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Acid-base & electrolytes > Acid-base > Flashcards

Acid-base Flashcards

1
Q

What is the name of the equation correlating pH to HCO3 and pCO2? What is the equation?

A

Henderson-Hasselbalch equation

pH = 6.1 + log (HCO3 / 0.03*pCO2)

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2
Q

What is the carbonic acid equation

A

CO2 + H2O = H2CO3 = H+ + HCO3-

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3
Q

Define base excess

A

Base excess is the amount of strong acid that needs to be added to 1L of fully oxygenated whole blood to restore the pH to 7.4 at 37°C with a pCO2 of 40 mmHg

(Calculated based on a human algorithm)

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4
Q

What does total CO2 indicate

A

The metabolic acid-base component (mostly HCO3). Usually 1-2 mmol/L higher than HCO3

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5
Q

What can falsely decrease the anion gap

A

Hypoalbuminemia

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6
Q

What is the degree of compensation expected for a metabolic acidosis / alkalosis

A
  • Acidosis: pCO2 decrease of 0.7 mmHg per 1 mmol/L decrease in HCO3 +/- 3 mmHg
  • Alkalosis: pCO2 increase of 0.7 mmHg per 1 mmol/L decrease in HCO3 +/- 3 mmHg
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7
Q

What is the expected degree of compensation for an acute / chronic respiratory alkalosis / acidosis

A
  • Acute respiratory acidosis: HCO3 increase of 0.15 mmol/L per 1 mmHg increase in pCO2 +/- 2 mmol/L
  • Chronic respiratory acidosis: HCO3 increase of 0.35 mmol/L per 1 mmHg increase in pCO2 +/- 2 mmol/L
  • Acute respiratory alkalosis: HCO3 decrease of 0.25 mmol/L per 1 mmHg increase in pCO2 +/- 2 mmol/L
  • Chronic respiratory alkalosis: HCO3 decrease of 0.55 mmol/L per 1 mmHg increase in pCO2 +/- 2 mmol/L
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8
Q

Why does hypokalemia contribute to metabolic alkalosis

A
  • Hypokalemia promotes renal acid loss
  • Hypokalemia promotes the efflux of K+ from intracellular to extracellular in exchange for H+
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9
Q

What is the prevalence of metabolic alkalosis and metabolic acidosis in cats and dogs

A

Metabolic acidosis: 43%
Metabolic alkalosis: 15%

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10
Q

What are alternative alkalinizing therapies

A
  • Trishydroxymethyl aminomethane (tromethamine = THAM)
  • Equimolar mixture of sodium bicarbonate and sodium carbonate (Carbicarb)
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11
Q

List 6 adverse effects of bicarbonate therapy

A
  • Paradoxical intracellular acidosis
  • Increased Hb affinity for O2 (left shift of dissociation curve)
  • Hypervolemia
  • Hypernatremia, hyperosmolarity
  • Ionized hypocalcemia and hypomagnesemia
  • Hypokalemia
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12
Q

Explain the paradoxical intracellular acidosis resulting from bicarbonate administration

A
  • HCO3- dissociates to CO2 and H2O
  • CO2 crosses the cell membranes and reaches the intracellular space (HCO3- does not)
  • In the cell the carbonic acid equation leads to CO2+H2O -> H+ + HCO3-
  • H+ accumulation causes intracellular acidosis
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13
Q

What is the bicarbonate deficit equation

A

HCO3 deficit (mmol) = 0.3 * body weight (kg) * base excess (mmol/L)

or HCO3 deficit (mmol) = 0.3 * body weight (kg) * (normal HCO3 - patient HCO3) (mmol/L)

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14
Q

What is the osmolality of sodium bicarbonate and the appropriate dilution

A

Osmolality of 2000 mOsm/L

Should be diluted 1:3 to be < 600 mOsm/L for peripheral administration

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15
Q

What point-of-care parameters can be assessed via an IO sample

A

BUN, TS, bilirubin, Na, Cl, glucose, blood gases (pH, PO2, pCO2) are similar to central venous

K and PCV/Hct not always reliable

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16
Q

What is the effect of body temperature on PO2 and PCO2

A

Increases in temperature will increase gas partial pressures (hypothermic patients will have lower PO2 and PCO2 in vivo than what is measured)

