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How To Read Acid Base
Disorder
Nanda, dr.Preodita, dr.
Irzan, dr.
Nova, dr.
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Stage I: Iden ti fy the Primary
Acid -Base Diso rder
Rule 1: An acid-base abnormality is present if
either the PaCO2 or the pH is outside thenormal range. (A normal pH or PaCO2 does
not exclude the presence of an acid-base
abnormality, as explained in Rule 3.)
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Rule 2: If the pH and PaCO2 are both abnormal,compare the directional change. If both changein the same direction (both increase ordecrease), the primary acid-base disorder is
metabolic, and if both change in oppositedirections, the primary acid-base disorder isrespiratory.
Example: Consider a patient with an arterial pH of7.23 and a PaCO2 of 23 mm Hg. The pH andPaCO2 are both reduced (indicating a primarymetabolic problem) and the pH is low (indicating
acidemia), so the problem is a primarymetabolic acidosis.
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Rule 3: If either the pH or PaCO2 is normal,
there is a mixed metabolic and respiratory
acid-base disorder (one is an acidosis and
the other is an alkalosis). If the pH is normal,
the direction of change in PaCO2 identifiesthe respiratory disorder, and if the PaCO2 is
normal, the direction of change in the pH
identifies the metabolic disorder.
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Example: Consider a patient with anarterial pH of 7.37 and a PaCO2 of 55mm Hg. The pH is normal, so there is amixed metabolic and respiratory acid-
base disorder. The PaCO2 is elevated,so the respiratory disorder is an acidosis,and thus the metabolic disorder must bean alkalosis. Therefore, this is acombined respiratory acidosis andmetabolic alkalosis. There is no primaryacid-base disorder in this situation; bothdisorders are equivalent in severity(which is why the pH is normal).
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Stage II: Evaluate Compensato ry
Responses
Rule 4: If there is a primary metabolic acidosisor alkalosis, use the measured serum
bicarbonate concentration in Equation 28.3 or
28.4 to identify the expected PaCO2. If themeasured and expected PaCO2 are
equivalent, the condition is fully compensated.
If the measured PaCO2 is higher than the
expected PaCO2, there is a superimposed
respiratory acidosis. If the measured PCO2 is
less than the expected PCO2, there is a
superimposed respiratory alkalosis.
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Example: Consider a patient with a PaCO2 of23 mm Hg, an arterial pH of 7.32, and a
serum HCO3 of 15 mEq/L. The pH isacidemic and the pH and PCO2 change inthe same direction, so there is a primarymetabolic acidosis. Equation 28.3 should
be used to calculate the expected PCO2:(1.5 15) + (8 2) = 30.5 2 mm Hg. Themeasured PaCO2 (23 mm Hg) is lowerthan the expected PaCO2, so there is an
additional respiratory alkalosis. Therefore,this condition can be described as aprimary metabolic acidosis with asuperimposed respiratory alkalosis
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Compensation for Metabo l ic Acidos is
Compensation for Metabo l ic A lkalos is
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Rule 5: If there is a respiratory acidosis or alkalosis, use
the PaCO2 to calculate the expected pH using
Equations 28.5 and 28.7 (for respiratory acidosis) orEquations 28.6 and 28.8 (for respiratory alkalosis).
Compare the measured pH to the expected pH to
determine if the condition is acute, partially
compensated, or fully compensated. For respiratoryacidosis, if the measured pH is lower than the expected
pH for the acute, uncompensated condition, there is a
superimposed metabolic acidosis, and if the measured
pH is higher than the expected pH for the chronic,
compensated condition, there is a superimposed
metabolic alkalosis. For respiratory alkalosis, if the
measured pH is higher than the expected pH for the
acute, uncompensated condition, there is a
superimposed metabolic alkalosis, and if the measured
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Example: Consider a patient with a PaCO2 of 23
mm Hg and a pH of 7.54. The PaCO2 and pH
change in opposite directions so the primary
problem is respiratory and, since the pH isalkalemic, this is a primary respiratory alkalosis.
The expected pH for an acute respiratory
alkalosis is described in Equation 28.6, and is7.40 + [0.008 (40 - 23)] = 7.54. This is the
same as the measured pH, so this is an acute,
uncompensated respiratory alkalosis. If the
measured pH was higher than 7.55, this would
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Respiratory
Acidosis
RespiratoryAlkalosis
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Thank You
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