watch The health care team monitors the effects of a diuretic, including its effect on electrolyte balance. A diuretic may cause electrolyte loss, whereas an I. Older adults, who are at risk for fluid and electrolyte imbalances, need careful monitoring because a diuretic can worsen an existing imbalance.

See How drugs affect nephron activity. When providing I. Improving your I. IQ To evaluate I. For more about I. For the electrolyte content of some commonly used I. Potassium is one of the major electrolytes inside the cell that leaks out into extracellular fluid after a major trauma, such as a burn. This puts the patient at risk for hyperkalemia.

Diuretics affect the kidneys by altering the reabsorption and excretion of: A. Diuretics generally affect how much water and sodium the body excretes. At the same time, other electrolytes such as potassium can also be excreted in urine. The main extracellular cation is: A. Answer: D. Sodium is the main extracellular cation. In addition to other functions, it helps regulate fluid balance in the body. In the nephron, most electrolytes are reabsorbed in the: A.

The proximal tubule reabsorbs most electrolytes from the filtrate. It also reabsorbs glucose, urea, amino acids, and water. Potassium is essential for conducting electrical impulses because it causes ions to: A. Answer: B. Potassium in the intracellular fluid causes ions to shift in and out of the cell, which allows electrical impulses to be conducted from cell to cell. Older adults are at increased risk for electrolyte imbalances because, with age, the kidneys have: A.

Older adults are at increased risk for electrolyte imbalances because their kidneys have fewer functioning nephrons, a decreased glomerular filtration rate, and a diminished ability to concentrate urine. Scoring If you answered all six questions correctly, congratulations! If you answered four or five correctly, great! You still have all the qualities of a well-balanced individual! If you answered fewer than four correctly, no need to feel too unbalanced!

Reference Lobo, D. Disorders of sodium, potassium, calcium, magnesium, and phosphate. In Basic concepts of fluid and electrolyte therapy pp. Melsungen, Germany: Medizinische Verlagsgesellschaft. A look at acids and bases The chemical reactions that sustain life depend on a delicate balance—or homeostasis—between acids and bases in the body. Even a slight imbalance can profoundly affect metabolism and essential body functions. Several conditions, such as infection or trauma, and medications can affect acid-base balance.

However, to understand this balance, you need to understand some basic chemistry. Understanding pH Understanding acids and bases requires an understanding of pH, a calculation based on the percentage of hydrogen ions in a solution as well as the amount of acids and bases. Acids consist of molecules that can give up, or donate, hydrogen ions to other molecules.

Carbonic acid is an acid that occurs naturally in the body. Bases consist of molecules that can accept hydrogen ions; bicarbonate is one example of a base. A solution that contains more base than acid has fewer hydrogen ions, so it has a higher pH. A solution with a pH above 7 is a base, or alkaline. A solution that contains more acid than base has more hydrogen ions, so it has a lower pH. A solution with a pH below 7 is an acid, or acidotic. Because arterial blood is usually used to measure pH, this discussion focuses on arterial samples.

Arterial blood is normally slightly alkaline, ranging from 7. A pH level within that range represents a balance between the percentage of hydrogen ions and bicarbonate ions. Generally, pH is maintained in a ratio of 20 parts bicarbonate to 1 part carbonic acid. A pH below 6. See Understanding normal pH. Understanding normal pH This illustration shows that blood pH normally stays slightly alkaline, between 7.

A pH below 7. Too low Under certain conditions, the pH of arterial blood may deviate significantly from its normal narrow range. In either case, a decrease in pH below 7. See Understanding acidosis. Understanding acidosis Acidosis, a condition in which pH is below 7. In either case, an increase in pH above 7. See Understanding alkalosis, page Understanding alkalosis Alkalosis, a condition in which pH is higher than 7.

A deviation in pH can compromise essential body processes, including electrolyte balance, activity of critical enzymes, muscle contraction, and basic cellular function. The body normally maintains pH within a narrow range by carefully balancing acidic and alkaline elements. The big three The body regulates acids and bases to avoid potentially serious consequences. These buffers instantly combine with the offending acid or base, neutralizing harmful effects until other regulators take over.

Renal regulation can take hours or days to restore normal hydrogen ion concentration. Regulation system 1: Buffers The body maintains a healthy pH in part through chemical buffers, substances that minimize changes in pH by combining with excess acids or bases. The main chemical buffers are bicarbonate, phosphate, and protein.

