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