ABG Analysis

Question 1

What is a normal ABG?

Answer 1

7.40/40/90/24

Question 2

A bicarb of 16 indicates a…

Answer 2

Metabolic acidosis is occurring if bicarb is less than 22 (pt may have acidemia or alkalemia depending on what other processes are going on, but there is at least a metabolic acidosis happening)

Note: A metabolic alkalosis is occurring if bicarb is greater than 26

Question 3

What is a normal anion gap?

Answer 3

3x albumin level (because albumin is the largest component of the anion gap)

Note: A patient with a normal albumin of 4 should have an anion gap of 12 or less. A pt with an albumin of 2 should have an anion gap of 6 or less.

Question 4

What is a normal pO2?

Answer 4

90 (80-100)

Question 5

What is a normal pCO2?

Answer 5

40 (35-45)

Question 6

What’s the fastest way to determine the primary acid-base disorder?

Answer 6

Look at the pH to determine whether it’s an acidemia or alkalemia

Then look at the pCO2 to determine whether it’s respiratory or not (if the pCO2 explains the pH, such as a high pCO2 with low pH in an acidemia, then it’s a respiratory process; if the pCO2 is moving in the same direction as the pH, such as a low pCO2 with a low pH in an acidemia, then it is a metabolic process)

Question 7

How do bicarb and pCO2 proportionally move in a metabolic acidosis?

Answer 7

In a metabolic acidosis, for every point of bicarb lost, pCO2 should go down by 1 point

In a metal pic alkalosis, for every point of bicarb gained, pCO2 should go up by 0.5 points

Note: The lung is more effective at breathing off CO2 by increasing tidal volume during an acidosis than it is at retaining CO2 during an alkalosis

Question 8

How do bicarb and pCO2 proportionally move in a respiratory acidosis?

Answer 8

In an acute respiratory acidosis, for every 10 points of pCO2 gained, bicarb should go up by 1 point

In a chronic respiratory acidosis, for every 3 points of pCO2 gained, bicarb should go up by 1 point

Note: It takes days for the kidneys to be able to make more bicarb so chronic processes are better compensated for

Question 9

How do bicarb and pCO2 proportionally move in a respiratory alkalosis?

Answer 9

In acute respiratory alkalosis, for every 5 points of pCO2 lost, bicarb should go down by 1 point

In a chronic respiratory alkalosis, for every 2 points of pCO2 lost, bicarb should go down by 1 point

Note: The kidneys are better at excreting bicarb to compensate for respiratory alkalosis than they are at producing bicarb to compensate for a respiratory acidosis

Question 10

When do you use winters formula?

Answer 10

To see if the lungs are adequately compensating for an acid-base disorder when there is a metabolic acidosis present

Note: If the pCO2 calculated with winters formula is within 2 points of the measured value, then compensation is said to be adequate

Question 11

What does it mean if the pCO2 calculated by winters formula is greater than the measures pCO2?

Answer 11

Then there is either a secondary respiratory alkalosis or a mixed acid-base disorder

Note: Compensation is not adequate (too much CO2 is being blown off)

Question 12

What does it mean if the measure pCO2 is greater than the pCO2 calculated by winters formula?

Answer 12

Then there is also a secondary respiratory acidosis or a mixed acid-base disorder

Note: Comensation is not adequate (too little CO2 is being blown off)

Question 13

What is winters formula?

Answer 13

If respiratory compensation is adequate then pCO2 should equal

(1.5 x bicarb) + 8 (give or take 2 points)

Question 14

What is the delta gap?

Answer 14

The change in the anion gap from what should be expected (normally 12)

So if a pt has an anion gap of 20, then their delta gap is 8

Note: The delta gap estimates how much bicarb is lost due to an anion gap metabolic acidosis

Question 15

How do you calculate the delta delta gradient?

Answer 15

The change in anion gap (AG - 12) MINUS the change in bicarb (24 - bicarb)

Note: For a pt with normal albumin, an anion gap of 20 and bicarb of 21 would give a delta delta gradient of 5 (8-3)

Question 16

What does a delta delta gradient of 0 indicate?

Answer 16

The change in the anion gap is completely accounted for by the loss of bicarb, so it is a simple metabolic acidosis (there is not an additional non anion gap metabolic acidosis/alkalosis going on)

Question 17

When should you calculate the delta delta?

Answer 17

When there is an anion gap metabolic acidosis to determine if there are any additional metabolic processes going on

Question 18

What does it mean if the delta delta gradient is greater than zero?

Answer 18

There is more bicarb than you would expect if there was just an anion gap metabolic acidosis

Note: There may be an additional metabolic alkalosis (vomiting, etc.), respiratory acidosis (opioid overdose, etc), or a non-acidic high anion gap stare (as can happen with excess penicillin)

Question 19

What does it mean if the delta delta gradient is less than 0?

