1. Acid-Base Calculations
10/7/2015

2. Goals Determine acid-base disturbance
Primary cause
Metabolic
Respiratory
Secondary causes
Determine if the compensation is appropriate or not
If the compensation is not appropriate, then look for secondary causes
Understand anion gap and how to calculate mixed acid/base problems with a focus on what is most common in the ICU
Understand respiratory compensation in the setting of metabolic acidemia by utilizing Winter’s formula
Things you will needs
Arterial blood gas
Bicarbonate on the arterial blood gas if calculated from the pCO2 of the arterial blood gas
Chemistry panel
Albumin

3. Physiology
Bicarbonate in your body is the primary buffering system for changes in pH
Any change in the acid-base status of your body that is not effectively taken care of by the bicarbonate buffering system results in compensation
Body pH: 7.38 to 7.42
What types of systems are available for compensation in your body?

4. Physiology Response to changes in acid-base status
Respiratory compensation
Faster response (seconds)
Decrease in pH leads to increase in ventilation
Increase in pH leads to decrease in ventilation
Example: Development of lactic acidosis from septic shock
Renal compensation
Slower response (days)
Decrease in pH leads to more retention of bicarbonate and production of more bicarbonate
Increase in pH leads to excretion of bicarbonate
Example: Severe COPD
In septic shock, lactic acidosis occurs (gap metabolic acidosis) dropping the pH. The fastest compensation system is the respiratory system which then causes the patient to breathe faster. Also seen in diabetic ketoacidosis.
In a COPD patient with chronic CO2 retention, the body tries to deal with the decrease in pH by producing more bicarbonate (renal). Thus a chronic COPD patient with CO2 retention will have a higher baseline bicarbonate.

5. Overview of Acid-Base Calculation
Determine if it is an acid or a base problem
pH < 7.4
Acid problem predominates
pH > 7.4
Base problem predominates
pH = 7.4
Absolutely nothing wrong
Or, more likely a mixed acid-base problem
Determine if the primary problem is a metabolic versus respiratory problem
Determine if there is an anion gap acidosis
Determine if there are secondary problems
In essence, is the compensation appropriate?
Most importantly, try to explain the problems you have determined using the history/physical and laboratory data
If not explained, other studies or history to determine what these problems are
Reviewing history is often helpful
Some problems require the tincture of time
Review of past ABGs and chemistry panels may be helpful
Re-evaluation may help elucidate more detail as the acid-base status changes

6. Overview of Acid-Base Calculation
pH from arterial blood gas
pH < 7.4 (acidemia)
pH > 7.4 (alkalemia)
Primary respiratory acidosis
pCO2 > 40 mmHg
Is this acute or chronic respiratory acidosis?
Is it appropriately compensated?
Is there another problem such as metabolic acidosis or alkalosis?
Primary metabolic acidosis
HCO3 < 24 mEq/L
Is it appropriately compensated (respiratory)?
Is this an anion gap acidosis, non-anion gap acidosis or both?
Primary respiratory alkalosis
pCO2 < 40 mmHg
Is this acute or chronic respiratory alkalosis?
Is it appropriately compensated?
Is there another problem such as metabolic acidosis or alkalosis?
Primary metabolic alkalosis
HCO3 > 24 mEq/L
Is it appropriately compensated (respiratory)?
Is there an anion gap acidosis?

7. Steps in Acid-Base Calculation
Determine the primary disturbance
Is it metabolic versus respiratory?
If respiratory, is it acute or chronic?
Determine if the compensation is appropriate for the primary disturbance
If the compensation is not appropriate, there must be a secondary process
For compensation, you will need to know the compensation formulas
Determine if there is an increase anion gap
You should always calculate the anion gap
Need to know the albumin
Determine if there is additional non-gap metabolic acidosis or metabolic alkalosis after taking the gap into account

8. Formula for Compensation in Primary Metabolic Acidosis
Expected compensation is increased ventilation given by Winter’s formula
Expected PaCO2 = (1.5 * [HCO3]) (Error is + 2)
Of note, [HCO3] is the measured bicarbonate

9. Formula for Compensation in Primary Metabolic Alkalosis
Expected compensation is decreased ventilation
Expected PaCO2 = (0.7 * [HCO3]) (Error is + 5)

10. Determining Compensation in Respiratory Acidosis/Alkalosis
Before determining the compensation, look at the pH of the arterial blood gas to determine if the process is acute or chronic in nature
Using the measured PaCO2, determine expected pH if it is acute and chronic.
This will help point you to whether it is acute versus chronic in a primary respiratory disturbance

