SM_173a: Acid Base

Question 1

The major buffer pair in bodily fluids is ______ and _______

Answer 1

The major buffer pair in bodily fluids is bicarbonate and carbonic acid

CO₂ + H₂O <-> H₂CO₃ <-> HCO₃^- + H⁺

Question 2

What is the Henderson-Hasselbach equation?

Answer 2

pH = 6.1 + log₁₀ ( [HCO₃^-) / [H₂CO₃] ) = 6.1 + log₁₀ ( [HCO₃^-) / (0.03 P_aCO₂) )

Question 3

Increased minute ventilation causes P_aCO₂ to _____, which _____ pH

Answer 3

Increased minute ventilation causes P_aCO₂ to decrease, which raises pH

(minute ventilation: V·_E = RR *V_T)

(HCO₃^- changes require renal compensation which requires time)

Question 5

Normal [H⁺} is ____, which corresponds to pH of ____

Answer 5

Normal [H⁺​} is 40 nEq/L, which corresponds to pH of 7.4

Question 6

What is the simplified version of the Henderson-Hasselbach equation?

Answer 6

X = 24 (P_aCO₂ / HCO₃^-) with the last two digits of pH equal to 80 - X

• Example: if [H⁺] is 40 nEq/L then 80-40 = 40 so pH is 7.40
• Example: if [H⁺] is 50 nEq/L then 80-50 = 30 so pH is 7.30

Question 7

What does each space in the electrolyte tree represent?

Answer 7

Question 8

How do you calculate the anion gap?

Answer 8

AG = Na - (Cl + HCO₃^-)

Question 9

Normal anion gap is ____ mEq/L

Answer 9

Normal anion gap is 10 ± 2 mEq/L

Question 10

______ or ______ increases the anion gap

Answer 10

Increase in unmeasured anions (such as with acid accumulation) or decrease in unmeasured cations increases the anion gap

(unmeasured means that the electrolyte or protein involved is not Na, Cl, or HCO₃₎

Question 11

______ or ______ decreases the anion gap

Answer 11

Decrease in unmeasured anions (such as with hypoalbuminemia) or increase in unmeasured cations decreases the anion gap

(unmeasured means that the electrolyte or protein involved is not Na, Cl, or HCO3)

Question 12

If pH and P_aCO₂ are on _____ sides of normal, think respiratory process

Answer 12

If pH and P_aCO₂​ are on opposite sides of normal, think respiratory process

Question 13

In respiratory acidosis, pH is ____ and P_aCO₂​ is ____

Answer 13

In respiratory acidosis, pH is decreased and P_aCO₂ is increased

(7.32/50 and 7.24/60)

Question 14

In respiratory alkalosis, pH is ____ and P_aCO₂​​ is ____

Answer 14

In respiratory alkalosis, pH is increased and P_aCO₂​​ is decreased

(7.48/30 and 7.56/20)

Question 15

For every change in P_aCO₂ of _____ mmHg, the pH changes in the _____ direction by _____

Answer 15

For every change in P_aCO₂ of 10 mmHg, the pH changes in the opposite direction by 0.08

Question 16

In response to a respiratory process, _____ compensation increases or decreases _____ so pH approaches normal

Answer 16

In response to a respiratory process, renal compensation increases or decreases serum bicarbonate so pH approaches normal

(can take 3 days)

(example of fully compensated respiratory alkalosis: patient / PE and increased minute ventilation for 3 days has 7.46/20 (14) on day 3 and 7.50/20 (16) on day 2)

Question 17

When pH and P_aCO₂ are on the ____ side of normal, think metabolic process

Answer 17

When pH and P_aCO₂ are on the same side of normal, think metabolic process

Question 18

In metabolic acidosis, pH is _____ and P_aCO₂ is _____ if there is expected respiratory compensation

Answer 18

In metabolic acidosis, pH is decreased and P_aCO₂​ is decreased if there is expected respiratory compensation

(low serum bicarbonate)

Question 19

In metabolic alkalosis, pH is _____ and P_aCO₂​ is _____ if there is expected respiratory compensation

Answer 19

In metabolic alkalosis, pH is increased and P_aCO₂​ is increased if there is expected respiratory compensation

(high serum bicarbonate)

Question 20

Low serum bicarbonate suggests ______

Answer 20

Low serum bicarbonate suggests metabolic acidosis

Question 21

High serum bicarbonate suggests _____

Answer 21

High serum bicarbonate suggests metabolic alkalosis

Question 22

For metabolic acidosis, quick compensatory _____ elevates pH

Answer 22

For metabolic acidosis, quick compensatory hyperventilation elevates pH

(example of metabolic acidosis with appropriate respiratory compensation: 7.20/20 (8) - with compensation the last 2 digits of pH roughly equal P_aCO₂)

Question 23

The two types of metabolic acidosis are _____ and _____

Answer 23

The two types of metabolic acidosis are elevated anion gap and non-elevated anion gap

Question 24

Winter’s formula provides expected _____ when _____ is present

Answer 24

Winter’s formula provides expected P_aCO₂ when metabolic acidosis is present

P_aCO₂ = 1.5 (bicarbonate) + 8 ± 2

Question 25

Metabolic acidosis follows ____ accumulation or ____ loss

Answer 25

Metabolic acidosis follows acid accumulation or bicarbonate loss

Question 26

In metabolic acidosis, acid accumulation increases _____ and therefore the _____

Answer 26

In metabolic acidosis, acid accumulation increases unmeasured anions and therefore the anion gap