Answer 19

There is less bicarb than you would have expected if there was only an anion gap metabolic acidosis

Note: There may be an additional non-gap acidosis (diarrhea, renal bicarb loss, etc) or a preexisting low anion gap state (hyperchloremia, para-proteinemia, etc)

Question 20

Common causes of anion gap metabolic acidosis

Answer 20

Methanol
Uremia
DKA
Paraldehyde
Isoniazid
Lactic acidosis
Ethylene glycol/alcohol
Salicylates (aspirin)

Question 21

Common causes of non gap metabolic acidosis

Answer 21

Loss of bicarb via:

Diarrhea
Renal tubular acidosis (especially type 1)
Carbonic anhydride inhibitors (such as acetazolamide)
Addison’s disease (unlike the other causes, which are associated with hypokalemia, Addisons is associated with hyperkalemia)

Question 22

Common causes of metabolic alkalosis

Answer 22

Vomiting (alkalosis responsive to NaCl)
Cushings disease (alkalosis not responsive to NaCl)

Question 23

Common causes of acute respiratory acidosis

Answer 23

COPD exacerbation

Opioid overdose

Question 24

Common causes of chronic respiratory acidosis

Answer 24

COPD (compensated)

Question 25

Common causes of an acute respiratory alkalosis

Answer 25

Hyperventilation:

Anxiety
Asthma exacerbation

Question 26

Common causes of chronic respiratory alkalosis

Answer 26

Pregnancy

Question 27

7.47/29/94/22 with AG of 12

Answer 27

Acute respiratory alkalosis

Bicarb should drop by 1 for every 5 points lost in pCO2 in acute respiratory alkalosis

Question 28

What is the fastest way to get a complete analysis of an ABG?

Answer 28

• determine the anion gap (if there is a gap you know there is an anion gap metabolic acidosis and you should calculate the delta delta gradient to see if there’s another process going on)
• look at the pCO2 and bicarb to determine whether the primary disorder is respiratory or metabolic
• look at the pH to determine whether there is an acidemia or alkalemia

Question 29

7.42/29/94/19 with AG of 11

Answer 29

Chronic respiratory alkalosis

Bicarb drops by 1 for every 2 pCO2 lost in chronic respiratory alkalosis

Question 30

7.26/60/55/26 with AG 10

Answer 30

Acute respiratory acidosis

Note: Bicarb should increase from 24 by 1 point for every 10 points gained in pCO2 in an acute respiratory acidosis (bicarb should increase by 1 for every 3 pCO2 gained if chronic)

Question 31

7.34/60/55/30 with AG 10

Answer 31

Chronic respiratory acidosis

Note: Bicarb should increase from 24 by 1 point for every 3 points gained in pCO2 in a chronic respiratory acidosis (bicarb should increase by 1 for every 10 pCO2 gained if acute)

Question 32

7.25/27/99/12 with AG 32

Answer 32

Well compensated anion gap metabolic acidosis with additional metabolic alkalosis

Delta delta gradient is (32-12) - (24-12) = 20-12 = 8 (more bicarb than expected)

Lungs are compensating appropriately for metabolic acidosis according to winters formula (which estimates that pCO2 should be 24-28, which it is)

Question 33

7.25/27/99/12 with AG 10

Answer 33

Primary metabolic acidosis

Lungs are well compensating because for every point drop in bicarb there is approximately 1 point drop in pCO2

Question 34

7.50/48/90/38 with AG 12

Answer 34

Well compensated primary metabolic alkalosis

pCO2 went up by 8 for an increase of 14 in bicarb (pCO2 should increase by 1 for every 2 point increase in bicarb if well compensated metabolic alkalosis)

Question 35

7.12/40/55/15 with AG 27

Answer 35

Anion gap metabolic acidosis

Additional metabolic alkalosis (delta delta gradient is 15-9 = 6, too much bicarb)

Additional respiratory acidosis (for bicarb drop of 9 there should have been a pCO2 drop of about 9, 1:1 ratio for metabolic acidosis; there must be excess CO2 due to an additional respiratory acidosis)

Question 36

7.44/20/75/15 with AG 30

Answer 36

Anion gap metabolic acidosis

Additional metabolic alkalosis (delta delta gradient is 18-9=9, too much bicarb)

Additional respiratory alkalosis (for a bicarb drop of 9 there should have been a pCO2 drop of 9, but there was a drop of 20 pCO2, so there must be an additional respiratory alkalosis)

Question 37

7.38/39/90/23 with AG 37

Answer 37

Anion gap metabolic acidosis

Additional metabolic alkalosis (delta delta gradient is 25-1 = 24, way too much bicarb around)

Good respiratory compensation (bicarb decrease of 1 should be associated with a pCO2 decrease of 1, which it is in this case)