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17
Q

What can falsely increase / decrease:
- PaO2
- PCO2
- Na
- iCa
- K
- Lactate
- Glucose

A
  1. PaO2
    - Increase: prolonged pre-analytical time with environmental exposure, air bubbles ; overdilution with liquid heparin
    - Decrease: environmental exposure when PaO2 supposed to be >160 (=atmospheric PaO2), leukocytosis
  2. PCO2:
    - Increase: -
    - Decrease: prolonged pre-analytical time with environmental exposure, air bubbles
  3. Na:
    - Increase: hypoproteinemia
    - Decrease: hyperlipidemia, hyperglycemia, hyperproteinemia
  4. iCa:
    - Increase: hypoproteinemia
    - Decrease: excess heparin, alkalemia (including post-sampling)
  5. K:
    - Increase: Hemolysis (esp Japanese / arctic breeds), thrombocytosis, leukocytosis
    - Decrease: -
  6. Lactate:
    - Increase: prolonged pre-analytical time, ethylene glycol metabolites
    - Decrease: -
  7. Glucose:
    - Increase: -
    - Decrease: prolonged pre-analytical time
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18
Q

What are the 3 determinants of acid-base balance in he Stewart approach

A
  • pCO2
  • Strong ion difference (SID): Na, Cl (Ca, K, Mg +/- lactate)
  • Nonvolatile weak acids (Atot): albumin, phosphate
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19
Q

What are the 5 possible metabolic acid-base abnormalities identified with the Stewart approach

A
  • Increased SID metabolic alkalosis
  • Decreased SID metabolic acidosis
  • Increased Atot metabolic acidosis
  • Decreased Atot metabolic alkalosis
  • Increased SIG metabolic acidosis
20
Q

What is the formula for the simplified SIG in cats and dogs? When does it need to be adjusted?

A

Dogs: SIG = albumin x 4.9 - AG
Cats: SIG = albumin x 7.4 - AG

In case of abnormal phosphorus, AG needs to be adjusted in the formula: AGp = AG + (2.52 -5.58*phosphorus)

21
Q

What is the general formula for SIG

A

SIG = SID - (HCO3 + Atot)

22
Q

How will SIG change with the addition of unmeasured anions? How about the simplified SIG?

A

SIG will increase (become more positive)

Simplified SIG will become more negative (because for some reason the formula is negative??)

23
Q

What are the parameters included in the semi-quantitative approach of metabolic acid-base disorders

A
  • Free water effect (Na)
  • Chloride effect
  • Albumin effect
  • Phosphorus effect
  • Lactate effect
24
Q

In the semi-quantitative approach, is a positive effect associated with alkalotic or acidotic influence

A

Alkalotic

25
Q

What are the simplified formulas of effects for the the semi-quantitative approach of metabolic acid-base disorders

A
  • Free water effect = (Na - mid-normal Na)/4
  • Chloride effect = mid-normal Cl - corrected Cl ; corrected Cl = Cl * (mid-normal Na / measured Na)
  • Phosphorus effect = (mid-normal P - P)/2
  • Albumin effect = 4*(mid-normal albumin - albumin)
  • Lactate effect = -1*lactate

Unmeasured ion effect = base excess - sum of effects

26
Q

What is the limitation of including lactate in the acid-base calculations

A

Lactate is not always associated with acidosis (when it is not due to anaerobic metabolism)

27
Q

What are possible unmeasured anions in the semi-quantitative approach

A

Ketoacids, sulphuric acid, ethylene glycol, salicylic acid, propylene glycol, metaldehyde

28
Q

What is the definition of a buffer

A

A molecule that can accept or donate protons and minimize a change in pH when H+ ions are added or removed from a solution

29
Q

Name physiologic buffers

A

Extracellular:
- Bicarbonate
- Phosphate - most important urinary buffer

Intracellular:
- Organic phosphates (2,3 -DPG, AMP, ATP, ADP)
- Proteins (ex: hemoglobin)

30
Q

What is the normal anion gap in dogs and cats

A

Dogs: 12-24 mEq/L

Cats: 13-27 mEq/L

31
Q

What are the mechanisms of metabolic acidosis in hypoadrenocorticism

A
  • Dilutional acidosis (hypoNa)
  • Decreased renal H+ excretion:
  • decreased aldosterone -> lack of activation of H+ transporter
  • decreased aldosterone -> decreased Na+ reabsorption -> positive tubular lumen -> inhibited H+ excretion
  • hyperK -> decreases NH4+ excretion)
  • Lactic acidosis
  • Hyperphosphatemia (from hypovolemia / decreased GFR)
32
Q

What are clinical scenarios where bicarbonate therapy is indicated? Not indicated?

