Electrolyte Disorders

Question 1

serum osmolality?

Answer 1

2(Na) + glucose/18 + BUN/2.8

when serum osmolality is lower than urine osmolality = hypotonic

Question 2

why is serum sodium low in dehydrated patient?

Answer 2

The low BV activates ADH secretion with avid water retention which
outperforms the retention of Na to yield hyponatremia, especially if the patient has increased free water intake. Almost all cases of hyponatremia (except for PP, etc) involve a relative excess of ADH, which may be appropriate or inappropriate.

Question 3

isotonic serum osmolality?

Answer 3

280-295 mosm

= pseudohyponatremia, due to extra fat and protein

• isotonic hyponatremia:
  1. hyperproteinemia (myeloma)
  2. hyperlipidemia (chylomicrons and triglycerides)

Question 4

hypotonic serum osmolality?

Answer 4

<280 moms/kg

• hypotonic hyponatremia: must evaluate volume status to decide if dehydrated

Question 5

hypertonic serum osmolality?

Answer 5

> 296 mosm/kg

• hypertonic hyponatremia = due to extra carbohydrates
  1. hyperglycemia
  2. mannitol, sorbitol glycerol, maltose
  3. radiocontrast agents
  4. ethylene glycol, methanol

Question 6

hypotonic hypovolemia hyponatremia

Answer 6

Una 20 = renal salt loss

1. diuretics, ACEIs
2. nephropathies
3. Addisons
4. partial obstruction
5. Type IV RTA

Question 7

hypotonic euvolemia hyponatremia

Answer 7

Una>20

1. SIADH
2. Pyschogenic polydipsis
3. hypothyroidism
4. idiosyncratic drug reaction (thiazides, ACEIs, NSAIDS)
5. adrenocorticotropin deficiency

Question 8

hypotonic hypervolemic hyponatremia

Answer 8

Una < 10

Edematous states:

1. CHF
2. liver disease
3. nephrotic syndrome
4. advanced kidney disease - though see Una>20

in liver disease see Hypoalbuminemia and decreased effective plasma volume with
increased sympathetic tone, decreased RBF and GFR, and increased
RAAS.

Question 9

EKG for dehydrated patients?

Answer 9

due to hypotonic hypovolemia hyponatremia –> increased RAAS –> increased ALDO –> hypokalemia, alkalosis

see flattened and inverted T waves with U waves

HCO3 is high due to contraction alkalosis

Question 10

high BUN/Cr ratio?

Answer 10

occurs in prerenal azotemia due to the decreased GFR with avid Na and H2O and BUN (passive) reabsorption in the proximal tubule. At the same time creatinine is actually secreted in the proximal tubule since the kidney is functioning normally. The result is a high BUN/creatinine ratio.

Question 11

causes of high BUN:Cr ratio?

Answer 11

High protein intake or breakdown:
Catabolic state
Catabolic drugs, ie tetracycline, steroids
GI bleed

Pre renal disease: 
         Dehydration
         CHF
         Shock
         Glomerulonephritis – looks like pre renal to the kidney! 

Post renal disease: should be able to feel on exam
Prostatic obstruction
Ureteral obstruction

Question 12

BUN/creatinine of 10/1

Answer 12

indicates either normal renal function (when creatinine is 1 or less) or intrinsic renal disease (when the creatinine is > 1), ie. BUN of 40 and creatinine of 4 = 40/4 = I0/1

Question 13

BUN/creatinine ratio > 10/1

Answer 13

BUN of 30 with creatinine of 1 = Prerenal (including glomerulo - nephritis), or postrenal azotemia, or catabolic state

Question 14

BUN/creatinine <10/1

Answer 14

(non-renal), ie low BUN seen in liver failure, malnutrition, overhydration, pregnancy, SIADH

Question 15

what are the causes of ATN?

Answer 15

urine sediment with “muddy brown” pigmented granular casts and renal tubular epithelial cells

Ischemia: 
     post op
     **shock, sepsis
     pancreatitis 
     **hypophosphatemia

Toxins: exogenous and endogenous
     aminoglycosides
     vancomycin
     cyclosporine
     radiographic contrast media

 **myoglobinuria
 hemoglobin
 hyperuricemia
 Bence Jones protein

Question 16

EKG seen during hyperkalemia?

Answer 16

prolonged PR interval with peaked T waves

Question 17

hypophosphatemia

Answer 17

causes decreased 2,3 DPG
2,3 DPG is necessary for Hgb to release O2, if not there then it results in hypoxia –> rhabdomylysis –> renal tubular necrosis