11. Formulas for Compensation in Acute Respiratory Acidosis
Primary acute respiratory acidosis
Determine the expected pH in acute respiratory acidosis
Decrease in pH below 7.4 = * [Increase in PaCO2 above 40]
Expected pH = 7.4 – { * [measured PaCO2 - 40] }
Note, if pH is not as low as expected for acute respiratory acidosis, then it is likely a case of chronic respiratory acidosis
Increase in HCO3 = 0.1 * [Increase in PaCO2 above 40]
Increase in HCO3 = 0.1 * [measure PaCO2 – 40]
Expected HCO3 = 24 + { 0.1 * [measure PaCO2 – 40] }

12. Formulas for Compensation in Chronic Respiratory Acidosis
Primary chronic respiratory acidosis
Determine the expected pH in chronic respiratory acidosis
Decrease in pH below 7.4 = * [Increase in PaCO2 above 40]
Expected pH = 7.4 – { * [measured PaCO2 - 40] }
Note, if pH is lower than expected for chronic respiratory acidosis, then it is likely a case of acute respiratory acidosis
Increase in HCO3 = 0.35 * [Increase in PaCO2 above 40]
Increase in HCO3 = 0.35 * [measure PaCO2 - 40]
Expected HCO3 = 24 + { 0.35 * [measure PaCO2 - 40] }

13. Formulas for Compensation in Acute Respiratory Alkalosis
Primary acute respiratory alkalosis
Determine the expected pH in acute respiratory alkalosis
Increase in pH above 7.4 = * [Decrease in PaCO2 above 40]
Expected pH = { * [40 - measured PaCO2] }
Note, if pH is not as high as expected for acute respiratory alkalosis, then it is likely a case of chronic respiratory alkalosis
Decrease in HCO3 = 0.2 * [Decrease in PaCO2 below 40]
Decrease in HCO3 = 0.2 * [40 - measured PaCO2]
Expected HCO3 = 24 - {0.2 * [40 - measured PaCO2] }

14. Formulas for Compensation in Chronic Respiratory Alkalosis
Primary chronic respiratory alkalosis
Determine the expected pH in chronic respiratory alkalosis
Increase in pH above 7.4 = * [Decrease in PaCO2 above 40]
Expected pH = { * [40 - measured PaCO2] }
Note, if pH is higher than expected for chronic respiratory alkalosis, then it is likely a case of acute respiratory alkalosis
Decrease in HCO3 = [0.5 to 0.7] * [Decrease in PaCO2 below 40]
Decrease in HCO3 = [0.5 to 0.7] * [40 - measured PaCO2]
Expected HCO3 = 24 - { [0.5 to 0.7] * [40 - measured PaCO2] }
Note: [0.5 to 0.7] is a range

15. Primary Metabolic Acidosis Calculations
pH < 7.4 and HCO3 < 24 mEq/L with pCO2 < 40 mmHg
Once we determine the primary process, we need to determine if there is appropriate respiratory compensation and if there is gap versus non-gap metabolic acidosis and other metabolic processes
Determine if there respiratory compensation is appropriate
Use Winter’s formula (expected pCO2 for metabolic acidosis)
If the respiratory compensation is not appropriate, then determine if there is a concomitant respiratory acidosis or respiratory alkalosis
If measured pCO2 < expected pCO2, then there is concomitant respiratory alkalosis (essentially, the patient is hyperventilating more than expected)
If measured pCO2 > expected pCO2, then there is concomitant respiratory acidosis (essentially, the patient is not ventilating as much as expected)
Determine if there is a gap metabolic acidosis
If there is no gap metabolic acidosis, then you are done at this point as there is only non-gap metabolic acidosis
If there is a gap metabolic acidosis, then determine if there is a concomitant non-gap metabolic acidosis or concomitant metabolic alkalosis

16. Gap versus Non-Gap Metabolic Acidosis
What is the difference between gap and non-gap metabolic acidosis?
Non-gap metabolic acidosis
Loss of naturally occurring bicarbonate in your system
Renal loss
Gastrointestinal losses through diarrhea
Gap metabolic acidosis
Production of anions that are higher than the normal levels in your body
Lactate
Ketones
Renal failure (Urea, sulfates, phosphates)
Production of anions that are not normally in your body
Ingestion of toxins
E.g., ethanol, methanol, ethylene glycol, formaldehyde, isoniazid, metformin
Initially comment on the importance of bicarbonate in your body to buffer against changes in pH (primarily against increased CO2)

