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 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 = 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