ABG Analysis

1. Arterial blood gas

2. What is an ABG
Arterial Blood Gas
Drawn from artery- radial, brachial, femoral
It is an invasive procedure.
Caution must be taken with patient on
anticoagulants.
Arterial blood gas analysis is an essential part
of diagnosing and managing the patient’s
oxygenation status, ventilation failure and acid
base balance.

3. What Is An ABG?
pH [H+
]
PCO2 Partial pressure
CO2
PO2 Partial pressure
O2
HCO3 Bicarbonate
BE Base excess

4. Acid/Base Balance
 The pH is a measurement of the acidity or alkalinity of the
blood.
 It is inversely proportional to the no. of (H+) in the blood.
 The normal pH range is 7.35-7.45.
Changes in body system functions that occur in an acidic
state decreases the force of cardiac contractions,
decreases the vascular response to catecholamines, and a
diminished response to the effects and actions of certain
medications.
An alkalotic state interferes with tissue oxygenation and
normal neurological and muscular functioning.
Significant changes in the blood pH above 7.8 or below 6.8
will interfere with cellular functioning, and if uncorrected,
will lead to death.

5. H2O + CO2  H2CO3  HCO3 + H+
Acid/Base Relationship

6. There are two buffers that work in
pairs
H2CO3 NaHCO3
Carbonic acid base bicarbonate
These buffers are linked to the
respiratory and renal compensatory
system
Buffers

7. The Respiratory buffer
response
• The blood pH will change acc.to
the level of H2CO3 present.
• This triggers the lungs to either
increase or decrease the rate
and depth of ventilation
• Activation of the lungs to
compensate for an imbalance
starts to occur within 1-3 minutes

8. The Renal Buffer Response
• The kidneys excrete or retain
bicarbonate(HCO3-).
• If blood pH decreases, the kidneys
will compensate by retaining HCO3
• Renal system may take from hours
to days to correct the imbalance.

9. ACID BASE DISORDER
Res. Acidosis
• is defined as a pH less than 7.35 with
a paco2 greater than 45 mmHg.
• Acidosis –accumulation of co2,
combines with water in the body to
produce carbonic acid, thus lowering
the pH of the blood.
• Any condition that results in
hypoventilation can cause respiratory
acidosis.

11. Causes
1. Central nervous system depression -medications
such as narcotics, sedatives, or anesthesia.
2. Impaired muscle function - spinal cord injury,
neuromuscular diseases, or neuromuscular
blocking drugs.
3. Pulmonary disorders such as atelectasis,
pneumonia, pneumothorax, pulmonary edema or
bronchial obstruction
4. Massive pulmonary embolus
5. Hypoventilation due to pain chest wall injury, or
abdominal pain.

12. Signs & symptoms of Respiratory
Acidosis
• Respiratory : Dyspnoea, respiratory
distress and/or shallow respiration.
• Nervous: Headache, restlessness and
confusion. If co2 level extremely high
drowsiness and unresponsiveness
may be noted.
• CVS: Tacycardia and dysrhythmias

13. Management
• Increase the ventilation.
• Causes can be treated rapidly include
pneumothorax, pain and CNS
depression r/t medication.
• If the cause can not be readily
resolved, mechanical ventilation.

14. Respiratory alkalosis
• Psychological responses, anxiety or fear.
• Pain
• Increased metabolic demands such as fever,
sepsis, pregnancy or thyrotoxicosis.
• Medications such as respiratory stimulants.
• Central nervous system lesions

15. Signs & symptoms
• CNS: Light Headedness, numbness,
tingling, confusion, inability to
concentrate and blurred vision.
• Dysrhythmias and palpitations
• Dry mouth, diaphoresis and tetanic
spasms of the arms and legs.

16. Management
• Resolve the underlying problem
• Monitor for respiratory muscle
fatigue
• When the respiratory muscle
become exhausted, acute
respiratory failure may ensue

17. Metabolic Acidosis
• Bicarbonate less than 22mEq/L with a
pH of less than 7.35.
• Renal failure
• Diabetic ketoacidosis
• Anaerobic metabolism
• Starvation
• Salicylate intoxication

18. Sign & symptoms
• CNS: Headache, confusion and
restlessness progressing to lethargy,
then stupor or coma.
• CVS: Dysrhythmias
• Kussmaul’s respirations
• Warm, flushed skin as well as nausea
and vomiting

19. Management
• Treat the cause
• Hypoxia of any tissue bed will produce
metabolic acids as a result of anaerobic
metabolism even if the pao2 is normal
• Restore tissue perfusion to the hypoxic
tissues
• The use of bicarbonate is indicated for
known bicarbonate - responsive acidosis
such as seen with renal failure

20. Metabolic alkalosis
• Bicarbonate more than 26m Eq /L with a pH
more than 7.45
• Excess of base /loss of acid can cause
• Ingestion of excess antacids, excess use of
bicarbonate, or use of lactate in dialysis.
• Protracted vomiting, gastric suction,excess
use of diuretics, or high levels of
aldesterone.

