1. ABG Interpretation & Acid-Base Disorders
Jeremy Barnett MD ~ Department of Critical Care
Amy Gutman ~ Department of Emergency Medicine

2. Outline ABG analysis steps Calculating anion gap Calculating delta gap
Differentials for specific acid-base disorders
A-a Gradients
-Objective slide

3. What Are You Ordering When Requesting An ABG?
PO2
PCO2 pH
Electrolytes (Na+, K+, Ca++)
Lactate
HCO3-
SaO2
Other calculated results

4. ABG Analysis Steps Is the pH acidemic or alkalemic?
Is the primary disorder respiratory or metabolic?
Is the compensation appropriate?
Is the compensation acute or chronic?
Is there an anion gap?
If there is a anion gap, does the delta-delta indicate an additional non-anion gap?
What is the differential diagnosis for the clinical process(es)?

5. Normal Arterial vs Venous Blood Gas ValuespH
7.25 – 7.45
7.31 – 7.41
pCO2
35-45mmHg
40-50mmHg
pO2
75-100mmHg
36-42mmHg
HCO3
22-26meQ/L BE
O2 Saturation
>95%
60-80%
Anion Gap
10-14
Albumin 4

6. ABG Interpretation Steps
Hypoxia?
Step 2
Acedemia or Alkalemia?
Step 3
Primary Disorder (Respiratory or Metabolic)?
Step 4
Compensation Appropriate, Acute & / or Chronic?
Step 5
Anion Gap?
Step 6
Delta Gap?
Step 7
Differential Diagnosis?

7. Step 1: Determine Adequacy of Oxygenation (PaO2)
?A-a Gradient
Is Patient Hypoxic (Normal: mmHg) ?
Hypoventilation
No Defect
Compensated
Defect
Other Defect NO
YES
?A-a Gradient
NORMAL
HIGH
NORMAL
HIGH

8. Step 2: Determine pH (Acidemic vs Alkalemic)
Normal pH range: 7.35–7.45
pH <7.4 = Acidosis / increased serum hydrogen ion concentration
pH >7.4 = Alkalosis / decreased serum hydrogen ion concentration
Both metabolic & respiratory abnormalities can alter pH
Respiratory & renal function strive to keep pH = 7.4
Minute ventilation responds quickly to metabolic acid-base problems
Renal bicarbonate excretion or retention takes days to compensate for respiratory acid-base problems
For our purposes, 3 questions:
Is abnormality respiratory or metabolic?
If respiratory, is it acute or chronic?
If metabolic, is respiratory system responding appropriately?

9. Step 3: Determine Primary Disorder
Acidemic (pH < 7.40)
Alkalemic (pH > 7.40)
PCO2
PCO2
> 40mmHg
< 40mmHg
> 40mmHg
< 40mmHg
Respiratory Acidosis
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Alkalosis

10. Primary Disorder
Disorder?
pH?
pCO2 or HCO3?
Respiratory Acidosis
pH low
pCO2 high
Metabolic Acidosis
HCO3 low
Respiratory Alkalosis
pH high
pCO2 low
Metabolic Alkalosis
HCO3 high
If pH & PCO2 going in opposite direction = Respiratory
If pH & PCO2 going in same direction = Metabolic
May not work with mixed disorders

11. Step 4: Appropriate Compensation? Chronic or Acute?
Acute Respiratory Acidosis
For every 10 increase in pCO2, HCO3 increases by 1, pH decreases 0.08
Acute Respiratory Alkalosis
For every 10 decrease in pCO2, HCO3 decreases by 2, pH increases 0.08
Chronic Respiratory Acidosis
For every 10 increase in pCO2, HCO3 increases by 4, pH decreases 0.03
Chronic Respiratory Alkalosis
For every 10 decrease in pCO2, HCO3 decreases by 5, pH increases 0.03
Partial Compensation: Change in pH between for every 10mmHg change in PCO2

12. Respiratory Component (PaCO2)?
Normal range: PaCO2 35–45 mmHg
Primary respiratory acidosis if pH <7.35 & HCO3– normal
PaCO2 >45mmHg: Respiratory compensation for metabolic alkalosis if pH >7.45 & HCO3– increased
PaCO2 <35mmHg: Primary respiratory alkalosis if pH >7.45 & HCO3– normal
Respiratory compensation for metabolic acidosis if pH <7.35 & HCO3– decreased