17. Anion Gap
Anion gap primarily reflects “unseen anions” (unseen anion-gap acids for the most part)
Important ones in the ICU
Lactate
Ketones
This is a concept and not an actual gap
The blood must be electrically neutral, otherwise it would fly toward an opposite charge
Electromagnetic exsanguination
Na + UC = Cl + HCO3 + UA
UC = unmeasured cations = Ca, K, Mg
UA = unmeasured anions = organic acids, phosphates, sulfates
Na – (Cl + HCO3) = UA - UC
Anion gap = Na – (Cl + HCO3) = UA – UC
Increases in unmeasured anions are more common, therefore a higher than expected anion gap is more common than a smaller than expected anion gap

18. Anion Gap Anion gap = Na – (Cl + HCO3) = UA – UC
A normal anion gap = 12 mEq/L + 2 mEq/L
Assuming a normal albumin = 4 g/dL
Therefore, a higher than normal anion gap reflects an increase in these unmeasured anions
Again, lactate and ketone are common ones
Other examples include ingestions
A lower than normal anion gap reflects an increase in the unmeasured cations
Increased immunoglobulins
Increased Ca, K, Mg
Lithium

19. Anion Gap Anion gap is influenced by albumin
A quick and dirty way of calculating the expected anion gap is to take the albumin x 3
Expected anion gap = albumin x 3
Normal albumin is 4 g/dL
Expected anion gap = 4 x 3 = 12 mEq/L
If there is no albumin noted, then assume that the anion gap is normal at 12 mEq/L
Example
Albumin = 2.5 mEq/L
Expected anion gap = 2.5 x 3 = 7.5 mEq/L (+ 2 mEq/L)
If the calculated (patient’s) anion gap (Na – Cl – HCO3) = 12 mEq/L, then there is a anion gap acidosis despite the fact that the calculated (patient’s) anion gap is 12 mEq/L which is considered the “normal” anion gap
Formal equation: Expected anion gap = 12 – [2.5 * (4.5 – measured albumin)]

20. Anion Gap
DAG (the difference between the calculated (patient’s) anion gap and the expected anion gap) helps to determine whether these is an underlying concomitant metabolic alkalosis or an additional non-gap metabolic acidosis in addition to the anion gap acidosis
DAG = calculated (patient’s) anion gap – expected anion gap
What is the significance of DAG?
Why bother calculating it?
What is its significance?

21. Anion Gap
Recall that if calculated AG > expected AG, then we have a situation where there is an increase in the unmeasured anions
AG = UA – UC = Na – (Cl + HCO3)
DAG = calculated anion gap – expected anion gap
Therefore, the DAG is the amount of unmeasured anion that is causing the anion gap acidosis
This is also the amount that the anion-gap acid lowers the patient’s bicarbonate from the normal value of 24 mEq/L
Metabolic acidosis of any kind lowers the bicarbonate

22. Anion Gap Example: Na 140, Cl 104, HCO3 24, albumin 4
Expected anion gap = albumin x 3 = 12 mEq/L
Calculated (patient’s) anion gap = 140 – 104 – 24 = 12 mEq/L
DAG = 12 – 12 = 0 mEq/L
The calculated (patient’s) anion gap of 12 mEq/L exactly matches the expected anion gap
In essence, there is no increase in unmeasured anions, therefore, there is no increase in anion-gap acid to lower the patient’s bicarbonate below 24 mEq/L
DAG is the amount of excess unmeasured anion there is
Adding back DAG to the patient’s HCO3 will determine the patient’s initial HCO3 before unmeasured anion (anion-gap acid) lowered the HCO3
This helps to determine whether there was an initial metabolic alkalosis or non-gap acidosis.

23. Anion Gap Example: Is there an anion gap acidosis?
Na 140, Cl 104, HCO3 18, albumin 3
Expected anion gap = albumin x 3 = 9 mEq/L
Calculated (patient’s) anion gap = 140 – 104 – 18 = 18 mEq/L
DAG = 18 – 9 = 9 mEq/L
How do we interpret the DAG?
The DAG is the amount of increased unmeasured anion
The DAG is the amount that this unmeasured anion lowers the patient’s initial HCO3.
What do we do with this number (DAG) to interpret its meaning?

