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ABG Interpretation

Arterial Blood Gas

Extracellular fluid pH is determined by the concentration of hydrogen ions
The concentration of hydrogen ions is determined by the balance between the partial pressure of CO2 and the concentration of bicarb

This relationship is expressed as:
[H+] (nEq/L) = 24 x (PCO2/HCO3)

Normal Values
PH = 7.34 to 7.44
PCO2 = 36 to 44 mm Hg
HCO3 = 22 to 26 mEq/L

Normal Regulation

Normally there is very tight control and a change in either PCO2 or HCO3 due to a pathological process results in a change in the other variable as a compensatory process.

Estimating Hydrogen Ion Concentration from pH

The change in hydrogen ion concentration with pH is curvilinear and therefore to mentally estimate accurately.

Roughly you can get away with assuming a linear relationship. Start with the fact that at pH 7.4 the proton concentration is 40. Then for each change in pH by 0.01 the hydrogen ion concentration will change by 1 nEq/L.

Examples:
PH 7.41 = Hydrogen concentration will decrease by 1 nEq/L which would put it as 39 nEq/L (40 – 1)

PH 7.32 = 40 (base value for 7.4) + 8 (1 neQ/L for each 0.01 change between 7.4 – 7.32) = 48

PH 7.52 = 40 - 12 = 28 nEq/L

Intuitively, you add to the hydrogen ion concentration as the pH drops.

A second way that is slightly more complex is also slightly more accurate because it more closely follows the true curvilinear relationship. When the blood is acidotic each 0.01 change in pH from 7.4 results in an increase in proton concentration by 1.25. When the blood is alkalotic, each 0.01 change in pH from 7.4 results in a decrease in proton concentration by 0.8.

Examples
PH 7.41 = Hydrogen concentration will decrease by 0.8 nEq/L which would put it as 39.2 nEq/L (40 – 0.8)

PH 7.32 = 40 (base value for 7.4) + (8 x 1.25) (1.25 neQ/L for each 0.01 change between 7.4 – 7.32) = 50

PH 7.52 = 40 - 12 = 30.4 nEq/L

As you can see there is little difference in the values obtained by the two equations until the extremes of pH are reached. Use the first method for pH values that are relatively close to 7.4 and use the second equation when the pH value is farther away.

7 Questions for ABG Interpretation

  • Is the ABG internally consistent?
  • Is there acidemia or alkalemia?
  • What is the primary base disorder?
  • What is the anion gap?
  • What is the delta gap?
  • Is there Compensation?
  • What is the cause?

 

Step 1: Is the ABG internally consistent?

Checking internal consistency is a fancy way to say, “do these numbers make sense?”

Use the equations above to convert pH to proton concentration. Then plug the numbers into the equation:

[H+] (nEq/L) = 24 x (PCO2/HCO3)

If both sides are equal the ABG is internally consistent. I’m not sure how much variation is expected and acceptable, but question an ABG that doesn’t match up.

Step 2: Is there acidemia or alkalemia?

 

If the pH value is less than 7.36 there is acidemia (acidic serum). If the pH is greater than 7.44 there is alkalemia (the serum is alkalotic). This helps defines the basic problem without delving into the cause.

If the pH is normal there may still be a mixed acid-base disorder.

Step 3: What is the Primary Base Disorder

 

  • Acidemia
    • If the HCO3 is decreased, then it is a metabolic acidosis
    • If the CO2 is increased, then it is a respiratory acidosis
  • Alkalosis
    • If the HCO3 is increased, then it is a metabolic alkalosis
    • If the CO2 is decreased, then it is a respiratory alkalosis

The HCO3 and CO2 should move in the same direction for a simple acid-base disorder. One is due to the primary pathophysiological problem and the other is due to compensation. Compensation will not entirely correct a disorder. If they are moving in opposite directions there are two acid-base disorders present.

Primary Base Disorders

Primary Disorder

HCO3

PCO2

Respiratory Acidosis

Increased

Increased

Respiratory Alkalosis

Decreased

Decreased

Metabolic Acidosis

Decreased

Decreased

Metabolic Alkalosis

Increased

Increased

Step 4: What is the anion gap?

The anion gap is calculated only when there is a metabolic acidosis. The anion gap calculates unmeasured anions. If there is a large amount of unmeasured anions then this is the cause of the metabolic acidosis. If the number of unmeasured anions is within normal limits then we can assume that the metabolic acidosis is due to a loss of bicarb.

Equations below are taken from Marino’s “The ICU Book”:

Positive ions = Negative ions
Na + Unmeasured Cations (UA) = Cl + HCO3 + Unmeasured Anions (UA)

Rearrange to get:
Na – (Cl + HCO3) = UA – UC
Na – (Cl + HCO3) = Anion gap (AG)

The normal range for the AG is 3 to 11 mEq/L.

Albumin usually makes up a large amount of the unmeasured anions. Therefore, in patients who are hypoalbuminemic the anion gap may be falsely normal. The AG should be adjusted for the albumin level.

True AG = Calculated AG + 2.5(4.5 – measured albumin (g/dL))

When all is said and done, if a patient has a metabolic acidosis with an elevated anion gap (> 11 mEq/L) then the patient is said to have an Anion Gap Metabolic Acidosis.

Step 5: What is the delta gap?

The delta gap is only calculated when there is an anion gap metabolic acidosis. The goal of the delta gap is to detect other concomitant metabolic disturbances.

When there is a solitary anion gap metabolic acidosis there is a 1 mmol/L fall in bicarbonate for every 1 mmol/L rise in the anion gap.

Δ Anion gap =  Δ [Bicarbonate]
Measured AG – 12 = 24 – Measured Bicarbonate

If the change in bicarbonate (right side of the equation) is higher than expected the patient may simultaneously have a normal anion gap metabolic acidosis.

If the change in the anion gap (left side of the equation) is higher than expected the patient may simultaneously have a metabolic alkalosis.

Step 6: Is there Compensation?

Changes in respiratory drive can occur very rapidly to compensate for metabolic disorders. Changes in HCO3 take place in the kidneys to compensate for respiratory disorders and take longer to establish. HCO3 is reabsorbed in the proximal tubules and this can be adjusted to increase or decrease the total amount of bicarbonate in the body. For the first 6 hours of a respiratory acid-base disorder there will be no compensation. At 6-12 hours acute compensation will develop. Chronic compensation develops over days.

Compensation will never completely correct the primary acid-base disorder.

Metabolic acid-base disorders

Disorder

Compensation

Metabolic Acidosis

PaCO2 = (1.5 x HCO3) + (8 ± 2)

Metabolic Alkalosis

PaCO2 = (0.7 x HCO3) + (21 ± 2)

Respiratory acid-base disorders


Disorder

Compensation

Acute Respiratory Acidosis

ΔpH = 0.008 x ΔPaCO2
[HCO3] = 24 + { (pCO2 - 40) / 10 }

Chronic Respiratory Acidosis

ΔpH = 0.003 x ΔPaCO2
[HCO3] = 24 + 4 { (pCO2 - 40) / 10}

Acute Respiratory Alkalosis

ΔpH = 0.008 x ΔPaCO2
[HCO3] = 24 - 2 { ( 40 - pCO2) / 10 }

Chronic Respiratory Alkalosis

ΔpH = 0.003 x ΔPaCO2
[HCO3] = 24 - 5 { ( 40 - pCO2 ) / 10 } ( range: +/- 2)

What does an “uncompensated” acid-base disorder imply? Mixed…

Step 7: What is the cause?

Interpreting a VBG

 

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