The ABC’s of ABG’s or How to read a blood gas without the Hassel(bach)

A blood gas interpretation is often a fear inducing “pimp” question. Probably because there is a so much packed into them and at some point, some basic math is needed. So, let’s try to unpack it a little so we have more method and less madness. I’m going to divide this up into 4 parts: The explanation (Part I), The calculation of the ABG (Part II), The Differential Diagnoses (Part III), The Practice (Part IV)

PART I – THE EXPLANATION

Blood gases are ruled by the often cited but never remembered: Henderson-Hasselbach equation:I think its easier to remember written in the ABCD form:

There are only FIVE RULES to understanding ABG’s

1. The primary disorder causes the pH
2. The primary disorder is moves with the pH in the resulted direction
   1. Metabolic disorders are like a boy band…They always changes in ONE DIRECTION (i.e pH goes in the same direction as the primary disorder)
   2. Credit to Joel Topf of the curbisders podcast for that dad joke…
3. Compensation occurs in the same direction as the primary disorder
4. The body can’t make a large number of anions so an anion gap always means a primary metabolic acidosis is present
5. A second primary disorder exists when the compensation doesn’t completely correct for the problem

PART II – THE CALCULATION OF THE ABG

The 4 steps in reading an ABG:

Step I: Determine the pH and Primary disorder:

–      If pH, HCO3^–, and pCO₂ are ONE DIRECTION then primary disorder is metabolic

e.g. All down:    <7.40, CO2 <40, HCO3 <24 = metabolic acidosis

e.g. All up:     >7.40, CO2 >40, HCO3 >24 = metabolic alkalosis

–    If pH, pCO2, HCO3 are in opposite directions then primary disorder is respiratory

Step II: Determine if there is a gap acidosis. If there is, then a  gap acidosis must be present

• Gap = Na (corrected for glucose) – (Cl + HCO3)
• PEARL: Remember to use the bicarb from the BMP/CMP not the ABG!
• An AG of >30 is very likely to have an AGMA
• An AG 20-29 then clinically 1/3 will not have metabolic acidosis
  • The Bicarb is CALCULATED in the ABG and MEASURED in the BMP
    • The pCO2/HCO3 ratio should always be checked
      • H+ = 24 x pCO2/HCO3-
  • Na is falsely low in hyperglycemia and must be corrected to get the correct Na. To correct do the following: For every 100 over 100 glucose add 1.6 to Na
  • Some people correct the  gap for albumin but you probably don’t need to. However, if you did it would be:
    • Corrected gap = AG + [2.5x(4-albumin)]

Step III: Determine Compensation (occurs in the same direction as the primary disorder)

• Remember pH is inversely related to pCO2 and directly proportional to HCO3
• If the HCO3 is low then the PCO2 should lower to compensate
• See Compensation question below
  • HOWEVER: YOU REALLY ONLY NEED TO KNOW THE WINTERS FORMULA HERE: PCO2 = 1.5 x HCO3 + 8 (± 2)
  • THIS IS BECAUSE YOU CAN’T HAVE A RESPIRATORY ACIDOSIS AND ALKALOSIS BUT CAN HAVE A METABOLIC ACIDOSIS AND ALKALOSIS
  • Some say if the last two digits of the pH = pCO2 then NO respiratory disturbance occurs (eg. pH = 7.40 and pCO2= 40 then no respiratory disturbance)

Step IV:  Calculate the excess (or Delta) Gap (that is take out the GAP):

• (Anion Gap – 12) + HCO3
• If Excess > 25 then underlying Metabolic Alkalosis
• If Excess < 23 then underlying Non-gapacidosis

USE THIS SHORTCUT:

Delta Gap =∆AG-∆HCO -=Na+-(Cl++HCO -)-12-(24-HCO -)

=Na+-Cl – 36

If the DG is significantly positive (>+6), a metabolic alkalosis  (IN ADDITION TO AGMA) is present because the rise in AG is more than the fall in HCO3-.

