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Interpretation of acid-base disorder

Dr. Reza DaliraniPediatric nephrologist Mofid children hospital

pH 7.35 - 7.45PaCO2 35 - 45 mm Hg

PaO2 80 - 100 mm Hg

SaO2 93 - 98%

HCO3¯ 22 - 26 mEq/L

Base excess -2.0 to 2.0 mEq/L

Normal Arterial Blood Gas Values

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.

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.

http://umed.med.utah.edu/MS2/renal/AcidBaseTables/img001.JPG

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)

Factors affect the accuracy of ABGType of syringe Analysis longer than 15 minPlacing the sample on icePresence of air bubbles in the sample (more

than 1-2 % blood volume) Pco2 & Po2

Use of heparin Pco2 & Hco3 (normal PH or lowered PH slightly)

Chang in temperature (T Pco2 & PH )

Factors affect the accuracy of ABGInternal 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, HCO3

- 10. 1=Yes 2=No

50=24x40/10=96??A diabetic child has the following ABGs:

pH 7.30, PaCO2 21, HCO3- 10. 1=Yes 2=No

50=24x21/10=50,4??

The Six Step Approach to Solving Acid-Base Disorders

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?

The Six Steps for Acid-Base Analysis

Step 1.

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.

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-

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

Six Steps for Acid-Base Analysis

Step 4A. Is the compensation adequate?

Rules of CompensationMetabolic 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

Six Steps for Acid-Base AnalysisStep 4B. Is the compensation adequate?Rules of CompensationAcute Respiratory Acidosis

Chronic Respiratory Acidosis

Six Steps for Acid-Base AnalysisStep 4C. Is the compensation adequate?

Rules of CompensationAcute Respiratory Alkalosis

Chronic Respiratory Alkalosis

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

Six Steps for Acid-Base AnalysisStep 6. Are there any other metabolic disturbances?

1. Calculate the anion gap: if AG 20 there is a primary metabolic acidosis (regardless of pH or HCO3)

2. 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 alkalosisIf sum < 23 there is an underlying nonanion gap metabolic acidosis

Example:Blood gas 7.50 / 20 / 15Na= 145, Cl = 100

AlkalemicLow 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 AlkalosisThis patient had a history of vomiting (met. alkalosis), poor oral intake (met. acidosis) and tachypnea secondary to bacterial pneumonia (resp. alkalosis)

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