Understanding and Interpreting ABGs

High Acuity Nursing

Youtube link

• https://www.youtube.com/watch?v=RegcYaJM304

​

Overview of Blood pH

• A solution with more base than acid has few hydrogen ions, so it has a higher pH; a pH above 7 is a base or alkaline
• A solution with more acid than base has more hydrogen ions, so it has a lower pH; a pH below 7 is an acid or acidotic
• We assess pt’s acid-base balance via pH of blood, typically arterial blood (but not always)

The basic numbers

• Arterial blood is slightly alkaline at 7.35-7.45
• pH < 7.35 = abnormally acidic (acids accumulate or based are lost)
• pH > 7.45 abnormally alkaline or alkalotic (bases accumulate or acids are lost)
• pH < 6.8 or >7.8 is usually fatal

​

Regulation/Compensation 1 – Chemical Buffers

• Chemical buffers include bicarb, phosphate, proteins (e.g., hemoglobin)
• They act immediately to protect tissues and cells
• Buffers combine with excess acid or base to neutralize harmful effects until other regulators take over
• Silent immediate response from our body

Regulation/Compensation 2 – Respiratory System

• Uses hypo/hyper ventilation to regulate excretion or retention of acids within minutes of a change in pH
• Lungs regulate blood levels of CO2
• Chemo receptors in medulla sense pH changes and vary rate and depth of breathing
• Breathing faster and deeper eliminates more CO2 and pH rises (so ideally when acidotic, RR increases to blow off more CO2 and pH corrects)
• If this corrects the pH, your body detects that pH changes and reduces CO2 excretion by breathing slower or less deeply

​

Regulation/Compensation 3 – Renal System

• Renal system excretes or retains acids and bases as needed
• Only kicks in after the first 2 systems fail to restore pH level
• Usually >6 hour delay
• Takes hours to days to restore pH balance

​

Remember

• PaCO2 and pH move in opposite directions
• If PaCO2 goes up, pH goes does (and vice versa)

The numbers and the Approach

1. Check pH – normal = 7.35-7.45 (< is acidotic; > is alkalotic)
2. Check PaCO2 – normal 35-45 mmHg (is it high or low?)
3. Check bicarb – normal 22-26 mEq/L (is it high or low?)
4. Look for signs of compensation- which value (PaCO2 or bicarb) more closely matches the change in pH?
5. Check PaO2 and SaO2
1. Is the PaO2 normal (80-100 mm Hg)? Is it low or high?
2. Is the SaO2 normal (95% - 100%)? Is it low or is it high?

​

​

Let’s try

1. Check pH 7.52
2. Check PaCO2 –31
3. bicarb - 23

​

Answer

• pH is high (alkalosis)
• PaCO2 low (leads to alkalosis)
• Bicarb normal
• PaCO2 more closely corresponds to the pH, making primary cause respiratory, with no metabolic compensation
• Answer: uncompensated respiratory alkalosis

​

Let’s Try

• pH 7.29
• PaCO2 17 mm Hg
• Bicarb 19 mEq/L

​

Answer

• Low pH indicates acidosis
• PaCO2 low (leads to alkalosis)
• Bicarb low (leads to acidosis)
• Bicarb more closely corresponds to the pH, making primary cause metabolic, resulting decrease in PaCO2 reflects partial respiratory compensation
• Answer: metabolic acidosis with compensatory respiratory alkalosis

Let’s Try�

• pH 7.33
• PaCO2 40 mm Hg
• Bicarb 20 mEq/L

Answer

• pH is low = acidosis
• PaCO2 is normal
• Bicarb is low (matching the pH)
• Primary cause is metabolic
• Uncompensated metabolic acidosis

Let’s Try

• pH 7.52
• PaCO2 47 mmHg
• Bicarb 36 mEq/L

Answer

• pH is alkalotic
• Both PaCO2 and bicarb have changed, but the bicarb matches the pH
• Elevated PaCO2 represents efforts of the respiratory system to compensate for the alkalosis by retaining CO2 (what would respiratory status look like?)
• Partially compensated metabolic alkalosis