abg

1. Blood Gas Analysis and it’s
Clinical Interpretation

2. Outline
1. Indications
2. Components of ABG
3. Discuss simple steps in analyzing ABGs
4. Calculate the anion gap
5. Calculate the delta gap
6. Differentials for specific acid-base disorders
7. Common Errors During ABG Sampling

3. ABG
 Test to assess the arterial blood gases
 Ph, PCo2 , PO2 , HCo3
 To assess the metabolic or respiratory

4. COMPONENTS OF THE ABG
 pH: Measurement of acidity or alkalinity, based on the hydrogen
(H+). 7.35 – 7.45
 Pao2 : The partial pressure oxygen that is dissolved in arterial
blood. 80-100 mm Hg.
 PCO2 : The amount of carbon dioxide dissolved in arterial blood.
35– 45 mmHg
 HCO3 : The calculated value of the amount of bicarbonate in the
blood. 22 – 26 mmol/L
 SaO2 : The arterial oxygen saturation > 95%
 pH,PaO2 ,PaCO2 , Lactate and electrolytes are measured Variables
 HCO3 (Measured or calculated)

5. Contd…
 Buffer Base:
 It is total quantity of buffers in blood including both
volatile(Hco3) and nonvolatile (as Hgb,albumin,Po4)
 Base Excess/Base Deficit:
 Amount of strong acid or base needed to restore
plasma pH to 7.40 at a PaCO2 of 40 mm Hg,at
37*C.
 Calculated from pH, PaCO2 and HCT
 Negative BE also referred to as Base Deficit
 True reflection of non respiratory (metabolic) acid
base status
 Normal value: -2 to +2mEq/L

6. CENTRAL EQUATION OF ACID-BASE
PHYSIOLOGY
 Henderson Hasselbach Equation:
 where [ H+] is related to pH by
 To maintain a constant pH, PCO2/HCO3- ratio should be
constant
 When one component of the PCO2/[HCO3- ]ratio is altered,
the compensatory response alters the other component in the
same direction to keep the PCO2/[HCO3- ] ratio constant
 [H+] in nEq/L = 24 x (PCO2 / [HCO3 -] )
 [ H+] in nEq/L = 10 (9-pH)

7. Compensatory response or regulation of
pH
By 3 mechanisms:
 Chemical buffers:
 React instantly to compensate for the addition or
subtraction of H+ ions
 CO2 elimination:
 Controlled by the respiratory system
 Change in pH result in change in PCO2 within minutes
 HCO3- elimination:
 Controlled by the kidneys
 Change in pH result in change in HCO3-
 It takes hours to days and full compensation occurs in 2-
5 days

8. Normal Values
Variable Normal Normal
Range(2SD)
pH 7.40 7.35 - 7.45
pCO2 40 35-45
Bicarbonate 24 22-26
Anion gap 12 10-14
Albumin 4 4

9. Steps for ABG analysis
1. What is the pH? Acidemia or Alkalemia?
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 check the delta gap?
7. What is the differential for the clinical processes?

10. 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 ????

11. 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

12. Step 2: What is the primary disorder?
What disorder is
present?
pH pCO2 HCO3
Respiratory
Acidosis
pH low high high
Metabolic Acidosis pH low low low
Respiratory
Alkalosis
pH high low low
Metabolic Alkalosis pH high high high

13. Contd….
Metabolic Conditions are suggested if
pH changes in the same direction as pCO2 or pH is
abnormal but pCO2 remains unchanged
Respiratory Conditions are suggested if:
pH changes in the opp direction as pCO2 or pH is abnormal
but HCO3- remains unchanged

