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Renal > Disorders of ECF Volume and Tonicity > Flashcards

Disorders of ECF Volume and Tonicity Flashcards

1
Q

What is the distribution of total body water in the following compartments?

  1. Intracellular
  2. Extracellular
  3. intravascular
  4. extravascular
A
  1. 2/3 (28L)
  2. 1/3 (14L)
  3. 1/4 of the extravascular water (3.5L)
  4. 3/4 of the extravascular water (10.5L)
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2
Q

What component of body water volume is sensed to determine Na handling?

A

the intravascular space is sensed

The dominant sensing mechanisms sense a component of the intravascular spaced called the EABV (effective arterial blood volume.

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3
Q

What does the kidney do if the EABV is low? high?

A

Low EABV - the kidney retains NaCl which stimulates ADH and reabsorbes water

High EABV- the kidney excretes NaCl to flush out excess volume

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4
Q

What happens if you add 210mM NaCl to the ECF?

A

The ECF increases in osmolarity.

  • This pulls water from the intracellular space to try to return to equilibrium.
  • the increased osmolarity releases ADH to insert aquaporins to resorb water
  • the urine is high in Na, but very concentrated
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5
Q

What happens if you add 1.5L of pure water to the ECF?

A

The ECF has decreased osmolarity.

  • Water will move into the intracellular space to return to equilibrium
  • because of the decreased osmolarity, ADH is NOT released and water is not reabsorbed in the CCD
  • Urine will be dilute with extra salts
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6
Q

What happens if you add 1.5L of normal saline to the ECF?

A

The osmolarity is maintained, but the volume of water is increased.

  • There is NO movement of fluid between intracellular space and extracellular space
  • the kidney excreted Na and H20 to reduce the body volume back to normal
  • the urine will be iso-osmotic
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7
Q

What is normal serum osmolarity?

A

280

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8
Q

What is the most reliable assessment of EABV?

How is the blood volume determined?

A

The physical exam is most reliable assessment.

  1. orthostatic/postural changes in BP - when the patient stands, if the diastolic pressure drops and pulse increases, this is a sign of low EABV.
  2. JVD
  3. skin turgor, dry mucous membranes, sunken eyes
  4. presence/absence of edema
  5. ascites
  6. pulmonary congestion
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9
Q

What are the limitations of the physical exam for assessing EABV?

A

Changes in pulse and blood pressure that would denote low EABV are insensitive and would only be present with MARKEDLY decreased volume.

Orthostatic/postural changes in BP/pulse are more sensitive however, patients with autonomic neuropathies may have postural changes in BP without being volume depleted

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10
Q

What is the most sensitive finding on physical exam for detecting low EABV?

A

If there is a 10% decrease in arterial volume and EABV, the patient will have a pulse increase of greater than or equal to 8 bpm upon standing.

Also

Decrease in diastolic pressure upon standing is always significant

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11
Q

What are the 4 major laboratory tests to assess low EABV?

A
  1. Spot urine Na, Cl, and creatinine
  2. BUN/Cr ratio
  3. Albumin levels
  4. Hct
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12
Q

If a person has low EABV, what should you see in terms of urine Na, Cl, and creatinine?
What is the effect on the fractional excretion of Na?

A

Urine Na and Cl should be low because they will be reabsorbed by the kidney
Creatinine should be normal because it is filtered but not really secreted or reabsorbed.

This will cause a decreased FEna

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13
Q

What is a limitation of spot urine analysis for determining EABV?
How is this limitation accounted for?

A

Spot urine measures the CONCENTRATION of Na, Cl and creatinine in the urine.
If the person drinks a lot of water, they will have a dilute urine making the Na a lower concentration and showing perceived low EABV.

The fractional excretion of NNa will factor out renal H20 handling by measuring urine/plasma ratios of Na to Creatinine.

Creatinine is filtered and has VERY little secreted by the kidney.
In a low EABV state, Na will be filtered, but then reasborbed along the nephron.
Comparing concentrations to each other will determine how much Na is being reasborbed.

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14
Q

What is the equation for FENa?

