Acid-Base Disorder analysis and interpretation through ABGs. Step wise approach to analysis

1. ABG
ANALYSIS & ACID-BASE DISORDERS
Dr. Haseeb Ahmed
2008-98
ZMU

2. STEPS FOR ABG ANALYSIS
1. What is the pH? Acidemic or Alkalemic?
2. What is the primary disorder present?
3. Is there appropriate compensation?
4. Is the compensation acute or chronic?
5. Is there an anion gap?
6. If there is a AG, what is the delta gap?
7. What is the differential for the clinical
processes?

3. STEP 1:
 Look at the pH: is the blood acidemic or alkalemic?
Variable Normal Range
pH - 7.37 - 7.43
pCO2 - 35-45 -
Bicarbonate - 22-26
NORMAL VALUES

4. 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

5. STEP 3: IS THERE APPROPRIATE
COMPENSATION?
 Respiratory Acidosis
 Acute: for every 10 increase in pCO2 -> HCO3 increases by 1
 Also know for every acute increase of 10 in pCO2 there is a
decrease of 0.08 in pH.
 Chronic: for every 10 increase in pCO2 -> HCO3 increases by
4
 Also know for every chronic increase of 10 in pCO2 there is a
decrease of 0.03 in pH
 Respiratory Alkalosis
 Acute: for every 10 decrease in pCO2 -> HCO3 decreases by
2
 Chronic: for every 10 decrease in pCO2 -> HCO3 decreases
by 5

6. STEP 3: IS THERE APPROPRIATE
COMPENSATION?
 Metabolic Acidosis
 Winter’s formula: pCO2 = 1.5[HCO3] + 8 ± 2 MEMORIZE
 Winter’s formula calculates the expected pCO2 in the setting
of metabolic acidosis. If the serum pCO2 > expected pCO2
then there is additional respiratory acidosis in which the
etiology needs to also be determined.
 Metabolic Alkalosis
 For every 10 increase in HCO3 -> pCO2 increases by 6

7. STEP 4: CALCULATE THE ANION GAP
 AG = Na – Cl – HCO3 (normal 12 ± 2)
 AG corrected = AG + 2.5[4 – albumin]
 If anion gap is greater than 20, a metabolic acidosis
is always present regardless of the pH or serum
bicarbonate concentration because the body is not
able to physically generate such a large anion gap
via purely compensatory mechanisms (i.e. even in
the setting of chronic respiratory alkalosis).
Therefore, there must be a primary metabolic
disorder present.

8. Differential for Anion Gap Metabolic Acidosis -
MUDPILERS
Methanol
Uremia
Diabetic ketoacidosis, starvation ketoacidosis, EtOH
ketoacidosis
Paraldehyde
INH, iron toxicity
Lactic acidosis
Ethylene glycol
Rhabdomyolysis
Salicylates

9. STEP 5: CALCULATE THE DELTA GAP
 Only need to calculate delta gap (excess anion gap)
when there is an anion gap present to determine
additional hidden metabolic disorders (nongap
metabolic acidosis or metabolic alkalosis)
 Delta gap = AG – 12 + HCO3 (normal 22-26)
 If delta gap > 26 -> additional metabolic alkalosis
 If delta gap < 22 -> additional nongap metabolic
acidosis
 If delta gap 22 – 26 -> no additional metabolic
disorders

10. THE DELTA GAP
 Delta gap is equivalent to excess anion gap. The principle
behind this formulation is that for each mMol of acid titrated
by the carbonic acid buffer system, 1 mMol of bicarbonate is
consumed as water and carbon dioxide and 1 mMol of
sodium salt of acid is formed. Therefore, each mMol
decrease in bicarbonate is accompanied by a mMol
increase in the anion gap.
 Delta bicarb = Delta anion gap. The sum of the new
(excess) anion gap and the remaining (measured)
bicarbonate should be equal to a normal bicarbonate
concentration. If the sum of the excess anion gap and the
measured bicarbonate value exceeds the normal
bicarbonate concentration, then an additional metabolic
alkalosis must be present. If the sum is less than normal,
there must be an additional nongap metabolic acidosis. If
the delta gap is equal to expected, there is no additional

11. NONGAP METABOLIC ACIDOSIS
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
For nongap metabolic acidosis, calculate the urine anion gap
UAG = UNA + UK – UCL
If UAG>0: renal problem
If UAG<0: nonrenal problem (most commonly GI)
In working kidneys: HCl + NH3 ↔ NH4CL, urine chloride increases,
UAG <0.

12. 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 Syndrome
Over-diuresis Exogenous corticosteroid
administration
Post-hypercapnia

13. 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

14. 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

15. CASE 1
65yo M with CKD presenting with nausea, diarrhea and acute
respiratory distress.
ABG 7.23/17/235 on 50% VM
Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1

16. CASE 1 ANSWER
 Primary metabolic acidosis – gap of 19
(uremia/renal failure causing gap met
acidosis), delta gap 14 -> additional non gap
metabolic acidosis (diarrhea). Winter’s
formula 18 -> no additional respiratory
acidosis.

17. CASE 2
60yo M with COPD on steroids presenting with worsening
SOB, hypoxia, and hypotension
ABG 7.38/54/
Na 134/ Cl 77/ HCO3= 33

18. CASE 2 ANSWER
 Primary respiratory acidosis with chronic
metabolic compensation (COPD). Gap = 24.
Gap metabolic acidosis (sepsis). Delta gap =
35 -> additional metabolic alkalosis from
exogenous steroids. Triple disorder

19. CASE 3
28yo F who is 28 weeks pregnant, diabetic, previous alcoholic
who recently stopped insulin and started binge drinking
ABG 7.60/21/
Na 136/ Cl 80/ HCO3 19

20. CASE 3 ANSWER
 Primary respiratory alkalosis with acute
metabolic compensation (pregnancy,
anxiety). Gap = 37 Gap metabolic acidosis
(DKA, alcoholic ketoacidosis). Delta gap 34 -
> additional metabolic alkalosis (vomiting).
Another triple ripple

21. CASE 4
17yo F with a history of depression is brought in altered to the
ED by her mother, who reports finding multiple empty
medication bottles around her.
ABG 7.50/20/
Na 140/ Cl 103/ HCO3 15

22. CASE 4 ANSWER
 Primary respiratory alkalosis with chronic
metabolic compensation (hyperventilation).
Gap = 22. Gap metabolic acidosis
(salicylates). Delta gap 25 -> no additional
metabolic disorders. ASA overdose

23. CASE 5
A 45yo F with Type 1 Diabetes is admitted with a gastroenteritis,
hyperglycemia and confusion.
ABG 7.10/50/102
BMP Na 145/Cl 100 / HCO3 15

24. CASE 5 ANSWER
 Primary respiratory acidosis (obtunded) with
gap metabolic acidosis , gap = 30 (DKA).
Delta gap 33 -> additional nongap metabolic
alkalosis (vomiting).