Abc acid base

DR.AHMED MOHAMED
ALBEYALY
Nephrology Specialist
‫بالدقهلية‬ ‫الصحة‬ ‫بمديرية‬ ‫الكلي‬ ‫ادارة‬ ‫مدير‬

What is an ABG?
• Arterial Blood Gas is a lab that shows us the
status of a patient’s chemical balance

Pemed www.pemed.com/lab/labanalz/labanalz.htm

ABGs: What You Get
• Arterial PO2
• Arterial PCO2
• Arterial pH
• Some electrolytes (e.g., Na+, K+, Ca++)
• Lactate
• [HCO3
-]
• SaO2
• Other assorted calculated results

Main Components of ABG
• pH-Measures hydrogen ion (H+) concentration
• PCO2-Partial pressure of CO2 in arteries
• HCO3-Measures serum bicarbonate

Normal Values of ABG
• pH-7.35-7.45
• PCO2-35-45mmHg
• HCO3-22-26 mEq/l

Abnormal ABG Readings
• The body will work hard to balance any
abnormal values within the acid-base system
• How the body works to do this depends on
the issue:
• CO2 is controlled by the lungs
• HCO3 is controlled by the kidneys

Compensated Vs Uncompensated
• If the body can work to balance the issue
through the lungs or kidneys it will result in a
normal pH
• Other values in the ABG will be ABNL however
do not matter since there not an overall effect
on the pH
• This is a compensated ABG

Step 1
• Acidosis (7.35 _ 7.45) Alkalosis
7.4
• Look at the pH-if abnormal you have an
uncompensated disturbance
• Now figure out if the disturbance is acidosis or
alkalosis-this will determine the treatment
• Acidosis is pH below7.35 and results from
adding H+ an acid
• Alkalosis is pH above 7.45 and results from
decrease in H+ an acid

Step 2
• HCO3 (Metabolic) = Alkalosis
Alkali Acidosis
• PCO2 (Respiratory) = Acidosis
Acidic Alkalosis

Step 2: What is the primary disorder?
What disorder is present? pH pCO2 or HCO3
Respiratory Acidosis pH low pCO2 high
Metabolic Acidosis pH low HCO3 low
Respiratory Alkalosis pH high pCO2 low
Metabolic Alkalosis pH high HCO3 high

Step 3
• Put all your findings together to determine the
overall imbalance
• Example: pH 7.29 PCO2 50 HCO3 26
• What is this patient’s condition?

Answer
• pH 7.29
• PCO2 50
• HCO3 26
• Respiratory acidosis

Acid-base Terminology
Acidemia: blood pH < 7.35
Acidosis: a primary physiologic process that, occurring alone, tends
to cause acidemia. Examples: metabolic acidosis from decreased
perfusion (lactic acidosis); respiratory acidosis from hypoventilation.
If the patient also has an alkalosis at the same time, the resulting
blood pH may be low, normal, or high.
Alkalemia: blood pH > 7.45
Alkalosis: a primary physiologic process that, occurring alone,
tends to cause alkalemia. Examples: metabolic alkalosis from
excessive diuretic therapy; respiratory alkalosis from acute
hyperventilation. If the patient also has an acidosis at the same
time, the resulting blood pH may be high, normal, or low.

Acid-base Terminology (cont.)
Primary acid-base disorder: One of the four acid-base disturbances that is
manifested by an initial change in HCO3
- or PaCO2. They are: metabolic
acidosis (MAc), metabolic alkalosis (MAlk), respiratory acidosis (RAc), and
respiratory alkalosis (RAlk). If HCO3
- changes first, the disorder is either
MAc (reduced HCO3
- and acidemia) or MAlk (elevated HCO3
- and
alkalemia). If PaCO2 changes first, the problem is either RAlk (reduced
PaCO2 and alkalemia) or RAc (elevated PaCO2 and acidemia).
Compensation: The change in HCO3
- or PaCO2 that results from the
primary event. Compensatory changes are not classified by the terms used
for the four primary acid-base disturbances. For example, a patient who
hyperventilates (lowers PaCO2) solely as compensation for MAc does not
have a RAlk, the latter being a primary disorder that, alone, would lead to
alkalemia. In simple, uncomplicated MAc the patient will never develop
alkalemia.

