The normal ranges for arterial blood gas values Approach to arterial blood gas interpretation Arterial blood gas abnormalities in special circumstances

1. Hanaa A. El Gendy
Associate Professor of Anesthesia and ICU
ASUH

2. The normal ranges for arterial blood gas values
Approach to arterial blood gas interpretation
Arterial blood gas abnormalities in special
circumstances

3. •Oxygenation
Represented by PaO2
• Ventilation
Represented by Pa Co2
• Acid Base Status
Represented by pH, HCO3 and base
deficit.

4. What information does ABG provide about the patient?
FIRST: ABG provide an assessment of the following:
1)PaO2: Amount of o dissolved in the blood, it give initial
information on efficiency of oxygenation.
2)PaCO2: Adequacy of ventilation is inversely proportional to
Paco2 (when ventilation increase PaCO2 decrease
and vice verse).
3) Acid base status (pH, HCO3, base deficit, anion gap).
4) Hb, Hct, oxygen saturation.
5) Electrolyte e.g. Na+, K+.

5. SECOND:
Calculation of Alveolar Gas Equation and A-a Gradient:
PAO2 = FiO2×(Bp-pH2O)-PaCO2/R.
= 21×(760-47)-40/0.8
= 100 mmHg.
A-a Gradient is alvealo-arterial O2 gradient.
A-a Gradient = PAO2 -PaO2
Ventilation-Perfusion abn
Affected by
-Age
-Fio2 ( A-a increase 5-7 mmHg/10 % ↑Fio2)
-PPV
A normal A–a gradient for a young adult non-smoker breathing air, is
between 5–10 mmHg.

6. 1) Arterial/alveolar
ratio(a/A)
PaO2/PAO2
PAO2 is calculated by the alveolar air equation:
PAO2 = FIO2 (PB – PH2O) –
PaCO2/0.8
Normal value for the a/A ratio is 0.8, meaning that 80% of the
alveolar oxygen is reaching the arterial system
2) PaO2/ FIO2 ratio
A person breathing FIO2 of 1.0 at sea level should have a PaO2 of
550 to 600 mmHg)
3) A-a gradient (on 100% oxygen)
PAO2 - PaO2
Where PAO2 is calculated by the alveolar air equation previously
presented
FIO2x500

7. Arterial/alveolar PCO2 Gradient (a-A PCO2)
Arterial PCO2 - Alveolar PCO2
Where Alveolar PCO2 is measured by means of end–tidal PCO2
Normal gradient is an alveolar PCO2 2 mmHg less than arterial,
Acute increase reflects increase in physiologic dead space

8. Sample source and collection:-
•Arterial blood sample is common utilized clinically
but venous blood may be useful in determining
acid base status. (Except in CHF and shock).
•Blood sample should be in heparin coated syringe.
•The sample should be analyzed as soon as
possible.
•Air bubble should be eliminated.
•The syringe should be capped and placed in ice.

9. Problem associated with obtaining ABG:
Arterial puncture may result in acute hyperventilation. To
minimize that: we should use local anesthetic with small
needle.
When would you withdraw ABG sample after beginning or stopping
O2 supplementation?
In absence of significant lung disease we should wait from 5-7
minutes before withdraw ABG sample while patient with
obstructive lung disease we should wait 25 min.
The blood sample should be analyzed within 15 minutes and be
stored at either room temperature 24-26 degree C or ice

10. Measurement
Arterial
blood
Mixed
venous
Venous
PH 7.36-7.44 7.30-7.40 7.30-7.40
PO2 80-100 35-40 30-50
PCO2 36-44 40-50 40-50
O2 saturation >95% 60%-80% 60%-85%
HCO3 22-26 22-26 22-28
Base difference
(deficit excess)
-2 to 2

11.  What is base deficit ?
The base deficit is the amount of acid or base
needed to titrate a serum PH back to normal at 37
degree while the Paco2 is held constant at 40
mmHg, thus eliminating the respiratory component
therefore the base deficit represent only the
metabolic component of acid base disorders.
Its +ve value indicate metabolic alkalosis,
While –ve value indicate metabolic acidosis.

12. The basic Bronnsted definition of acids and bases
are:
An acid is a species having a tendency to lose
proton
A base is a species having a tendency to accept
proton
.

