ABG ANALYSIS IN 6 STEPS

ABC of ABG

Dr. Padmesh

• The 6 Easy Steps to basic ABG Analysis: • 1. Is the pH normal? Acidic or Alkaline?

• 2. Is the pCO2 normal? Acidic or Alkaline?

• 3. Is the HCO3 normal? Acidic or Alkaline?

• 4. Match the CO2 or the HCO3 with the pH

• 5. Does the CO2 or the HCO3 go the opposite direction of the pH?

• 6. Are the pO2 and the O2 saturation normal?

• Step 1: Analyze the pH • Normal blood pH is 7.4 (range 7.35 to 7.45). • pH < 7.35 acidic. • pH > 7.45 alkaline.

• If it falls into the normal range, label what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic.

• Step 2: Analyze the pCO2 :• Normal pCO2 levels = 35-45mmHg. • Below 35 is alkaline, • Above 45 is acidic.

• Step 3: Analyze the HCO3:• Normal HCO3 level is 22-26 mEq/L. • If the HCO3 is below 22, the patient is acidotic. • If the HCO3 is above 26, the patient is alkalotic

• Step 4: Match the CO2 or the HCO3 with the pH• If the pH is acidotic, and the CO2 is acidotic, then

the acid-base disturbance is being caused by the respiratory system.

Therefore, we call it a respiratory acidosis.

• However, if the pH is alkalotic and the HCO3 is alkalotic, the acid-base disturbance is being caused by the metabolic (or renal) system. Therefore, it will be a metabolic alkalosis.

• Step 5: Does the CO2 or HCO3 go the opposite direction of the pH?

• If so, there is compensation by that system. • For example, the pH is acidotic, the CO2 is

acidotic, and the HCO3 is alkalotic: The CO2 matches the pH making the primary acid-base disorder respiratory acidosis. The HCO3 is opposite of the pH and would be evidence of compensation from the metabolic system.

• Step 6: Analyze the pO2 and the O2 saturation.• If they are below normal there is evidence of

hypoxemia.

• NORMAL VALUES:

• Interpretation of Values:

Hypoxia Hyperoxia

Examples:

• Example 1:• Step 1: pH < 7.35 acidotic. • Step 2: CO2 > 45 acidotic.

• Step 3: HCO3 is normal.

• Step 4: CO2 matches pH, because both are acidotic. Therefore the imbalance is respiratory acidosis.

• Step 5: HCO3 is normal, therefore there is no compensation. If the HCO3 is alkalotic (opposite direction) then compensation would be present.

• Step 6: PaO2 and O2 sat are low indicating hypoxemia.

• Full diagnosis: Uncompensated respiratory acidosis with hypoxemia. • This patient has an acute respiratory disorder.

• Example 2:• Step 1: pH > 7.45 alkalotic.

• Step 2: CO2 < 35 alkalotic.

• Step 3: HCO3 is normal.

• Step 4: CO2 matches pH, because both are alkalotic. Therefore imbalance is respiratory alkalosis.

• Step 5: HCO3 is normal, therefore there is no compensation. If the HCO3 is acidotic (opposite direction) then compensation would be present.

• Step 6: PaO2 and O2 sat are normal indicating normal oxygenation.

• The full diagnosis for this blood gas is: Uncompensated respiratory alkalosis.

• Example 3:• Step 1: pH < 7.35 acidotic.

• Step 2: CO2 is normal.

• Step 3: HCO3 < 22 acidotic.

• Step 4: HCO3 matches pH, because both are acidotic. Therefore the imbalance is metabolic acidosis.

• Step 5: CO2 is normal, therefore there is no compensation. If the CO2 is alkalotic (opposite direction) then compensation would be present.

• Step 6: PaO2 and O2 sat are normal indicating normal oxygenation.

• Full diagnosis: Uncompensated metabolic acidosis.

• Example 4:• Step 1: pH > 7.45 alkalotic.

• Step 2: CO2 is normal.

• Step 3: HCO3 > 26 alkalotic.

• Step 4: HCO3 matches the pH, because they are both alkalotic. Therefore the imbalance is metabolic alkalosis.

• Step 5: CO2 is normal, therefore there is no compensation. If the CO2 is acidotic (opposite direction) then compensation would be present.

• Step 6: PaO2 and O2 sat are normal.

• Full diagnosis: Uncompensated metabolic alkalosis.

• Example 5:• Step 1: pH < 7.35 acidotic. • Step 2: CO2 < 35 alkalotic.

• Step 3: HCO3 < 22 acidotic.

• Step 4: HCO3 matches pH, because both are acidotic. Therefore imbalance is metabolic acidosis.

• Step 5: CO2 is alkalotic and goes the opposite direction of the pH, so there is compensation. Because the pH is not in the normal range the compensation is called partial.

• Step 6: PaO2 and O2 sat are low indicating hypoxemia.

• Full diagnosis: Partially-compensated metabolic acidosis with hypoxemia.

• Expected Compensation for simple Acid-base dis: Disorder Primary Compensatory Magnitude of expected event event compensation

Met Acidosis Bicarb pCO2 For every 1 meq/L decrease in bicarb, pCO2 decreases by 1-1.5 mm Hg

Met Alkalosis Bicarb pCO2 For every 1 meq/L increase in bicarb, pCO2 increases by 0.5-1 mm Hg

Resp Acidodis pCO2 Bicarb For every 1 mm Hg increase in pCO2, Bicarb increases by 0.1 – 0.4 meq/L

Resp Alkalosis pCO2 Bicarb For every 1 mm Hg decrease in pCO2, Bicarb decreases by 0.1- 0.4 meq/L

• Appropriate Compensation During Simple Acid-Base Disorders:

• DISORDER EXPECTED COMPENSATION • Metabolic acidosis PCO2 = 1.5 × [HCO3

-] + 8 ± 2• Metabolic alkalosis For each 10 mEq/L increase in serum

[HCO3-] , PCO2 increases by 7 mm Hg .

