Slide 1: Today we will be discussing acid base imbalances and ABG
interpretations. An acid or base imbalance is a change in the normal value of
a person’s pH that may result when the acid or base load overwhelms the
excretory capacity of the body. This can result when a patient has an
abnormal renal or respiratory function. Essentially, the patient can have too
much acid or base in the body in which the normal body processes have
difficulty eliminating. Our agenda for this lecture will be to define acid-base
disorder, identify normal acid-base values, discuss simple acid-base
disorders, discuss appropriate acid-base compensation, discuss step-wise
approach to analyzing the ABG, and identify common etiologies of acid-base
disorders.
Slide 2: What is normal? The pH should be between 7.35 and 7.45. A patient
has an acidemia when the pH less than 7.35 and alkalemia when the pH is
greater than 7.45. A normal pCO2 is between 35 and 45 and this is indicative
the respiratory system. A normal HCO3 (bicarb) is between 22 and 26 and this
is indicative of the metabolic system. A normal PaO2 is 80 to 100.
Slide 3: What happens when it is abnormal?
The clinical disturbances associated with acid base metabolism are defined by
the HCO3 and CO2 buffer system. In an acidosis, you’ll note an increase in
hydrogen ions because of the increase in CO2 or decrease in bicarb. In an
alkalosis there will be a reduction of hydrogen ions as evidenced by a
decrease in the pCO2 or increase in he HCO3. An acronym to help remember
this is ROME or respiratory opposite, metabolic equal. If the pH is less than
7.35 and the Co2 is higher than 45, this is indicative of a respiratory acidosis.
The arrows will be opposite. If the pH is less than 7.35 and the bicarb is less
than 22, this is indicative of a metabolic acidosis and the arrows will be equal
or pointing in the same direction.
Slide 4: Let’s practice with uncompensated ABGs. So when the ph is above
7.45 we have an alkalosis. For the 2 examples with alkalosis we have a pH of
7.55 so our arrows are pointed up. The first example, our Co2 is 25. We know
that a normal Co2 should be between 35-45 so we will place a down arrow.
The bicarb of 24 is normal. Given the arrows are opposite, we have a
respiratory alkalosis. The second example of a pH of 7.55 has a Co2 40 which
is normal, and a bicarb of 30. We know that a normal bicarb is 22-26, so we
will place an up arrow. Given the Ph is higher than normal, and our bicarb is
higher, the arrows are equal, so we have a metabolic alkalosis.
The next 2 are examples of acidosis as our pH is less than 7.35 so we will
place down arrows on both of these. The first example shows a Co2 of 75
, which is high. Our arrows are opposite so this ABG is a respiratory acidosis.
The next example with a Ph of 7.13 shows a normal Co2 but the bicarb is 9
which is low. This is an example of a metabolic acidosis as both arrows are
pointing down.
Slide 5: It will be important that you know and understand how a patient will
compensate for their acid base disorder. With respiratory acidosis, the cell
buffering will occur primarily through hemoglobin and proteins which leads to
the production of HCO3 within the kidneys.
If the CO2 remains persistently elevated, then there is an increased excretion
of titratable acid and ammonium by the kidneys leading to the development of
HCO3. With respiratory alkalosis, cellular buffering occurs in which
bicarbonate is combined to hydrogen ions. Ultimately, there is
a decrease in the overall plasma bicarb. The kidneys over time will increase
bicarbonate excretion as well as various other components to help decrease
overall plasma bicarbonate. With Metabolic acidosis, compensation is
attempted by decreasing the pCO2.
How is this accomplished? The patient will become tachypneic to blow off
extra CO2. The patient attempts to do this by increasing their tidal volume or
rate of breathing. With Metabolic alkalosis, compensation is attempted by
increasing the pCO2. There is a decrease alveolar ventilation. The patient
may take shallow or less breaths to hold onto more CO2.
It’s important to note that the body does not overshoot when attempting to
compensate.
Remember the goal of compensation is to return the pH as close to
physiologically normal as possible. If the pH is in a normal range the ABG is
interpreted as compensated. If it has not return to the normal range, then the
ABG is interpreted as partially or uncompensated.
