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.
3. Why Order an ABG?
Aids in establishing a diagnosis
Helps guide treatment plan
Aids in ventilator management
Improvement in acid/base management allows
for optimal function of medications
Acid/base status may alter electrolyte levels
critical to patient status/care
4. What Is An ABG?
pH [H+
]
PCO2 Partial pressure
CO2
PO2 Partial pressure O2
HCO3 Bicarbonate
SaO2 Oxygen Saturation
5. Basic principles
• Human bodies were designed to maintain:
pHpH 7.35-7.457.35-7.45
PaO2PaO2 80-10080-100
mmHgmmHg
PaCO2PaCO2 35-4535-45
mmHgmmHg
HCO3HCO3 22-2622-26
mmHgmmHg
6. Arterial Blood Gas Interpretation
pH: negative log of H+
concentration
In blood:
•Normal range: 7.35 - 7.45
•Acidosis = pH less than 7.35
•Alkalosis = pH greater than 7.45
•A pH < 7.0 or > 7.8 can cause death
7. • PaCO2: partial pressure of CO2 dissolved in the
arterial plasma
– Normal: 35 - 45 mm Hg
– Is regulated in the lungs
– A primary respiratory problem is when PaCO2 is:
– > 45 mm Hg = respiratory acidosis
– < 35 mm Hg = respiratory alkalosis
• HCO3 will be normal (22 - 26 mEq/L)
8. Compensation
• Body attempts to recover from primary problem
and return to homeostasis
• Primary metabolic acidosis breathe faster to
compensate (blow off CO2) by creating a
respiratory alkalosis state
• This would be labeled as: Metabolic acidosis
with a compensatory respiratory alkalosis
e.g: pH 7.30, PaCO2 = 28 & HCO3 = 15
PaCo2 & HCO3 below normal? Yes! Compensation!
9. Acid/Base Balance
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.
10. 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
11. • Limit pH changes when strong acids/bases are
introduced
• Addition of a strong acid is partly neutralized by
the weak base
HB (weak acid) H+ (strong acid) B- (conjugate weak base)
12. Extracellular vs. Intracellular
buffering
H+ buffering upon entering the extracellular and intracellular
fluid
• Immediate buffering by HCO3- extracellularly
• H+ takes 2-3 hrs to enter the cell
• Then buffered by intracellular shifts in electolytes
•Cl- follows H+ into the cell to maintain
electroneutrality
•Na+ and K+ move out of the cell therefore
increasing K+ and Na+ in the extracellular fluid
• This all occurs to help maintain the PH near 7.4
14. Respiratory Buffer Response
The blood pH will change acc. to the level of
H2CO3present.
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
15. 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
16. Acid/Base Disorders
1- Respiratory 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.
17. Causes
1. CNS depression: medications such as
narcotics, sedatives, or anesthesia.
2. Impaired muscle function: spinal cord
injury, NM diseases, or NM 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.
18. Signs & symptoms of Respiratory
Acidosis
Respiratory: Dyspnea, respiratory distress
and/or shallow respiration.
Nervous: Headache, restlessness and
confusion. If Co2 level extremely high
drowsiness and unresponsiveness may be
noted.
CVS: Tachycardia and dysrhythmias
19. 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 MV
20. 2- Respiratory alkalosis
Is defined as a pH more than 7.35 with a
paco2 less than 45 mmHg, due to:
Psychological responses, anxiety or fear.
Pain
Increased metabolic demands such as
fever, sepsis, pregnancy or thyrotoxicosis.
Medications such as respiratory stimulants.
CNS lesions
21. Signs & symptoms
CNS: Light Headedness, numbness, tingling,
confusion, inability to concentrate and blurred
vision.
CVS: Dysrhythmias and palpitations
Dry mouth, diaphoresis and tetanic
spasms of the arms and legs.
22. Management
Resolve the underlying problem
Monitor for respiratory muscle fatigue
When the respiratory muscle become
exhausted, acute respiratory failure may
ensue
23. 3- Metabolic Acidosis
Is Bicarbonate less than 22mEq/L with
a pH of less than 7.35.
Caused by:
oRenal failure
oDiabetic ketoacidosis
oAnaerobic metabolism (lactic acidosis)
oStarvation
oSalicylate intoxication
24. 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
25. 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
26. 4- Metabolic alkalosis
Is Bicarbonate more than 26m Eq /L
with a pH more than 7.45
Causes:
o Excess of base /loss of acid
o Ingestion of excess antacids, excess use of
bicarbonate, or use of lactate in dialysis.
o Protracted vomiting, gastric suction,
hypochloremia, excess use of diuretics, or high
levels of aldosterone.
27. Signs/symptoms
CNS: Dizziness, lethargy disorientation,
seizures & coma.
M/S: weakness, muscle twitching, muscle
cramps and tetany.
Nausea, vomiting and respiratory
depression.
It is difficult to treat.
28. Components of ABGpH
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 HCO3 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)
29. Stepwise approach to ABG
Step 1: Acidemic or Alkalemic?
Step 2: Is the primary disturbance respiratory
or metabolic?
Step 3: Assess 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
30. • Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory
or metabolic?
