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ARTERIAL BLOOD GAS
ANALYSIS
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
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 monitoringNeed for multiple ABGs
Where to place -- the optionsRadialFemoral BrachialDorsalis PedisAxillary
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
AcidemiaAcidosis
Respiratory CO2
Metabolic HCO3
BASESAlkalemiaAlkalosis
Respiratory CO2
Metabolic HCO3
RESPIRATORY ACIDOSIS ph, CO2, Ventilation Causes
CNS depressionPleural diseaseCOPD/ARDSMusculoskeletal disordersCompensation 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 hemorrhageSalicylate and Progesterone drug usageAnxiety lung complianceCirrhosis of the liverSepsis
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
AcidosisM - MethanolU - UremiaD - DKAP - Paraldehyde I - INHL - Lactic AcidosisE - Ehylene GlycolS - Salicylate
Non Gap Metabolic AcidosisHyperalimentatio
nAcetazolamideRTA (Calculate
urine anion gap)DiarrheaPancreatic Fistula
METABOLIC ALKALOSIS pH, HCO3 PCO2 by 0.7 for every 1mEq/L in
HCO3
CausesVomitingDiureticsChronic diarrheaHypokalemiaRenal 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 / 22Na- 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