ABG analysis & Acid-Base Disorders
2012
Outline
1.
2.
3.
4.
Discuss simple steps in analyzing ABGs
Calculate the anion gap
Calculate the delta gap
Differentials for specific acid-base disorders
Steps for ABG analysis
1.
2.
3.
4.
5.
6.
7.
What is the pH? Acidemia or Alkalemia?
What is the primary disorder present?
Is there appropriate compensation?
Is the compensation acute or chronic?
Is there an anion gap?
If there is a AG check the delta gap?
What is the differential for the clinical processes?
Normal Values
Variable
pH
Normal Range
7.35 - 7.45
pCO2
35-45
Bicarbonate
22-26
Anion gap
10-14
Albumin
4
Step 1:

Look at the pH: is the blood acidemic or alkalemic?

EXAMPLE :
65yo M with CKD presenting with nausea, diarrhea and acute
respiratory distress




ABG :ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1
ACIDMEIA OR ALKALEMIA ????
EXAMPLE ONE
ABG 7.23/17/235 on 50% VM
 BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr
5.1
 Answer PH = 7.23 , HCO3 7
 Acidemia

Step 2: What is the primary disorder?
What disorder is
present?
pH
pCO2 or HCO3
Respiratory Acidosis
pH low
pCO2 high
Metabolic Acidosis
pH low
HCO3 low
Respiratory Alkalosis
pH high
pCO2 low
Metabolic Alkalosis
pH high
HCO3 high
EXAMPLE




ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.
PH is low , CO2 is Low
PH and PCO2 are going in same directions then its most likely
primary metabolic will check to see if there is a mixed disoder.
Step 3-4: Is there appropriate
compensation? Is it chronic or acute?

Respiratory Acidosis



Acute: for every 10 increase in pCO2 -> HCO3 increases by 1 and
there is a decrease of 0.08 in pH MEMORIZE
Chronic: for every 10 increase in pCO2 -> HCO3 increases by 4
and there is a decrease of 0.03 in pH
Respiratory Alkalosis


Acute: for every 10 decrease in pCO2 -> HCO3 decreases by 2 and
there is a increase of 0.08 in PH MEMORIZE
Chronic: for every 10 decrease in pCO2 -> HCO3 decreases by 5
and there is a increase of 0.03 in PH
Step 3-4: Is there appropriate
compensation? Is it acute or chronic ?

Metabolic Acidosis
 Winter’s formula: pCO2 = 1.5[HCO3] + 8 ± 2 MEMORIZE
 If serum pCO2 > expected pCO2 -> additional respiratory
acidosis

Metabolic Alkalosis
 For every 10 increase in HCO3 -> pCO2 increases by 6
EXAMPLE




ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.
Winter’s formula : 17= 1.5 (7) +8 = 18.5
So correct compensation so there is only one
disorder Primary metabolic
Step 5: Calculate the anion gap




AG = Na – Cl – HCO3 (normal 12 ± 2)
AG corrected = AG + 2.5[4 – albumin]
If there is an anion Gap then calculate the
Delta/delta gap (step 6). Only need to calculate
delta gap (excess anion gap) when there is an anion
gap to determine additional hidden metabolic
disorders (nongap metabolic acidosis or metabolic
alkalosis)
If there is no anion gap then start analyzing for
non-anion acidosis
EXAMPLE

Calculate Anion gap



ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/ Albumin 4.
AG = Na – Cl – HCO3 (normal 12 ± 2)
123 – 97 – 7 = 19

No need to correct for albumin as it is 4
Step 6: Calculate the different needed
formulas





Delta gap = (actual AG – 12) + HCO3
Adjusted HCO3 should be 24 (+_ 6) {18-30}
If delta gap > 30 -> additional metabolic alkalosis
If delta gap < 18 -> additional non-gap metabolic
acidosis
If delta gap 18 – 30 -> no additional metabolic
disorders
EXAMPLE : Delta Gap
ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/ Albumin 4.


Delta gap = (actual AG – 12) + HCO3
 (19-12) +7 = 14
 Delta gap < 18 -> additional non-gap metabolic
acidosis
 So Metabolic acidosis anion and non anion gap

Metobolic acidosis: Anion gap acidosis
EXAMPLE: WHY ANION GAP?

65yo M with CKD presenting with nausea, diarrhea and acute
respiratory distress




ABG :ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1
So for our patient for anion gap portion its due to BUN
of 119 UREMIA
But would still check lactic acid
Nongap metabolic acidosis
For non-gap metabolic acidosis, calculate the urine anion gap
UAG = UNA + UK – UCL
If UAG>0: renal problem
If UAG<0: nonrenal problem (most commonly GI)
Causes of nongap metabolic acidosis - DURHAM
Diarrhea, ileostomy, colostomy, enteric fistulas
Ureteral diversions or pancreatic fistulas
RTA type I or IV, early renal failure
Hyperailmentation, hydrochloric acid administration
Acetazolamide, Addison’s
Miscellaneous – post-hypocapnia, toulene, sevelamer, cholestyramine ingestion
EXAMPLE : NON ANION GAP ACIDOSIS

65yo M with CKD presenting with nausea, diarrhea and acute
respiratory distress



ABG :ABG 7.23/17/235 on 50% VM
BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1
Most likely due to the diarrhea
Metabolic alkalosis


Calculate the urinary chloride to differentiate saline
responsive vs saline resistant
Must be off diuretics in order to interpret urine chloride
Saline responsive UCL<10
Saline-resistant UCL >10
Vomiting
If hypertensive: Cushings, Conn’s, RAS,
renal failure with alkali administartion
NG suction
If not hypertensive: severe hypokalemia,
hypomagnesemia, Bartter’s, Gittelman’s,
licorice ingestion
Over-diuresis
Exogenous corticosteroid administration
Post-hypercapnia
Respiratory Alkalosis
Causes of Respiratory Alkalosis
Anxiety, pain, fever
Hypoxia, CHF
Lung disease with or without hypoxia – pulmonary embolus, reactive
airway, pneumonia
CNS diseases
Drug use – salicylates, catecholamines, progesterone
Pregnancy
Sepsis, hypotension
Hepatic encephalopathy, liver failure
Mechanical ventilation
Hypothyroidism
High altitude
Respiratory Acidosis
Causes of respiratory acidosis
CNS depression – sedatives, narcotics, CVA
Neuromuscular disorders – acute or chronic
Acute airway obstruction – foreign body, tumor, reactive airway
Severe pneumonia, pulmonary edema, pleural effusion
Chest cavity problems – hemothorax, pneumothorax, flail chest
Chronic lung disease – obstructive or restrictive
Central hypoventilation, OSA
Steps for ABG analysis
1.
2.
3.
4.
5.
6.
7.
What is the pH? Acidemic or Alkalemic?
What is the primary disorder present?
Is there appropriate compensation?
Is the compensation acute or chronic?
Is there an anion gap?
If there is a AG, what is the delta gap?
What is the differential for the clinical processes?