Arterial Blood Gas Analysis

What is an ABG?
• The Components
• pH / PaCO2 / PaO2 / HCO3 / O2sat / BE
• Desired Ranges
• 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?
• 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
• 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

• Assessment of status via bicarbonate-carbon 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


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 Ü hypochloremia)
• pH ¯by 0.03 for 10mmHg ­in CO2


Respiratory Alkalosis
• ­pH, ¯CO2, ­Ventilation
• ¯ CO2 Ü ¯ HCO3 (­Cl to balance charges Ü hyperchloremia)
• Causes
• Intracerebral hemorrhage
• Salicylate and Progesterone drug usage
• Anxiety Ü ¯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
Ü PCO2 = 1.5(HCO3) +8 ± 2
The Causes
• Metabolic Gap Acidosis
• M - Methanol
• U - Uremia
• D - DKA
• P - Paraldehyde
• I - INH
• L - Lactic Acidosis
• E - Ehylene Glycol
• S - Salicylate


Metabolic Alkalosis
• ­pH, ­HCO3
• ­PCO2 by 0.7 for every 1mEq/L ­ in HCO3
• Causes
• 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
• 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

• Anion Gap = 137 - (90 + 22) = 25
Ü anion gap metabolic acidosis
• Winters Formula = 1.5(22) + 8 ± 2
= 39 ± 2
Ü compensated
• Delta Gap = 25 - 10 = 15
15 + 22 = 37
Ü 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
Ü anion gap metabolic acidosis
• Winters Formula = 1.5 (17) + 8 ± 2
= 34 ± 2
Ü uncompensated
• Delta Gap = 19 - 10 = 9
9 + 17 = 26
Ü 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

• Anion Gap = 135 - (98 + 15) = 22
Ü anion gap metabolic acidosis
• Winters Formula = 1.5 (15) + 8 ± 2
= 30 ± 2
Ü compensated
• Delta Gap = 22 - 10 = 12
12 + 15 = 27
Ü 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
Ü uncompensated