SM 159a, SM 173a - Acid Base Flashcards
What does the isohydric principle state?
How does this apply to the buffer systems in the body?
The acid/base ratio of a buffer is determined by Ka and [H+]
Changes in [H+] will change the acid/base ratio of all buffer systems in a solution.
However, since these systems are in equillibrium, we only need to know about one system to calcualte the pH of the solution
The slope of the buffer line is defined by ______________
The slope of the buffer line is defined by the nonbicarbonate buffer systems
What are the two types of metabolic acidosis?
-
Anion Gap Metabolic Acidosis: AG > 20
- Metabolic acidosis is due to acid accumulation
(ex: MUD PILES)
- Metabolic acidosis is due to acid accumulation
-
Non-Anion Gap Metabolic Acidosis: AG < 20
- Metabolic acidosis is due to bicarbonate loss
(ex: USED CARS) - AG gap is normal becaue Cl- has increased to compensate
- Metabolic acidosis is due to bicarbonate loss
Would the bicarbonate buffere system function in the absense fo gas exchange and ventilation?
Why or why not?
No;
We must continually breathe off CO2 in order for the bicarbonate system to function.
Excreting CO2 pulls the reaction to the left (via Le Chatelier), thus minimizing the effects of added H+
What system compensates for metabolic alkylosis or acidosis?
The respiratory system
Compensates almost instantly
Which patients are least likely to have adequate respiratory compensation for metabolic acidosis or alkylosis?
Mechanically ventilated patients
What is the Henderson-Hasselbalch equation?
Which systems buffer metabolic acidosis or alkylosis?
Bicarbonate and non-bicarbonate buffer systems
In general, what causes metabolic acidosis?
Addition of a fixed acid or loss of a base
May be primary or secondary
- Primary: acid-base process occurrs independently
- Secondary: acid-base process occurs in response to a primary process (ex: the process is compensatory to restore pH to normal)
For every 10 mmHg of PaCO2 change, pH will change by units in the opposite direction.
For every 10 mmHg of PaCO2 change, pH will change by 0.08 units in the opposite direction.
A change in PaCO2 of _______ will result in a 0.08 unit change in pH
A change in PaCO2 of 10 mmHg will result in a 0.08 unit change in pH
What process is occcuring if pH and PaCO2 are both increased?
Metabolic Alkylosis
(With expected compensatory respiratory acidosis)
A decreasein unmeasuredcations will ________ the anion gap
A decreasein unmeasuredcationswillincrease the anion gap
Would increasing hemoglobin concentration increase or decrease the buffer value of the nonbicarbonate buffers in the body as a whole?
Adding hemoglobin would increase the buffer vaule
Hemoglobin acts as a buffer, so more of it would help the body resist changes to pH
If we have buffers, why do we need respiratory compensation for primary metabolic alkylosis or acidosis?
Buffers reduce, but do not prevent changes in pH due to primary metabolic processes.
- The small change in pH stimulates peripheral chemoreceptors
- Ventilation increases to compensate for decreased pH
- Goal is to decrease PCO2
- Ventilation decreases to compensate for increased pH
- Goal is to increase PCO2
- Ventilation increases to compensate for decreased pH
A decreasein unmeasuredanions will ________ the anion gap
A decreasein unmeasuredanions will decrease the anion gap
A patient has ABG of 7.47/47 (34)
What acid/base process is present?
Metabolic alkylosis
What is the difference between a primary and secondary metabolic acid-base process?
- Primary: acid-base process occurrs independently
- Secondary: acid-base process occurs in response to a primary process (ex: the process is compensatory to restore pH to normal)
What steps should you take when evaluating a patient with an acid-base process?
- Look at arterial blood gas
Acidosis or alkylosis?
Metabolic or respiratory? - a) If the patient is in metabolic acidosis, calculate the anion gap
AG > 20 = acid add-on state
AG < 20 = bicarbonate-wasting state
b) If the patient is in metabolic aklylosis, evaluate for Cl- responsive (Low Cl-, Na+, FeNa in urine, low urine output) or CL- unresponseive - Perform a delta-delta analysis to uncover underlying bicarbonate excess or bicarbonate wasting states
Low serum HCO3 indicates the presence of which acid-base process?
Metabolic acidosis
Metabolic acidosis -> Increased [H+]
Increased [H+] -> Le Chatelier to the left; this uses up HCO3
Where are Na+, Cl-, and HCO3 in the electrolyte tree?
What process is occuring if pH is increased and PaCO2 is decreased?
Respiratory Alkylosis
What enzyme catalyzes the bicarbonate buffer system?
Carbonic anhydrase
Catalyzes H2O + CO2 -> H2CO3
How would the buffer line change if hemoglobin concentration in the body were increased?
Increasing hemoglobin would increase the buffering power of the non-bicarbonate buffer systems in the body.
The slope of the line would be steeper -> less change in pH, even when [HCO3-] changes
In general, what causes metabolic alkylosis?
Addition of a base or loss of an acid
May be primary or secondary
- Primary: acid-base process occurrs independently
- Secondary: acid-base process occurs in response to a primary process (ex: the process is compensatory to restore pH to normal)
In general, what causes respiratory acidosis or alkylosis?
A primary change in PaCO2 due to hypoventilation (acidosis) or hyperventilation (alkylosis)
The rapid intravenous infusion of HCO3- into a 70 kg male initially increased arterial pH from 7.40 to 7.51 and [HCO3-] from 24 to 32 mEq/L. However, over the next 3 hr the arterial [HCO3-] decreased to
27 mEq/L and pH decreased to 7.45.
What caused the fall in HCO3-and pH over the 3 hr period?