What fractional excretion would indicated a low EABV?

A

FENa = (Una x Pcr)/(Pna x Ucr) x100%

FE below 0.5-1% would indicate a low effective arterial blood volume

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15
Q

What are the limitations of FE for determining EABV?

A
  1. Primary renal Na retention can lead to low FE, but the EABV may be normal
  2. Diuretics, Addison’s (hypoaldosteronism) and other renal diseases can lead to over excretion of Na and Cl that can mask a low EABV
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16
Q

What does the plasma BUN/Cr ratio tell you about the volume status?

A

Low EABV leads to increases in the BUN (>20) and the uric acid (>5) and increases in EABV lead to decreases in BUN and uric acid

The BUN/Cr ratio of >20 suggest low EABV

(*normal BUN/Cr is 10)

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17
Q

What are the limitations of using BUN increases for determining low EABV?

A

BUN can be increased by decreases in GFR or by increases in protein metabolism.

Uric acid can be influenced by changes in purine metabolism.

This can complicate using plasma BUN and uric acid solely as determinants of EABV.

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18
Q

While the regulation of ECFV occurs by the bodies handling of _________, the tonicity of the ECF occurs by regulation of ____________/

A

ECFV = regulating NaCl

ECF tonicity = regulating the bodies handling of H20

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19
Q

What is the difference in solute composition between the extracellular and intracellular fluid?

A

Na and its salts are confined to the ECF
K is typically confined intracellularly

Despite the different compositions, when discussing disorders of osmolarity, the water is TBW.
This means that if extracellular Na is low, intracellular K will be low as well .

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20
Q

What is the difference between osmolarity and tonicity?

Which is more clinically important?

A

Osmolarity is the number of particles per kg of solution. (concentration of different substances).

Tonicity ONLY measures those particles that have the capacity to affect hydrodynamics (move water) because they cannot penetrate the cell membrane

Tonicity is more clinically important`

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21
Q

How is plasma osmolarity measured clinically?
How can it be calculated?
What should the calculated value typically be?
What does a difference in the two values mean?

A

Clinically it can be measured by an osmometer.
It is calculated with the equation:

Osm = 2[Na] = [glucose]/18 + [BUN]/2.8

Normal osmolarity is 280-290

A difference in the values of more than 10 means that an non-effective osmol must be present. This means that a substance is present that can freely diffuse across cell membranes, thus not affecting tonicity, but affecting measured osmolarity.

ex.
1. methanol
2. ethanol
3. ethylene glycol
4. acetone
5. urea

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22
Q

Is urea an effective or ineffective osmole?

A

It is an ineffective osmol meaning that it can freely equilibrate across membranes.

Patients with renal failure and very elevated BUN will have no fluid shifts because the urea will equilibrate, NOT pull water

23
Q

What are the 3 most relevant effective osmols?

A
  1. Glucose
  2. Na
  3. Mannitol
24
Q

Describe the regulation of tonicity as you move through the nephron.
(what substances are able to leave which parts of the nephron tubule)

A
  1. GFR
  2. H20 and Na are removed down to the tip of the loop of Henle
  3. Salt is removed in the ascending limb (water is impermeable–> urinary dilution)
  4. CCD - salt and water are removed depending on presence of mineralocorticoids and tubular Na concentration (Na) and ADH (H20)
25
Q

What hormone is responsible for the regulation of plasma tonicity?
Where is it released from and when?

A

ADH (AVP, vasopressin) is released from the hypothalamus at

  • tonicity above 280 osm.
  • low volume (decreased EABV)

ADH then attaches to V2R on the basolateral side of the renal CCD and activates AQP2 to insert on the luminal side. This allows water to be reabsorbed from the kidney to decrease the osmolarity.

26
Q

When is ADH released in response to low EABV? What is a limitation of ADH for replenishing volume?

A

ADH is only used by the body to restore volume in emergency states where NaCl retention cannot retain EABV.
The limitation is that water will equilibrate between the body cavities so 2/3 will move to the intracellular space and only 1/3 will replenish the decreased EABV and ECFV.
This is inefficient

27
Q

In general the marker for hypotonicity is __________________.

