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Clinical Pathology > Acid-Base > Flashcards

Acid-Base Flashcards

1
Q

In what 2 ways is acid-base status of a patient determined? What is measured and calculated in each?

A
  1. biochemistry profile - measured = TCO2; calculated anion gap —> NO INFORMATION ON RESP ACID-BASE DISTURBANCES OR COMPENSATION
  2. blood gas profile - measured = pH, pCO2, pO2; calculated = HCO3-, base excess
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2
Q

What is TCO2?

A

estimate of plasma bicarbonate concentration, with 95% of it accounting in the form of HCO3-

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3
Q

What are the 4 contributors to blood pH?

A
  1. pCO2 - changes with ventilation
  2. addition or removal of acids (H+) - lactated metabolites of ethylene glycol or loss of H+
  3. strong ion movements - Na, K, Cl
  4. serum proteins, phosphates, and other weak acids
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4
Q

Why is pH maintenance important?

A

proper pH is necessary for the maintenance of protein structure, metabolism, and enzyme conformation/action

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5
Q

What keeps hydrogen ion concentration in check? What are 3 examples?

A

buffers and regulatory systems

  1. bicarbonate system: HCO3-, pCO2
  2. non-bicarbonate system: PO4, NH3-, Hgb, albumin, plasma proteins
  3. renal excretion (H+) and alveolar ventilation (CO2)
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6
Q

What are the 3 classifications of acid-base disturbances?

A
  1. metabolic acidosis - titrational vs. secretional metabolic acidosis
  2. metabolic alkalosis
  3. mixed - both metabolic acidosis and alkalosis
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7
Q

How is pH interpreted on biochemistry? Metabolic disturbances?

A

ALKALEMIA = increased pH
ACIDEMIA = decreased pH

METABOLIC ACIDOSIS = decreased HCO3-/TCO2
METABOLIC ALKALOSIS = increased HCO3-/TCO2

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8
Q

What is base excess?

A

characterizes the overall metabolic acid-base status independent of the respiratory acid-base status (accounts for all arterial bases

  • the amount of bases that must be added or removed to return the pH to normal
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9
Q

What does HCO3- represent on biochemistry profiles?

A

amount of CO2 gas that is released from plasma/serum when mixed with a strong acid

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10
Q

What is an anion gap? How is it calculated?

A

the difference in cations and anions that is used to further characterize metabolic acidosis and approximate unmeasured anions and/or cations in circulation

AG = [Na + K] - [Cl + HCO3]

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11
Q

What does an increased anion gap indicate?

A

metabolic acidosis with a high number of unmeasured anions:

  • Ketones
  • Lactate
  • Uremic acid
  • Ethylene glycol metabolites
  • salicylates
  • hyperalbuminemia
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12
Q

What does metabolic acidosis with an elevated anion gap mean?

A

titrational metabolic acidosis

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13
Q

What does a decreased anion gap indicate? What are 2 causes?

A

metabolic alkalosis with increased cations —> RARE, not clinically significant

  1. decreased unmeasured anions - hypoalbuminemia
  2. increased unmeasured cations - hyperalbuminemia and hypermagnesemia
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14
Q

What is the law of electroneutrality?

A

the sum of all positively charged ions (cations) must be equal to the sum of all negatively charged ions (anions) in the body

  • HCO3- is excreted into the urine and Cl- is retained/reabsorbed
  • Na+ is reabsorbed, so H+ or K+ is excreted
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15
Q

What machinery is used for blood gas data? What blood is preferred? What tube should it be collected in?

A

electrochemical analyzers (in-house common)

arterial blood allows for complete evaluation of blood oxygenation; venous blood is common an adequate, too

whole blood collected into heparinized tube

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16
Q

What should be avoided so that pH of blood is not altered before blood gas data is collected? When should it be processed?

A

avoid exposure of blood to room temperature and gas bubbles during collection

process within 10 mins or put on ice and process within an hour if not immediate

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17
Q

What happens on blood gas analysis if there is a delay in sample processing? Air exposure?