13. Body Buffer system
Hydrogen Ion Homeostasis
-About 50 to100 m mol of hydrogen ions are
released from cells into extracellular fluid each
day
*Hydrogen ion concentration [H+] is maintained
between about 35 and 45 nano molL.
40nmol/L=pH 7.4))

14. Is a process by which a strong acid (or base) is
replaced by a weaker one, with a consequent
reduction in the number of free hydrogen ions
and therefore the change in PH
HCl + NaHCO3 = H2CO3 + NaCl
Strong acid buffer weak acid neutral salt

15. What is PH?
PH is –ve log of H+ concentration.
P=protenz (strength-power), H=H+ concentration.
Relationship between pH & [H+]
pH [H+]
(nanomoles/l)
6.8 158
6.9 125
7.0 100
7.1 79
7.2 63
7.3 50
7.4 40
7.5 31
7.6 25
7.7 20
7.8 15
or more simply: The Henderson
equation:
[H+] = 24 x ( pCO2 / [HCO3] )
Henderson-Hasselbalch Equation
pH = pK’a + log ([HCO3] / 0.03 x pCO2)
The PKa of an acid is the PH at which
it is exactly half-dissociated

16. Compensation is the body’s way of restoring a
normal blood pH (7.36-744)
Remember: Acid + Base  Neutrality
Compensation DOES NOT treat the root of the
problem – the reason for the acid-base
imbalance is STILL THERE!!!

17. The body has three means to try to compensate
for an acid-base imbalance
(( Seconds-least efficient•Chemical
• Respiratory
15 min-mod efficient – HCO3/CO2 ≈ 20)-3(
• Renal
5 d-max efficient – HCO3/CO2 ≈ 20)–12 hs)

18. Chemicals within the blood act within seconds
to correct respiratory or metabolic imbalances
Used up quickly – not effective long-term
Chemical buffers in the blood include
Bicarbonate
Phosphate
Proteins

19. Used to compensate for metabolic imbalances
only
Chemoreceptors respond to changes in H+
concentrations  alters respiratory rate and
depth
Remember CO2 is an acid

20. Used to compensate for respiratory imbalances
Remember: HCO3
- is a base
Kidneys respond to changes in blood pH
Excrete H+ and retain HCO3
- when acidemia is present
Retain H+ and excrete HCO3
- when alkalemia is
present

21. The body is very smart and will
not overcompensate for an
imbalance

22. An acid-base imbalance will be compensated for
in one of three ways
Uncompensated
Partially compensated
Fully compensated

23. Uncompensated
Body has made no attempt to correct the acid-base
imbalance
Partially compensated
Body is attempting to correct the imbalance
Blood pH remains abnormal in spite of the attempt

24. Fully compensated
The body is correcting the imbalance
Blood pH is normal
Other blood gas values remain abnormal until the
root cause is treated and corrected

25. PH abnormal
Either PaCO2 OR HCO3
- abnormal
All other values normal
If PaCO2 is abnormal
Problem is respiratory
If HCO3
- is abnormal
Problem is metabolic

26. Uncompensated
respiratory acidosis
PH < 7.36
PaCO2> 44
HCO3
-WNL
Uncompensated
respiratory alkalosis
PH ˃ 7.44
PaCO2˂ 36
HCO3
-WNL
Remember that CO2 is an acid and that the more of it there is the worse is the acidemia.
Notice that with uncompensated respiratory, the HCO3 is normal – this is because the
body has not began to compensate for the alterations in CO2

27. Uncompensated
metabolic acidosis
pH ˂ 7.36
PaCO2WNL
HCO3
- ˂ 22
Uncompensated
metabolic alkalosis
pH > 7.44
PaCO2WNL
HCO3
- > 26
Remember that HCO3 is a base and that the more of it there is the more
alkalotic you will be. Notice that in the case of uncompensated metabolic
the PaCO2 is normal indicating that the body has not began to compensate.

