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

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)

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

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

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

Basic Concepts: Paul L. Marino 3rd edition

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

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)

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

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

Primary Acid-Base Disorders and Associated compensatory changes

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 HCO3 22-26 normal or↑ Respiratory Alkalosis: pH↑ 7.45 CO2↓35 HCO3↓ 22-26 or normal

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

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

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

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,

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

Let's Review ABGs

Questions? References: 1. Marino, Paul. The ICU Book 3rd Edition 2. A primer on Arterial Blood Gas Analysis by Andrew M. Luks, MD https://courses.washington.edu/med610/abg/abg_primer. html 3. Frendl G., Urman Richard D. Pocket Notebook ICU, 2013