This system relies on a series of chemical reactions in which pairs of weak acids and bases such as carbonic acid and bicarbonate combine with stronger acids such as hydrochloric acid and bases to weaken them. Decreasing the strength of potentially damaging acids and bases reduces the danger those chemicals pose to pH balance. The kidneys assist the bicarbonate buffer system by regulating production of bicarbonate. Feeling better with phosphate Like the bicarbonate buffer system, the phosphate buffer system depends on a series of chemical reactions to minimize pH changes.

Phosphate buffers react with either acids or bases to form compounds that slightly alter pH, which can provide extremely effective buffering. This system proves especially effective in renal tubules, where phosphates exist in greater concentrations. Plenty of protein Protein buffers, the most plentiful buffers in the body, work inside and outside cells. Behaving chemically like bicarbonate buffers, protein buffers bind with acids and bases to neutralize them.

In red blood cells, for instance, hemoglobin combines with hydrogen ions to act as a buffer. Regulation system 2: Respiration The respiratory system serves as the second line of defense against acid-base imbalances. The lungs regulate blood levels of carbon dioxide CO2 , a gas that combines with water to form carbonic acid. Increased levels of carbonic acid lead to a decrease in pH. Chemoreceptors in the medulla of the brain sense those pH changes and vary the rate and depth of breathing to compensate.

Breathing faster or deeper eliminates more carbon dioxide from the lungs. The more carbon dioxide that is lost, the less carbonic acid that is made and, as a result, pH rises. The body detects that pH change and reduces carbon dioxide excretion by breathing slower or less deeply. See Carbon dioxide and hyperventilation, page Check for success To assess the effectiveness of ventilation, look at the partial pressure of carbon dioxide in arterial blood PaCO2. A normal PaCO2 level in the body is 35 to 45 mm Hg.

PaCO2 values reflect carbon dioxide levels in the blood. As those levels increase, so does PaCO2. Twice as good As a buffer, the respiratory system can maintain acid-base balance twice as effectively as chemical buffers because it can handle twice the amount of acids and bases. Although the respiratory system responds to pH changes within minutes, it can restore normal pH only temporarily.

The kidneys are responsible for long-term adjustments to pH. Regulation system 3: Kidneys The kidneys serve as yet another mechanism for maintaining acid-base balance in the body. They can reabsorb acids and bases or excrete them into urine. They can also produce bicarbonate to replenish lost supplies. Such adjustments to pH can take the kidneys hours or days to complete. As with other acid-base regulatory systems, the effectiveness of the kidneys changes with age.

See Acid-base balance across the life span. Ages and stages Acid-base balance across the life span The effectiveness of the systems that regulate acid-base balance vary with age. Also, the respiratory system of an older adult may be compromised and, therefore, less able to regulate acid-base balance. The kidneys also have a part in the regulation of the bicarbonate level, which is a reflection of the metabolic component of acid-base balance. Normally, the bicarbonate level is reported with arterial blood gas ABG results.

The kidneys keep working If the blood contains too much acid or not enough base, pH drops and the kidneys reabsorb sodium bicarbonate. The kidneys also excrete hydrogen along with phosphate or ammonia. Although urine tends to be acidic because the body usually produces slightly more acids than bases, in such situations, urine becomes more acidic than normal. The reabsorption of bicarbonate and the increased excretion of hydrogen causes more bicarbonate to be formed in the renal tubules and eventually retained in the body.

The bicarbonate level in the blood then rises to a more normal level, increasing pH. Ups and downs of acids and bases If the blood contains more base and less acid, pH rises. The kidneys compensate by excreting bicarbonate and retaining more hydrogen ions. As a result, urine becomes more alkaline and blood bicarbonate level drops. Conversely, if the blood contains less bicarbonate and more acid, pH drops. Altogether now The body responds to acid-base imbalances by activating compensatory mechanisms that minimize pH changes.

Returning the pH to a normal or near-normal level mainly involves changes in the component—metabolic or respiratory—not primarily affected by the imbalance. If the body compensates only partially for an imbalance, pH remains outside the normal range. If the body compensates fully or completely, pH returns to normal. Respiratory helps metabolic. If metabolic disturbance is the primary cause of an acid-base imbalance, the lungs compensate in one of two ways. When a lack of bicarbonate causes acidosis, the lungs increase the rate of breathing, which blows off carbon dioxide and helps raise the pH to normal.

