Question

Acid-base balance of the blood

Mr. Un. Metabalki was brought to the ER. His arterial blood results contained the following: pH = 7.54, pCO2=40 mmHg, HCO3 - =34 mEq/L.

Mrs. I. Donno entered ER comatose and with shallow infrequent breaths. Her friend said she might have taken an overdose of a narcotic drug. Her arterial blood values were: pH = 7.18, pCO2=80 mmHg, HCO3 - =31 mEq/L.

It has been a busy day, but your shift is nearly over. It looks like your last patient will be Norman O’Normally. His arterial blood results are: pH = 7.38, pCO2=33 mmHg, HCO3 - =19 mEq/L.

Oops! One more patient’s lab results were found at the bottom of the pile. Before you go home, interpret her arterial blood data: pH = 7.43, pCO2=32 mmHg, HCO3 - =20 mEq/L

Answer 1

Steps in identification of the acid-base disorder are:

1. Identify validity of the ABG
2. See the pH and identify acidic of alkaline
3. Check the pCO2 and HCO3 values
   1. if pH is low ( acidic)
      1. pCO2 high - respiratory acidosis
      2. HCO3 low - metabolic acidosis
   2. If the pH is high - alkalosis
      1. pCO2 low - respiratory alkalosis
      2. HCO3 high - Metabolic alkalosis
4. determine the compensatory - complete or incomplete

example 1 pH - 7.54 , pCO2 - 40 mmHg, HCO3 - 34 meq/l

Step 1 - validity

Calculate the H+ ion can compare with the pH

H+ = 24 * (pCO2 / HCO3) = 24 * 40 / 34 = 28

The pH 7.54 represent approximately = 80 - 54( last two digits of the pH) = 26 H+ ion. This value is similar to the value we calculated (28) therefore, this ABG is valid

Step 2: pH = 7.54 ( normal pH = 7.35 - 7.45)

this pH is higher than normal, therefore - This is alkaline

Step 3: - lets see the pCO2 and HCO3 -

• In metabolic alkalosis - the HCO3 and pH both are high ( both move in the same direction)
• In respiratory alkalosis - the pCO2 is low and pH is high ( they move in the opposite direction)
• In this case, HCO3 is 34 meq/l ( normally HCO3 - 24 meq/l)
• Therefore, they both moved in the same direction
• This is metabolic alkalosis

Step 4: Compensatory response -

• The compensatory response to metabolic alkalosis is respiratory acidosis
• In this case the pCO2 is normal ( 40)
• The expected PCO2 for the HCO3 = 0.7( HCO3) + 21 (+/-5)
• 0.7 ( 40) + 21
• 2.8+21 = 23.8
• Therefore, there measure pCO2 (40 mmHg) is more than expected pCO2
• This means this is uncompensated metabolic alkalosis and there is the presence of another acid/ base disorder that is respiratory acidosis

Complete diagnosis - Uncompensated metabolic alkalosis with respiratory acidosis

Example 2 ;pH - 7.18, pCO2 - 80 mmHg , HCO3 - 31 meq/l

Step 1 - validity

Calculate the H+ ion can compare with the pH

H+ = 24 * (pCO2 / HCO3) = 24 * 80 / 31 = 61.9 = 62

The pH 7.18 represent approximately = 80 - 18( last two digits of the pH) = 62 H+ ion. This value is similar to the value we calculated (62) therefore, this ABG is valid

Step 2: pH = 7.18 ( normal pH = 7.35 - 7.45)

this pH is lower than normal, therefore - This is acidosis

Step 3: - let us see the pCO2 and HCO3 -

• In metabolic acidosis - the HCO3 and pH both are low ( they move in the same direction)
• In respiratory acidosis - the pCO2 is high and pH is low ( they move in the opposite direction)
• In this case,
• Therefore, pH is low and pCO2 is high ( they move in the opposite direction)
• This is respiratory alkalosis

Step 4: Compensatory response -

• The compensatory response to respiratory acidosis is metabolic alkalosis
• The expected HCO3 for acute respiratory acidosis
  • 24 + {(actual pCO2 - 40) / 10}
  • 24 + {(80-40) / 10}
  • 24 + {40/10}
  • 24 + 4
  • 28 (+/- 2) = 30
• The measured HCO3 is equal to the expected HCO3 - no additional acid-base disorder

Complete diagnosis - Uncompensated respiratory acidosis

Example 3  pH - 7.38 , pCO2 - 33 mmHg , HCO3 - 19 meq/l

Step 1 - validity

Calculate the H+ ion can compare with the pH

H+ = 24 * (pCO2 / HCO3) = 24 * 33 / 19 = 41.6= 42

The pH 7.38 represent approximately = 80 - 38( last two digits of the pH) = 42 H+ ion. This value is similar to the value we calculated (62) therefore, this ABG is valid

Step 2: pH = 7.38 ( normal pH = 7.35 - 7.45)

this pH is normal but close to acidosis

Step 3: - let us see the pCO2 and HCO3 -

• In metabolic acidosis - the HCO3 and pH both are low ( they move in the same direction)
• In respiratory acidosis - the pCO2 is high and pH is low ( they move in the opposite direction)
• In this case,
• Therefore, pH is low and HCO3 is low ( they move in the same direction)
• This is metabolic acidosis

Step 4: Compensatory response -

• The compensatory response to metabolic acidosis is respiratory acidosis. In this case, the pH is corrected so it is compensated
• The expected PCO2 for the HCO3 = 0.7( HCO3) + 21 (+/-5)
• 0.7 ( 19 ) + 21
• 13.3 + 21
• 34.3
• The measured PCO2 is equal to the expected PCO2 - therefore no additional acid-base disorder

Complete diagnosis -Compensated metabolic acidosis

Example 4 - pH - 7.43 , pCO2 - 32 mmHg , HCO3 - 20 meq/l

Step 1 - validity

Calculate the H+ ion can compare with the pH

H+ = 24 * (pCO2 / HCO3) = 24 * 32 / 20 = 38.4

The pH 7.43 represent approximately = 80 - 43( last two digits of the pH) = 37 H+ ion. This value is similar to the value we calculated ) therefore, this ABG is valid

Step 2: pH = 7.43 ( normal pH = 7.35 - 7.45)

this pH is normal but close to alkalosis

Step 3: - let us see the pCO2 and HCO3 -

• In metabolic alkalosis - the HCO3 and pH both are high ( both move in the same direction)
• In respiratory alkalosis - the pCO2 is low and pH is high ( they move in the opposite direction)
• In this case,
• Therefore, pH is high and pCO2 is low ( they move in the opposite direction)
• This is respiratory acidosis

Step 4: Compensatory response -

• The compensatory response to respiratory alkalosis is metabolic acidosis. In this case, the pH is corrected so it is compensated
• The expected HCO3 for the pCO2 =
• 24 - 2 {(40 - pCO2) / 10 }
• 24 -2 {40 - 32) / 10}
• 22.4 (+/- 2)
• 20.4
• The measured HCO3 is equal to the expected HCO3 - therefore no additional acid-base disorder

Complete diagnosis -Compensated respiratory alkalosis