Arterial Blood Gas Analysis
Overview
What is an ABG?
• The Components
– pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
• Desired Ranges
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pH - 7.35 - 7.45
PaCO2 - 35-45 mmHg
PaO2 - 80-100 mmHg
HCO3 - 21-27
O2sat - 95-100%
Base Excess - +/-2 mEq/L
Why Order an ABG?
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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
Logistics
• When to order an arterial line -– Need for continuous BP monitoring
– Need for multiple ABGs
• Where to place -- the options
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Radial
Femoral
Brachial
Dorsalis Pedis
Axillary
Acid Base Balance
• The body produces acids daily
– 15,000 mmol CO2
– 50-100 mEq Nonvolatile acids
• The lungs and kidneys attempt to maintain
balance
Acid Base Balance
• Assessment of status via bicarbonatecarbon dioxide buffer system
– CO2 + H2O <--> H2CO3 <--> HCO3- + H+
– ph = 6.10 + log ([HCO3] / [0.03 x PCO2])
The Terms
• ACIDS
– Acidemia
– Acidosis
• Respiratory
CO2
• Metabolic
HCO3
• BASES
– Alkalemia
– Alkalosis
• Respiratory
CO2
• Metabolic
HCO3
Respiratory Acidosis
• ph, CO2, Ventilation
• Causes
– CNS depression
– Pleural disease
– COPD/ARDS
– Musculoskeletal disorders
– Compensation for metabolic alkalosis
Respiratory Acidosis
• Acute vs Chronic
– Acute - little kidney involvement. Buffering via
titration via Hb for example
• pH by 0.08 for 10mmHg  in CO2
– Chronic - Renal compensation via synthesis and
retention of HCO3 (Cl to balance charges 
hypochloremia)
• pH by 0.03 for 10mmHg in CO2
Respiratory Alkalosis
• pH, CO2, Ventilation
•  CO2   HCO3 (Cl to balance charges 
hyperchloremia)
• Causes
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Intracerebral hemorrhage
Salicylate and Progesterone drug usage
Anxiety  lung compliance
Cirrhosis of the liver
Sepsis
Respiratory Alkalosis
• Acute vs. Chronic
– Acute - HCO3 by 2 mEq/L for every 10mmHg  in
PCO2
– Chronic - Ratio increases to 4 mEq/L of HCO3 for
every 10mmHg  in PCO2
– Decreased bicarb reabsorption and decreased
ammonium excretion to normalize pH
Metabolic Acidosis
• pH, HCO3
• 12-24 hours for complete activation of
respiratory compensation
• PCO2 by 1.2mmHg for every 1 mEq/L HCO3
• The degree of compensation is assessed via
the Winter’s Formula
 PCO2 = 1.5(HCO3) +8  2
The Causes
• Metabolic Gap
Acidosis
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M - Methanol
U - Uremia
D - DKA
P - Paraldehyde
I - INH
L - Lactic Acidosis
E - Ehylene Glycol
S - Salicylate
• Non Gap Metabolic
Acidosis
– Hyperalimentation
– Acetazolamide
– RTA (Calculate urine
anion gap)
– Diarrhea
– Pancreatic Fistula
Metabolic Alkalosis
• pH, HCO3
• PCO2 by 0.7 for every 1mEq/L  in HCO3
• Causes
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Vomiting
Diuretics
Chronic diarrhea
Hypokalemia
Renal Failure
Mixed Acid-Base Disorders
• Patients may have two or more acid-base
disorders at one time
• Delta Gap
Delta HCO3 = HCO3 + Change in anion gap
>24 = metabolic alkalosis
The Steps
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Start with the pH
Note the PCO2
Calculate anion gap
Determine compensation
Sample Problem #1
• An ill-appearing alcoholic male presents
with nausea and vomiting.
– ABG - 7.4 / 41 / 85 / 22
– Na- 137 / K- 3.8 / Cl- 90 / HCO3- 22
Sample Problem #1
• Anion Gap = 137 - (90 + 22) = 25
 anion gap metabolic acidosis
• Winters Formula = 1.5(22) + 8  2
= 39  2
 compensated
• Delta Gap = 25 - 10 = 15
15 + 22 = 37
 metabolic alkalosis
Sample Problem #2
• 22 year old female presents for attempted
overdose. She has taken an unknown amount
of Midol containing aspirin, cinnamedrine,
and caffeine. On exam she is experiencing
respiratory distress.
Sample Problem #2
• ABG - 7.47 / 19 / 123 / 14
• Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17
• ASA level - 38.2 mg/dL
Sample Problem #2
• Anion Gap = 145 - (109 + 17) = 19
 anion gap metabolic acidosis
• Winters Formula = 1.5 (17) + 8  2
= 34  2
 uncompensated
• Delta Gap = 19 - 10 = 9
9 + 17 = 26
 no metabolic alkalosis
Sample Problem #3
• 47 year old male experienced crush injury at
construction site.
• ABG - 7.3 / 32 / 96 / 15
• Na- 135 / K-5 / Cl- 98 / HCO3- 15 / BUN- 38 /
Cr- 1.7
• CK- 42, 346
Sample Problem #3
• Anion Gap = 135 - (98 + 15) = 22
 anion gap metabolic acidosis
• Winters Formula = 1.5 (15) + 8  2
= 30  2
 compensated
• Delta Gap = 22 - 10 = 12
12 + 15 = 27
 mild metabolic alkalosis
Sample Problem #4
• 1 month old male presents with projectile
emesis x 2 days.
• ABG - 7.49 / 40 / 98 / 30
• Na- 140 / K- 2.9 / Cl- 92 / HCO3- 32
Sample Problem #4
• Metabolic Alkalosis, hypochloremic
• Winters Formula = 1.5 (30) + 8  2
= 53  2
 uncompensated