14. 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

15. EXPECTED CHANGES IN ACID-BASE DISORDERS
Primary Disorder Expected Changes
Metabolic acidosis PCO2 = 1.5 × HCO3 + (8 ± 2)
Metabolic alkalosis PCO2 = 0.7 × HCO3 + (21 ± 2)
Acute respiratory acidosis delta pH = 0.008 × (PCO2 - 40)
Chronic respiratory acidosis delta pH = 0.003 × (PCO2 - 40)
Acute respiratory alkalosis delta pH = 0.008 × (40 - PCO2)
Chronic respiratory alkalosis delta pH = 0.003 × (40 - PCO2)
From: THE ICU BOOK - 2nd Ed. (1998) [Corrected]

16. Step 3-4: Is there appropriate
compensation? Is it chronic or acute?
 Respiratory Acidosis
 Acute (Uncompensated): for every 10 increase in pCO2 -> HCO3
increases by 1 and there is a decrease of 0.08 in pH
 Chronic (Compensated): for every 10 increase in pCO2 -> HCO3
increases by 4 and there is a decrease of 0.03 in pH
 Respiratory Alkalosis
 Acute (Uncompensated): for every 10 decrease in pCO2 -> HCO3
decreases by 2 and there is a increase of 0.08 in PH
 Chronic (Compensated): for every 10 decrease in pCO2 -> HCO3
decreases by 5 and there is a increase of 0.03 in PH
1 4
2 5
10
 Partial Compensated: Change
in pH will be between 0.03 to
0.08 for every 10 mmHg
change in PCO2

17. Step 3-4: Is there appropriate
compensation?
 Metabolic Acidosis
 Winter’s formula: Expected pCO2 = 1.5[HCO3] + 8 ± 2
OR
 pCO2 = 1.2 ( HCO3)
 If serum pCO2 > expected pCO2 -> additional respiratory
acidosis and vice versa
 Metabolic Alkalosis
 Expected PCO2 = 0.7 × HCO3 + (21 ± 2)
OR
 pCO2 = 0.7 ( HCO3)
 If serum pCO2 < expected pCO2 - additional respiratory
alkalosis and vice versa

18. 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 ±2 = 18.5(16.5 –
20.5)
 So correct compensation so there is only one
disorder Primary metabolic

19. Step 5: Calculate the anion gap
 AG used to assess acid-base status esp in D/D of
met acidosis
  AG &  HCO3
- used to assess mixed acid-base
disorders
 AG based on principle of electroneutrality:
 Total Serum Cations = Total Serum Anions
 Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4
+ Protein + Organic Acids)
 Na + UC = HCO3 + Cl + UA
 Na – (HCO3 + Cl) = UA – UC
 Na – (HCO3 + Cl) = AG
 Normal =12 ± 2

20. Contd…
 AG corrected = AG + 2.5[4 – albumin]
 If there is an anion Gap then calculate the
Delta/delta gap (step 6) to determine
additional hidden nongap metabolic acidosis
or metabolic alkalosis
 If there is no anion gap then start analyzing
for non-anion gap acidosis

21. EXAMPLE
 Calculate Anion gap
 ABG 7.23/17/235 on 50% VM
 BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/ Albumin
2.
 AG = Na – Cl – HCO3 (normal 12 ± 2)
123 – 97 – 7 = 19
 AG corrected = AG + 2.5[4 – albumin]
= 19 + 2.5 [4 – 2]
= 19 + 5 = 24

22. Step 6: Calculate Delta Gap
 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

23. 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

24. Metobolic acidosis: Anion gap acidosis

25. 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

26. Nongap metabolic acidosis
 For non-gap metabolic acidosis, calculate the urine anion
gap
 URINARY AG
Total Urine Cations = Total Urine Anions
Na + K + (NH4 and other UC) = Cl + UA
(Na + K) + UC = Cl + UA
(Na + K) – Cl = UA – UC
(Na + K) – Cl = AG
 Distinguish GI from renal causes of loss of HCO3 by estimating
Urinary NH4+ .
 Hence a -ve UAG (av -20 meq/L) seen in GI, while +ve value (av
+23 meq/L) seen in renal problem.
UAG = UNA + UK – UCL
Kaehny WD. Manual of Nephrology 2000; 48-62

