Electrolytes & Acid-Base Balance

Question 1

Control of Cell Excitability
K controls…
Ca controls…

Answer 1

RMP (-90)

Threshold (-60)

Question 2

Does hypocalcemia or hypercalcemia lead to an increase in nerve and muscle excitability?

Answer 2

Hypocalcemia - threshold (-60) becomes more negative and shifts toward the RMP (-90)
*Parathyroid glands removed - hypocalcemia - risk of larynospasm

Question 3

Does hypokalemia or hyperkalemia cause cells to become more excitable?

Answer 3

Hyperkalemia - RMP (-90) becomes more positive and moves toward threshold (-60)
*Makes sense why we give Ca in the setting of hyperkalemia (increase threshold)

Question 4

Does alkalosis or acidosis cause a decrease in free ionized calcium levels?

Answer 4

Acute respiratory alkalosis
Causes a functional hypocalcemia (total calcium doesn’t change, free ionized active calcium levels decrease)
Hyperventilate - H concentration decreases - H breaks away from proteins to become active - now there is more protein to bind to ionized Ca

Question 5

Therapies for Treating Hyperkalemia

Answer 5

1. Give Ca in order to make threshold more positive (RMP is more positive)
2. Give HCO3 - alkaline urine = increases K secretion, increased HCO3 - decreased H (metabolic alkalosis) - H shifts OUT cells + K shifts IN cells
3. Hyperventilate - respiratory alkalosis - H shifts OUT cells + K shifts IN cells
4. Insulin + glucose - stimulates the Na-K pump
5. Beta-2 agonist - stimulates the Na-K pump
6. Dialyze

Question 6

For each 10 mmHg decrease in PaCO2, serum K decreases by ____mEq/L.

Answer 6

0.5 mEq/L

Question 7

Why do you see PVCs with hypokalemia?

Answer 7

Hypokalemia - RMP hyperpolarizes - excitability is decreased
Exception: purkinje cells more readily depolarize to cause the PVCs

Question 8

Hyperventilation causes what 2 electrolyte abnormalities?

Answer 8

1. Hypokalemia

2. Hypocalcemia (functional)

Question 9

Can complete compensation be achieved if there is metabolic acidosis or metabolic alkalosis?

Answer 9

NO

If it is completely compensated, then it is a respiratory disturbance

Question 10

What is the fundamental event in the kidney’s regulation of acid-base balance?

Answer 10

Na-H exchange
Permits Bicarb to be reabsorbed
Permits acids to be excreted (H, NH3 to NH4 diffusion trapping)

Question 11

Is HCO3 normally excreted?

Answer 11

NO
90% is reabsorbed from the PCT
10% is reabsorbed in later segments

Question 12

Describe the Na-H exchange.

Answer 12

Na is being reabsorbed
HCO3 is being reabsorbed
H is being secreted in urine
Carbonic anhydrase plays a role

Question 13

Anion Gap Formula

What is the normal anion gap?

Answer 13

AG = Na - Cl + HCO3 
HCO3 = Na - Cl - AG 
Normal = 12 (this accounts for the unmeasured anions) 

Diagnosis of metabolic acidosis
High AG - uremia, lactic acidosis, ketoacidosis, ingested acids
Normal AG = hyperchloremic acidosis diarrhea, carbonic anhydrase inhibitors

Question 14

What does acidosis do to neuronal activity? What does acidosis do to the seizure threshold?

Answer 14

Depresses neuronal activity

Raises the seizure threshold - more difficult to have a seizure

Question 15

What is the acid-base status of the hypothermic patient?

Answer 15

Metabolic acidosis

Question 16

What is the major intracellular buffer?

Answer 16

Proteins

Question 17

The kidneys excrete H as what 2 types of acids?

Answer 17

1. H2PO4

2. NH4

Question 18

What is the most common cause of normal anion gap metabolic acidosis?

Answer 18

Diarrhea

Question 19

What % of body weight is water?

Answer 19

60%

Question 20

What fraction of total body water is found in the extracellular space?

Answer 20

1/3

Question 21

Sodium salts represent what % of all electrolytes in the ECF?

Answer 21

90%

Question 22

Sodium concentration is regulated by what 2 factors?

Answer 22

1. Thirst
2. ADH

• ADH - osmolality - CONCENTRATION of Na
• Aldosterone - volume - AMOUNT of Na

Question 23

What is the most common electrolyte disturbance in the hospitalized patient?

Answer 23

Hyponatremia < 135

If 120, severe s/s

Question 24

Which hormone works faster: ADH or aldosterone?

Answer 24

ADH 5-10 mins

aldosterone 30-45 min

Question 25

What is the fastest way to correct hyperkalemia?

Answer 25

Sodium bicarb
Then insulin-glucose, beta 2 agonists
*Calcium is fast, BUT remember calcium does NOT correct hyperkalemia - it stabilizes membrane excitability

Question 26

Does acute respiratory alkalosis/metabolic alkalosis cause hyperpolarization or depolarization of the RMP?

Answer 26

Hyperpolaization
H moves from cells to plasma
K moves from plasma to cells
Hypokalemia (more K now wants to leave the cell) - RMP becomes more negative

Question 27

What is the hallmark of hypocalcemia?

Answer 27

Neuronal membrane irritability

Question 28

Chvostek’s sign is associated with what?

Answer 28

Hypocalcemia

Contraction of the facial muscle produced by tapping the facial nerve

Question 29

Which ion both resembles and antagonizes calcium?

Answer 29

Magnesium
Membrane stabilize like Ca
*An important function of Mag is the regulation of the release of Ach from nerve terminals

Question 30

What are the s/s of severe hyperphosphatemia?

Answer 30

S/S of HYPOcalcemia

Question 31

What is the most common cause of hypocalcemia?

Answer 31

Hypoalbuminemia