A

Hyponatremia (low Na with excess of free H20)

28
Q

Before doing a workup on a patient with hyponatremia, you must first check their tonicity.
What are 2 possible causes of hyponatremia with:
1. high plasma osmolarity
2. normal plasma osmolarity
3. What is the next step of they are hypotonic?

A

Make sure that they are actually hypotonic.

If they have normal tonicity, it may be:

  1. pseudohyponatremia
  2. hyperglycemia after AVP and H20 involvement

If they have high tonicity with hyponatremia, it may be due to:

  1. Hyperglycemia
  2. Gammaglubulin Therapy (hyperkalemia, hyponatremia)

If they are hyponatremic and hypotonic, the next step is check the urine osmolarity.

29
Q

When you have a hyponatremic, hypotonic patient your next step is the check urine osmolarity.
What is the likely cause if their urine osmolarity is low (<100, dilute urine)?

A

primary polydypsia- they are drinking too much water (20-25 liters a day)

  1. psychogenic
  2. lesion in the hypothalamus
30
Q

When you have a hyponatremic, hypotonic patient and they have concentrated urine (>200), what is the next thing to evaluate?

A

They are able to concentrate the urine which means that ADH is working to reabsorb water.
The next thing to check is the EABV.

31
Q

If you have a hyponatremic, hypotonic patient with concentrated urine, what are likely scenarios if the EABV is normal?

A

SIADH
Hypoaldosteronism (Addison’s)
Glucocorticoid deficiency
Drugs (anti-psychotics, SSRIs)

32
Q

If you have a hyponatremic, hypotonic patient with concentrated urine, and a LOW EABV, what is the next thing to evaluate?
What if it is high?
What if it is low?

A

You next evaluate the ECFV.

If it is high, you are suspicious of edematous disorders.

If it is low, you are suspicious of:

  1. diarrhea
  2. mineralocorticoid deficiency (salt wasting)
  3. diuretics
33
Q

A patient presents to you with hyponatremia. When measuring the plasma osmolarity, you find it to be normal. What are the 2 potential causes? How would you differentiate?

A
  1. Pseudohyponatremia- the patient will have elevated triglycerides or proteins in their serum
  2. Late hyperglycemia (*after ADH and water reabsorption), but they will still have relatively high glucose
34
Q

A hyponatremic patient presents to your office. They have high plasma osmolarity, but no osmolarity gap. What are the 2 most likely causes and how do you differentiate them?

A
  1. Hyperglycemia- the elevated glucose is causing water to extract from cells and is diluting the Na concentration in the extracellular space
  2. Gammaglobulin studies- these studies are performed with sucrose which extracts H20 from cells, diluting the Na concentration. They will also have elevated K (as this is pulled out of the cell with the water via solvent drag)
35
Q

A hyponatremic, hypotonic patient presents to your office. After performing urinalysis, you see that the urine has a very low osmolarity (<100).
What should you be suspicious of?

A

This person is peeing out a lot of water and still has water overload in the body.
This means they most likely have psychogenic polydypsia (they drink too much water)

36
Q

A hyponatremic, hypotonic patient presents to your office. They have concentrated urine (>200). You know the next step is to assess their EABV.
The EABV is normal.
What is the suspected cause of the hyponatremia/hypotonicity?

A

EABV is normal meaning that they shouldn’t be reabsorbing Na, Cl or water.
They are hypotonic meaning that they shouldn’t be making ADH to reabsorb water either.

Because the urine is concentrated, it appears that they are INFACT absorbing water. This means that ADH is acting inappropriately and the patient probably has:

SIADH

(other possible explanations include: glucocorticoid deficiency, hypothyroidism, SSRI, anti-psych)

37
Q

A patient presents with hyponatremia and hypotonicity. They have concentrated urine, and a low EABV. You calculate their ECFV and find that it is low. What are likely causes of the problem?

