Arterial Blood Gas Analysis

Vanessa Klee MSIV

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 GapDelta 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 = 1515 + 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 = 99 + 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 = 1212 + 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