• Step 3: Assess 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
31. Step:1
Assess the pH –acidotic/alkalotic
If above 7.5 – alkalotic
If below 7.35 – acidotic
32. • Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance
respiratory or metabolic?
• Step 3: Assess 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
33. 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.
Step 2:
34. 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
35. • Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory
or metabolic?
• Step 3: Assess 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
36. Step 3:
Assess pao2 < 80 mm Hg - Hypoxemia
For a respiratory disturbance : acute, chronic
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)
37. Example 1:
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
SO Follow the steps
• pH is low – acidosis
• paCO2 is high – in the opposite direction of the pH.
• Hco3 is Normal.
Respiratory Acidosis
38. Example 2:
Fifty five years old male patient admitted with recurring
bowel obstruction has been experiencing intractable
vomiting for the last several hours. His ABG is:
pH : 7.50
paCO2: 42
HCO3 : 33
Metabolic alkalosis
(IV fluids, measures to reduce the excess
base)
40. • Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory
or metabolic?
• Step 3: Assess 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
41. Anion GAP
Step 4:
Calculation of AG is useful approach to
analyze metabolic acidosis: N = 12 +/- 2
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.
42. - High anion gap occurs in:
lactic acidosis, diabetic ketoacidosis, renal failure (sulfates, phosphates &
urates accumulate), alcohol abuse & some drugs (aspirin, Fe, INH, phenformin)
-Low anion gap occurs in:
hypoalbuminemia Albumin is a negatively charged protein and its loss from
the serum results in the retention of other negatively charged ions
-Normal anion gap occurs in:
hyperchloremic acidosis (Cl- increase in response to lowered HCO3-) in:
oGIT loss of HCO3− (diarrhea) (N.B. vomiting causes hypochloraemic alkalosis)
oRena loss of HCO3− (i.e. proximal renal tubular acidosis (type 2 RTA)
oRenal dysfunction (i.e. distal renal tubular acidosis (type 1 RTA)
oIngestions: Ammonium chloride and Acetazolamide, ifosfamide,
Hyperalimentation fluids (i.e. total parenteral nutrition)
oSome cases of ketoacidosis, particularly during rehydration with Na+
containing IV solutions.
oAlcohol (such as ethanol) can cause a high anion gap acidosis in some
patients, but a mixed picture in others due to concurrent metabolic alkalosis.
oMineralocorticoid deficiency (Addison's disease)
43. • Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory
or metabolic?
• Step 3: Assess 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
44. COMPENSATION
Step 5:
A patient can be uncompensated or
partially compensated pH remains
outside the normal range
pH has returned within normal range fully
compensated though other values may be
still abnormal
45. Determine if there is a compensatory
mechanism working to try to correct the pH.
i.e: primary respiratory acidosis increased
PaCO2 and decreased pH. Compensation
occurs when the kidneys retain HCO3.
46. 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.
48. Assess the HCO3
The pH and the HCO3 moving in the same
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.
49. 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
51. Example 3:
• Male patient was admitted, he was a 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
52. Example 4:
• Male patient with COPD. His ABG is:
• pH : 7.35
• PaCO2 : 55
• HCO3 : 30
• PaO2 : 90
Fully compensated Respiratory
Acidosis
53. Quiz 1
24 years old male patient with a history of
drug abuse, brought to ER cyanotic
• pH 7.08
• PaCO2 80
• PaO2 37
• HCO3 24
54. 24 years old male patient with of drug abuse,
brought to ER cyanotic
• pH 7.08
• PaCO2 80
• PaO2 37
• HCO3 24
Acidemic or Alkalemic?
Acidemic
55. 24 years old male patient with a history of drug
abuse, brought to ER cyanotic
• pH 7.08
• PaCO2 80
• PaO2 37
• HCO3 24
pH in relation to PaCO2 and HCO3?
56. 24 years old male patient with a history of drug
abuse, brought to ER cyanotic
• pH 7.08
• PaCO2 80
• PaO2 37
• HCO3 24
primarily respiratory
57. 24 years old male patient with a history of drug
abuse, brought to ER cyanotic
• pH 7.08
• PaCO2 80
• PaO2 37
• HCO3 24
Is the compensation adequate?
58. PaCO2 increased by 40
For every 10 increase you would expect 1
increase in HCO3
Expected HCO3 would be ~28
59. • 24 years old male patient with a history of drug
abuse, brought to ER cyanotic
– pH 7.08
– PaCO2 80
– PaO2 37
– HCO3 24
• Acidemic, primarily respiratory, but mild
component of metabolic
• Also hypoxemic
• Narcotic OD
60. Quiz 2
42 years old female patient with DM, presents
with 4 days of unwell
• pH 7.23
• PaCO2 27
• PaO2 118
• HCO3 12
61. Quiz 2
42 years old female patient with DM, presents
with 4days of unwell
• pH 7.23
• PaCO2 27
• PaO2 118
• HCO3 12
Acidemia, metabolic
DKA, Na 135, Cl 99
AG = Na – Cl – HCO3 = 135 – 111 = 24
62. Quiz 3
71 male patient with COPD, c/o SOB
• pH 7.21
• PaCO2 75
• PaO2 41
• HCO3 30
68. 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
69. It’s not magic understanding
ABG, it just takes a little
practice!