A

They person probably has:

  1. Diarrhea
  2. diuretic
  3. mineralocorticoid deficiency

We know this because the low EABV should cause Na to reabsorbed from the tubule of the kidney, however, the hyponatremia is showing us that this is not the case for the patient. This leads us to believe that there is a problem with reabsorption rooting in: excessive GI loss, diuretic not allowing Na to be reabsorbed or lack of CCD MR

38
Q

A patient presents with hyponatremia, hypotonicity and they have concentrated urine. The EABV is low, but the ECFV is high. What is the likely problem?

A

They person most likely had an edematous disorder

39
Q

What are the 3 hallmarks of a patient with SIADH?

A
  1. hyponatremia
  2. concentrated urine
  3. increase [Na] in the urine
40
Q

What MUST be absent in order for a patient to develop hypernatremia?

A

The drive for thirst must be absent, or the person must not have access to water.

When [Na] is high due to low H20, when the osmolarity first increases, ADH is activated. If this is overwhelmed, thirst kicks in

41
Q

If a person is hypernatremic AND hypovolemic, what are likely causes?

A
  1. Renal loss
    - Osomotic = hyperglycemia, mannitol, relief of acute obstruction
    - Diuretics
  2. Extrarenal loss- GI tract (diarrhea/vomit)
42
Q

If a person has hypernatremia and isovolemia, what are likely causes?

A
  1. Renal loss
    - Central DI
    - nephrogenic DI
  2. extrarenal loss
    - respiratory
    - skin
43
Q

If a person has hypernatremia and hypervolemia, what are likely causes?

A
  1. Mineralocorticoid excess
  2. Iatrogenic
    - hypertonic saline
    - after MI with hypertonic NaHCO3
44
Q

If a patient is having diarrhea or is vomiting, what is likely to happen to their hydration status and volume?

A

They will be hypernatremic and hypovolemic.

45
Q

What are 4 extrarenal situations that can cause hypovolemic hypernatremia?

A
  1. vomiting
  2. diarrhea
  3. burns
  4. fever
46
Q

Enhanced renal water loss&raquo_space; Na loss can occur under what 3 situations?
How does a patient with this present?

A
  1. osmotic diuresis
  2. Central DI
  3. Nephrogenic DI

The patient will present with polydypsia and polyuria

47
Q

How can you differentiate central DI from nephrogenic DI?

What are common causes of each?

A

Central- ADH is not released from posterior pituitary

  • tumors, granulomas
  • infection
  • sarcoidosis
  • trauma (most common)

Nephrogenic- ADH levels are fine but urine is inappropriately dilute

  • urinary obstruction
  • low K
  • high Ca
  • amyloidosis
48
Q

How do you differentiate primary polydypsia, central DI and nephrogenic DI?

A
  1. Water deprivation
    - PP will concentrate urine, the other 2 will not
  2. Exogenous ADH
    - CDI will concentrate urine, NDI will not
49
Q

What is the effect of acute and chronic hyponatremia on cell size?
What does this mean for treatment strategy?

A

Acute hyponatremia- cells will swell to restore osmotic balance (this can cause brain edema)

Chronic- cells transport K, Cl and AA out to decrease intracellular osmols and causing water to move out to restore brain size

Na should be restored at the rate that it fell using hypertonic saline.
Acute- quickly
Chronic- no faster than 12mEq/day (usually 6-8)

50
Q

What is the negative effect of restoring Na too quickly in chronic hyponatremia?

A

The cells (which have transported osmoles out of the cell and have returned to normal size) will lose even more water now and shrink.

This can cause osmotic demyelination syndrome (pontine myelinolysis)

51
Q

What are examples of acute hyponatremia (<48hrs)?

A
  1. marathon runner that drinks water
  2. hypotonic saline
  3. ecstasy (triggers ADH and thirst at the same time)
52
Q

What equation determines how much Na is required to restore a person with hyponatremia?
What do you add if volume expansion is occuring during the Na restoration?

A

Na/h = [Na]/h x TBW

If volume expansion, add furosemide

53
Q

What should the rate of correction for chronic hypernatremia be?

A

Water should be administered at a rate that leads to half correction in 24 hours.

TBWcurrent X Na current = TBW normal x Na normal

Renal (11 decks)

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