Arterial Blood Gas Analysis

1
Arterial Blood Gas Analysis

• Overview

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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

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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

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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

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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

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Acid Base Balance

• Assessment of status via bicarbonate-carbon
  dioxide buffer system
• CO2 H2O lt--gt H2CO3 lt--gt HCO3- H
• ph 6.10 log (HCO3 / 0.03 x PCO2)

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The Terms

• ACIDS
• Acidemia
• Acidosis
• Respiratory
• ?CO2
• Metabolic
• ?HCO3

• BASES
• Alkalemia
• Alkalosis
• Respiratory
• ?CO2
• Metabolic
• ?HCO3

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Respiratory Acidosis

• ?ph, ?CO2, ?Ventilation
• Causes
• CNS depression
• Pleural disease
• COPD/ARDS
• Musculoskeletal disorders
• Compensation for metabolic alkalosis

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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

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Respiratory Alkalosis

• ?pH, ?CO2, ?Ventilation
• ? CO2 ? ? HCO3 (?Cl to balance charges ?
  hyperchloremia)
• Causes
• Intracerebral hemorrhage
• Salicylate and Progesterone drug usage
• Anxiety ? ?lung compliance
• Cirrhosis of the liver
• Sepsis

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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

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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
  Winters Formula
• ? PCO2 1.5(HCO3) 8 ? 2

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The Causes

• Metabolic Gap Acidosis
• 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

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Metabolic Alkalosis

• ?pH, ?HCO3
• ?PCO2 by 0.7 for every 1mEq/L ? in HCO3
• Causes
• Vomiting
• Diuretics
• Chronic diarrhea
• Hypokalemia
• Renal Failure

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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
• gt24 metabolic alkalosis

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The Steps

• Start with the pH
• Note the PCO2
• Calculate anion gap
• Determine compensation

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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

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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

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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.

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Sample Problem 2

• ABG - 7.47 / 19 / 123 / 14
• Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17
• ASA level - 38.2 mg/dL

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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

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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

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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

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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

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Sample Problem 4

• Metabolic Alkalosis, hypochloremic
• Winters Formula 1.5 (30) 8 ? 2
• 53 ? 2
• ? uncompensated