When an excess of bicarbonate causes alkalosis, the lungs decrease the rate of breathing, which retains carbon dioxide and helps lower pH. And vice versa If the respiratory system disturbs the acid-base balance, the kidneys compensate by altering levels of bicarbonate and hydrogen ions. When PaCO2 is high a state of acidosis , the kidneys retain bicarbonate and excrete more acid to raise the pH.

When PaCO2 is low a state of alkalosis , the kidneys excrete bicarbonate and hold on to more acid to lower the pH. If PaCO2 rises, then pH falls, and vice versa. Diagnosing imbalances A number of tests are used to diagnose acid-base disturbances. Arterial blood gas analysis An ABG analysis is a diagnostic test in which a sample of blood obtained from an arterial puncture can be used to assess the effectiveness of breathing and overall acid-base balance.

See Taking an ABG sample. However, the angle of penetration varies. For the radial artery the artery most commonly used , the needle should enter bevel up at a degree angle, as shown below. For the brachial artery, the angle should be 60 degrees; for the femoral artery, 90 degrees. Keep in mind that ABG analysis should be used only in conjunction with a full patient assessment. An ABG analysis involves several separate test results, only three of which relate to acid-base balance: pH, PaCO2, and bicarbonate level.

The ABCs of ABGs Recall that pH is a measure of the hydrogen ion concentration of blood; PaCO2 is a measure of the partial pressure of carbon dioxide in arterial blood, which indicates the effectiveness of breathing. PaCO2 levels move in the opposite direction of pH levels. Other information routinely reported with ABG results includes partial pressure of oxygen dissolved in arterial blood PaO2 and arterial oxygen saturation SaO2. After age 60 years, the PaO2 may drop below 80 mm Hg without signs and symptoms of hypoxia. See Quick look at ABG results. Is it normal 7. Is it normal 35 to 45 mm Hg , low, or high?

Which value PaCO2 or bicarbonate more closely corresponds to the change in pH? Is the PaO2 normal 80 to mm Hg , low, or high? Step 1: Check the pH First, check the pH level. This figure forms the basis for understanding most other figures. If pH is abnormal, determine whether it reflects acidosis below 7. Then figure out whether the cause is respiratory or metabolic. Then determine whether the abnormal result corresponds with a change in pH.

For example, if the pH is high, you would expect the PaCO2 to be low hypocapnia , indicating that the problem is respiratory alkalosis. Conversely, if the pH is low, you would expect the PaCO2 to be high hypercapnia , indicating that the problem is respiratory acidosis caused by hypoventilation. Step 3: Watch the bicarbonate Next, examine the bicarbonate level. This value provides information about the metabolic aspect of acid-base balance. Then determine whether the abnormal result corresponds with the change in pH.

For example, if pH is high, you would expect the bicarbonate level to be high, indicating that the problem is metabolic alkalosis. Conversely, if pH is low, you would expect the bicarbonate level to be low, indicating that the problem is metabolic acidosis. Memory jogger Remember, bicarbonate and pH increase or decrease together. When one rises or falls, so does the other. Partial compende the normal range. Compensation involves opposites.

For instance, if results indicate primary metabolic acidosis, c sation, on the other hand, occurs when pH remains outsiompensation will come in the form of respiratory alkalosis. The low pH indicates acidosis. However, the PaCO2 is low, which normally leads to alkalosis, and the bicarbonate level is low, which normally leads to acidosis.

The bicarbonate level, then, more closely corresponds with the pH, making the primary cause of the problem metabolic. The resultant decrease in PaCO2 reflects partial respiratory compensation. A low PaO2 represents hypoxemia and can cause hyperventilation.

The PaO2 value also indicates when to make adjustments in the concentration of oxygen being administered to a patient. See Inaccurate ABG results. Anion gap You may also come across a test result called the anion gap. See Crossing the great anion gap. Earlier chapters discuss how the strength of cations positively charged ions and anions negatively charged ions must be equal in the blood to maintain a proper balance of electrical charges. The anion gap result helps you differentiate among various acidotic conditions. Crossing the great anion gap This illustration represents the normal anion gap.

