Acid-Base Flashcards
In what 2 ways is acid-base status of a patient determined? What is measured and calculated in each?
- biochemistry profile - measured = TCO2; calculated anion gap —> NO INFORMATION ON RESP ACID-BASE DISTURBANCES OR COMPENSATION
- blood gas profile - measured = pH, pCO2, pO2; calculated = HCO3-, base excess
What is TCO2?
estimate of plasma bicarbonate concentration, with 95% of it accounting in the form of HCO3-
What are the 4 contributors to blood pH?
- pCO2 - changes with ventilation
- addition or removal of acids (H+) - lactated metabolites of ethylene glycol or loss of H+
- strong ion movements - Na, K, Cl
- serum proteins, phosphates, and other weak acids
Why is pH maintenance important?
proper pH is necessary for the maintenance of protein structure, metabolism, and enzyme conformation/action
What keeps hydrogen ion concentration in check? What are 3 examples?
buffers and regulatory systems
- bicarbonate system: HCO3-, pCO2
- non-bicarbonate system: PO4, NH3-, Hgb, albumin, plasma proteins
- renal excretion (H+) and alveolar ventilation (CO2)
What are the 3 classifications of acid-base disturbances?
- metabolic acidosis - titrational vs. secretional metabolic acidosis
- metabolic alkalosis
- mixed - both metabolic acidosis and alkalosis
How is pH interpreted on biochemistry? Metabolic disturbances?
ALKALEMIA = increased pH
ACIDEMIA = decreased pH
METABOLIC ACIDOSIS = decreased HCO3-/TCO2
METABOLIC ALKALOSIS = increased HCO3-/TCO2
What is base excess?
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
What does HCO3- represent on biochemistry profiles?
amount of CO2 gas that is released from plasma/serum when mixed with a strong acid
What is an anion gap? How is it calculated?
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]
What does an increased anion gap indicate?
metabolic acidosis with a high number of unmeasured anions:
- Ketones
- Lactate
- Uremic acid
- Ethylene glycol metabolites
- salicylates
- hyperalbuminemia
What does metabolic acidosis with an elevated anion gap mean?
titrational metabolic acidosis
What does a decreased anion gap indicate? What are 2 causes?
metabolic alkalosis with increased cations —> RARE, not clinically significant
- decreased unmeasured anions - hypoalbuminemia
- increased unmeasured cations - hyperalbuminemia and hypermagnesemia
What is the law of electroneutrality?
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
What machinery is used for blood gas data? What blood is preferred? What tube should it be collected in?
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
What should be avoided so that pH of blood is not altered before blood gas data is collected? When should it be processed?
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
What happens on blood gas analysis if there is a delay in sample processing? Air exposure?
DELAY = decreased pH and PO2
AIR EXPOSURE = increased pH and PO2, decreased PCO2 and HCO3-
(caused by continued glycolysis of RBCs)
What analytes are measured and calculated on blood gas analysis?
MEASURED - pH (H+ ions), pCO2 (respiratory), electrolytes (Ca, Mg, etc.), pO2
CALCULATED - HCO3-, base excess
Why isn’t base excess commonly used to determine metabolic differences? What do different values mean?
doesn’t really add information —> can be used to guide fluid bicarbonate therapy
- < 0 = metabolic acidosis
- > 0 = metabolic alkalosis
What do levels of pCO2 above/below RI indicate?
ABOVE RI = hypercapnia/hypercarbia, usually from hypoventilation —> RESPIRATORY ACIDOSIS
BELOW RI = hypocapnia/hypocarbia, usually from hyperventilation (panting) —> RESPIRATORY ALKALOSIS
What is the normal blood pH?
7.35 - 7.45 —> 7.4 average
What are the compensatory mechanisms used to control respiratory and metabolic changes that alter pH?
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
What occurs during acidemia/alkalemia? How are they compensated?
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)
What are the 5 steps to approaching blood gas data?
- evaluate pH
- evaluate HCO3-
- evaluate pCO2
- determine what mechanism (resp/met) is primary and which is compensatory
- determine if compensation is adequate —> pH 7.4-7.6
How can the mechanism of acidosis/alkalosis be determined as primary or compensatory?
- 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
When are compensatory mechanisms not seen in pH differences?
if the process is very acute
What are 4 common causes of respiratory acidosis? What values differ? How is it compensated?
retention of CO2 —> hypoventilation
- anesthesia
- diffuse pulmonary disease
- intrathoracic lesions
- CNS disease
decreased pH (acidemia) and increased pCO2 (acidosis) —> secondary metabolic alkalosis by retaining HCO3-
What are 4 common causes of respiratory alkalosis? What values differ? How is it compensated?
release of CO2 —> hyperventilation
- hypoxemia
- pain, anxiety
- hyperthermia
- drugs that stimulate medullary respiratory center
increased pH (alkalemia) and decreased pCO2 (alkalosis) —> secondary metabolic acidosis by renal retention of H+
What are the 2 mechanisms of metabolic acidosis?
- titrational metabolic acidosis - increase in acid
- secretional metabolic acidosis - loss of base
What is titrational metabolic acidosis? What differential needs to be considered?
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)
What is secretional metabolic acidosis? What are 4 differentials?
HCO3- is lost through bicarbonate-rich fluids (NaHCO3 or KHCO3), resulting in the retention of HCl
- GI loss from diarrhea
- intestinal ileus
- salivation in ruminants
- urinary loss by tubules
What are the 2 types of metabolic acidosis compensation?
- SHORT-TERM: respiratory alkalosis by increased ventilation
- LONG-TERM: increased renal excretion of H+
What are the 3 biochemical findings in titrational metabolic acidosis?
- decreased TCO2
- elevated anion gap
- Cl- within RI
What are the 3 biochemical findings in secretional metabolic acidosis?
- decreased TCO2
- normal anion gap
- Cl- within RI or increased due to excess of Na+ (hyperchloremic metabolic acidosis)
What are the 2 mechanisms of metabolic alkalosis?
- loss of acid (HCl-rich fluids)*
- increase in base
What are 3 differentials related to excessive renal loss of H+ causing metabolic alkalosis?
- diuretics
- hypokalemia that stimulated H+, K+ ATPase pump —> K+ retention, H+ secretion, HCO3- generation
- chronic respiratory acidosis
What are the differentials in hypochloremic metabolic alkalosis in monogastrics and ruminants?
MONOGASTRIC: severe gastric vomiting, pyloric outflow obstruction
RUMINANTS: sequestration of fluid in the abomasum due to ileus or displacement
What are 4 findings associated with metabolic alkalosis?
- increased TCO2
- normal anion gap
- decreased Cl- in excess of Na+
- hypochloremic metabolic alkalosis
What are the 2 compensatory responses to metabolic alkalosis?
- SHORT-TERM: respiratory acidosis by decreased ventilation
- LONG-TERM: increased renal retention of H+
What is paradoxcial aciduria? What causes it? How is it treated?
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
What are 3 common cases where mixed acid-base disturbances should be considered?
- high anion gap with normal TCO2
- Cl- is lost in excess of Na+, but TCO2 is normal
- Cl- is high in excess of Na+, but TCO2 is normal
A diabetic ketoacidotic dog is vomiting and has a high anion gap with normal TCO2 and low Cl-. What is likely going on?
titrational metabolic acidosis AND hypochloremic metabolic alkalosis —> mixed disturbance
- acidosis from ketones
- alkalosis from vomiting
What are mixed acid-base disturbances? When should patients be suspected of this?
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
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?
- 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
