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Project - USMLE Step 3 > Fluids, Electrolytes, and Acid-Base Disorders > Flashcards

Fluids, Electrolytes, and Acid-Base Disorders Flashcards

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USMLE® Step 1 flashcards
1
Q

What % of body weight is water in men? In women? What two factors cause total body water % to decrease?

A

60% of men’s total body weight = water,
50% of women’s total body weight = water
Age and obesity cause total body water to decrease over time

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2
Q

How much of total body water is intracellular fluid and how much is extracellular fluid?

A

2/3 is ICF and 1/3 is ECF

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3
Q

How much of body’s weight is water in intracellular fluid? How much of body’s weight is water in extracellular fluid?

A 
ICF = 40% of body weight (2/3 * 60)
ECF = 20% of body weight (1/3 * 60)
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4
Q

What compartment is plasma and interstitial fluid apart of?

A

ECF

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5
Q

What two forces determine fluid shift in the body?

A

Hydrostatic pressure and oncotic pressure

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6
Q

True/False: Skin turgor and mucous membrane appearance are good indicators of volume status?

A

False

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7
Q

What is normal urine output in infants and what is normal urine output in adults?

A 
Infants = 1.0 mL/kg/hr
Adults = .5-1.0 mL/kg/hr
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8
Q

What causes the body to be hypervolemic on an overall level but intravascularly depleted and why?

A

Anything that causes hypoalbuminemia (nephrotic syndrome or liver disease) will cause fluid to shift to third space and out of the vasculature.

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9
Q

What fluids can be used to increase intravascular volume? (3)

A
  1. Normal saline (unless the patient has CHF)
  2. D51/2NS
  3. Lacted Ringer
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10
Q

D51/2NS has what that can help prevent muscle breakdown but should be used in caution in what patients?

A

It has glucose to help prevent muscle breakdown, however, it should be used in caution in diabetics

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11
Q

What is the standard maintenance fluid?

A

D51/2NS

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12
Q

What is D5W used for? (2) and how much of it remains intravascularly?

A
  1. dilute powdered medications
  2. Sometimes indicated in correcting hypernatremia

Only 1/12th remains intravascularly because a large amount of it distributes to total body water compartment

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13
Q

What electrolyte does lacted ringer contain and what is its use?

A

Used as replacement for intravascular volume, not a maintenance fluid, contains potassium (don’t use if hyperkalemia is suspected)

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14
Q

What are three broad causes of hypovolemia?

A
  1. Third spacing due to ascites, effusions, bowel obstructions, crush injuries, burns
  2. significant loss
  3. Inadequate intake
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15
Q

What are some clinical features of hypovolemia?

  1. CNS findings?
  2. Cardiovascular findings?
  3. Skin?
  4. Urine output?
  5. Renal conditions?
A
  1. CNS findings: mental status changes, sleepiness, apathy, coma
  2. Cardiovascular: (due to decrease in plasma volume): orthostatic hypotension, tachycardia, decreased pulse pressure, decreased CVP, decreased PCWP
  3. Skin: Poor turgor, hypothermia, dry tongue, pale extremities
  4. Oliguria
  5. Acute renal failure (prerenal azotemia lab findings)
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16
Q

What % increase is seen in hematocrit for each liter of volume depletion in a patient?

A

3% increase for each liter of deficit in a patient

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17
Q

How is hypovolemia treated?

A
  1. Correct volume deficit
    • Use bolus to achieve euvolemia. Begin with isotonic solution (lactated ringer, or NS). Maintain urine output at 0.5 to 1.0 mL/kg/hr.
  2. Maintenance fluid - D51/2NS with 20 mEq KCL/L is most common adult maintenance fluid
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18
Q

How is maintenance fluid calculated?

A

4/2/1 rule:
4 mL/kg for first 10 kg, 2 mL/kg for next 10 kg, 1 mL/kg for every 1 kg over 20

Example) 70kg person = 410 = 40; 210 = 20, 1*50 = 50; total = 110 ml/hr

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19
Q

What are two broad causes of hypervolemia?

A
  1. Iatrogenic (parenteral overhydration)

2. Fluid-retaining states: CHF, nephrotic syndrome, cirrhosis, ESRD

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20
Q

What are the clinical features of hypervolemia?

A
  1. Weight gain
  2. Peripheral edema
  3. Ascites
  4. Pulmonary edema
  5. JVD
  6. Pulmonary rales
  7. Low hematocrit and albumin concentration
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21
Q

How is hypervolemia treated?

A
  1. Fluid restriction

2. Judicious use of diuretics

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22
Q

Hyponatremia in the blood causes water to flow which way?

A

Water from the blood flows into cells, causing them to expand

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23
Q

Hypernatremia in the blood causes water to flow which way?

A

Water from cells flows into cells, causing them to shrink

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24
Q

How is serum osmolality calculated?

A

2*Sodium + Glucose/18 + BUN/2.8

25
Q

What is normal serum osmolality?

A

Approximately 280-295 mOsm/kg

26
Q

How do loop diuretics work?

A

They inhibit sodium reabsorption in the thick ascending loop of henle (Na+, K+, Cl- transporter inhibition)

27
Q

How do thiazide diuretics work?

A

They inhibit Na+ Cl- cotransporter in the distal tubule.

28
Q

Where does the majority of sodium reabsorption occur?

A

In the proximal tubule of the kidney

29
Q

What effect does aldosterone have on the kidney?

A

Aldosterone increases sodium reabsorption and potassium secretion from the late distal tubules

30
Q

Plasma hypertonicity (above >295 mOsm/kg) causes what? (2)

A

Activation of osmoreceptors in the hypothalamus which stimulates thirst; secretion of ADH

31
Q

How does ADH work?