The gap is calculated by adding the chloride level and the bicarbonate level and then subtracting that total from the sodium level. Potassium is generally omitted because it occurs in such low, stable amounts. The gap between the two measurements represents the anions not routinely measured, including sulfates, phosphates, proteins, and organic acids such as lactic acid and ketone acids.

Increases can occur with acidotic conditions characterized by higher than normal amounts of organic acids. Such conditions include lactic acidosis and ketoacidosis. The anion gap remains normal for certain other conditions, including hyperchloremic acidosis, renal tubular acidosis, and severe bicarbonate-wasting conditions, such as biliary or pancreatic fistulas and poorly functioning ileal loops. Acidosis or alkalosis? Is it normal, high, or low? For example, metabolic acidosis can lead to compensation by respiratory alkalosis.

Together they yield information about oxygen status. Quick quiz 1. PaCO2 level indicates the effectiveness of: A. PaCO2 reflects how well the respiratory system is helping to maintain acid- base balance. The kidneys respond to acid-base disturbances by: A. If your patient is breathing rapidly, his body is attempting to: A. High carbon dioxide levels in the blood, measured as PaCO2, cause a drop in pH. Chemoreceptors in the brain sense this decrease and stimulate the lungs to hyperventilate, causing the body to eliminate more carbon dioxide.

A low PaCO2 means less carbon dioxide acid is in the blood, which raises pH. When pH is raised, the bicarbonate level also increases. The laboratory reports the following ABG results for your patient: pH, 7. You interpret these results as: A. Because PaCO2 is normal and bicarbonate is low matching the pH , the primary cause of the problem is metabolic. A colleague hands you these ABG results: pH, 7.

The pH is alkalotic. Although both PaCO2 and bicarbonate have changed, the bicarbonate matches the pH. The elevated PaCO2 represents the efforts of the respiratory system to compensate for the alkalosis by retaining carbon dioxide. You did a great job covering all the bases and acids! References Appel, S. Understanding acid-base balance. Find out how to interpret values and steady a disturbed equilibrium in an acutely ill patient. Nursing , 38, 9— Rogers, K. Understanding arterial blood gases.

Journal of Perioperative Practice, 23 9 , — A look at fluid volume Blood pressure is related to the amount of blood the heart pumps and the extent of vasoconstriction present. Certain types of pressure, such as pulmonary artery pressure PAP and central venous pressure CVP , are measured through specialized catheters. These measurements also help assess fluid volume status.

To maintain the accuracy of whatever blood pressure measurement system you use, periodically compare the readings of automated and direct measurement systems with manual readings. Cuff measurements A simple blood pressure measurement, taken with a stethoscope and a sphygmomanometer, is still one of the best tools for assessing fluid volume. A sizeable task To measure blood pressure accurately, you must first make sure the cuff is the correct size. In position Position the arm so that the brachial artery is at heart level. To position the blood pressure cuff properly, wrap it snugly around the upper arm.

For children, place the lower border closer to the antecubital fossa. Most cuffs have a reference mark to help you position the bladder. After positioning the cuff, palpate the brachial artery using your index finger, and place the bell of the stethoscope directly over the point where you can feel the strongest pulsations. See Positioning a blood pressure cuff. Positioning a blood pressure cuff This photograph shows how to properly position a blood pressure cuff and stethoscope bell. Listen up Once you have the stethoscope and cuff in place, use the thumb and index finger of your other hand to turn the screw on the rubber bulb of the air pump and close the valve.

Then pump air into the cuff while auscultating over the brachial artery and continue pumping air until the gauge registers at least 10 mm Hg above the level of the last audible sound. Next, carefully open the air pump valve and slowly deflate the cuff. In an observational study on blood donors, Lyon et al. Significant differences were found between the IVCd e before and after blood donation and between IVCd i before and after donation 5.

In patients treated for hypovolemia, Zengin et al. The IVCd was measured ultrasonographically by M-mode in the subxiphoid area and the RVd was measured in the third and fourth intercostals spaces before and after fluid resuscitation. As compare with healthy volunteers average diameters in hypovolemic patients of the IVC during inspiration and expiration, and right ventricule diameter were significantly lower. After fluid resuscitation, there was a significant increase in mean IVC diameters during inspiration and expiration as well as in the right ventricule diameter [ 35 ].