13. Metabolic Component (HCO3)?
Normal HCO3– range 22–26mmol/L
HCO3 <22mmol/L: Primary metabolic acidosis if pH <7.35
Renal compensation for respiratory alkalosis if pH >7.45
HCO3 >26mmol/L: Primary metabolic alkalosis if pH >7.45
Renal compensation for respiratory acidosis if pH <7.35

14. Base Excess ~ Degree of Metabolic Disturbance
Bicarbonate poor indicator of acid-base disturbance as affected by respiratory & metabolic components
Base excess (-2 to +2mEq/L)
Dose of acid or alkali to return blood to normal pH
Examples:
Abnormal base excess with normal AG
Normal AG metabolic acidosis (hypercholoremia, GI HCO3 loss, RTA)
Normal base excess with abnormal AG
Lactic acidosis, HAGMA with pre-existing metabolic alkalosis
HAGMA masked by hypoalbuminemia (uncorrected AG)
ASA toxicity: respiratory alkalosis + increased AG metabolic acidosis

15. Appropriateness of Respiratory Response to Metabolic Acidosis
Predicted Change in PCO2 = (1.5 x HCO3) + 8
If patient’s PCO2 is roughly this value, the response is appropriate
If patient’s PCO2 higher than this value, they are failing to compensate

16. Winter’s Formula ~ Predicting pCO2 in Metabolic Acidosis
In metabolic acidosis the expected pCO2 estimated from HCO3
pCO2 (actual) > pCO2 (predicted) = additional respiratory acidosis (“mixed acidosis”)
pCO2 (actual) < pCO2 (predicted) = respiratory alkalosis
Expected pCO2 = (1.5 x HCO3)

17. Step 5: Calculate Anion Gap
AG = Na+ – (HCO3- + Cl-)
Alternative: (Na+ + K) – (HCO3- + Cl-)
Normal 12 ± 2
If albumin <4 add 2.5 to AG for every decrease of 1
Corrected = AG [4 – albumin]
If no AG, evaluate for non-anion gap acidosis
If there is an AG, calculate delta gap (“excess anion gap”)
Calculate delta gap if AG to determine “hidden” metabolic disorders (i.e. non-AG metabolic acidosis, metabolic alkalosis)

18. Step 6: Calculate Delta Gap
Compares increase in anion gap to the decrease in HCO3
Delta gap = (Actual AG – 12) + HCO3
Delta gap >30 = additional metabolic alkalosis
Delta gap < 18 = additional non-gap metabolic acidosis
Delta gap 18–30 = no additional metabolic disorders
Must memorize how to calculate the delta gap
Just read off the slide

19. Example Delta Gap Calculation
ABG 7.23 / 17 / 235 on 50% ventimask
BMP Na 123 / Cl 97/ HCO3 7/ Albumin 4
Delta gap = (actual AG – 12) + HCO3
= (19-12) + 7 = 14
Delta gap <18 = Additional non-AG metabolic acidosis
= Metabolic acidosis anion & non-anion gaps

20. Delta Ratio & Delta Gap ~ High Anion Gap Metabolic Acidosis (HAGMA)
Evaluate if high AG metabolic acidosis
Delta Ratio = AG = (AG – 12) HCO3 = (24 – HCO3)
Delta Gap = (Actual AG – 12) + HCO3
< Hyperchloremic Non-AG metabolic acidosis
0.4– HAGMA + NAGMA
1-2 Pure HAGMA
>2 Metabolic Acidosis w/ pre- existing elevated HCO3 (metabolic alkalosis or respiratory acidosis)
DG > 30 Additional metabolic alkalosis
DG < 18 Additional non-anion gap metabolic acidosis
DG 18–30 No additional metabolic disorder

21. Expected Change in Acid-Base Disorders
Primary Disorder
Expected Change
Metabolic acidosis
PCO2 = 1.5 × HCO3 + (8 ± 2)
Metabolic alkalosis
PCO2 = 0.7 × HCO3 + (21 ± 2)
Acute respiratory acidosis
Delta pH = × (PCO2 - 40)
Chronic respiratory acidosis
Delta pH = × (PCO2 - 40)
Acute respiratory alkalosis
Delta pH = × (40 - PCO2)
Chronic respiratory alkalosis
Delta pH = × (40 - PCO2)

