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ACID-BASE BALANCE ABG INTERPRETATION

DR. NABIL BASHIR

RESPIRATORY SYSTEM/BIOCHEMISTRY

Henderson–Hasselbalch equation

where:

pH: is the negative logarithm of molar concentration of hydrogen ions.

pKa H2CO3: is the logarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1.

[HCO−3] is the molar concentration of bicarbonate in the blood plasma

[H2CO3] is the molar concentration of carbonic acid .

However, since the carbonic acid concentration is directly proportional to the partial pressure of carbon dioxide the equation can be rewritten as follows

DEFINITIONS

• LOW PH: ACIDEMIA

• HIGH PH: ALKALEMIA

• HIGH CO2: ACIDIFYING EFFECT

• LOW CO2: ALKALINIZING EFFECT

• HIGH HCO3: ALKALINIZING EFFECT

• LOW HCO3: ACIDIFYING-EFFECT

STEP 1: pH

pH: NORMAL RANGE: 7.35-7.45

IF pH >7.45, ALKALEMIA

IF pH < 7.35, ACIDEMIA

STEP 2: CO2

NORMAL RANGE: 35-45 mmHg or (4.7-6.0)

ACIDIFYING: CO2 >45

ALKALINIZING: CO2< 35

INTERPRETATION

COMBINE STEP 1 AND 2

STEP1: PH LOW: ACIDEMIA

STEP2: CO2 HIGH: ACIDIFYING

BOTH IN THE SAME DIRECTION, SO

THE PROBLEM IS RESPIRATORY ACIDOSIS

STEP 1: PH HIGH: ALKALEMIA

STEP 2: CO2 LOW: ALKALINIZING

THE PROBLEM IS RESPIRATORY ALKALOSIS

PH : LOW, ACIDEMIA

CO2: LOW, ALKALINIZING

PH AND CO2 ARE IN OPPOSITE DIRECTION, SO THE PROBLEM IS METABOLIC ACIDOSIS

PH : HIGH, ALKALEMIA

CO2: HIGH, ACIDIFYING

PH AND CO2 ARE IN OPPOSITE DIRECTION, SO THE PROBLEM IS IS METABOLIC ALKALOSIS

Step 3: HCO3

NORMAL RANGE: 22-26 MMOL/L

HCO3>26, ALKALINIZING

HCO3<22, ACIDIFYING

PH :LOW, ACIDEMIA

CO2: HIGH, ACIDIFYING

HCO3: HIGH, ALKALINIZING

HCO3 IS IN OPPOSITE DIRECTION, ALKALINIZING- TO COMPENSATE .

THE PROBLEM IS RESPIRATORY ACIDOSIS WITH PARTIAL COMPENSATION

PH: LOW, ACIDIC

CO2: LOW, ALKALINIZING

HCO3: LOW, ACIDIFYING

PH AND HCO3 GO IN THE SAME DIRECTION, THEN THE PROBLEM IS METABOLIC ACIDOSIS WITH PARTIAL COMPENSATION BY CO2

PH: HIGH, ALKALINE

CO2: LOW, ALKALINIZING

HCO3: LOW, ACIDIFYING

SINCE PH AND CO2 GO IN THE SAME DIRECTION, SO THE PROBLEM IS RESPIRATORY ALKALOSIS WITH PARTIAL COMPENSATION BECAUSE HCO3 IS IN THE OPPOSITE DIRECTION

PH; HIGH, ALKALINE

CO2: HIGH, ACIDIFYING

HCO3: HIGH, ALKALINIZING

PROBLEM: METABOLIC ALKALOSIS WITH PARTIAL COMPENSATION

NORMAL PH?

WHAT HAPPENS WHEN THE PH IS NORMAL?

1. THE PATIENT DOES NOT HAVE AND ACID BASE PROBLEM WITH NORMAL PH, CO2 AND HCO3, OR

2. THERE IS ACID BASE DISORDER WITH NO COMPENSATION

PH: NORMAL

CO2: HIGH, ACIDIFYING

HCO3: HIGH, ALKALINIZING

FULLY COMPENSATED RESPIRATORY ACIDOSIS OR FULLY COMPENSATED METABOLIC ALKALOSIS.

YOU HAVE TO RELATE THIS PICTURE TO THE CLINICAL CASE, IT IS MORE PROBABLY TO BE RESPIRATORY ACIDOSIS BCZ THE COPD PATIENTS RETAIN CO2 AND BECOME ACEDEMIC SO HCO3 WILL RISE AND STAY HIGH FOR COMPENSATION

PH: NORMAL

CO2: LOW, ALKALANIZING

HCO3: LOW, ACIDIFYING

COULD BE RESPIRATORY ALKALOSIS OR METABOLIC ACIDOSIS WITH COMPLETE COMPENSATION.

DECISION DEPENDS ON CLINICAL CASE

EXAMPLES

PH: 7.3 (7.35-7.45)

CO2: 9 (4.7-6.0)

HCO3: 30 (22-26)

RESPIRATORY ACIDOSIS WITH PARTIAL COMPENSATION

PH: 7.5, ALKALINE

CO2: 2.7 (4.7-6.0), ALKALINIZING

HCO3: 20 (22-26), ACIDIFYING

RESPIRATORY ALKALOSIS WITH PARTIAL COMPENSATION

PH: 7.3, ACIDEMIA

CO2: 3.0, LOW, ALKALINIZING

HCO3: 18, LOW, ACIDIFYING

METABOLIC ACIDOSIS WITH PARTIAL COMPENSATION

PH: 7.37, NORMAL

CO2: 8, ACIDIFYING

HCO3: 33, ALKALINIZING

DIAGNOSIS?

PH: 7.2, ACIDEMIA

CO2: 9, ACIDIFYING

HCO3:26

RESPIRATORY ACIDOSIS WITHOUT COMPENSATION

PH: 7.50, ALKALEMIA

CO2: 2.2, ALKALINIZING

HCO3: 24, NORMAL

DIAGNOSIS: RESPIRATORY ALKALOSIS WITH NO COMPENSATION

PH: 7.10, ACIDEMIA

CO2: 4.8, NORMAL

HCO3: 9, LOW, ACIDIFYING

DIAGNOSIS: METABOLIC ACIDOSIS WITHOUT COMPENSATION

PH: 7.51, ALKALEMIA

CO2: 3.3, ALKALINIZING

HCO3: 23, NORMAL

DIAGNOSIS: RESPIRATORY ALKALOSIS WITH NO COMPENSATION

PH: 7.11, ACIDENIA

CO2: 4.9, NORMAL

HCO3: 8, LOW ACIDIFYING

DIAGNOSIS: METABOLIC ACIDOSIS WITHOUT COMPENSATION

PH: 7.30, ACIDEMIA

CO2: 3, ALKALINIZING

HCO3: 15, ACIDIFYING

DIAGNOSIS: METABOLIC ACIDOSIS WITH PARTIAL COMPENSATION

PH: 7.27, ACIDEMIA

CO2: 2.5, ALKALINIZING

HCO3: 7, ACIDIFYING

DIAGNOSIS: METABOLIC ACIDOSIS WITH PARTIAL COMPENSATION

PH: 7.50, ALKALEMIA

CO2: 7.5, ACIDIFYING

HCO3: 33, ALKALIZING

DIAGNOSIS: METABOLIC ALKALOSIS WITH PARTIAL COMPENSATION

PH: 7.20, ACIDEMIA

CO2: 7, ACIDIFYING

HCO3: 15, ACIDIFYING

DIAGNOSIS: MIXED METABOLIC AND RESPIRATORY ACIDOSIS