24. Anion Gap Example: Is there an anion gap acidosis?
Na 140, Cl 104, HCO3 18, albumin 3
DAG = 18 – 9 = 9 mEq/L
Add back the DAG to the patient’s HCO3
This will then tell us the patient’s initial HCO3 before the unmeasured anion (anion-gap acid) had a chance to lower the patient’s HCO3
Adding back the DAG of 9 mEq/L to the HCO3 of 18, we obtain 27 mEq/L
How do we interpret this result?
Before the unmeasured anion (anion-gap acid) was able to lower the patient’s HCO3, the patient’s HCO3 was 27 mEq/L
Therefore, the patient had a baseline metabolic alkalosis prior to the anion- gap acidosis
As an aside, this increased baseline HCO3 can also be due to previous chronic respiratory acidosis with metabolic (renal) compensation
Best assessed by previous ABGs and history

25. Anion Gap Final Thoughts on Anion Gap
You should always calculate an anion gap to determine if there is an anion gap metabolic acidosis
Once you determine if there is an anion gap metabolic acidosis, then you should determine if the patient has concomitant non-gap metabolic acidosis or concomitant metabolic alkalosis
Make sure to correct the expected anion gap in hypoalbuminemia

26. Example 1 Arterial blood gas Chemistry panel Albumin 2.5 g/dL
pH 7.25
pCO2 25 mmHg
Chemistry panel
Na 129
K 4.2
Chloride 105
HCO3 10
BUN 51
Creatinine 4.6
Albumin 2.5 g/dL
Weight 69 kg

27. Example 1 What do we do first?
pH = 7.25
Is this a primary acidotic or alkalotic problem?
Acidosis because pH < 7.4
Is this a primary metabolic acidosis or primary respiratory acidosis
HCO3 < 24 consistent with a primary metabolic acidosis
pCO2 < 40 which is NOT consistent with primary respiratory acidosis
What next?

28. Example 1
Next, we need to calculate if there was an appropriate respiratory compensation
Empirically, we expect that the patient will try to blow off CO2 if they have an underlying metabolic acidosis
This is termed respiratory compensation to try to normalize the pH (though it never quite gets back to 7.4)
Winter’s formula

29. Example 1 Winter’s formula
Tells us the expected respiratory compensation given the measured HCO3
pCO2 (compensated) = 1.5 [HCO3] + 8 (+ 2 mmHg)
In our example, the HCO3 was 10 mEq/L and pCO2 was 25 mmHg (measured from ABG)
pCO2 (compensated) = 1.5 * = 23 mmHg
Therefore, in our example, the pCO2 (compensated) = 23 mmHg which is very close to the measured pCO2
What does this mean?
Our patient has appropriate respiratory compensation for the metabolic acidosis that they are experiencing

30. Example 1 Next, calculate whether there is an anion gap
Recall albumin 2.5 g/dL
First, compute expected anion gap correcting for albumin
Expected anion gap = 2.5 x 3 = 7.5 mEq/L
Calculated (patient’s) anion gap = 129 – 105 – 10 = 14 mEq/L
Is there an anion gap metabolic acidosis?
Because the calculated anion gap is greater than the expected anion gap by 2 mEq/L, there is an anion gap acidosis
What next?

31. Example 1 Calculate the DAG
DAG = calculated (patient’s) AG – expected AG
DAG = 14 – 7.5 = 6.5 mEq/L
How do we interpret DAG and what do we do with it?
Add the DAG to the patient’s HCO3
DAG + HCO3 = = 16.5 mEq/L
This is the amount that the unmeasured anion (anion-gap acid) lowers the patient’s initial HCO3
How do we interpret this?
This is the patient’s HCO3 before the unmeasured anion (anion-gap acid) had a chance to lower it
Therefore, before the patient was affected by the anion-gap acidosis, the patient had a non-gap metabolic acidosis because 16.5 mEq/L is lower than 24 mEq/L
Recall that metabolic acidosis (even non-gap) lowers the bicarbonate
Non-gap metabolic acidosis usually due to gastrointestinal losses or renal tubular acidoses

32. Example 1 The patient has a primary metabolic acidosis
The metabolic acidosis is a combined gap and non-gap metabolic acidosis
The patient has appropriate respiratory compensation
More importantly, now that you have identified the acid/base problem, you need to find out why?
In this case, the patient had acute kidney injury
But think of what else could cause the anion gap metabolic acidosis and consider appropriate workup