Primary Acid-base Disorders:
Respiratory Alkalosis
Respiratory alkalosis - A primary disorder where the first change is a
lowering of PaCO2, resulting in an elevated pH. Compensation
(bringing the pH back down toward normal) is a secondary lowering
of bicarbonate (HCO3) by the kidneys; this reduction in HCO3
- is not
metabolic acidosis, since it is not a primary process.
Primary Event Compensatory Event
HCO3
- ↓HCO3
-
↑ pH ~ ------- ↑ pH ~ --------
↓ PaCO2 ↓ PaCO2

Respiratory Acidosis
Respiratory acidosis - A primary disorder where the first change is
an elevation of PaCO2, resulting in decreased pH. Compensation
(bringing pH back up toward normal) is a secondary retention of
bicarbonate by the kidneys; this elevation of HCO3
- is not metabolic
alkalosis since it is not a primary process.
HCO3
- ↑ HCO3
-
↓ pH ~ --------- ↓ pH ~ ---------
↑PaCO2 ↑ PaCO2

Metabolic Acidosis
Metabolic acidosis - A primary acid-base disorder where the first
change is a lowering of HCO3
-, resulting in decreased pH.
Compensation (bringing pH back up toward normal) is a secondary
hyperventilation; this lowering of PaCO2 is not respiratory alkalosis
since it is not a primary process.
↓ HCO3
- ↓HCO3
-
↓ pH ~ ------------ ↓ pH ~ ------------
PaCO2 ↓ PaCO2

Metabolic Alkalosis
Metabolic alkalosis - A primary acid-base disorder where the first change is
an elevation of HCO3
-, resulting in increased pH. Compensation is a
secondary hypoventilation (increased PaCO2), which is not respiratory
acidosis since it is not a primary process. Compensation for metabolic
alkalosis (attempting to bring pH back down toward normal) is less
predictable than for the other three acid-base disorders.
↑ HCO3
- ↑HCO3
-
↑ pH ~ ------------ ↑ pH ~ ---------
PaCO2 ↑PaCO2

Metabolic Acid-base Disorders:
Some Clinical Causes
METABOLIC ACIDOSIS ↓HCO3
- & ↓ pH
- Increased anion gap
• lactic acidosis; ketoacidosis; drug poisonings (e.g., aspirin, ethylene
glycol, methanol)
- Normal anion gap
• diarrhea; some kidney problems (e.g., renal tubular acidosis,
interstitial nephritis)
METABOLIC ALKALOSIS ↑ HCO3
- & ↑ pH
Chloride responsive (responds to NaCl or KCl therapy): contraction alkalosis,
diuretics, corticosteroids, gastric suctioning, vomiting
Chloride resistant: any hyperaldosterone state (e.g., Cushing’s syndrome,
Bartter’s syndrome, severe K+ depletion)

RESPIRATORY ACIDOSIS ↑PaCO2 & ↓pH
Central nervous system depression (e.g., drug overdose)
Chest bellows dysfunction (e.g., Guillain-Barré syndrome, myasthenia
gravis)
Disease of lungs and/or upper airway (e.g., chronic obstructive lung
disease, severe asthma attack, severe pulmonary edema)
RESPIRATORY ALKALOSIS ↓PaCO2 & ↑ pH
Hypoxemia (includes altitude)
Anxiety
Sepsis
Any acute pulmonary insult (e.g., pneumonia, mild asthma attack, early
pulmonary edema, pulmonary embolism)
Respiratory Acid-base Disorders:
Some Clinical Causes

Mixed Acid-base Disorders
In chronically ill respiratory patients, mixed disorders are probably
more common than single disorders, e.g., RAc + MAlk, RAc + Mac,
Ralk + MAlk.
In renal failure (and other conditions) combined MAlk + MAc is also
encountered.
Always be on the lookout for mixed acid-base disorders. They can
be missed!