A

DELAY = decreased pH and PO2

AIR EXPOSURE = increased pH and PO2, decreased PCO2 and HCO3-

(caused by continued glycolysis of RBCs)

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18
Q

What analytes are measured and calculated on blood gas analysis?

A

MEASURED - pH (H+ ions), pCO2 (respiratory), electrolytes (Ca, Mg, etc.), pO2

CALCULATED - HCO3-, base excess

19
Q

Why isn’t base excess commonly used to determine metabolic differences? What do different values mean?

A

doesn’t really add information —> can be used to guide fluid bicarbonate therapy

  • < 0 = metabolic acidosis
  • > 0 = metabolic alkalosis
20
Q

What do levels of pCO2 above/below RI indicate?

A

ABOVE RI = hypercapnia/hypercarbia, usually from hypoventilation —> RESPIRATORY ACIDOSIS

BELOW RI = hypocapnia/hypocarbia, usually from hyperventilation (panting) —> RESPIRATORY ALKALOSIS

21
Q

What is the normal blood pH?

A

7.35 - 7.45 —> 7.4 average

22
Q

What are the compensatory mechanisms used to control respiratory and metabolic changes that alter pH?

A

RESP = when changes in HCO3- occur, the lungs compensate by altering ventilation to change pCO2 - FAST

MET = when changes in pCO2 occur, the kidneys compensate by retaining or excreting HCO3- - SLOW

23
Q

What occurs during acidemia/alkalemia? How are they compensated?

A

ACIDEMIA:
- pCO2 increased (respiratory acidosis): HCO3- is increased by kidney (metabolic alkalosis)
- HCO3- decreased (metabolic acidosis): pCO2 is decreased by lung ventilation change (respiratory alkalosis)

ALKALEMIA:
- pCO2 decreased (respiratory alkalosis): HCO3- decreased by kidney (metabolic acidosis)
- HCO3- is increased (metabolic alkalosis): pCO2 is increased by lung ventilation change (respiratory acidosis)

24
Q

What are the 5 steps to approaching blood gas data?

A
  1. evaluate pH
  2. evaluate HCO3-
  3. evaluate pCO2
  4. determine what mechanism (resp/met) is primary and which is compensatory
  5. determine if compensation is adequate —> pH 7.4-7.6
25
Q

How can the mechanism of acidosis/alkalosis be determined as primary or compensatory?

A
  • primary mechanism if driving pH away from 7.4
  • compensatory mechanism moves in the opposite direction
  • if a difference is not noted, it is likely a mixed disturbance

combined abnormalities between HCO3- and pCO2 —> least abnormal value is almost always due to compensation response

26
Q

When are compensatory mechanisms not seen in pH differences?

A

if the process is very acute

27
Q

What are 4 common causes of respiratory acidosis? What values differ? How is it compensated?

A

retention of CO2 —> hypoventilation

  1. anesthesia
  2. diffuse pulmonary disease
  3. intrathoracic lesions
  4. CNS disease

decreased pH (acidemia) and increased pCO2 (acidosis) —> secondary metabolic alkalosis by retaining HCO3-

28
Q

What are 4 common causes of respiratory alkalosis? What values differ? How is it compensated?

A

release of CO2 —> hyperventilation

  1. hypoxemia
  2. pain, anxiety
  3. hyperthermia
  4. drugs that stimulate medullary respiratory center

increased pH (alkalemia) and decreased pCO2 (alkalosis) —> secondary metabolic acidosis by renal retention of H+

29
Q

What are the 2 mechanisms of metabolic acidosis?

A
  1. titrational metabolic acidosis - increase in acid
  2. secretional metabolic acidosis - loss of base
30
Q

What is titrational metabolic acidosis? What differential needs to be considered?