33. Example 1 What about the non-gap metabolic acidosis?
How do we determine the source of the non-gap metabolic acidosis?
In general, once you have a non-gap metabolic acidosis, you should determine if the loss of the bicarbonate is renal in nature of gastrointestinal (non-renal) in nature.
An alternate cause could be ingestion of compounds like hydrochloride acid
History helps as a history of ongoing diarrhea could point to non-gap metabolic acidosis secondary to gastrointestinal losses

34. Example 1
Urine anion gap helps to determine if the non-gap metabolic acidosis is secondary to renal versus non-renal losses
Motivation: If the kidneys are functioning normally, you would expect the kidneys to reabsorb more urine HCO3 in a situation where there is non-gap metabolic acidosis
The urine anion gap allows us to assess whether the kidneys are appropriately reabsorbing the urine HCO 3 or inappropriately losing urine HCO3.
Urine anion gap = Urine Na + Urine K – Urine Cl
Urine HCO3 is not routinely measured as HCO3 in aqueous solution becomes CO2 thus lowering its concentration when collected as a lab
Again, urine anion gap is a surrogate for unmeasured anions just like the serum anion gap
However, in this case, you are interested in the unmeasured HCO3 in urine (the thing that you actually want to measure)

35. Urine Anion Gap Urine anion gap = Urine Na + Urine K – Urine Cl
Urine Na + Urine K + Unmeasured Urine Cations = Urine Cl + Unmeasured Urine Anions
Urine Na + Urine K + Urine UC = Urine Cl + Urine UA
UC = unmeasured cation
UA = unmeasured anion
Urine Na + Urine K – Urine Cl = Urine UA – Urine UC
Urine anion gap = Urine UA – Urine UC

36. Urine Anion Gap Urine anion gap = Urine Na + Urine K – Urine Cl
Urine anion gap = Urine UA – Urine UC
Urine anion gap = UAG
In the case where there is non-gap metabolic acidosis, the kidneys would be expected to reabsorb more urine HCO3 if the kidneys are functioning appropriately.
HCO3 is an anion (part of the unmeasured anions).
Therefore, with properly functioning kidneys, in non-gap metabolic acidosis, the kidneys should try to reabsorb more urine HCO3 thus making the Urine UA < Urine UC
This results in a negative urine anion gap (UAG)
With normal kidneys in non-gap metabolic acidosis, you expect the urine anion gap < 0
I.E., No inappropriate loss of urine HCO3
Thus if UAG < 0, then we know that the loss of HCO3 has to be non-renal in nature (e.g., gastrointestinal loss)

37. Urine Anion Gap
Conversely, in the case of non-gap metabolic acidosis, if there is a UAG > 0, then the kidneys are not functioning appropriately and losing HCO3 through the urine
In non-gap metabolic acidosis, if UAG > 0, then the kidneys are losing HCO3 inappropriately
Recall, UAG = Urine UA – Urine UC
Note that HCO3 is an anion, thus if UAG > 0, it suggests that there is loss of unmeasured anion (in this case HCO3)

38. Example 1 (continued)
Recall that so far, we have a gap metabolic acidosis and non-gap metabolic acidosis
Arterial blood gas
pH 7.25
pCO2 25 mmHg
Chemistry panel
Na 129
K 4.2
Chloride 105
HCO3 10
BUN 51
Creatinine 4.6
Albumin 2.5 g/dL

39. Example 1 (continued)
We have to determine the source of the gap metabolic acidosis
However, we also have to determine the source of the non- gap metabolic acidosis
To do this, we order urine Na, urine K and urine Cl
If the resultant UAG > 0, then we suspect that the loss of the HCO3 is through the kidneys
If, however, the resultant UAG < 0, then we suspect that the loss of the HCO3 is non-renal in nature (e.g., gastrointestinal)

40. Example 1a
Now, what if we had the previous example but the measured pCO2 from the ABG was 30 mmHg, and Winter’s formula using a HCO3 of 10 mEq/L shows that the compensated pCO2 should be as follows:
pCO2 (compensated) = 1.5 * = 23 mmHg
What does this tell us about the respiratory compensation?
The patient is NOT appropriately compensated for the metabolic acidosis and has an underlying additional respiratory acidosis
In essence, the patient is NOT ventilating enough to compensate for the metabolic acidosis
Because the measured pCO2 is 30 mmHg, which is above the expected compensated pCO2 of 23 mmHg (calculated from Winter’s formula)