27. 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 14
 AG = 123 – 97-14 = 12
 Most likely due to the diarrhea

28. 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

29. 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<25 Saline-resistant UCL >25
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

30. 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

31. Case1.
 7.27/58/60 on 5L, HCO3
- 26, anion gap is
12, albumin is 4.0
 1. pH= Acidemia (pH < 7.4)
 2.CO2= Acid (CO2>40)
 Opposite direction so Primary disturbance =
Respiratory Acidosis
 3 &4: Compensation : Acute or chronic? ACUTE
 CO2 has increased by (58-40)=18
 If chronic the pH will decrease 0.05 (0.003 x 18 = 0.054)
 pH would be 7.35
 If acute the pH will decrease 0.14 (0.008 x 18 = 0.144)
pH would be 7.26.

32. Contd.
 5: Anion gap –N/A
 6: There is an acute respiratory acidosis, is there
a metabolic problem too?
 ΔHCO3
- = 1 mEq/L↑/10mmHg↑pCO2
 The pCO2 is up by 18  so it is expected that the HCO3
-
will go up by 1.8. Expected HCO3
- is 25.8, compared to
the actual HCO3
- of 26, so there is no additional
metabolic disturbance.
 Dx-ACUTE RESPIRATORY ACIDOSIS

33. Case.2
 7.54/24/99 on room air, HCO3
- 20, anion
gap is 12, albumin is 4.0.
 1: pH= Alkalemia (pH > 7.4)
 2.CO2= Base (CO2<40)
 pH & pCO2 change in opposite Direction So
Primary disturbance = Respiratory Alkalosis
 3 &4: Compensation ? acute or chronic? ACUTE
 ΔCO2 =40-24=16
 If chronic the pH will increase 0.05 (0.003 x 16 = 0.048)
 pH would be 7.45
 If acute the pH will increase 0.13(0.008 x 16 = 0.128)
pH would be 7.53

34. Contd…
 5:Anion gap – N/A
 6: There is an acute respiratory alkalosis, is there
a metabolic problem too?
 ΔHCO3
- = 2 mEq/L↓/10mmHg↓pCO2
 The pCO2 is down by 16  so it is expected that the
HCO3
- will go down by 3.2. Expected HCO3
- is 20.8,
compared to the actual HCO3
- of 20, so there is no
additional metabolic disturbance.
 Dx-ACUTE RESPIRATORY ALKALOSIS

35. Case-3
 7.58/55/80 on room air, HCO3
- 46, anion gap is
12, albumin is 4.0. Ucl -20
 1: pH= Alkalemia(pH > 7.4)
 2:CO2= Acid (CO2>40)
 Same direction so Primary disturbance = Metabolic
Alkalosis
 3&4: Compensation:
 ∆ pCO2=0.7 x ∆ HCO3
-
 The HCO3
- is up by 22.CO2 will increase by 0.7x22 = 15.4.
Expected CO2 is 55.4, compared to the actual CO2 of 55,
therefore there is no additional respiratory disturbance.

36. contd
 5: No anion gap is present; and no adjustment
needs to be made for albumin. Metabolic
Alkalosis
 Urinary chloride is 20 meq/l (< 25 meq/l)so
chloride responsive, have to treat with Normal
saline.
Dx-METABOLIC ALKALOSIS

37. Case-4
 7.46/20/80 on room air, HCO3
- 16, anion
gap = 12, albumin = 4.0
 1: pH = Alkalemia (pH > 7.4)
 2:CO2 = Base (CO2<40)
 So Primary disturbance = Respiratory Alkalosis
 3 &4: Compensation? acute or chronic? Chronic
 ΔCO2 =40-20= 20.
 If chronic the pH will increase 0.06 (0.003 x 20 = 0.06) 
pH would be 7.46.
 If acute the pH will increase 0.16 (0.008 x 20 = 0.16) pH
would be 7.56.