- HCO3-
- Equilibrated throughout the extracellular space
- Non-bicarbonate buffers
- This wil inrease CO2
- pH
- Respiratory compensation for metabolic alkylosis = hypoventilation
- However, increased CO2 due to the bicarbonate infusion will stimulate central chemoreceptors to increase respiration. This limits respiratory compensation of increased pH.
What limits respiratory compensation for primary metabolic alkylosis?
The compensatory response to metabolic acidosis = respiratory alkylosis, achieved by hypoventilation
This compensatory response is limited by hypoxemia; If the body hypoventilates to PaO2 < 60 mmHg, carotoid chemoreceptor activtion will work to increase ventilation, thus counteracting the compensatory respiratory acidosis.
What is the most important buffer system in the body?
The bicarbonate buffer system
How do you calculae the Anion Gap?
What is the expected value?
AG = [Na+] - [Cl-] - [HCO3]
Expected value = 10 +/- 2
A change in PaCO2 of 10 mmHg will result in a ________ change in pH in
A change in PaCO2 of 10 mmHg will result in a 0.08 unit change in pH
An increase in unmeasured cations will ________ the anion gap
An increase in unmeasured cations will decrease the anion gap
How would the buffer line change if hemoglobin concentration in the body were decreased?
Decreasing hemoglobin would decrease the buffering power of the non-bicarbonate buffer systems in the body.
The slope of the line would be less steep; changes in [HCO3-] would have a large effect on pH
Which systems buffer respiratory acidosis or alkylosis?
Non-bicarbonate bufferes only
How do you perform a delta-delta analysis?
- Calculate the ΔAG
* *ΔAG** = AG - Expected AG = AG - 10 - Add ΔAG + HCO3
If ΔAG + HCO3 > 30 => additional bicarbonate excess state
If ΔAG + HCO3 < 20 => additional bicarbonate wasting condition
If ΔAG + HCO3 < 20 => No other underlying acid/base processes are present
Which protein contributes most signfiicantly to the anion gap?
Albumin
Albumin is negatively chagrged
Hypoalbuminemia = smaller anion gap
Hyperalbuminemia = larger anion gap
How can you tell if a metabolic acid/base process is appropriately compensated?
Winter’s formula:
If PaCO2 = 1.5 * [HCO3] + 8 , the metabolic process is perfectly compensated
OR
If the last 2 digist of PCO2 are the same as the last 2 digits of pH, the metabolic process is perfectly compensated
High serum HCO3 indicates the presence of which acid-base process?
Metabolic alkylosis
Metabolic alkylosis -> decreased [H+]
Decreased [H+] -> Le Chatelier to the right; this produces HCO3
What process is occuring if pH is decreased and PaCO2 is increased?
Respiratory Acidosis
For every 10 mmHg of PaCO2 change, pH will change by 0.08 units in the direction.
For every 10 mmHg of PaCO2 change, pH will change by 0.08 units in the opposite direction.
What are the two types of metabolic alkylosis?
- Cl- responsive metabolic alkylosis
- Primary problem: Increase in serum bicarbonate
- Usually associated with volume depletion
- The kidney wants to resorb Na and Cl to reabsorb H2O
- However, when it runs out of Cl-, it chooses to reabsorb HCO3- to restore volume
- This comes at the cost of increased pH
- Giving NaCl infusion can restore the patient to normal
- Primary problem: Increase in serum bicarbonate
- Cl- unresponsive metabolic alkylosis
- Problem = too much aldosterone or not not enough K+
- Promotes H+ secretion in the renal epithelial cells
- Problem = too much aldosterone or not not enough K+
For every mmHg of PaCO2 change, pH will change by 0.08 units in the opposite direction.
For every 10 mmHg of PaCO2 change, pH will change by 0.08 units in the opposite direction.
What is the relationship of pH and [H+]?
pH = -log[H+]
If a patient has metabolic acidosis with an elevated anion gap, what is the cause?
Acid add-on state: MUD PILES
(The patient ate something they weren’t supposed to, or they are in lactic or ketoacidosis)
What process is occcuring if pH and PaCO2 are both decreased?
Metabolic acidosis
(with expected compensatory respiratory alkylosis)
How long does it take for the kidney to begin to compensate for respiratory acidosis or alkylosis?
How can you tell if compensation has occurred?
~ 3 days
- Compensated Respiratory Acidosis
- pH is higher than expected for the increase in PaCO2
- Compensated Respiratory Alkylosis
- pH is lower than expected for the decreaed PaCO2
What is the Henderson-Hasselbalch equation for the bicarbonate buffer system?
pH = 6.1 + log { [HCO3-] / (0.03 * PaCO2) }
- Normal values
- HCO3- = 24
- PaCO2 = 40
- => pH = 7.4
A patient has ABG of 7.47/47 (34), characteristic of metabolic alkylosis.
What would you expect to see if this were a Cl- responsive metabolic alkylosis?
- Low urine output
- Low levels of the following in the urine:
- Cl-
- Na+
- FeNa
Cl- responsive metabolic alkylosis is usually associated with a a low volume state; the body is reabsorbing bicarbonate to try to restore pH
If a patient has metabolic acidosis without an elevated anion gap, what is the cause?
Bicarbonate wasting state
USED CARS
(Noteably: Saline administration, Diarrhea, Spironolactone)
If there were no non-bicarbonate buffers, which line would represent the buffer line?
Line (a)
An increase in unmeasured anions will ________ the anion gap
An increase in unmeasured anionswillincrease the anion gap
What is the “buffer value” of a buffer?
The buffer value refers to the moles of H+ that must be added or removed from the solution to produce a 1 pH unit change
The highe the value, the stronger the buffer