41. Example 1b
Now, what if we had the previous example but the measured pCO2 from the ABG was 18 mmHg, and Winter’s formula using a HCO3 of 10 mEq/L shows that the compensated pCO2 should be as follows:
pCO2 (compensated) = 1.5 * = 23 mmHg
What does this tell us about the respiratory compensation?
The patient is overcompensated for the metabolic acidosis and has an underlying addition respiratory alkalosis
In essence, the patient is hyperventilating over the expected respiratory compensation for the metabolic acidosis
Because the measured pCO2 is 18 mmHg, which is less than the expected compensated pCO2 of 23 mmHg (calculated from Winter’s formula)

42. Example 2
A patient with normal renal function developed a small bowel obstruction and a nasogastric tube was placed
Previous appendectomy
No other significant medical problems
No previous history of smoking
Several days into admission, he aspirated and became hypotensive and hypoxemic
Labs were drawn after the patient became hypotensive
A code blue was called and the patient was intubated using paralytics
After intubation, the patient remained hypotensive and was unresponsive to fluid resuscitation while awaiting central venous access and initiation of vasopressors

43. Example 2 Initial labs were drawn prior to intubation ABG
pH 7.29
pCO2 30 mmHg
Chemistry panel
Na 132
Chloride 90
HCO3 20
Albumin 4

44. Example 2 What is the primary acid/base problem? Metabolic acidosis
pH = 7.29 < 7.40
Therefore, this is an acid problem
HCO3 = 20 < 24
Therefore, primary metabolic acidosis
pCO2 = 30 mmHg < 40 mmHg
Therefore, not primary respiratory acidosis
What next?

45. Example 2
Next, use Winter’s formula to determine if there is appropriate respiratory compensation
pCO2 (compensated) = 1.5 * = 38 mmHg
Recall that the patient measured pCO2 = 30 mmHg
What does this mean?
The patient has a respiratory alkalosis in addition to the metabolic gap acidosis
What is the respiratory alkalosis due to?

46. Example 2
Potential causes of the respiratory alkalosis in this patient are
Sepsis
Infection
Hypoxemia
Other potential causes:
Anxiety
Pain
Liver failure
Neurological disorders

47. Example 2
Next, calculate the anion gap to determine if there is a gap metabolic acidosis
Albumin = 4 g/dL
Expected anion gap = 4 x 3 = 12 mEq/L
Calculated (patient’s) anion gap = Na – Cl – HCO3
Calculated (patient’s) anion gap = 132 – 90 – 20 = 22 mEq/L
Is there a gap metabolic acidosis?
Yes, the calculated anion gap of 22 mEq/L is greater than the expected anion gap of 12 mEq/L
What next?

48. Example 2
Use DAG to determine if there is an underlying metabolic alkalosis or non-gap acidosis in addition to the anion gap acidosis
DAG = 22 – 12 = 10 mEq/L
What do we do with this DAG and how do we interpret it?
The DAG is the amount that the gap acidosis lowers the patient’s initial HCO3
Therefore, we need to add the DAG to the patient’s HCO3 to determine the patient’s initial HCO3 before the anion gap acidosis.
Add the DAG of 10 mEq/L to the patient’s HCO3 of 20 mEq/L
Therefore, prior to the anion gap acidosis, the patient’s HCO3 was 30 mEq/L
What does this tell us about the patient’s acid/base status prior to the anion gap acidosis?
The patient had a metabolic alkalosis prior to the anion gap acidosis

49. Example 2 What is the acid/base disturbance that we have?
A primary metabolic gap acidosis with a metabolic alkalosis and respiratory alkalosis
But this is not what’s important. We need to know why?
What is the cause of the metabolic gap acidosis?
Likely lactic acidosis from the hypotension/sepsis around the aspiration event
What is the cause of the metabolic alkalosis?
Metabolic alkalosis from vomiting and nasogastric suctioning as the patient had small bowel obstruction
What is the respiratory alkalosis from?
Likely secondary to sepsis and hypoxemia given that the patient is both septic and had hypoxemic from aspiration