Conversely, if the DG is significantly negative (<-6), then a hyperchloremic (non-gap) acidosis (IN ADDITION TO AGMA) is present because the rise in AG is less than the fall in HCO3-.

*You could stop at the above step at get most of the way there*

OPTIONAL FURTHER STEP:

Step IVa: Calculate “correction equations” to find the second primary disorder

• Is there ENOUGH compensation to make up for the primary disorder (Is there a SECOND PRIMARY disorder?) ∆= Delta = Change
• Correction equations can be made into a mnemonic (not a great one but kinda) if you remember things alphabetically (metabolic then respiratory, acidosis then alkalosis, acute then chronic) and the numbers  1.5 – 8 = 7, 1,2,3,4:
  • Metabolic acidosis:           pCO2 =  1.5 × HCO₃ + 8 ± 2 (Winter’s formula)
  • Metabolic alkalosis:           ∆ pCO2 = 9x [∆ HCO3] OR                                         (pCO2 =0.9x HCO3+9±5) [Narins]
  • (Acute) Respiratory acidosis:   ∆pCO2:∆HCO3 changes 10:1
  • (Acute) Respiratory alkalosis:  ∆pCO2:∆HCO3 changes 10:2
• (Chronic) Respiratory acidosis:         ∆pCO2:∆HCO3 changes 10:4
• (Chronic) Respiratory alkalosis:       ∆pCO2:∆HCO3 changes 10:3
• Alternately remembered as: The RESPIRATORY corrections table 

	∆pCO2 : ∆HCO3
	Acidosis	Alkalosis
Acute	10:1	10:2
Chronic	10:4	10:3
	FOR EVERY RISE OF 10 pCO2 HCO3 WILL RISE BY: 1 or 4	FOR EVERY FALL OF 10 pCO2 HCO3 WILL RISE BY: 2 or 3

SUMMARY

STEP I:   LOOK AT THE PH (>7.40 is alkalosis, <7.40 is acidosis)
STEP II:  CALCULATE THE ANION GAP FOR A METABOLIC GAP ACIDOSIS
STEP III: CALCULATE THE DELTA GAP (Na-Cl-36; if <6 NAGMA
          if >6 there is a metabolic alkalosis)
STEP IV:  CALCULATE FOR A COMPENSATION TO SEE ADDITIONAL PROCESS 

PART III – THE DIFFERENTIAL DIAGNOSES

1. ANION GAP METABOLIC ACIDOSIS: (CAT MUDPILES OR GOLDMARK)

         Old: CAT MUDPILES                                                       New: GOLDMARK

C	CO, CN		G	Glcyols (ethylene and propylene)
A	AKA		O	5-oxoproline (Pyroglutamic Acid) [from chronic acetaminophen toxicity]
T	Toluene		L	L-Lactic acidosis
M	Methanol		D	D-Lactic acidosis (short gut syndromes)
U	Uremia		M	Methanol
D	DKA		A	ASA
P	PARALDEHYDE, Pyroglutamic Acid, Phenphormin, Paraquat, Propylene Glycol		R	Renal Failure
I	INH, Fe, Ibuprofen (large doses)		K	Ketosis (DKA/AKA)
L	Lactate
E	Ethylene glycol
S	Salicylates

2. NON GAP METABOLIC ACIDOSIS: (HARD)

H	Hyperchloraemia
A	Acetazolamide, Addison’s
R	RTA’S
D	Diarrhea from ileostomies, fistulas

Note:

Use urinary anion gap [= (Na+ + K+) – Cl-] to differentiate between GI and renal causes

The remaining significant ions are NH4+ or  HCO3-

Renal causes increase HCO3- excretion thus increased urinary AG

GI causes increase NH4+ excretion thus decreased urinary AG

 

3. LOW GAP (NOT ACTUALLY AN ACIDOSIS): [3-LAMB]’s:

3-L	Lytes (Ca,K, Na, Mg), Lipids, Lithium
A	Albumin
M	Multiple Myeloma (IgG – cationic; IgA is anionic)
B	Bromide, polymyxin B