A

HCO3- lost due to titration of organic acid (it titrates the salt of the acid it titrates)

increased nonvolatile acids - Ketones, Lactate, Uremic acids (phosphates, sulfated), Ethylene glycol metabolites (toxins)

31
Q

What is secretional metabolic acidosis? What are 4 differentials?

A

HCO3- is lost through bicarbonate-rich fluids (NaHCO3 or KHCO3), resulting in the retention of HCl

  1. GI loss from diarrhea
  2. intestinal ileus
  3. salivation in ruminants
  4. urinary loss by tubules
32
Q

What are the 2 types of metabolic acidosis compensation?

A
  1. SHORT-TERM: respiratory alkalosis by increased ventilation
  2. LONG-TERM: increased renal excretion of H+
33
Q

What are the 3 biochemical findings in titrational metabolic acidosis?

A
  1. decreased TCO2
  2. elevated anion gap
  3. Cl- within RI
34
Q

What are the 3 biochemical findings in secretional metabolic acidosis?

A
  1. decreased TCO2
  2. normal anion gap
  3. Cl- within RI or increased due to excess of Na+ (hyperchloremic metabolic acidosis)
35
Q

What are the 2 mechanisms of metabolic alkalosis?

A
  1. loss of acid (HCl-rich fluids)*
  2. increase in base
36
Q

What are 3 differentials related to excessive renal loss of H+ causing metabolic alkalosis?

A
  1. diuretics
  2. hypokalemia that stimulated H+, K+ ATPase pump —> K+ retention, H+ secretion, HCO3- generation
  3. chronic respiratory acidosis
37
Q

What are the differentials in hypochloremic metabolic alkalosis in monogastrics and ruminants?

A

MONOGASTRIC: severe gastric vomiting, pyloric outflow obstruction

RUMINANTS: sequestration of fluid in the abomasum due to ileus or displacement

38
Q

What are 4 findings associated with metabolic alkalosis?

A
  1. increased TCO2
  2. normal anion gap
  3. decreased Cl- in excess of Na+
  4. hypochloremic metabolic alkalosis
39
Q

What are the 2 compensatory responses to metabolic alkalosis?

A
  1. SHORT-TERM: respiratory acidosis by decreased ventilation
  2. LONG-TERM: increased renal retention of H+
40
Q

What is paradoxcial aciduria? What causes it? How is it treated?

A

acidic urinary pH when there is metabolic alkalosis

  • hypchloremia causes HCO3-, instead of Cl-, to be resorbed along with Na+
  • Na+ resorption is coupled with secretion of H+, which acidicfies the urine

correcting NaCl deficit with appropriate fluids

41
Q

What are 3 common cases where mixed acid-base disturbances should be considered?

A
  1. high anion gap with normal TCO2
  2. Cl- is lost in excess of Na+, but TCO2 is normal
  3. Cl- is high in excess of Na+, but TCO2 is normal
42
Q

A diabetic ketoacidotic dog is vomiting and has a high anion gap with normal TCO2 and low Cl-. What is likely going on?

A

titrational metabolic acidosis AND hypochloremic metabolic alkalosis —> mixed disturbance

  • acidosis from ketones
  • alkalosis from vomiting
43
Q

What are mixed acid-base disturbances? When should patients be suspected of this?

A

2 primary acid-base disturbances are occurring simultaneously

electrolyte disturbances and/or….

  • disease associated with an acid-base disturbance, yet has a normal bicarbonate concentration or pH
  • compensatory response is moving in the wrong direction
44
Q

CASE: 2 day old female calf is septic. The patient has diffuse diarrhea (scours) and pneumonia. What is going on based on the biochemistry profile and blood gas analysis?

A
  • low pH = acidemia
  • high pCO2 = primary respiratory acidosis
  • low HCO3- = primary metabolic acidosis
  • 2 primary changes are driving the acidemia, not a primary change with compensatory change

high anion gap supports titrational metabolic acidosis, (lactate) and sodium/chloride are decreased proportionally —> no acid-base change

Clinical Pathology (33 decks)

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