1. Acid-base and ABG interpretation
Carlos A. Camal Sanchez DNP
NCLIN 411

2. Normal parameters: pH = 7.35 – 7.45 PCO2 = 35 – 45 mm Hg
This is your acid, your respiratory component (takes 45s to 2 min for lungs to compensate)
HCO3 = 22 – 26 mEq/L
This is your base, your metabolic component (takes 6 to 12 hrs fully developed after a few days for kidneys to compensate)

3. CO2 and HCO3 have a reverse reaction and equilibrium
CO2 + H2O  H2CO3  H + HCO3 This is the way kidney excrete or retain HCO3 Or
CO2 + H2O  H2CO3  H + HCO3 This is the way to eliminate CO2 through the lungs

4. Basic Concepts:
Hydrogen Ion [H+] in extracellular fluid is determined by balance between:
-Partial pressure of carbon dioxide (PCO2) Normal 40mm Hg arterial blood
-Concentration of Bicarbonate (HCO3) Normal 24 mEq/L arterial blood

5. Basic Concepts:
For example Hydrogen ion concentration expressed as follows in nanoequivalents per liter because represents amount, and expressd in pH units:
[H+] (nEq/L) = 24 x (PCO2 / HCO3)
= 24x (40/24) = 40 nEq/L

6. Basic Concepts:
Paul L. Marino 3rd edition

7. Basic Concepts: There is a relationship between pH and [H+]
A normal [H+] of 40 nEq/L corresponds to a pH of 7.40

8. Normal parameters: pH = 7.35 – 7.45 PCO2 = 35 – 45 mm Hg
PCO2 = 35 – 45 mm Hg
This is your acid, your respiratory component (takes 45 s to 2 min for lungs to compensate)
HCO3 = 22 – 26 mEq/L
– This is your base, your metabolic component (takes 6 to 12 hrs fully developed after a few days for kidneys to compensate)

9. For example:  of PCO2 refers to respiratory acidosis
 of PCO2 refers to respiratory alkalosis
 of HCO3 refers to metabolic alkalosis
 of HCO3 refers to metabolic acidosis
Any changes of values outside the normal range, you may use the suffix
-emia
-Acidemia pH < 7.36
-Alkalemia pH >7.44

10. Remember that pH requires a constant PCO2 / HCO3 ration of 40 nEq/L

11. Primary Acid-Base Disorders and Associated compensatory changes

12. Respiratory Compensation
Peripheral chemoreceptors located:
In the carotid body at carotid bifurcation in the neck
Metabolic acidosis stimulate the chemoreceptors
- ventilation and  arterial PCO2 (PaCO2)
Metabolic alkalosis silences the chemoreceptors
- ventilation and  arterial PaCO2
Respiratory Acidosis:
pH ↓ 7.35 CO2↑ 45 HCO normal or↑
Respiratory Alkalosis:
pH↑ 7.45 CO2↓35 HCO3↓ or normal

13. Metabolic Compensation
A primary change in PaCO2 takes place in the kidneys
And adjustment in HCO3 reabsorption in the proximal tubules
 PaCO2 (respiratory acidosis) results in  HCO3
 PaCO2 (respiratory alkalosis) results in decreased HCO3

14. Systematic approach to Acid-Base interpretation:
1. pH normal? pH low acidemia, pH high alkalemia
2. PCO2 normal? If pH and PCO2 are opposite direction, therefore it is primary respiratory disorder
-If both pH and PaCO2 same direction  or ; therefore is primary metabolic disorder
3. HCO3 normal? HCO3 < 22; therefore the patient is acidotic
HCO3 >26, then patient is alkalotic
4. If PaCO2 and HCO3 opposite direction of pH:
Compensation when:
- HCO3 is opposite from pH, as evidence of compensation from metabolic system
-pH acidotic, PaCO2 acidotic, and HCO3 alkalotic

15. Why Acid-Base interpretation and Nursing Consideration:
Vomiting, What GI Content is loss?
Acid
 Diarrhea, What GI content loss?
HCO3

16. Causes of respiratory Acidosis?
Patient will not breathe D/T CNS depression
Analgesia, sedatives: (naloxone to reverse) Or
Head injury
Brainstem stroke
CNS infection
Neuromuscular weakness/illness:
-Myasthenia gravis
-Guillain-barre
-ALS
Chronic lung disease:
CODP, emphysema,

17. Causes of respiratory Alkalosis:
Pain, anxiety, fever
ASA toxicity
Pregnancy
Liver failure
Mechanical ventilation

18. Let's Review ABGs

19. Questions?
References: 1. Marino, Paul. The ICU Book 3rd Edition 2. A primer on Arterial Blood Gas Analysis by Andrew M. Luks, MD html 3. Frendl G., Urman Richard D. Pocket Notebook ICU, 2013