28. Occur when compensation mechanisms are activated,
but have not had sufficient time to normalize the
blood pH
NOTE: Some people say that there is no such thing as
“partially” compensated – but it is indicative of a
part of the process called compensation

29. pH is abnormal
Both PaCO2 and HCO3
- are abnormal in the same
direction (increased or decreased from normal)
If PaCO2 is high (↑ acid), HCO3
- will also be high (↑
alkaline) to neutralize the environment
If PaCO2 is low (↓ acid), HCO3
- will also be low (↓
alkaline) to neutralize the environment

30. Partially Compensated
Respiratory Acidosis
pH< 7.36
PaCO2> 44
HCO3
-> 26
Partially Compensated
Respiratory Alkalosis
pH˃7.44
PaCO2˂ 36
HCO3
-˂ 22
In the case of Partially Compensated Resp Acidosis, the pH is low, indicating an
acid environment…when you look at the PaCO2, it too is acidic, which is how you
know that you have a respiratory acidosis. With the HCO3 being high, you can
deduce that the body is raising its base to counteract the acid represented by the pH;
therefore, partially compensated respiratory acidosis.

31. Partially Compensated
Metabolic Acidosis
pH< 7.36
PaCO2˂ 36
HCO3
-˂ 22
Partially Compensated
Metabolic Alkalosis
pH˃7.44
PaCO2> 44
HCO3
-> 26
With partially compensated metabolic acidosis, you notice first that the pH is low
(acidosis).Ask yourself, which number is representative of an acid condition. In this
case it is the low base (HCO3), so you know you have a metabolic acidosis. You know
it is partially compensated because the PaCO2 is low indicating that CO2 (an acid) is
being lost from the body to correcfor the low pH.

32. Compensated
Respiratory Acidosis
WNLPH closer to 7.36
PaCO2> 44
HCO3-> 26
Compensated
Respiratory Alkalosis
PH closer to 7.44 WNL
PaCO2< 36
HCO3-˂ 22
In compensated respiratory acidosis, the pH tends to range between 7.35 and 7.39 – still
acidic,But in the normal pH range. When you look at the PaCO2, you notice that it is
high (acidic), butThe HCO3 is also high, indicating that the body has compensated and
normalized the low pH.

33. Compensated Metabolic
Acidosis
PH closer to 7.36 WNL
PaCO2< 36
HCO3-˂ 22
Compensated Metabolic
Alkalosis
PH closer to 7.44
PaCO2> 44
HCO3-> 26

34. Occur when patient has both metabolic and
respiratory disorders that cause an acid-base
imbalance
Examples:
Diabetic KetoAcidosis (metabolic acidosis) with
decreased respiratory drive (respiratory acidosis)
Severe vomiting (metabolic alkalosis) with high fever
(respiratory alkalosis)

35. pH will be near normal
PaCO2 and HCO3
- will be abnormal
PaCO2 will be high with low HCO3
- (both tend
toward acid side)
PaCO2 will be low with high HCO3
- (both tend
toward base side)

36. Mixed acidosis
pH˂7.36
PaCO2> 44
HCO3
- ˂ 22
Mixed alkalosis
pH˃7.44
PaCO2< 36
HCO3
-> 26
Notice with the mixed acidosis that you have an acidic pH (less than 7.35, with other
Parameters indicating an acid environment. High PaCO2 (too much acid). Low HCO3
(too little base – an acidic environment). This is classic mixed acidosis.

37. How is the patient?
Will provide useful clues to help with interpretation of the
results
TEN Steps for interpretation of ABG
STEP 1
STEP 2
Assess oxygenation
The PaO2 should be > 10 kPa (75 mmHg) breathing air
inspired concentration
Assessed by FiO2, SaO2, PaO2, Hb Ideal to keep FiO2 < .4/.5, PaO2 60-90
mmHg,
The SpO2 can be used to titrate FiO2; goal is >90%
Desired FiO2 = PaO2 desired X FiO2 known
PaO2 known

38. STEP 3
Determine if:
Acidemia (PH<7.36) OR Alkalemia(PH >7.44) is
present.

39. Identify primary disturbance:
PHIncrease Decrease
Alkalosis Acidosis
Look at PCO2
Increased Decrease Increased Decreased
Metabolic
Alkalosis
Respiratory
Alkalosis
Respiratory
acidosis
Metabolic
acidosis
STEP 4

