Arterial blood gas analysis
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Arterial Blood Gas Analysis. Overview. What is an ABG?. The Components pH / PaCO 2 / PaO 2 / HCO 3 / O 2 sat / BE Desired Ranges pH - 7.35 - 7.45 PaCO 2 - 35-45 mmHg PaO 2 - 80-100 mmHg HCO 3 - 21-27 O 2 sat - 95-100% Base Excess - +/-2 mEq/L. Why Order an ABG?.

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Arterial Blood Gas Analysis

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Arterial Blood Gas Analysis

Overview


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


Acid Base 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

  • 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

    • 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

    • 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

    • 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


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


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