1. با نام و یاد خدا

2. Interpretation of acid-base disorder
Dr. Reza Dalirani Pediatric nephrologist Mofid children hospital

3. Normal Arterial Blood Gas ValuespH
PaCO mm Hg
PaO mm Hg
SaO %
HCO3¯ mEq/L
Base excess to 2.0 mEq/L

4. Acid-base Terminology
Acidemia: blood pH < 7.35
Acidosis: a pathophysiologic process that tend to acidify body fluids.
If the patient also has an alkalosis at the same time, the resulting blood pH may be low, normal, or high.

5. Acid-base Terminology
Alkalemia: blood pH > 7.45
Alkalosis: : a pathophysiologic process that tend to alkanize body fluids.
If the patient also has an acidosis at the same time, the resulting blood pH may be high, normal, or low.

7. Principles of Acid-base balance interpretation
E.C.F. acceptable pH range maintained by :
1) Chemical buffers
react very rapidly (< 1 sec)
2) Respiratory regulation
reacts rapidly (sec to min)
3) Renal regulation
reacts slowly (min to hr)

8. Factors affect the accuracy of ABG
Type of syringe
Analysis longer than 15 min
Placing the sample on ice
Presence of air bubbles in the sample (more than 1-2 % blood volume) Pco & Po2
Use of heparin Pco2 & Hco3 (normal PH or lowered PH slightly)
Chang in temperature (T Pco2 & PH )

9. Factors affect the accuracy of ABG
Internal consistency of ABGs ?
[H+] = 24 x PaCO2/HCO3-
80-last two digit of PH= 24 x PaCO2/HCO3-
A woman with renal failure has the following ABGs:
pH 7.30, PaCO2 40, HCO =Yes 2=No
50=24x40/10=96??
A diabetic child has the following ABGs:
pH 7.30, PaCO2 21, HCO =Yes 2=No
50=24x21/10=50,4??

10. The Six Step Approach to Solving Acid-Base Disorders

11. Step 1: Acidemic, alkalemic, or normal?
Step 2: Is the primary disturbance respiratory or metabolic?
Step 3: For a primary respiratory disturbance, is it acute or chronic?
Step 4: For a metabolic disturbance, is the respiratory system compensating OK?
Step 5: For a metabolic acidosis, is there an increased anion gap?
Step 6: For an increased anion gap metabolic acidosis, are there other derangements?

12. The Six Steps for Acid-Base Analysis
Look at the pH  Acidemia or Alkalemia? Whichever side of 7.40 the pH is on, the process that caused it to shift to that side is primary.

13. The six steps for Acid-Base Analysis
Step 2. Is the primary process metabolic or respiratory ?
If ΡaCO2 is correlated with the changes of pH, the problem is respiratory
Acidosis → ↑ CO2
Alkalosis→ ↓ CO2
If HCO3- (base or "alkali") is correlated with the abnormal pH the problem is metabolic
Alkalosis → ↑ HCO3-
Acidosis → ↓ HCO3-

14. Six Steps for Acid-Base Analysis
Step 3. If the primary process is respiratory, is it acute or chronic?
Acute Respiratory Acidosis
Chronic Respiratory Acidosis
Acute Respiratory Alkalosis
Chronic Respiratory Alkalosis

15. Six Steps for Acid-Base Analysis
Step 4A. Is the compensation adequate?
Rules of Compensation
Metabolic Acidosis
PaCO2 should fall by 1 to 1.5 mm Hg x the fall in plasma [HCO3] . ΔHCO3 x 1.2= ΔCO2 or Expected PCO2 = 1,5 Χ (HCO3-) + (8 ± 2)=2 last digits of pH
Metabolic Alkalosis

16. Six Steps for Acid-Base Analysis
Step 4B. Is the compensation adequate? Rules of Compensation Acute Respiratory Acidosis Chronic Respiratory Acidosis

17. Six Steps for Acid-Base Analysis
Step 4C. Is the compensation adequate? Rules of Compensation Acute Respiratory Alkalosis Chronic Respiratory Alkalosis

18. Six Steps for Acid-Base Analysis
Step 5. Is there an anion gap?
Calculate the anion gap (AG).
Na+ - (Cl- + HCO3- )> 12 ?
Principle:
If the AG > 20 mmol/L, there is a metabolic acidosis with elevated AG. The body does not generate large AGs to compensate for a primary disorder

20. Six Steps for Acid-Base Analysis
Step 6. Are there any other metabolic disturbances?
Calculate the anion gap: if AG  20 there is a primary metabolic acidosis (regardless of pH or HCO3)
Calculate the excess anion gap(Δ 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

22. Example: 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)

23. 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