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ARTERIAL BLOOD GAS
ANALYSIS
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WHAT IS AN ABG? The Components
pH / PaCO2 / PaO2 / HCO3 / O2sat / BE Desired Ranges
pH - 7.35 - 7.45PaCO2 - 35-45 mmHgPaO2 - 80-100 mmHgHCO3 - 21-27O2sat - 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 monitoringNeed for multiple ABGs
Where to place -- the optionsRadialFemoral BrachialDorsalis PedisAxillary
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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 <--> H2CO3 <--> HCO3- + H+
ph = 6.10 + log ([HCO3] / [0.03 x PCO2])
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THE TERMS ACIDS
AcidemiaAcidosis
Respiratory CO2
Metabolic HCO3
BASESAlkalemiaAlkalosis
Respiratory CO2
Metabolic HCO3
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RESPIRATORY ACIDOSIS ph, CO2, Ventilation Causes
CNS depressionPleural diseaseCOPD/ARDSMusculoskeletal disordersCompensation 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 hemorrhageSalicylate and Progesterone drug usageAnxiety lung complianceCirrhosis of the liverSepsis
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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 Winter’s Formula
PCO2 = 1.5(HCO3) +8 2
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THE CAUSES Metabolic Gap
AcidosisM - MethanolU - UremiaD - DKAP - Paraldehyde I - INHL - Lactic AcidosisE - Ehylene GlycolS - Salicylate
Non Gap Metabolic AcidosisHyperalimentatio
nAcetazolamideRTA (Calculate
urine anion gap)DiarrheaPancreatic Fistula
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METABOLIC ALKALOSIS pH, HCO3 PCO2 by 0.7 for every 1mEq/L in
HCO3
CausesVomitingDiureticsChronic diarrheaHypokalemiaRenal Failure
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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
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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 / 22Na- 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 = 1515 + 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 = 99 + 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 = 1212 + 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