50. Primary Metabolic Alkalosis Calculations
Primary metabolic acidosis
pH > 7.4 and HCO3 > 24 mEq/L with pCO2 > 40 mmHg
Once we determine the primary process, we need to determine if there is appropriate respiratory compensation
Determine if there respiratory compensation is appropriate
If the respiratory compensation is not appropriate, then determine if there is a concomitant respiratory acidosis or respiratory alkalosis
If measured pCO2 < expected pCO2, then there is concomitant respiratory alkalosis (essentially, the patient is hyperventilating more than expected)
If measured pCO2 > expected pCO2, then there is concomitant respiratory acidosis (essentially, the patient is not ventilating as much as expected)
Determine if there is a gap metabolic acidosis
Use Urine [Cl] concentration to help determine if chloride-responsive or chloride resistant

51. Example 3 Arterial blood gas Chemistry panel pH 7.64 pCO2 32 mmHg
Na 135
K 4.2
Chloride 95
HCO3 33
BUN 20
Creatinine 1.0

52. Example 3 What is the primary disturbance?
Is it appropriately compensated?
Is there an anion gap?
If primary metabolic alkalosis, use urine [Cl] concentration to determine if it is chloride-responsive or chloride resistant

53. Example 3 Alkalemia: pH > 7.4 What is the primary disturbance?
In this case, HCO3 > 24 mEq/L and PaCO2 < 40 mmHg
So there is both metabolic alkalosis and respiratory alkalosis
Which one is the primary cause of the pH of 7.6?

54. Example 3
Is metabolic alkalosis or respiratory alkalosis the primary cause of the pH of 7.6?
In this case, it is metabolic alkalosis
How do we determine that?
Assume that the primary problem is respiratory alkalosis?
Is it possible that a PaCO2 of 32 mmHg can raise the pH from 7.4 to 7.6?
Assume that this is an acute respiratory alkalosis. How much would an acute change of PaCO2 from 40 mmHg (normal) to 32 mmHg change the pH by?
Recall that for every 1 mmHg decrease in PaCO2, the pH increases by mmHg in the acute setting.
Thus a PaCO2 decrease from 40 mmHg to 32 mmHg, would cause a (8) * (0.008) = increase in the pH from 7.4 to 7.464
Of note, if acute respiratory alkalosis can not account for the pH of 7.6, then chronic respiratory alkalosis can not either because the change in pH of chronic respiratory changes is less than acute respiratory changes
Acute respiratory alkalosis can not account for this pH of 7.6 and thus the primary process responsible for the increase in pH to 7.6 must be metabolic alkalosis

55. Example 3
Now that we know the primary process is metabolic alkalosis, what next?
We need to determine if the respiratory compensation is appropriate.
Expected PaCO2 = (0.7 * [HCO3]) (Error is + 5)
Expected PaCO2 = (0.7 * 33) (Error is + 5)
Expected PaCO2 = 43.1 mmHg (+ 5)
As you will recall, the measured PaCO2 = 32 mmHg. The patient has overventilated from the expected compensated PaCO2 of 43.1 mmHg to the actual measured PaCO2 of 32 mmHg.
Thus, the patient has respiratory alkalosis on top of the primary metabolic alkalosis.
But then again, we already knew this without going through this calculation

56. Example 3
Finally, we should also determine whether there is a gap metabolic acidosis
Na 135, Cl 95, HCO3 33
Since there is no albumin, we assume an anion gap of 12
Anion gap = 135 – 95 – 33 = 7
Thus, there is no an additional anion gap metabolic acidosis
Therefore, this ABG is consistent with a primary metabolic alkalosis with a concomitant respiratory alkalosis

57. Example 3
Now that we know there is a primary metabolic alkalosis, we need to determine if it is chloride-responsive or chloride resistant
To do this, we need to measure the urine chloride concentration
Chloride responsive metabolic alkalosis
Urine Cl concentration < 20 mmol/L
In this case, the body is depleted of chloride and the kidneys are working to try to “conserve” chloride thus reducing the amount that is excreted in urine
In this case, the patient would be responsive to resuscitation with saline
Chloride resistant metabolic alkalosis
Urine Cl concentration > 20 mmol/L
In this case, the patient would not be responsive to resuscitation with saline

58. Causes of Metabolic Alkalosis
Chloride Responsive
Chloride Resistant
GI loss of H+
Vomiting, nasogastric suctioning
Chloride rich diarrhea
Villous Adenoma
Renal loss of H+
Diuretics
Hypovolemia
Primary hyperaldosteronism
Increased corticosteroid activity
Increase renin activity
Iatrogenic
Excessive sodium bicarbonate infusion
Excessive acetate infusion (e.g., TPN)
Excessive lactate infusion (e.g., Ringer’s lactate)
Milk-alkali syndrome