38. Contd….
 5: Anion gap – N/A
 6: There is a chronic respiratory alkalosis, is there
a metabolic problem also?
 Chronic: ΔHCO3
- = 4 mEq/L↓/10mmHg↓pCO2
 The pCO2 is down by 20  so it is expected that the
HCO3
- will go down by 8. Expected HCO3
- is 16, therefore
there is no additional metabolic disorder.
 Dx-CHRONIC RESPIRATORY ALKALOSIS

39. Case-5
 7.19/35/60 on 7L, HCO3
- 9, anion gap = 18,
albumin = 4.0
 1: pH = Acidemia (pH < 7.4)
 2:CO2= Base (CO2<40)
 So Primary disturbance: Metabolic Acidosis
 3&4: Compensation ?
∆ pCO2=1.2 x ∆ HCO3
-
 CO2 will decrease by 1.2 (∆HCO3
-)  1.2 (24-9) 18. 40 – 18=
22 Actual CO2 is higher than expected Respiratory Acidosis
 5: Anion Gap = 18 (alb normal so no correction necessary)

40. Contd…..
6: Delta Gap:
 Delta gap = (actual AG – 12) + HCO3
= (18-12) + 9
= 6 + 9 = 15 which is<18 Non-AG Met Acidosis
 Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION GAP
METABOLIC ACIDOSIS with RESPIRATORY ACIDOSIS

41. Case-6
 7.54/80/65 on 2L, HCO3
- 54, anion gap
12,albumin = 4.0 , Ucl 40 meq/l
 1: pH = Alkalemia (pH > 7.4)
 2:CO2= Acid (CO2>40)
 So Primary disturbance: Metabolic Alkalosis
 3&4: Compensation?
∆ pCO2=0.7 x ∆ HCO3
-
 CO2 will increase by 0.7 (∆HCO3
-)  0.7 (54-24) 2140
+ 21 = 61 Actual CO2 is higher than expected
Respiratory Acidosis

42. Contd….
 5: Anion Gap = 12 (alb normal so no correction
necessary)
 Urinary chloride is 40 meq/l (> 25 meq/l)so
chloride resistant. So treatment would be disease
specific and repletion of potassium
 Dx-METABOLIC ALKALOSIS with RESPIRATORY
ACIDOSIS

43. Case-7
 7.6/30/83 on room air, HCO3
- 28, anion gap = 12, albumin =
4.0
 1: pH = Alkalemia (pH > 7.4)
 2:CO2= Base (CO2<40)
 SoPrimary Disturbance: Metabolic Alkalosis
 3&4: Compensation ?
∆ pCO2=0.7 x ∆ HCO3
-
 CO2 will increase by 0.7 (∆HCO3
-)  0.7 (28-24) 2.8 40 + 2.8 = 42.8
Actual CO2 is lower than expected Respiratory Alkalosis
 Anion Gap = 12 (alb normal so no correction necessary)
 See urinary chloride for further Dx.
 Dx-METABOLIC ALKALOSIS with RESPIRATORY ALKALOSIS

44. Case-8
 A 50 yo male present with sudden onset of SOB with
following ABG 7.25/46/78 on 2L, HCO3
- 20, anion gap = 10,
albumin = 4.0
 1: pH = Acidemia (pH < 7.4)
 2:CO2= Acid (CO2>40)
 So Primary disturbance: Respiratory Acidosis
 3 &4: If respiratory disturbance is it acute or chronic?
ACUTE
 ∆ CO2 = 46-40= 6
 If chronic the pH will decrease 0.02 (0.003 x 6 = 0.018) 
pH would be 7.38
 If acute the pH will decrease 0.05 (0.008 x 6 = 0.048)
pH would be 7.35.