Slide 6: Let’s practice interpreting compensated ABGs. Again using the
ROME method, we place arrows in the direction of the abnormality. For
compensated ABGs, you will have a normal PH. For these, I place a half
arrow in the direction of the Ph from 7.40. If the Ph is greater than 7.4 I place
a half arrow up, and if it is less than 7.4, I place a half arrow down.
interpretations. An acid or base imbalance is a change in the normal value of
a person’s pH that may result when the acid or base load overwhelms the
excretory capacity of the body. This can result when a patient has an
abnormal renal or respiratory function. Essentially, the patient can have too
much acid or base in the body in which the normal body processes have
difficulty eliminating. Our agenda for this lecture will be to define acid-base
disorder, identify normal acid-base values, discuss simple acid-base
disorders, discuss appropriate acid-base compensation, discuss step-wise
approach to analyzing the ABG, and identify common etiologies of acid-base
disorders.
Slide 2: What is normal? The pH should be between 7.35 and 7.45. A patient
has an acidemia when the pH less than 7.35 and alkalemia when the pH is
greater than 7.45. A normal pCO2 is between 35 and 45 and this is indicative
the respiratory system. A normal HCO3 (bicarb) is between 22 and 26 and this
is indicative of the metabolic system. A normal PaO2 is 80 to 100.
Slide 3: What happens when it is abnormal?
The clinical disturbances associated with acid base metabolism are defined by
the HCO3 and CO2 buffer system. In an acidosis, you’ll note an increase in
hydrogen ions because of the increase in CO2 or decrease in bicarb. In an
alkalosis there will be a reduction of hydrogen ions as evidenced by a
decrease in the pCO2 or increase in he HCO3. An acronym to help remember
this is ROME or respiratory opposite, metabolic equal. If the pH is less than
7.35 and the Co2 is higher than 45, this is indicative of a respiratory acidosis.
The arrows will be opposite. If the pH is less than 7.35 and the bicarb is less
than 22, this is indicative of a metabolic acidosis and the arrows will be equal
or pointing in the same direction.
Slide 4: Let’s practice with uncompensated ABGs. So when the ph is above
7.45 we have an alkalosis. For the 2 examples with alkalosis we have a pH of
7.55 so our arrows are pointed up. The first example, our Co2 is 25. We know
that a normal Co2 should be between 35-45 so we will place a down arrow.
The bicarb of 24 is normal. Given the arrows are opposite, we have a
respiratory alkalosis. The second example of a pH of 7.55 has a Co2 40 which
is normal, and a bicarb of 30. We know that a normal bicarb is 22-26, so we
will place an up arrow. Given the Ph is higher than normal, and our bicarb is
higher, the arrows are equal, so we have a metabolic alkalosis.
The next 2 are examples of acidosis as our pH is less than 7.35 so we will
place down arrows on both of these. The first example shows a Co2 of 75
, which is high. Our arrows are opposite so this ABG is a respiratory acidosis.
The next example with a Ph of 7.13 shows a normal Co2 but the bicarb is 9
which is low. This is an example of a metabolic acidosis as both arrows are
pointing down.
Slide 5: It will be important that you know and understand how a patient will
compensate for their acid base disorder. With respiratory acidosis, the cell
buffering will occur primarily through hemoglobin and proteins which leads to
the production of HCO3 within the kidneys.
If the CO2 remains persistently elevated, then there is an increased excretion
of titratable acid and ammonium by the kidneys leading to the development of
HCO3. With respiratory alkalosis, cellular buffering occurs in which
bicarbonate is combined to hydrogen ions. Ultimately, there is
a decrease in the overall plasma bicarb. The kidneys over time will increase
bicarbonate excretion as well as various other components to help decrease
overall plasma bicarbonate. With Metabolic acidosis, compensation is
attempted by decreasing the pCO2.
How is this accomplished? The patient will become tachypneic to blow off
extra CO2. The patient attempts to do this by increasing their tidal volume or
rate of breathing. With Metabolic alkalosis, compensation is attempted by
increasing the pCO2. There is a decrease alveolar ventilation. The patient
may take shallow or less breaths to hold onto more CO2.
It’s important to note that the body does not overshoot when attempting to
compensate.
Remember the goal of compensation is to return the pH as close to
physiologically normal as possible. If the pH is in a normal range the ABG is
interpreted as compensated. If it has not return to the normal range, then the
ABG is interpreted as partially or uncompensated.
Slide 6: Let’s practice interpreting compensated ABGs. Again using the
ROME method, we place arrows in the direction of the abnormality. For
compensated ABGs, you will have a normal PH. For these, I place a half
arrow in the direction of the Ph from 7.40. If the Ph is greater than 7.4 I place
a half arrow up, and if it is less than 7.4, I place a half arrow down.
