Met 2: Nephro; Lec. 13 Disorders Of Salt And Water

Front 1

Are most hyponatremias due to a deficiency of sodium?

Back 1

No, most are due to a relative excess of water.  Very few are actually due to a sodium deficiency.

Front 2

What three organ systems are most often implicated in a high volume state (edema)?

Back 2

1) Kidneys: eg. kidney failure with reduced GFR and Na retention or nephrotic syndrome.

2) Cardiac failure with Na retention,

3) Liver failure with Na retention.

(don't forget pregnancy)

Front 3

Name three processes that can produce a low volume state.

Back 3

Hemmorage,
GI disease with salt and water loss,
Diuretic use,
Burns with skin fluid/Na loss,
Renal tubular diseases with partial failure of Na absorption.

Front 4

What does hyper-osmolality (above 280 mOsm/kg) usually manifest itself as, clinically?
What does it normally represent, physiologically?

Back 4

It usually manifests itself as hypernatremia and is a result of disproportionate loss of free water with relative sodium retention.  (e.g. with a deficit of ADH)

Front 5

Why does the total body potassium affect the serum [Na+]?

Back 5

Because water is in osmotic equilibrium, and potassium will preferentially go to the intracellular space, it will influence the amount of sodium in the serum (for instance, by pushing Na out of cells and into the extracellular space).

Front 6

Why do you need caution when treating a hyponatremic patient that's also hypokalemic?

Back 6

You want to increase [Na+] slowly to avoid a demylenating event, but if you also replace K at the same time, it will drive water into the cells, increasing [Na+] more than you calculated when only considering adding Na.

So, it's hard to know exactly what could happen."

Front 7

What are the two main processes that can generate a hypernatremia?

Back 7

- Unreplaced water/hypotonic fluid loss: e.g. sweat, vomitting, DI, osmotic diuresis like in DM.
- Salt overload in excess of water: hypertonic saline or bicarbonate administration; infant formula errors.

Front 8

What does hypernatremia indicate about the patient?

Back 8

either an impairment in the patient's thirst mechanism or the ability to respond to thirst

Front 9

What is a common cause of pseudohyponatremia? (i.e. with an acutally normal osmolarity)

Back 9

If it is administered without water, the result will be a mild hypernatremia as water is driven into the cells, following the potassium.

Front 10

What is a common cause of pseudohyponatremia? (i.e. with an acutally normal osmolarity)

Back 10

The common cause is an excess of mannitol or glucose in the serum, most often due to an uncontrolled DM.  The increased serum osmolality from the sugars drive water out of the cells, diluting [Na].

Front 11

How much does an increase in glucose actually affect the normal [Na+]?  (give numbers)"

Back 11

for every mM increase in blood glucose there will be a 3-4 mEq decrease in serum [Na]

Front 12

What is the most useful clinical approach for patients that are hyponatremic? (i.e. initial determination)

Back 12

Beginning by determining the volume status, whether the patient is:
Clinically dry,
Volume overloaded, or
Normal volume status.

Front 13

How can you estimate the volume status of a patient, clinically?  (give four sources)"

Back 13

JVP
BP including postural changes
skin turgor
presence of edema
presence of ascites
of findings of HF

Front 14

What is the cause of water retention in hyponatremic patients with volume depletion?

(i.e. despite having low plasma osm)

Back 14

It is an appropriate secretion of AVP in response to reduced circulating volume, as sensed by the baroreceptor cells.  

teleologically it may be useful to think of it as the body retaining fluid to maintain adequate BP at the cost of osmoregulation.

Front 15

How do thiazide diuretics lead to volume-depleted hyponatremias in some patients?

Back 15

Thiazide diuretics inhibit NaCl reabsorption in the distal convoluted tubule, the main diluting site of the nephron. Thus, thiazide diuretics interfere with maximum dilution of urine because sodium (Na+) excretion is increased along with diminished free-water excretion.  

This is maintained by the body's appropriate ADH response to the volume depletion, further impairing free water excretion and maintaining the hyponatremia.

Front 16

Why is ADH secretion increased in hyponatremic patients with volume overload?

Back 16

Although the total body Na and volume are high, the degree of filling in the arterial vascular bed is low (effective circulating volume), leading to the same physiological response as a hypovolemic hyponatremia.

(e.g. as in patients with forward cardiac failure)

Front 17

what is the cause of clinically dry hyponatremia?

And clinically wet hyponatemia?

Back 17

Tricked you! They are the same ADH is turned on so you aren't getting rid of the excess water

when clinically wet effective circulating volume is reduced

when clinically dry true circulating volume is reduced

Front 18

what conditions cause high volume hyponatremia?

Back 18

HF
cirrhosis
nephrotic syndrome

Front 19

What are four important causes of euvolemic hyponatremia?

Back 19

Pregnancy (resets osmostat or causes vasodilation)
Syndrome of Inappropriate ADH (SIADH), can be caused by drugs, CNS damage, cancer, or infection.
Hypocortisolism/Addison's disease.
Hypothyroidism.

Front 20

what does not maximally dilute urine tell you about
the hyponatemia?

and what is tricky about euvolumic hyponatremia?

Back 20

It tells you that ADH is present, which is to say that it effectively tells you nothing because ADH activation is the mechanism behind every category of hyponatremia

euvolumic hyponatremia is likely a lie. Patients are volume overloaded it's just that the volume is in the ICS and therefore undetectable.