Approach to Arterial Blood Gases
McMaster University: Approach to ABG's -
Focusing on ABGs for Hypoxia, Resp Acidosis & Alkalosis
Source: Internal Medicine Residency Training Program: Housestaff Survival Guide. McMaster University Department of Medicine 6th Ed. Available at the University Book store for $16.00 - extremely helpful algorithms and basics for internal medicine rotation in clerkship year!
A: Know the components of an ABG
B: Know your normal values
pH: 7.40 (7.35 - 7.45)
pCO2: 40 mm Hg (35-40 mm Hg)
pO2 = 97% (normal), >80% clinically acceptable on 21% room air oxygen. If patient is on ventilator, between 60-90
[HCO3]: 24 mmol/L (21-28 mmol/L)
Anion Gap (AG) = Na - (Cl + HCO3) = 12 +/- 2 mEq/L [this is only relevant for metabolic acidosis]
Acid solution pH less than 7.35
Alkaline solution pH is greater than 7.45
Why is acid-base balance important?
If the blood pH is too acidic or too alkaline, thiswill change normal physiology and lead to dysfunction of important body functions. For example, an acidic pH in the blood decreases the force of cardiac contractions. If there is alkalotic pH, this will interfere with tissue perfusion (another word for "oxygenation"). Significant changes to pH can lead to death.
C: Understand the basic physiology behind acids and bases
What happens when blood pH becomes too acidic or too alkaline?
The two key body systems that help keep pH maintained at an optimal level are the lungs, and the kidneys. We will focus specifically on the lungs to keep things simple.
Respiratory Acidosis: hypoventilation
Respiratory Alkalosis: hyperventilation
D: Use an organized approach to Interpreting ABGs
Step 1: Is the pH acidic or alkalitic?
- pH < 7.35 = acidic
- pH > 7.45 = alkaline
- Assess the pCO2 level. You can tell that the cause of the disturbance is pH is due to the lungs/respiratory (hypoventilation, hyperventilation) if pH and pCO2 move in opposite directions.
- if pH < 7.35 (decreased), the pCO2 should increase
- if pH > 7.45 (increased), the pCO2 should decrease
- Look at the pO2 value, if < 80 mm Hg, then hypoxemia is present
- If acute: then you'll notice a change in pH in addition to changes in pO2
- if patient is chronically hypoxemic, then the body has time to compensate. the pH is within an acceptable range (7.35 - 7.45) but the pO2 continues to be less than 80 mm Hg.