21. Signs/symptoms
• CNS: Dizziness, lethargy
disorientation, siezures & coma.
• M/S: weakness, muscle twitching,
muscle cramps and tetany.
• Nausea, vomiting and respiratory
depression.
• It is difficult to treat.

22. 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
B.E:
The base excess indicates the amount of excess or insufficient
level of bicarbonate. -2 to +2mEq/L
(A negative base excess indicates a base deficit in blood)
SaO2:The arterial oxygen saturation.
>95%

23. Stepwise approach to ABG
• Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory or
metabolic?
• Step 3. Asses to Pa O2. A value below 80mm Hg
indicates Hypoxemia. For a respiratory
disturbance, determine whether it is acute or
chronic.
• Step 4. For a metabolic acidosis, determine
whether an anion gap is present.
• Step 5. Assess the normal compensation by the
respiratory system for a metabolic disturbance

24. STEPS TO AN ABG
INTERPRETATION
• Step:1
• Assess the pH –acidotic/alkalotic
• If above 7.5 – alkalotic
• If below 7.35 – acidotic

25. Contd…..
• Step 2:
• Assess the paCO2 level.
• pH decreases below 7.35, the paCO2
should rise.
• If pH rises above 7.45 paCO2 should fall.
• If pH and paCO2 moves in opposite
direction – primary respiratory
problem.

26. contd
• Step:2
• Assess HCO3 value
• If pH increases the HCO3 should also
increase
• If pH decreases HCO3 should also
decrease
• They are moving in the same direction
• primary problem is metabolic

27. • Step 3
Assess pao2 < 80 mm Hg - Hypoxemia
For a resp. disturbance : acute, chronic
The differentiation between A/C &
CHR.respiratory disorders is based on whether
there is associated acidemia / alkalemia.
If the change in paco2 is associated with the
change in pH, the disorder is acute.
In chronic process the compensatory process
brings the pH to within the clinically acceptable
range ( 7.30 – 7.50)

28. • J is a 45 years old female admitted with the severe
attack of asthma. She has been experiencing
increasing shortness of breath since admission three
hours ago. Her arterial blood gas result is as follows:
• pH : 7.22
• paCO2 : 55
• HCO3 : 25
• Follow the steps
• pH is low – acidosis
• paCO2 is high – in the opposite direction of the pH.
• Hco3 is Normal.
• Respiratory Acidosis
• Need to improve ventilation by oxygen therapy,
mechanical ventilation, pulmonary toilet or by
administering bronchodilators.

29. • EXAMPLE 2: Mr. D is a 55 years old
admitted with recurring bowel
obstruction has been experiencing
intractable vomiting for the last
several hours. His ABG is:
• pH : 7.5
• paCO2 :42
• HCO3 : 33
• Metabolic alkalosis
• Management: IV fluids, measures to
reduce the excess base

30. Respiratory
acidosis
pH PaCo2 HC03
normal
Respiratory
Alkalosis
normal
Metabolic
Acidosis
normal
Metabolic
Alkalosis
normal

31. BASE EXCESS
• Is a calculated value estimates the
metabolic component of an acid based
abnormality.
• It is an estimate of the amount of
strong acid or base needed to correct
the met. component of an acid base
disorder (restore plasma pH to 7.40at
a Paco2 40 mmHg)

32. Formula
• With the base excess is -10 in a 50kg
person with metabolic acidosis mM of
Hco3 needed for correction is:
= 0.3 X body weight X BE
= 0.3 X 50 X10 = 150 mM

33. Anion GAP
Step 4
• Calculation of AG is useful approach to
analyse metabolic acidosis
AG = (Na+ + K+) – (cl- + Hco3-)
• * A change in the pH of 0.08 for each 10
mm Hg indicates an ACUTE condition.
* A change in the pH of 0.03 for each 10
mm Hg indicates a CHRONIC condition.