• Analytical errors like increased Na+ (most common), increased viscosity, iodide, increased triglycerides)
• Decrease in anions (albumin, dilution)
• Increase in cations (multimyeloma (IgG – is a cation; IgA is an anion), hyperkalemia, hypercalcemia, hypermagnesemia, lithium, polymixin B)
• Bromide OD (causes falsely elevated chloride measurements)

4. METABOLIC ALKALOSIS:

Alkaline Input

• Bicarbonate Infusion
• Hemodialysis
• Calcium Carbonate
• Parenteral Nutrition

Proton Loss

• GI Loss (vomiting, NG suction)
• Renal loss
• Diuretics
• Mineralocorticoids

5. RESPIRATORY ACIDOSIS: (CLIMB)

C	CO2 overproduction (Malignanty Hyperthermia) or CNS Depression (Trauma or Toxins)
L	Lung obstruction/injury (Upper or Lower)
I	Inadequate ventilation
M	Myopathies
B	OBesity – Pickwickian syndrome

 6. RESPIRATORY ALKALOSIS: (Only 2 general causes)

– Stimulated Respiratory Drive

– Hypoxemia

PART IV PRACTICE PROBLEMS:

HERE ARE THE (ABBREVIATED) STEPS AGAIN:

Step I: Determine the pH and Primary disorder:

• If pH, HCO3^–, and pCO₂ are ONE DIRECTION then primary disorder is metabolic

Step II: Determine if there is a gap acidosis.

• Gap = Na (corrected for glucose) – (Cl + HCO3)

Step III:  Calculate the excess (or Delta) Gap:

• Na – Cl – 36
• If Excess > 6 then underlying Metabolic Alkalosis
• If Excess < 6 then underlying Non-AGMA

Step IV: Determine Compensation (same direction as the primary disorder)

 

Practice Problems:

1. pH 7.50 / pCO2 20 / HCO3 15 / Na 140 / Cl 103
2. pH 7.40 / pCO2 40 / HCO3 24 / Na 145 / Cl 100
3. pH 7.10 / pCO2 50 / HCO3 15 / Na 145 Cl 100
4. pH 7.37 / pCO₂ 18 / HCO₃ 10
5. pH 7.50 / pCO₂ 48 / HCO₃ 36
6. pH 7.35 / pCO₂ 56 / HCO₃ 30
7. pH 7.56 / pCO₂ 22 / HCO₃ 23
8. pH 7.14 / pCO₂ 18 / HCO₃ 8 / Na 134 / Cl 104
9. pH 7.45 / pCO₂ 17 / HCO₃ 12 / Na 139 / Cl 114

Practice Answers (Primary disorder is listed first)

1. Respiratory Alkalosis and Anion Gap Metabolic Acidosis (e.g. aspirin overdose)
2. Gap Acidosis AND metabolic alkalosis (e.g. A vomiting renal failure patient)
3. Primary Respiratory alkalosis, Gap Acidosis AND metabolic alkalosis
4. Metabolic acidosis, predicted pCO₂ = 23, Respiratory alkalosis
5. Metabolic alkalosis, pCO₂ 48
6. Respiratory acidosis HCO₃ acute: 26, HCO₃ chronic 29, Metabolic alkalosis
7. Respiratory alkalosis, HCO₃ acute: 20, HCO₃ chronic 16, Metabolic alkalosis
8. Metabolic acidosis that is a gap acidosis with an additional non-gap acidosis
9. Respiratory alkalosis with both a gap and non-gap metabolic acidosis.

References: 

1: Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical
approach. Medicine (Baltimore). 1980 May;59(3):161-87. PubMed PMID: 6774200.

2:  Baillie JK. Simple, easily memorised ‘rules of thumb’ for the rapid assessment of physiological compensation for respiratory acid-base disorders. Thorax 2008;63:289-290 doi:10.1136/thx.2007.09122

3. Haber RJ. A practical approach to acid-base disorders. West J Med. 1991
Aug;155(2):146-51. Review. PubMed PMID: 1843849; PubMed Central PMCID:
PMC1002945.