Pediatric Lectures

1. Arterial Blood Gas Analysis
Overview

2. 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

3. 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

4. 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

5. Acid Base Balance
• Assessment of status via bicarbonate-
carbon dioxide buffer system
– CO2 + H2O <--> H2CO3 <--> HCO3
-
+ H+

6. The Terms
• ACIDS
– Acidemia
– Acidosis
• Respiratory
↑CO2
• Metabolic
↓HCO3
• BASES
– Alkalemia
– Alkalosis
• Respiratory
↓CO2
• Metabolic
↑HCO3

7. Respiratory Acidosis
• ↓ph, ↑CO2,↓Ventilation
• Causes
– CNS depression
– Pleural disease
– COPD/ARDS
– Musculoskeletal disorders
– Compensation for metabolic alkalosis

8. 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

9. 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

10. 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

11. 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

12. 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

13. Metabolic Alkalosis
• ↑pH, ↑HCO3
• ↑PCO2by 0.7 for every 1mEq/L ↑ in HCO3
• Causes
– Vomiting
– Diuretics
– Chronic diarrhea
– Hypokalemia
– Renal Failure

14. Mixed Acid-Base Disorders
• Patients may have two or more acid-base
disorders at one time
• Delta Gap
Delta HCO3 = HCO3 + Change in anion gap
>24 = metabolic alkalosis

15. The Steps
• Start with the pH
• Note the PCO2
• Calculate anion gap
• Determine compensation