59. Primary Respiratory Acidosis Calculations
Use pH to determine if it is acute versus chronic
Determine if there is appropriate compensation
Determine if there is an anion gap metabolic acidosis

60. Example 4 ABG Chemistry pH 7.34 PaCO2 60 mmHg Na 140 mEq/L
Cl 100 mEq/L
HCO3 31 mEq/L
Albumin 3 g/dL

61. Example 4 Acidemia: pH < 7.4
What is the primary process responsible?
Respiratory acidosis because PaCO2 > 40 mmHg and HCO3 > 24 mEq/L
Determine if the process is acute or chronic?
Look at the pH

62. Example 4 Determine if the process is acute or chronic? Look at the pH
Assume that it is an acute respiratory acidosis
Recall that for every 1 mmHg increase in PaCO2, there is a decrease in pH in acute respiratory acidosis
Assuming an acute increase of PaCO2 from 40 mmHg (normal) to 60 mmHg (measured), there would be a decrease of (0.008 * [60 mmHg – 40 mmHg] ) in pH
Decrease in pH = (0.008) * (60 mmHg – 40 mmHg)
Decrease in pH = 0.16
If the change in PaCO2 is acute, the pH would have dropped by 0.16, thus the pH would have been 7.4 – 0.16 or 7.24.
Because the measured pH was 7.34 and an acute process would have resulted in a pH of 7.24, this is not acute respiratory acidosis
Therefore, it is likely a case of chronic respiratory acidosis

63. Example 4 Determine if the process is acute or chronic? Look at the pH
Let us verify that it is chronic respiratory acidosis
For chronic respiratory acidosis, a 1 mmHg increase in PaCO2 results in a decrease in pH
Decrease in pH = (0.003) * (60 mmHg – 40 mmHg)
Decrease in pH = 0.06
Therefore, the expected pH would be 7.4 – 0.06 or 7.34
Since this exactly matches the measured pH of 7.34, it is consistent with chronic respiratory acidosis
Just as we expected

64. Example 4
Now that we know that the primary process is chronic respiratory acidosis, we need to determine if the compensation is appropriate.
Going back to the compensation rules, we see that in chronic respiratory acidosis, the HCO3 increases by 0.35 * (Increase in PaCO2)
Increase in HCO3 = (0.35) * (60 mmHg – 40 mmHg)
Increase in HCO3 = 7 mEq/L
Recall that this is an increase above normal: 24 mEq/L
Expected compensated HCO3 = 24 mEq/L + 7 mEq/L = 31 mEq/L
The measured HCO3 = 31 mEq/L
Thus, because the measured HCO3 is equal to the expected compensated HCO3, the compensation is appropriate

65. Example 4
Finally, we should calculate the anion gap to ensure that there is not an underlying anion gap metabolic acidosis
Anion gap = Na – Cl – HCO3
Anion gap = 140 – 100 – 31 = 9 mEq/L
Recall that the albumin was 3 g/dL
Therefore, there is no concomitant anion gap metabolic acidosis

66. Example 4a
Assume that we have the same problem with chronic respiratory acidosis
Assume again that the expected compensated HCO3 = 31 mEq/L
In addition, assume that the measured HCO3 was noted to be 20 mEq/L
In this case, the metabolic compensation is not appropriate as the measured HCO3 is lower than expected for compensation
Therefore, in this case, the patient would have a metabolic acidosis on top of the primary chronic respiratory acidosis
Again, you would need to calculate an anion gap to determine if the metabolic acidosis is secondary to anion gap metabolic acidosis

67. Example 4b
Assume that we have the same problem with chronic respiratory acidosis
Assume again that the expected compensated HCO3 = 31 mEq/L
In addition, assume that the measured HCO3 was noted to be 38 mEq/L
In this case, the metabolic compensation is not appropriate as the measured HCO3 is higher than expected for compensation
Therefore, in this case, the patient would have a metabolic alkalosis on top of the primary chronic respiratory acidosis
Again, you would need to calculate an anion gap to determine if there is an anion gap metabolic acidosis

68. Primary Respiratory Alkalosis Calculations
Use pH to determine if it is acute versus chronic
Determine if there is appropriate metabolic compensation
Determine if there is an anion gap metabolic acidosis
The calculations are similar to that noted above in respiratory acidosis so no examples are provided.
Note, however, that the compensation formulas are slightly different