Diuretics, especially loop diuretics, remain a valid therapeutic alternative for relieving symptoms and improving pathophysiological states of fluid overload such as congestive heart failure and in patients with AKI. At this time, there is no evidence that favors ultrafiltration over diuretic use in volume overload patients with or without AKI in terms of less progression of AKI, improved clinical outcomes or reduce incidence of AKI [ 36 ].

What should be the goal of urine output when using diuretics to manage fluid overload? Diuretics could be either administered by bolus or using a continuous infusion. There has been controversy about which of these strategies is better; some authors advocate that diuretic infusion is superior to boluses since urinary output could be maintain easily [ 41 ]. In one study diuretic infusion was associated with greater diuresis and this was achieved with a lesser dose [ 42 ]; infusion was also associated with fewer adverse events such as worsening AKI, hypokalemia, and ototoxicity.

Since common electrolyte disturbances could be encountered during diuretic therapy, it is important to monitor electrolytes levels and also to assess acid-based status. In order to avoid hypokalemia, administration of oral potassium it is easy. Measuring urinary potassium concentration and calculating the daily losses of potassium, which require replacement is a strategy that can be used to estimate daily potassium requirements.

Another strategy is the use of potassium-sparing diuretics like spironolactone. Hypomagnesemia is frequently found during diuretic therapy, magnesium replacement can be achieved either intravenously or orally, typically with 20—30 mmoL per day. Finally in some patients, chloride losses exceed sodium losses and hypochloremic metabolic alkalosis develops; this is usually corrected with the administration of potassium chloride and magnesium chloride.

A recent comprehensive review have shown that torsemide and bumetanide have more favorable pharmacokinetic profiles than furosemide, and in the case of torsemide it could be more efficacious than furosemide in patients with heart failure decreased mortality, decrease hospitalizations, and improved New York Heart Association functional classification. In AKI patients, as compared with torsemide the use of furosemide was associated with a significant improvement in urine output.

Moreover, two trials comparing bumetanide with furosemide showed conflicting results [ 44 ]. Finally, in patients with AKI the response to furosemide may be reduced due to multiple mechanisms including a reduced tubular secretion of furosemide and blunted response of Na-K-2Cl co-transporters at the loop of Henle [ 45 ]. This reduced response to furosemide in AKI patients often requires the use of higher doses that may increase the risk of ototoxicity, especially as the clearance of furosemide is severely reduced in AKI.

High doses of furosemide may also result in myocardial dysfunction secondary to furosemide induced vasoconstriction [ 46 ]. Accurate management of fluid balance becomes obligatory with the ultimate goal of improving pulmonary gas exchange and organ perfusion while maintaining stable hemodynamic parameters. The optimal renal replacement therapy for patients with AKI and fluid overload has not been defined yet and there is still an ongoing debate.

Some large observational studies have suggested that CRRT is an independent predictor of renal recovery among survivors [ 48 — 50 ]. In the absence of definite data to support the use of particular type of renal replacement therapy, one should consider CRRT and IHD as complementary therapies. Relatively small surface-area filters can be employed with reduced heparin doses since low ultrafiltration and blood flow rates are required, [ 51 ]. Continuous veno-venous hemofiltration CVVH is another CRRT technique that allows meticulous, minute-to-minute control of fluid balance by providing continuous fluid, electrolyte, and toxin clearance.


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  8. Machine fluid balance refers to the total balance over h period of fluids administered by the CRRT machine dialysate or replacement fluid or both depending on the technique and fluids removed by the CRRT machine spent dialysate or ultrafiltrate or both depending on the technique. Tri-sodium citrate was added at the arterial catheter port with ionized calcium levels been measured post-filter. Post-filter ionized calcium levels were used to adjust tri-sodium citrate flow rates. Pre-filter BUN value was measured after the infusion of tri-sodium citrate and after pre-dilution replacement fluid Qr , thus accounting for the pre-dilutional effect.

    A target effluent volume was adjusted by hourly modifying substitution fluid rate Qs to achieve a negative, zero, or positive fluid balance. Qb, blood flow rate; Qd, dialysate flow rate; Qr, replacement fluid rate; Quf, total ultrafiltration rate; Qnet, net fluid removal rate. The ultimate goal is to preserve tissue perfusion, optimizing fluid balance by effectively removing fluid without compromising the effective circulating fluid volume; therefore, meticulous monitoring of fluid balance is critical for all patients [ 52 ].