45. Contd…
 Anion Gap = 10 (alb normal so no correction necessary)
 6: There is an acute respiratory acidosis, is there a metabolic
problem too?
 ∆ HCO3
- = 1 mEq/L↑/10mmHg↑pCO2
 The HCO3
- will go up 1mEq/L for every 10mmHg the pCO2goes up
above 40
 The pCO2 is up by 6  so it is expected that the HCO3
- will go up by 0.6.
Expected HCO3
- is 24.6, compared to the actual HCO3
- of 20. Since the
HCO3
- is lower than expected Non-Anion Gap Metabolic Acidosis
(which we suspected).
 Dx-RESPIRATORY ACIDOSIS with NON-ANION GAP
METABOLIC ACIDOSIS

46. Case-9
 7.15/22/75 on room air, HCO3
- 9, anion gap = 10, albumin =
2.0
 1: pH = Acidemia (pH < 7.4)
 2:CO2= Base (CO2<40)
 So Primary disturbance: Metabolic Acidosis
 3&4:∆ Compensation ?
pCO2=1.2 x ∆ HCO3
-
 Expected pCO2 = 1.2 x ∆ HCO3
- 1.2 (24 -9)  1.2 (15)
18. The expected pCO2is 22mmHg. The actual pCO2 is
22, which is expected, so there is no concomitant
disorder.

47. Contd….
 5: Anion Gap = 10
 AGc = 10 + 2.5(4-2) = 15  Anion Gap Metabolic
Acidosis
 6: Delta Gap:
 Delta gap = (actual AG – 12) + HCO3
= (15-12) + 9
= 3+ 9 = 12 which is<18 Non-AG Met
Acidosis
 Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION
GAP METABOLIC ACIDOSIS

48. Delayed Analysis
 Consumptiom of O2 & Production of CO2
continues after blood drawn
Iced Sample maintains values for 1-2 hours
Uniced sample quickly becomes invalid within 15-
20 minutes
PaCO2  3-10 mmHg/hour
PaO2 
pH  d/t lactic acidosis generated by glycolysis
in R.B.C.

49. Parameter 37 C (Change
every 10 min)
4 C (Change
every 10 min)
 pH 0.01 0.001
 PCO2 1 mm Hg 0.1 mm Hg
 PO2 0.1 vol % 0.01 vol %
Temp Effect On Change of ABG Values

50. AIR BUBBLES
:
1. PO2 150 mmHg & PCO2 0 mm Hg in air bubble(R.A.)
2. Mixing with sample, lead to  PaO2 &  PaCO2
To avoid air bubble, sample drawn very slowly and
preferabily in glass syringe
Steady State:
Sampling should done during steady state after change in
oxygen therepy or ventilator parameter
Steady state is achieved usually within 3-10 minutes

51. Leucocytosis :
  pH and Po2 ; and  Pco2
 0.1 ml of O2 consumed/dL of blood in 10
min in pts with N TLC
 Marked increase in pts with very high
TLC/plt counts – hence imm chilling/analysis
essential
EXCESSIVE HEPARIN
Dilutional effect on results  HCO3
- & PaCO2
Only .05 ml heperin required for 1 ml blood.
So syringe be emptied of heparin after flushing or only dead
space volume is sufficient or dry heperin should be used

52.  TYPE OF SYRINGE
1. pH & PCO2 values unaffected
2. PO2 values drop more rapidly in plastic syringes (ONLY
if PO2 > 400 mm Hg)
 Differences usually not of clinical significance so plastic
syringes can be and continue to be used
Risk of alteration of results  with:
1. size of syringe/needle
2. vol of sample
 HYPERVENTILATION OR BREATH HOLDING
May lead to erroneous lab results

53. FEVER OR HYPOTHERMIA
1. Most ABG analyzers report data at N body temp
2. If severe hyper/hypothermia, values of pH &
PCO2 at 37 C can be significantly diff from pt’s
actual values
3. Changes in PO2 values with temp also predictable
Hansen JE, Clinics in Chest Med 10(2), 1989 227-237
 If Pt.’s temp < 37C
Substract 5 mmHg Po2, 2 mmHg Pco2 and Add
0.012 pH per 1C decrease of temperature