A

Binds to V2 receptors in renal collecting ducts and water channels are synthesized and more water is reabsorbed

32
Q

What causes ADH secretion to be suppressed?

A

Increasing plasma tonicity

33
Q

Below which amount of sodium concentration, do symptoms of hyponatremia begin?

A

<120 mEq/L

34
Q

How many different broad categories of hyponatremia are there?

A

3, hypotonic hyponatremia, hypertonic hyponatremia, and isotonic hyponatremia

35
Q

How is hypotonic hyponatremia characterized?

A

It is characterized by hyponatremia along with decreased serum osmolality <280 mOsm/kg

36
Q

What are three broad categories of hypotonic hyponatremia?

A
  1. Hypovolemic hypotonic hyponatremia
  2. Euvolemic hypotonic hyponatremia
  3. Hypervolemic hypotonic hyponatremia
37
Q

What are the symptoms and causes of hypervolemic hypotonic hyponatremia?

A

Symptoms are edema, JVD, crackles in the lungs, and causes are water-retaining states such as CHF, nephrotic syndrome, and liver disease

38
Q

What are the symptoms and broad causes (2) of hypovolemic hypotonic hyponatremia?

A

Symptoms are sunken eyes, dry mucous membranes, skin tenting, and causes are extrarenal salt loss or renal salt loss.

Extrarenal salt loss shows up as low urine sodium (<10).

Renal salt loss shows up as high urine sodium (>20)

39
Q

What are some causes of extrarenal salt loss?

A

Diarrhea, vomiting, diaphoresis, third spacing

40
Q

What are some causes of renal salt loss?

A

Excessive diuretic use, low aldosterone, and acute tubular necrosis

41
Q

What are the symptoms and causes of euvolemic hypotonic hyponatremia?

A

No symptoms of hypovolemia or hypervolemia.

Causes are RATS
Renal Tubular acidosis
Addison's Dz
Thyroid Dz
SIADH
42
Q

What is the serum osmolality for isotonic hyponatremia and what are the causes? Isotonic hyponatremia aka?

A

Aka psuedohyponatremia. Caused by anything that increases triglycerides or proteins in the blood. Hyponatremia + normal serum osmolality (280-295)

43
Q

What are the causes and serum osmolality for hypertonic hyponatremia?

A

Serum osmolality >295

Caused by hyperglycemia, mannitol and glycerol in the blood

44
Q

Draw out the evaluation of hyponatremia

A

See SUTM pg 308

45
Q

What are the clinical features of hyponatremia?

A

Neurological symptoms predominate: cerebral edema, headache, delirum, irritability, muscle twitching, weakness, hyperactive deep tendon reflexes

Hypertension

Oliguria progressing to anuria

46
Q

How does hyperglycemia affect serum sodium levels?

A

For every 100 mg/dL increase in blood glucose level above normal, the serum sodium decreases by 3 mEq/L.

47
Q

How is isotonic and hypertonic hyponatremia’s managed?

A

Diagnose and treat the underlying disorder

48
Q

How is hypotonic hyponatremia managed?

A
  1. Mild cases (Na+ 120-130): withhold free water, allow patient to re-equilibrate naturally.
  2. Moderate cases (Na+ 110 to 120): loop diuretics given with saline
  3. Severe cases (Na+ <110): give hypertonic saline to increase serum sodium concentration by 1-2 mEq/L/hr until symptoms improve.

DO NOT INCREASE serum sodium by more than 8 mmol/L during the first 24 hours because it may lead to central pontine demyelination

49
Q

How is hypernatremia defined? (Plasma Na+ concentration)

A

Na+ > 145 mmol/L

50
Q

What are two broad causes for hypernatremia?

A

Water loss or sodium infusion

51
Q

What are three broad categorizations and causes of hypernatremia? (Describe each category)

A
  1. Hypovolemic hypernatremia (sodium stores are depleted, but more water loss than sodium loss)
    Causes: Renal loss or Extrarenal loss
  2. Isovolemic hypernatremia (sodium stores are normal, water loss)
    Cause: Diabetes Inspidus
  3. Hypervolemic hypernatremia (excess sodium)
    Causes: cushing syndrome, exogenous corticosteroids, iatrogenic causes
52
Q

Rapid correction of hypernatremia leads to what?

Rapid correction of hyponatremia leads to what?

A

Rapid correction of hypernatremia leads to cerebral edema

Rapid correction of hyponatremia leads to central pontine demyelination

53
Q

What are some clinical features of hypernatremia?

A

Neurological symptoms predominate: altered mental status, restlessness, weakness, focal neurological deficits, confusion, seizures, coma

Salivation increases, tissue and mucous membranes are dry

54
Q

What is a good lab test for hypernatremia?

A

Urine osmolality > 800 mOsm/kg

55
Q

What is formula for calculating free water deficit?

A
  1. Water Deficit = TBW (1- actual Na+/desired Na+)
56
Q

How is hypovolemic hypernatremia treated?

A

Give isotonic saline to restore hemodynamics. Correction of hypernatremia can wait until patient is hemodynamically stable, then replace free water deficit

57
Q

How is isovolemic hypernatremia treated?

A

Since cause is usually diabetes inspidus, treatment may require vasopressin

58
Q

How is hypervolemic hypernatremia treated?

A

Give diuretics (furosemide) and D5W to remove excess sodium. Dialyze patients with renal failure

Project - USMLE Step 3 (82 decks)

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