1. Evaluation and Analysis of Acid-Base Disorders

2. Acid-Base Analysis, What do You Need?
Blood gas (pH, CO2)
Serum chemistry (Na, Cl, HCO3)
Calculator
30 seconds

3. ABG: 7.40 / 40 / 80 / 24 pH
PaCO2
PaO2
HCO3

4. pH= 7.40 (7.35 - 7.45) PCO2 = 40 (35 - 45) HCO3 = 24 (22 - 26)
Acid-Base Normals:
pH= 7.40 ( )
PCO2 = 40 ( )
HCO3 = 24 ( )

5. Acidemic vs. Alkalemic pH < 7.35 = Acidemic
pH > 7.45 = Alkalemic

6. Rule 1
Look at the pH. Whichever side of 7.40 the pH is on, the process (CO2, HCO3) that caused it to shift that way is the primary abnormality.
Principle: The body does not fully compensate for a primary acid-base disorder

7. Keep It Simple: CO2 = Acid HCO3 = Base CO2 =  pH (acidemia)
 CO2 =  pH (alkalemia)
HCO3 = Base
 HCO3 =  pH (alkalemia)
 HCO3 =  pH (acidemia)

8. Four Primary Disorders:
PCO2 < 35 = respiratory alkalosis
PCO2 > 45 = respiratory acidosis
HCO3 < 22 = metabolic acidosis
HCO3 > 26 = metabolic alkalosis
Can have mixed pictures with compensation
Can have up to 3 abnormality simultaneously (1 respiratory + 2 metabolic)
The direction of the pH will tell you which is primary!

9. Simple Acid-Base Disorders

10. Example # 1: Blood gas: 7.50 / 29 / 22
Alkalemic
Low PCO2 is the primary (respiratory alkalosis)
No metabolic compensation = acute process
Acute Respiratory Alkalosis

11. Acute Respiratory Alkalosis

12. Example # 2: Blood gas: 7.25 / 60 / 26
Acidemic
Elevated CO2 is primary (respiratory acidosis)
No metabolic compensation= acute process
Acute Respiratory Acidosis

13. Acute Respiratory Acidosis

14. Example # 3: Blood gas: 7.34 / 60 / 31
Acidemic
Elevated CO2 is primary (respiratory acidosis)
Metabolic compensation has occurred = chronic process
Chronic Respiratory Acidosis with Metabolic Compensation*
*true metabolic compensation takes 3 days (72hrs)

15. Chronic Respiratory Acidosis with Metabolic Compensation

16. Example # 4: Blood gas: 7.50 / 48 / 36
Alkalemic
Elevated HCO3 is primary (metabolic alkalosis)
Respiratory compensation has occurred = acute /chronic ?
Metabolic Alkalosis with Respiratory Compensation*
*Respiratory compensation takes only minutes

17. Metabolic Alkalosis with Respiratory Compensation

18. Example # 5: Blood gas: 7.20 / 21 / 8
Acidemic
Low HCO3 Is primary (metabolic acidosis)
Respiratory compensation is present
Metabolic Acidosis with Respiratory Compensation

20. Anion Gap (AG):
The calculated difference between the positively charged (cations) and negatively charged (anions) electrolytes in the body:
AG= Na+ - (Cl- + HCO3 -)
Normal AG = 12 ± 2 (10 – 14)

21. Rule 2
Calculate the anion gap. If the anion gap is  20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentration
Principle: The body does not generate a large anion gap to compensate for a primary disorder (anion gap must be primary)

22. Rule 3
Calculate the excess anion gap (total anion gap – normal anion gap) and add this value to the measured bicarbonate concentration:
if the sum is > than normal bicarbonate (> 30) there is an underlying metabolic alkalosis
if the sum is less than normal bicarbonate (< 23) there is an underlying nonanion gap metabolic acidosis
Excess AG = Total AG – Normal AG (12)
Excess AG + measured HCO3 = > 30 or < 23?

23. Mixed Acid-Base Disorders

24. Remember the Rules
Look at the pH: (< or > 7.40?) whichever caused the shift (CO2 or HCO3) is the primary disorder
Calculate the anion gap: if AG  20 there is a primary metabolic acidosis (regardless of pH or HCO3)
Calculate the excess anion gap, add it to HCO3:
Excess AG = Total AG – Normal AG (12)
Excess AG + HCO3 = ?
If sum > 30 there is an underlying metabolic alkalosis
If sum < 23 there is an underlying nonanion gap metabolic acidosis

25. Example # 1 Blood gas: 7.50 / 20 / 15 Na= 140, Cl = 103
Alkalemic
Low CO2 is primary (respiratory alkalosis)
Partial metabolic compensation for chronic condition?
AG = 22 (primary metabolic acidosis)
Excess AG (AG – 12) + HCO3 = 25 (no other primary abnormalities)
Respiratory Alkalosis and Metabolic Acidosis
The patient ingested a large quantity of ASA and had both centrally mediated resp. alkalosis and anion gap met. Acidosis associated with salicylate overdose

26. Example # 2 Blood gas: 7.40 / 40 / 24 Na= 145, Cl= 100
pH normal
AG = 21 (primary metabolic acidosis)
Excess AG (AG – 12) + HCO3 = 33 ( underlying metabolic alkalosis)
Metabolic Acidosis and Metabolic Alkalosis
This patient had chronic renal failure (met. acidosis) and began vomiting (met. alkalosis) as his uremia worsened. The acute alkalosis of vomiting offset the chronic acidosis of renal failure = normal pH

27. Example # 3 Blood gas 7.50 / 20 / 15 Na= 145, Cl = 100
Alkalemic
Low CO2 is primary (respiratory alkalosis)
AG = 30 (primary metabolic acidosis)
Excess AG (AG – 12) + HCO3 = 33 (underlying metabolic alkalosis)
Respiratory alkalosis, Metabolic Acidosis and Metabolic Alkalosis
This patient had a history of vomiting (met. alkalosis), poor oral intake (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)

28. How Many Primary Abnormalities Can Exist in One Patient?
Three primary abnormalities is the max because a person cannot simultaneously hyper and hypoventilate
One patient can have both a metabolic acidosis and a metabolic alkalosis – usually one chronic and one acute

29. Example # 4 Blood gas: 7.10 / 50 / 15 Na= 145, Cl= 100
Acidemic
High CO2 and low HCO3- both primary (respiratory acidosis and metabolic acidosis)
AG = 30 (metabolic acidosis is anion gap type)
Excess AG + HCO3 = 33 (underlying metabolic alkalosis)
Respiratory Acidosis, Metabolic Acidosis and Metabolic Alkalosis
This is an obtunded patient (resp. acidosis) with a history of emesis (metabolic alkalosis) and lab findings c/w diabetic ketoacidosis (metabolic acidosis w/ gap)

30. Example # 5 Blood gas: 7.15 / 15 / 5 Na= 140, Cl= 110
Acidemic
Low HCO3- primary (metabolic acidosis)
AG= 25 (metabolic acidosis is anion gap type)
Excess AG + HCO3 = 18 (underlying nonanion gap metabolic acidosis)
Anion Gap and Nonanion gap Metabolic Acidosis
Diabetic ketoacidosis was present (anion gap met. acidosis). Patient also had a hyperchloremic nonanion gap met. acidosis secondary to failure to regenerate bicarbonate from ketoacids lost in the urine.

31. Conclusions:
To do accurate acid-base evaluations you need both blood gas and serum chemistry
Use a systematic approach
Remember the 3 rules
“normal” blood gases may not be normal
It is important to identify all the underlying acid-base in order to appropriately treat the patient