A

Indicated: HCO3- loss (GI with diarrhea / renal with RTA)

Not indicated: DKA, lactic acidosis

33
Q

What were the results of the BICAR-ICU trial on the use of bicarbonates in critically ill patients with metabolic acidosis

A
  • No benefit of using NaHCO3 on 28-day mortality except for patients with AKI
  • Decreased need for RRT and vasopressors in patients receiving NaHCO3
  • More alkalosis, hypernatremia, and hypocalcemia in patients receiving NaHCO3 but no life threatening complications
34
Q

What is the name of the ventilation pattern caused by severe metabolic acidosis

A

Kussmaul breathing

35
Q

In the kidney, which mechanisms regulate HCO3 reabsorption?

A
  1. Increase in filtered HCO3 –> increased reabsorbed HCO3
    *** in metabolic alkalosis, filtered load may exceed reabsorptive capacity
  2. Hypercapnia –> more H+ secreted => more HCO3 reabsorbed
  3. Hypocapnia –> less H+ secreted => less HCO3 reabsorbed
  4. ECF contraction –> renin –> aldosterone –> H+ secretion –> HCO3 reabsorption
  5. ECF expansion –> decreased HCO3 reabsorption
  6. Angiotensin II –> increases HCO3 reabsorption
36
Q

In the kidney, which mechanisms regulate H+ secretion?

A
  1. Aldosterone –> increases H+ secretion via the H+-ATPase pump in the intercalated cells
  2. Diffusion trapping: H+ combines with NH3 in the lumen and is excreted as NH4+
    ** in acidosis, there is an adaptive increase in NH3 synthesis
  3. Hyperkalemia inhibits NH3 synthesis –> decreased H+ excretion
  4. Hypokalemia stimulates NH3 synthesis
37
Q

How can IV fluid therapy resolve alkalosis?

A

By adressing contraction alkalosis

Volume contraction activated RAAS system and aldosterone promotes reabsorption of HCO3 in the renal tubule –> contraction alkalosis

38
Q

How can respiratory alkalosis can cause signs of hypoacalcemia.

A

H+ and Ca2+ compete for binding sites on plasma proteins.

Decreased [H+] causes increased protein binding of Ca2+ and decreased free ionized Ca2+.

39
Q

What is the difference between base excess and standard base excess

A

Standard base excess is calculated for a Hgb of 5 g/dL (anemic blood) to account for the fact that Hgb has to buffer both the intravascular space and extravascular space.
SBE is supposed to be more representative of the acid-base status of the whole extra-cellular fluid vs. only blood.

40
Q

What are the major unmeasured ions in the normal animal?

A

Albumin & phosphorus

  • Anion Gap not reliable with abnormal albumin
41
Q

List 5 adverse effects of metabolic acidosis

A
  • Decreased myocardial contractility
  • Arterial vasodilation
  • Impaired coagulation
  • Increased work of breathing
  • Decreased renal and hepatic blood flow
  • Insulin resistance
  • Altered CNS function
42
Q

Why should sodium bicarbonate be administered slowly?

A

If administered rapidly, may cause vasodilation and increase ICP

43
Q

What is the advantage of SIG over AG?

A

SIG is independent of changes in albumin and is therefore more reliable in hypoalbuminemia

44
Q

What is the corrected chloride formula?

A

Measured Cl x (normal Na/measured Na)

45
Q

What is a limitation of the non traditional approaches to acid/base disturbances?

A

Do not recognize compensatory changes to respond disorders –> all changes are considered pathologic

Acid-base & electrolytes (3 decks)

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