    Another option for treating patients with fluid overload are the new smaller and more portable devices like the Aquadex FlexFlow System Baxter Healthcare.

    How to approach NBME/USMLE questions

    In patients with heart failure, Costanzo et al. Changes in renal function and the day mortality were similar in both groups. Several complications like congestive heart failure, pulmonary edema, delayed wound healing, tissue breakdown, and impaired bowel function are associated with fluid overload. Fluid overload has also been related to increased mortality. The optimal assessment of volume status in critically ill patients is of vital importance particularly during the early management of these patients.

    One key aspect of fluid overload management is to maintain hemodynamic stability and optimize organ function. Loop diuretics are frequently used as the initial therapy to treat critically ill patients with fluid overload; nevertheless, diuretics have limited effectiveness due to several factors such as underlying acute kidney injury that contribute to diuretic resistance.

    Renal replacement therapies are often required for optimal volume management in critically ill patients with fluid overload. Fluid accumulation, survival and recovery of kidney function in critically ill patients with acute kidney injury. Kidney Int. Outcome in children receiving continuous venovenous hemofiltration.

    Pediatric patients with multi-organ dysfunction syndrome receiving continuous renal replacement therapy.

    Therapeutic Uses

    Effect of fluid overload and dose of replacement fluid on survival in hemofiltration. Pediatr Nephrol. Comparison of two fluid-management strategies in acute lung injury. N Engl J Med. Effects of intravenous fluid restriction on postoperative complications: comparison of two perioperative fluid regimens: a randomized assessor-blinded multicenter trial. Ann Surg. Fluid balance and acute kidney injury.

    Nat Rev Nephrol. Lancet Infect Dis. Kellum JA, Lameire N. Kidney Int Suppl Mehta RL, Bouchard J. Controversies in acute kidney injury: effects of fluid overload on outcome. Contrib Nephrol. Improved survival in ARDS patients associated with a reduction in pulmonary capillary wedge pressure. Effect of intraoperative fluid management on outcome after intraabdominal surgery. Myocellular and interstitial edema and circulating volume expansion as a cause of morbidity and mortality in heart failure. J Card Fail.

    Edema and acute renal failure. Semin Nephrol. Bouchard J, Mehta RL. Fluid balance issues in the critically ill patient. Schrier RW, Wang W. Acute renal failure and sepsis. The importance of fluid management in acute lung injury secondary to septic shock. Fluid resuscitation in septic shock: a positive fluid balance and elevated central venous pressure are associated with increased mortality. Crit Care Med. Fluid overload as a biomarker of heart failure and acute kidney injury.

    Does this dyspneic patient in the emergency department have congestive heart failure? Bedside cardiovascular examination in patients with severe chronic heart failure: importance of rest or inducible jugular venous distension. The selection is not exhaustive. Clinical science Sodium is the most important extracellular cation and plays an important role in maintaining the body's extracellular fluid volume.

    Hypernatremia Hypovolemic hypernatremia Dehydration e. Clinical features are primarily neurological and depend on the severity of the sodium imbalance. Osmotic myelinolysis Definition : Damage to the myelin sheath of nerves in the CNS caused by a sudden rise in the osmolarity of blood. Central pontine myelinolysis Most common type of osmotic myelinolysis Affects central region of the pons Extrapontine myelinolysis Affects cerebellum , lateral geniculate body , thalamus , putamen , cortical, and subcortical white matter Causes Rapid correction of chronic hyponatremia Clinical features Altered level of consciousness , coma Locked-in syndrome Impaired cranial nerve function : dysarthria , dysphagia , diplopia Worsening quadriparesis Treatment : supportive care The symptoms of pontine myelinolysis appear 2 to 6 days after the correction of hyponatremia!

    Others Intracranial hemorrhage Cerebral edema Noncardiogenic pulmonary edema Rhabdomyolysis Bone fractures References: [11] We list the most important complications. Prepare and succeed on your medical exams.

    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES
    USMLE STEP 1 REVIEW: FLUIDS AND ELECTROLYTES

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