Ex: patient with alcoholic ketoacidosis

7.20/20 (8) 135 | 119

8

AG = 135 - (119+8) = 27

Question 27

Anion gap ≥ 20 mEq/L is highly suggestive of metabolic acidosis from _____

Answer 27

Anion gap ≥ 20 mEq/L is highly suggestive of metabolic acidosis from acid accumulation

MUDPILES: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid, lactic acidosis, ethylene glycol, salicylates

Question 28

Causes of metabolic acidosis from acid accumulation are described by the mnemonic _____

Answer 28

Causes of metabolic acidosis from acid accumulation are described by the mnemonic MUDPILES

M: methanol

U: uremia

D: diabetic ketoacidosis

P: propylene glycol

I: isoniazid

L: lactic acidosis

E: ethylene glycol

S: salicylates

Question 29

Metabolic acidosis caused by _____ involves NO anion gap increase and instead involves an increase in _____

Answer 29

Metabolic acidosis caused by bicarbonate loss involves NO anion gap increase and instead involves an increase in chloride

(hyperchloremic b/c chloride has increased)

Example: patient with diarrhea and loss of bicarbonate-rich fluid

7.20/20 (8) 137 | 119

8

AG = 137 - (119+8) = 10

Question 30

Non-elevated anion gap is highly suggestive of metabolic acidosis from _____

Answer 30

Non-elevated anion gap is highly suggestive of metabolic acidosis from bicarbonate loss

(increased chloride)

(USED CARS: uretero-enterostomy w/ bicarb-rich ostomy fluid, saline administration, endocrine disorders such as adrenal insufficiency, diarrhea, carbonic anhydrase inhibitors such as acetazolamide, ammonium chloride, renal tubular acidosis, spironolactone)

Question 31

Causes of metabolic acidosis from bicarbonate loss are described by the mnemonic _____

Answer 31

Causes of metabolic acidosis from bicarbonate loss are described by the mnemonic USED CARS

U: uretero-enterostomy w/ bicarb-rich ostomy fluid

S: saline administration

E: endocrine disorders (e.g. adrenal insufficiency)

D: diarrhea

C: carbonic anhydrase inhibitors (acetazolamide)

A: ammonium chloride

R: renal tubular acidosis

S: spironolactone

Question 32

Metabolic alkalosis with appropriate respiratory compensation involves _____ in both pH and P_aCO₂ and is primarily caused by _____

Answer 32

Metabolic alkalosis with apprpriate respiratory compensation involves increase in both pH and P_aCO₂ and is primarily caused by elevated serum bicarbonate

(causes hypoventilation, which increases P_aCO₂ and drops pH closer but completely to normal)

Example: 7.47/47 (34) 140 | 96

34

Question 33

The two types of metabolic alkalosis are _____ and _____

Answer 33

The two types of metabolic alkalosis are chloride responsive and chloride non-responsive

Question 34

Chloride responsive metabolic alkalosis is associated with _____

Answer 34

Chloride responsive metabolic alkalosis is asssociated with decreased extracellular volume

Question 35

_____ can cause chloride responsive metabolic alkalosis

Answer 35

Vomiting w/ volume depletion can cause chloride responsive metabolic alkalosis

Volume depletion signals the kidney to reabsorb NaCl and H₂O:

• If patient is chloride depleted from vomiting HCl, then NaHCO₃ is reabsorbed with H₂O to restore intravascular volume at the cost of alkalosis
• Body chooses to restore volume over maintaining normal pH -> urine output low, urine Na and Cl and FeNa low

Question 36

Chloride unresponsive metabolic alkalosis results from _____

Answer 36

Chloride unresponsive metabolic alkalosis results from excess aldosterone or hypokalemia

• Excess aldosterone causes H⁺ secretion in renal epithelial cells by activating Na/H exchange
• Hypokalemia causes intracellular shift of H⁺ for K⁺

Question 37

If each mEq/L of acid is titrated by the carbonic acid buffer system, serum bicarbonate will drop by ____

Answer 37

If each mEq/L of acid is titrated by the carbonic acid buffer system, serum bicarbonate will drop by 1 mEq/L

(example: if acid elevates AG to 22 mEq/L from 10 mEq/L, serum bicarobnate should drop from 24 mEq/L to 12 mEq/L)

Question 38

Delta anion gap should ____ delta bicarbonate unless other conditions are present

Answer 38

Delta anion gap should equal delta bicarbonate unless other conditions are present

Question 39

If serum bicarbonate is significantly lower than expected by delta-delta analysis, consider ______

Answer 39

If serum bicarbonate is significantly lower than expected by delta-delta analysis, consider a concurrent bicarbonate wasting condition (i.e. a non-elevated anion gap metabolic acidosis)

Question 40

If serum bicarbonate is significantly higher than expected by delta-delta analysis, consider ______

Answer 40

If serum bicarbonate is significantly higher than expected by delta-delta analysis, consider a concurrent bicarbonate excess state (i.e. a concurrent metabolic alkalosis)

Question 41

Delta-delta analysis is most helpful when anion gap is _____ because it allows one to identify a ______

Answer 41

Delta-delta analysis is most helpful when anion gap is elevated because it allows one to identify a concurrent non-elevated anion gap metabolic acidosis or metabolic alkalosis

Question 42

What are the steps for interpreting acid base problems?

Answer 42

1. Check electrolytes
2. Calculate anion gap
3. Perform delta-delta analysis