34. • K etoacidosis
• U remia
• S epsis
• S alicylate & other drugs
• M ethanol
• A lcohol (Ethanol)
• L actic acidosis
• E thylene glycol
REMEMBER

35. COMPENSATION
• Step 5
• A patient can be uncompensated or
partially compensated or fully
compensated
• pH remains outside the normal range
• pH has returned within normal range-
fully compensated though other values
may be still abnormal
• Be aware that neither the system has
the ability to overcompensate

36. 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

37. 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

38. Step 5 cont…
Determine if there is a compensatory
mechanism working to try to correct the
pH.
ie: if have primary respiratory acidosis will
have increased PaCO2 and decreased pH.
Compensation occurs when the kidneys
retain HCO3.
ABG Interpretation

39. Assess the PaCO2
• In an uncompensated state – when the pH
and paCO2 moves in the same direction: the
primary problem is metabolic.
• The decreasing paco2 indicates that the lungs
acting as a buffer response (blowing of the
excess CO2)
• If evidence of compensation is present but
the pH has not been corrected to within the
normal range, this would be described as
metabolic disorder with the partial
respiratory compensation.

41. Assess the HCO3
• The pH and the HCO3 moving in
the opposite directions, we would
conclude that the primary
disorder is respiratory and the
kidneys acting as a buffer
response: are compensating by
retaining HCO3 to return the pH to
normal range.

42. Example 3
• Mrs. H is admitted, he is kidney
dialysis patient who has missed his
last 2 appointments at the dialysis
centre his ABG results:
• pH : 7.32
• paCo2 : 32
• HCO3 : 18
• Pao2 : 88
• Partially compensated metabolic
Acidosis

43. Example 4
• Mr. K with COPD.His ABG is:
• pH : 7.35
• PaCO2 : 48
• HCO3 : 28
• PaO2 : 90
• Fully compensated Respiratory
Acidosis

44. Example 5
• Mr. S is a 53 year old man presented to
ED with the following ABG.
• pH : 7.51
• PaCO2 : 50
• HCO3 : 40
• Pao2 : 40 (21%O2)
• He has metabolic alkalosis
• Acute respiratory alkalosis (acute
hyperventilation).

45. FULLY COMPENSATED
pH paco2 Hco3
Resp.Acidosis Normal
but<7.40
Resp.Alkalosis Normal
but>7.40
Met. Acidosis Normal
but<7.40
Met. Alkalosis Normal
but>7.40

46. Partially compensated
pH paco2 Hco3
Res.Acidosis
Res.Alkalosis
Met. Acidosis
Met.Alkalosis

47. ~ PaCO2 – pH Relationship
80 7.20
60 7.30
40 7.40
30 7.50
20 7.60

48. Precautions
 Excessive Heparin Decreases bicarbonate and
PaCO2
 Large Air bubbles not expelled from sample PaO2
rises, PaCO2 may fall slightly.
 Fever or Hypothermia, Hyperventilation or breath
holding (Due to anxiety) may lead to erroneous lab
results
 Care must be taken to prevent bleeding

49. 2SD NORMAL CL.ACCEPTABLE
• PH 7.35 – 7.45 7.30 – 7.50
• PCO2 35 – 45 30 – 50
• PO2 97 >80
(ON 21% O2)
(ON VENTILATOR) 60 – 90
• HCO3 22-26 24 - 28

50. Take Home Message:
Valuable information can be gained from an
ABG as to the patients physiologic condition
Remember that ABG analysis if only part of the patient
assessment.
Be systematic with your analysis, start with ABC’s as always
and look for hypoxia (which you can usually treat quickly),
then follow the four steps.
A quick assessment of patient oxygenation can be achieved
with a pulse oximeter which measures SaO2.

51. It’s not magic understanding
ABG’s, it just takes a little
practice!

52. Practice ABG’s
1. PaO2 90 SaO2 95 pH 7.48 PaCO2 32 HCO3 24
2. PaO2 60 SaO2 90 pH 7.32 PaCO2 48 HCO3 25
3. PaO2 95 SaO2 100 pH 7.30 PaCO2 40 HCO3 18
4. PaO2 87 SaO2 94 pH 7.38 PaCO2 48 HCO3 28
5. PaO2 94 SaO2 99 pH 7.49 PaCO2 40 HCO3 30
6. PaO2 62 SaO2 91 pH 7.35 PaCO2 48 HCO3 27
7. PaO2 93 SaO2 97 pH 7.45 PaCO2 47 HCO3 29
8. PaO2 95 SaO2 99 pH 7.31 PaCO2 38 HCO3 15
9. PaO2 65 SaO2 89 pH 7.30 PaCO2 50 HCO3 24
10. PaO2 110 SaO2 100 pH 7.48 PaCO2 40 HCO3 30

53. What is going on?

54. Answers to Practice ABG’s
1. Respiratory alkalosis
2. Respiratory acidosis
3. Metabolic acidosis
4. Compensated Respiratory acidosis
5. Metabolic alkalosis
6. Compensated Respiratory acidosis
7. Compensated Metabolic alkalosis
8. Metabolic acidosis
9. Respiratory acidosis
10.Metabolic alkalosis

55. Any Questions?