• Appropriate Compensation During Simple Acid-Base Disorders:

• DISORDER EXPECTED COMPENSATION • Respiratory acidosis • Acute : For each 10-mm Hg increase in PCO2 , [HCO3

-] increases by 1.

• Chronic : For each 10-mm Hg increase in PCO2 , [HCO3-]

increases by 3.5 .• Respiratory alkalosis • Acute: For each 10mm Hg decrease in PCO2 , [HCO3

-] falls by 2.• Chronic : For each 10mm Hg decrease in PCO2,[HCO3

-] falls by 4

• EXAMPLE 6:• pH = 7.55, • pCO2 = 14 mmHg, • Bicarb = 10 meq/L

• Since CO2 matches pH, disorder is RESPIRATORY, that is, RESPIRATORY ALKALOSIS.

• Since Bicarb goes in opposite direction to pH, it is compensatory. But compensation is only partial,since pH is not in normal range.

AlkalosisCO2 is also AlkaloticBicarb is Acidotic

• EXAMPLE 6:• pH = 7.55, • pCO2 = 14 mmHg, • Bicarb = 10 meq/L

• Fall in pCO2 = 40-14 = 26• Expected fall in Bicarb = 0.2 x 26 = 5.2 meq/L• However,actual fall in Bicarb = 24-10 = 14

• Since actual fall in Bicarb is more than expected, there is Metabolic Acidosis also.

Respiratory Alkalosis+ Metabolic Acidosis

(MIXED DISORDER)

• EXAMPLE 7:• pH = 7• pCO2 = 30 mmHg• Bicarb = 6 meq/L

• Since Bicarbonate matches pH, disorder is METABOLIC, hence, METABOLIC ACIDOSIS.

• Since pCO2 goes in opposite direction to pH, it is compensatory. Only Partial compensation as pH is not normal.

AcidosisAlkalotic

Acidotic

• EXAMPLE 7:• pH = 7• pCO2 = 30 mmHg• Bicarb = 6 meq/L

• Fall in Bicarb = 24-6 = 18 meq/L• Expected fall in pCO2 = 18 x 1.2 = 21.6 mmHg• However, actual fall in pCO2 is only 40-30 = 10 mmHG

• Therefore, actual fall in pCO2 is much lesser than expected. Hence, pCO2 is actually higher, which means there is RESPIRATORY ACIDOSIS also.

Metabolic Acidosis+ Respiratory Acidosis

(MIXED DISORDER)

• Relationship between pO2 and SpO2 :

Mnemonic: 40, 50, 60, 70, 80, 90

pO2 SpO2 40 70 50 80 60 90 90 99

• ANION GAP:• Anion gap indicates the presence or absence of increased levels of

unmeasured anions (proteins, phosphates, SO4, organic anions)

(Na + K + Ca + Mg + unmeasured cations) = (Cl + HCO3 + unmeasured anions)

AG = Measured Cations – Measured Anions

• AG =[ Na – (Cl + HCO3) ] = [ 140 - (104+24) ] = 12 meq/L• AG =[ (Na + K ) – (Cl + HCO3) ] = [ (140 + 4) - (104+24) ] = 16 meq/L

• ANION GAP:• Anion gap indicates the presence or absence of increased levels of

unmeasured anions (proteins, phosphates, SO4, organic anions)

(Na + K + Ca + Mg + unmeasured cations) = (Cl + HCO3 + unmeasured anions)

AG = Measured Cations – Measured Anions

• AG = [ Na – (Cl + HCO3) ] = 8 - 12 meq/L• AG = [ (Na + K ) – (Cl + HCO3) ] = 12 - 20 meq/L

• ANION GAP: - Increased Anion Gap Metabolic Acidosis: -Addition of acid load Increase in unmeasured anions

- Normal Anion Gap Metabolic Acidosis: -Loss of bicarbonate. -However, this loss of bicarbonate is compensated by corresponding increase in chloride,so that the anion gap remains unchanged.

AG = [ Na – ( Cl + HCO3 ) ]

(Hyperchloremic Met.Acidosis)

Net effect is Normal Anion gap

• CLASSIFICATION OF ANION GAP:

• High Anion gap M.A Normal Anion gap M.A High MUD PILES Normal USED CARP

• Methanol -Ureterostomy• Uremia -Small bowel fistula• DKA -Extra Chloride• Paraldehyde -Diarrhea• IEM, Iron, INH -Carb.anhydrase inhibitor• Lactic acidosis -Adrenal insufficiency• Ethanol, ethylene glycol -RTA• Salicylates -Pancreatic fistula

• RESPIRATORY FAILURE:• Type I Respiratory Failure: ( Diffusion defect; CO2 diffusion is better; eg: Pnemonia, Pulmonary edema)

• pO2 , but normal pCO2

• Type II Respiratory Failure: ( Respiratory gases not reaching alveoli for proper gaseous exchange; eg: HMD , Respiratory paralysis in Polio )

• pO2 and pCO2

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