40. STEP 5:
Look at the direction of the change of HCO3/PCO2:
•If it is in the same direction it is either partial or compensated change.
•But if it is in the opposite direction so it is mixed change.
STEP 6:
Calculate rate of change of Hco3 and co2
(Expected compensation)
Disturbance Response Expected change
Metabolic acidosis ↓Paco2 1.2×(24-HCO3 measured)
Metabolic alkalosis ↑Paco2 0.7×(HCO3-24)
Acute respiratory acidosis ↑Hco3 0.1×(PaCO2-40)
Chronic respiratory acidosis ↑ Hco3 0.4×(PaCO2-40)
Acute respiratory alkalosis ↓ Hco3 0.2×(40-PaCO2)
Chronic respiratory alkalosis ↓ Hco3 0.4×(40-PaCO2)

41. N.B.:
There is no over correction or compensation in acid base
balance → if the compensatory response is more or less than
expected → it is mixed acid base disorder
N.B.:
• In respiratory disturbance arterial pH change 0.08 for every 10
mmHg change in PCO2.
• In metabolic disturbance arterial pH change 0.1 for every 6 meq /l
change in HCO3.

42. Disturbance Primary Abnormality Compensation Cause
Metabolic
Acidosis
Excess endogenous
acid depletes
bicarbonate
Hyperventilation lowers
pCO2,
Kidney excretes excess
H+ and forms more
HCO3
-
Renal failure
Ketosis
Increased lactic acid
Diarrhea
Respiratory
Acidosis
Inefficient excretion
of CO2 by the
lungs
Formation of excess
HCO3
- by kidney
Chronic pulmonary
Diseases (COPD), such as
emphysema
Acute problems, such as
pneumonia, airway
obstruction, drugs such as
opiates, congestive heart
failure
Metabolic
Alkalosis
Excess plasma
bicarbonate
Kidneys excrete excess
HCO3
- and form less
HCO3
- and NH4,
Lungs hypoventilate
Loss of gastric juice
Chloride depletion
Hypokalemia
Increased
corticosteroid
Increased ingestion of
antacids
Respiratory
Alkalosis
Hyperventilation
lowers pCO2
Increased excretion of
bicarbonate by kidney
Hyperventilation, such as
with severe anxiety,
fever, head injuries
Stimulation of resp.
center by drugs
Central nervous system
diseases

43. 43

44. 44

45. 45

46. 46

47. The Anion Gap
[(Na+) + (K+)] – [(Cl-) + (HCO3
-)]
The normal anion gap is 12meq ± 4.
Causes ofHyperchloremic Acidosis
↑ GIT loss of HCO3 as in: diarrhea, high output fistula
(pancreatic, biliary or small intestinal).
↑ renal HCO3 loss as in: RTA(I, II), CAIs, hypoaldosteronism.
TPN.
Large amount of HCO3 free fluid
↑ CL containing acids.
Wide Anion Gap Acidosis
Keto Acidosis
Uremia
Lactic Acidosis
Salicylism
Toxins : Methanol,Paraldehyde,Ethylene glycol

49. If there is metabolic acidosis calculate the anion
gap
Anion Gap = Na+ - (Cl- + HCO3
-) = 12meq ± 4.
• Corrected anion gap = observed anion gap + 2.5
(normal albumin - measured albumin).
• If the anion gap ↑ proceed to step 8.
STEP 7

50. If the anion gap metabolic acidosis is present we should evaluate
for additional metabolic disorder because the elevation of anion gap
above normal ∆ AG = (AG-12) should be buffered by HCO3.
Adding ∆AG to current HCO3 will yield the corrected Hco3 which should be
normal value 24 meq/l unless there is another disorder present.
Corrected HCO3 = current HCO3 (measured) +∆A.G
(Normal value 24 meq/l)
•If corrected HCO3 >24 → metabolic alkalosis is also present
•If corrected HCO3 <24 → a non gap metabolic acidosis is also present
•If corrected HCO3 = 24 → it is pure gap metabolic acidosis.
STEP 8

51. In case of metabolic alkalosis: measure urinary Cl- concentration
Final step:
Be sure that the interpretation of blood gas is consistent and
correlated with the clinical picture of the patient.
STEP 9
(CL responsive ( UC < 15 mEq/L
CL resistant ( UC ˃ 25 mEq/L)