4. Acid-Base Interpretation Flashcards
ABGs will be tested on exam. You need to understand the 4 major acid-base abnormalities as well as states of compensation (____, ____, ____).
Uncompensated, partial compensation, and full compensation. Not only will you need to know how to interpret the ABG, but you will also need to know what the clinical implications are for the pt and the treatment, if there are any. A quick review of acid-base interpretation is given in Table 4-4.
Normal ABG parameters and Absolute Normal
pH
pCO2
HCO3
BE
PO2
SaO2
pH normal 7.35 -7.45 (absolute norm = 7.40)
pCO2: 35 - 45 mmHg (40 mmHg)
HCO3: 22 - 26 mmol/L (24 mmol/L)
BE: -2 to +2 (0)
PO2: 80 - 100 mmHg
SaO2: 95 - 99% (on room air)
Remember that the pH represents the…
hydrogen ion (H+) concentration of the blood.
Due to the Henderson-Hasselbalch equation, when the H+ is increased, the ___
pH decreases, and when the H+ concentration is decreased, the pH increases. There is an inverse relationship bt the H+ and the pH so don’t get confused!
Think of the PaCO2 as an
acid. When it increases, there is acidosis. When it decreases, there is alkalosis.
-The PaCO2 is controlled by the lungs. It is the
RESPIRATORY parameter.
-The lungs can change the PaCO2 within minutes
(rapid change).
Think of HCO3 as a
base. When it is greater than normal, alkalosis may be present. When it is less than normal, acidosis may be present.
-The HCO3 is controlled by the kidneys. It is the
METABOLIC parameter.
-The kidneys alter the HCO3 over hours to days (slow
change).
Respiratory acidosis: pH, primary change, compensatory change
ph: <7.35
primary change: increased PaCO2
compensatory change: increased HCO3
Metabolic acidosis: pH, primary change, compensatory change
pH: < 7.35
primary change: decreased HCO3
compensatory change: decreased PaCO2
Metabolic acidosis may also be evaluated by the anion gap and the venous CO2 (which will be lower than normal in the presence of metabolic acidosis).
Respiratory alkalosis: pH, primary change, compensatory change
pH: >7.45
primary change: decreased PaCO2
compensatory change: decreased HCO3
Metabolic alkalosis: pH, primary change, compensatory change
pH: >7.45
primary change: increased HCO3
compensatory change: increased PaCO2
In the presence of FULL compensation, the pH will
enter the normal range.
Anion Gap
is the difference between positive and negative anions. In most instances of metabolic acidosis, there is an increase in the anion gap. In several types of metabolic acidosis, tho, the anion gap remains normal.
You will NOT need to calculate the anion gap for the exam but you should know what is normal:
(Na + K) - (Cl + HCO3)
Normal Anion Gap is
5 -15 mEq/L
The anion gap is helpful in determining the ___
cause and/or response to treatment for metabolic acidosis (Table 4-6)
Problems associated with an Increase in the Anion Gap (high anion gap metabolic acidosis)
Think “Kussmaul”
-Ketoacidosis
-Uremia
-Salicylate intoxication
-Methanol toxicity
-Alcoholic ketosis
-Unmeasured osmoses: ethylene glycol, paraldehyde
-Lactic acidosis: shock, hypoxemia
Problems Associated with a normal Anion Gap Metabolic Acidosis:
-Saline Infusion (hyperchloremic acidosis)
-TPN
-Diarrhea
-Acute renal failure, sometimes chronic
Acid-Base Compensation:
see tables 4-7, 4-8, 4-9, 4-10
Compensation is the body’s way of attempting to return the pH to normal (7.35 -7.45).
-Uncompensated
-Partial compensation
-Full compensation: RARE in critically ill, suspect a mixed
disorder if present.
Example of uncompensated Resp Acidosis
pH: 7.30
PaCO2: 50
HCO3: 24
Example of Partial Compensation Resp Acidosis
pH: 7.32
PaCO2: 50
HCO3: 29
Example of Full Compensation Resp Acidosis
pH: 7.35
PaCO2: 50
HCO3: 31
Example of Uncompensated Resp Alakalosis
pH: 7.55
PaCO2: 25
HCO3: 22
Example of Partial Compensation Resp Alakalosis
pH: 7.5
PaCO2: 25
HCO3: 18
Example of Full Compensation Resp Alakalosis
pH: 7.45
PaCO2: 25
HCO3: 16
Example of Uncompensated Metabolic Acidosis
pH: 7.30
PaCO2: 35
HCO3: 16
Example of Partial Compensation Metabolic Acidosis
pH: 7.32
PaCO2: 33
HCO3: 16
Example of Full Compensation Metabolic Acidosis
pH: 7.35
PaCO2: 30
HCO3: 16
Example of uncompensated Metabolic Alakalosis
pH: 7.50
PaCO2: 45
HCO3: 32
Example of Partial Compensation Metabolic Alakalosis
pH: 7.47
PaCO2: 48
HCO3: 32
Example of Full Compensation Metabolic Alakalosis
pH: 7.45
PaCO2: 50
HCO3: 32
Combined acid-base d/o: seldom seen on exam
Occurs when 2 single disorders are present simultaneously to produce the same abnormality:
-Combined respiratory and metabolic acidosis:
pH: 7.21
PaCO2: 50
HCO3: 12
-Combined respiratory and metabolic alkalosis:
pH: 7.59
PaCO2: 30
HCO3: 33
Combined mixed-base disorders - seldom seen on exam
-Simple acid-base d/o result from a single process, such as metabolic acidosis.
-In many critically ill pts, multiple acid-base disturbances exist concurrently and result in complex, mixed acid-base d/o
-for ex., a pt with septic shock may present with respiratory alkalosis AND metabolic acidosis.
-Complex formulas can be applied to determine whether the compensating parameter (PaCO2 or HCO3) has compensated more than predicted for the primary problem, indicating a mixed d/o is present. You won’t need to know these formulas for exam.
Systematic assessment of acid-base
- Evaluate the pH –>determine whether it is normal,
acidotic, or alkalotic. - Evaluate respiratory parameters and then renal
parameters –> determine which, if either, is abnormal. - Determine the state of compensation.
- Evaluate for a mixed d/o.
- Assess oxygenation (PaO2, SaO2).
pH 7.48
PaCO2 32
HCO3 24
Interpretation:
Respiratory Alkalosis, no compensation
pH 7.32
PaCO2 48
HCO3 25
Interpretation:
Respiratory Acidosis, no compensation
pH 7.30
PaCO2 38
HCO3 18
Interpretation:
Metabolic Acidosis, no compensation
pH 7.28
PaCO2 60
HCO3 29
Interpretation:
Partially compensated Respiratory Acidosis
pH 7.49
PaCO2 40
HCO3 30
Interpretation:
Metabolic Alkalosis, no compensation
pH 7.28
PaCO2 70
HCO3 33
Interpretation:
Partially compensated respiratory acidosis
pH 7.50
PaCO2 49
HCO3 38
Interpretation
Partially compensated Metabolic Alkalosis
pH 7.31
PaCO2. 32
HCO3 15
Interpretation:
Partially compensated Metabolic Acidosis
pH 7.30
PaCO2. 50
HCO3 25
Interpretation:
Uncompensated Respiratory Acidosis
pH 7.48
PaCO2 40
HCO3 30
Interpretation:
Metabolic Alkalosis, no compensation
pH 7.38
PaCO2: 80
HCO3 47
Interpretation:
Mixed disorder: respiratory acidosis and metabolic alkalosis
