Fluid-Electrolyte Balance and Disturbances

Question 1

_________ controls threshold.

Answer 1

ionized calcium

Question 2

A _______ in plasma calcium concentration leads to an increase in nerve and muscle excitability because the threshold shifts towards the resting potential.

Answer 2

decrease (hypocalcemia)

Question 3

______ % of Calcium are in bones/teeth, the _____ % that is called ionized calcium is what controls all of the threshold potentials.

Answer 3

99%; 1%

Question 4

Can s\s of hypocalcemia be elicited when the patient hyperventilates?

Answer 4

Yes. hyperventilation causes a resp alkalosis. Ionized calcium decreases, thus eliciting s\s of hypocalcemia.

Question 5

Name in order of the fastest to slowest for treatment of hyperkalemia. (7)

Answer 5

1) give Ca++ (fastest, but does not correct the K+); only takes 1-2 min
2) administer HCO3-; takes 3-5 min; HCO3- decreases H+ concentration in plasma (metabolic alkalosis)–> H+ shifts out of cells to buffer alkalosis, and in exchange K+ shifts into cells
3) hyperventilate; 10-15 min; H+ concentration in plasma decreases (resp alkalosis)–> H+ shifts out to buffer the alkalosis in exchange for K+
4) give insulin-glucose–> insulin stimulates Na-K+ pump, driving K+ into cells; insulin also opens glucose channels; glucose is given with insulin to prevent hypoglycemia; ~30 min to work
5) administer a B2-agonist (stimulates Na-K pump); ~30 min to work
6) Give Kayexalate; min–> hours–> days
7) Dialyze patient–> min–> hours–> days
* loop diuretics take approx… 25-45 min to work

Question 6

For each _____mmHg decrease in PaCO2, serum K+ decreases ______ mEq/L.

Answer 6

10; 0.5

Question 7

Normal RMP for nerve cells is _____mV.

Answer 7

-70

Question 8

Normal RMP for muscle cells is _____mV.

Answer 8

-90

Question 9

Normal threshold for muscle cells is _____mV.

Answer 9

-60

Question 10

Cardioplegia solution used during CABG surgery has a high concentration of _____, and the cardiac cells depolarize to a level between threshold and 0mV.

Answer 10

K+; action potential is elicited and contraction occurs–> after that there is no electrical activity and Na+ gates are shut in inactivated state until normal K+ is restored

Question 11

Why are PVCs seen with hypokalemia?

Answer 11

hypokalemia leads to a decrease in excitability d\t hyperpolarization of the SA and AV node–> but there are ventricular Purkinje fibers that more readily depolarize to threshold and PVCs occur

Question 12

How does hyperventilation lead to the lowering of ionized calcium?

Answer 12

Both H+ and Ca+ bind to plasma proteins-> hyperventilation lowers H+ concentration causing protein to release H+ (law of mass action), thus freeing up Prot- which can then bind ionized calcium and correspondingly lower plasma concentration of ionized calcium

Question 13

S\S of what two electrolyte abnormalities may be manifested in the hyperventilated patient?

Answer 13

hypokalemia and hypocalcemia
*remember that with hyperventilation there may develop a TRUE hypokalemia b\c K+ gets pushed into the cells in exchange for H+ to buffer the resp alkalosis; but true hypocalcemia does NOT develop–> it just binds to the free proteins- (total calcium, ionized plus nonionized, does not change)–> so s\s of hypocalcemia may still develop b\c there is a decrease in FREE ionized calcium