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