23. Non-Anion Gap Metabolic Acidosis
HARD-UP
FUSED-CARS
Hyperalimentation, Hydrochloric Acid
Fistula
Acetazolamide, Addison’s
Ureterogastric conduits
RTA I, IV, Renal disease
Saline administration
Diarrhea
Endocrine (hyperparathyroid)
Uretero-Pelvic Shunt
Post hypocapnia
Carbonic Anhydrase inhibitors
RTA
Spironolactone
For non-AG metabolic acidosis, calculate urine anion gap
UAG = UNa+ + UK+ – UCL-
If UAG>0: renal problem
If UAG<0: non-renal problem

24. Anion Gap Metabolic Acidosis
CAT - MUDPILES
Carbon monoxide, Cyanide, Congenital heart failure
Aminoglycosides
Tolulene, Theophylline
Methanol
Uremia
DKA, alcoholic ketosis, starvation ketosis
Propylene glycol, Paracetamol, Paraldehyde
Isoniazid, Iron, Inborn errors of metabolism
Lactic acidosis
Ethylene glycol, Ethanol
Salicylates

25. Osmolar Gap
Elevated osmolar gap >10 provides indirect evidence for presence of an abnormal low molecular weight solute (e.g. ethanol, methanol & ethylene glycol)
Osmolar gap = Osmolality (mOsm/kg) – Osmolarity (mOsm/L)
Osmolality is measured, Osmolarity is calculated
Osmolarity = (1.86 x Na+) + glucose + urea + 9  (mmol/l)
Osmolarity = (1.86 x Na+) + glucose/18 + BUN/ (mg/dl)

26. HAGMA

27. Acidosis Mechanism = Gap vs Non-Gap
Increased Anion Gap
Normal Anion Gap
Increased Acid Production
Lactic acidosis
Ketoacidosis
Ingestions: methanol, ASA, ethylene glycol, tolulene (early)
Loss of Bicarbonate
GI losses (diarrhea)
Type 2 Proximal RTA
Ketoacidosis treatment
Carbonic Anhydrase inhibitors
Ureteral diversion (loop ileostomy)
Decreased Renal Acid Excretion
CKD
CKD with tubular dysfunction but intact GFR
RTA: Type 1 Distal, Type 4

28. Must be off diuretics in order to interpret urine chloride
Metabolic Alkalosis
Determine urinary Cl- 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 HTN: Cushings, Conn’s, RAS, renal failure with alkali administration
NG suction
If not HTN: hypomagnesemia, hypokalemia, Bartter’s, Gittelman’s, licorice ingestion
Over-diuresis
Exogenous corticosteroids
Post-hypercapnia
For metabolic alkalosis , check urine cholride (must be off diuretics)
Urine chloride < 10 implies responsivenss to saline : extracelluar fluid volume depletion
Urine chloride >10 implies resistance to sailne : severe poatssium depletion , mineralcorticoid excees syndrome Etc

29. Respiratory Alkalosis
Systemic
Central
Pulmonary / cardiac
Anxiety, pain, fever
Respiratory Center
Hypoxia
Fever
Ischemia
Lung disease with/ without hypoxia: PE, reactive airway, pneumonia
Drugs: ASA, catecholamines, progesterone
CNS diseases
CHF
Pregnancy
High altitude
Hypotension
Sepsis
Mechanical Ventilation
Liver failure
Hypothyroid

30. Respiratory Acidosis CNS depression: sedatives, narcotics, CVA
Neuromuscular disorders: acute or chronic
Acute airway obstruction
Severe pneumonia, pulmonary edema, pleural effusion
Chest cavity problems: hemothorax, pneumothorax, flail chest
Chronic lung disease: obstructive or restrictive
Central hypoventilation, OSA

31. Alveolar–Arterial (A-a) Gradient
Measure of difference between alveolar & arterial O2 concentration
Normal = 5-10mmHg, plus age-related changes
Hypoxemia DDX by assessing integrity of alveolar capillary unit
In high altitude, arterial O2 low but only because alveolar O2 also low
In ventilation perfusion mismatch (PE, right-to-left shunt), O2 not effectively transferred from alveoli to blood resulting in elevated A-a gradient
A-a Gradient = PAO2 – PaO2
Normal < [Age/ 4] + 4

32. Summary Stepwise analysis
Clinical suspicion + labs = differential diagnosis
ABG is a set of numbers… use as part of the “big picture” not an isolated value