Expected changes in pH and HCO3
- for a 10-mm Hg change in PaCO2
resulting from either primary hypoventilation (respiratory acidosis) or
primary hyperventilation (respiratory alkalosis):
ACUTE CHRONIC
Resp Acidosis
pH ↓ by 0.07 pH ↓ by 0.03
HCO3
- ↑ by 1* HCO3
- ↑ by 3 - 4
Resp Alkalosis
pH ↑ by 0.08 pH ↑ by 0.03
HCO3
- ↓ by 2 HCO3
- ↓ by 5
* Units for HCO3
- are mEq/L

Predicted changes in HCO3
- for a directional change in
PaCO2 can help uncover mixed acid-base disorders.
a) A normal or slightly low HCO3
-
in the presence of hypercapnia
suggests a concomitant metabolic acidosis, e.g., pH 7.27, PaCO2 50
mm Hg, HCO3
-
22 mEq/L. Based on the rule for increase in HCO3
-
with hypercapnia, it should be at least 25 mEq/L in this example;
that it is only 22 mEq/L suggests a concomitant metabolic acidosis.
b) A normal or slightly elevated HCO3
-
in the presence of hypocapnia
suggests a concomitant metabolic alkalosis, e.g., pH 7.56, PaCO2 30
mm Hg, HCO3
-
26 mEq/L. Based on the rule for decrease in HCO3
-
with hypocapnia, it should be at least 23 mEq/L in this example; that
it is 26 mEq/L suggests a concomitant metabolic alkalosis.

How much oxygen is in the blood?
PaO2 vs. SaO2 vs. CaO2
OXYGEN PRESSURE: PaO2
Since PaO2 reflects only free oxygen molecules dissolved in plasma and not those bound to hemoglobin,
PaO2 cannot tell us “how much” oxygen is in the blood; for that you need to know how much oxygen is also
bound to hemoglobin, information given by the SaO2 and hemoglobin content.
OXYGEN SATURATION: SaO2
The percentage of all the available heme binding sites saturated with oxygen is the hemoglobin oxygen
saturation (in arterial blood, the SaO2). Note that SaO2 alone doesn’t reveal how much oxygen is in the
blood; for that we also need to know the hemoglobin content.
OXYGEN CONTENT: CaO2
Tissues need a requisite amount of O2 molecules for metabolism. Neither the PaO2 nor the SaO2 provide
information on the number of oxygen molecules, i.e., how much oxygen is in the blood. (Neither PaO2 nor
SaO2 have units that denote any quantity.) Only CaO2 (units ml O2/dl) tells us how much oxygen is in the
blood; this is because CaO2 is the only value that incorporates the hemoglobin content. Oxygen content
can be measured directly or calculated by the oxygen content equation:
CaO2 = (Hb x 1.34 x SaO2) + (.003 x PaO2)

ABG Practice
• pH 7.36
• PaCO2 43
• HCO3 22
• Interpretation?
A. Normal
B. Respiratory acidosis
C. Compensated respiratory
acidosis
D. Respiratory Alkalosis
E. Compensated respiratory
alkalosis
F. Metabolic acidosis
G. Compensated metabolic
acidosis
H. Metabolic alkalosis
I. Compensated metabolic
alkalosis
Answer: A. Normal

ABG Practice
• pH 7.45
• PaCO2 30
• HCO3 19
• Interpretation?
Answer: E. Compensated Respiratory Alkalosis
PaCO2 is low
HCO3 is low
A. Normal
C. Compensated respiratory acidosis
E. Compensated respiratory alkalosis
G. Compensated metabolic acidosis
I. Compensated metabolic alkalosis

ABG Practice
• pH 7.52
• PaCO2 43
• HCO3 31
• Interpretation?
Answer: H. Metabolic Alkalosis
PaCO2 is normal
HCO3 is high
A. Normal

ABG Practice
• pH 7.3
• PaCO2 72
• HCO3 24
• Interpretation?
Answer: B. Respiratory Acidosis
PaCO2 is high
HCO3 is normal
A. Normal

ABG Practice
• pH 7.35
• PaCO2 28
• HCO3 18
• Interpretation?
Answer: G. Compensated Metabolic Acidosis
PaCO2 is low
HCO3 is low
A. Normal

ABG Practice
• pH 7.51
• PaCO2 28
• HCO3 23
• Interpretation?
Answer: D. Respiratory Alkalosis
PaCO2 is normal
HCO3 is low
A. Normal

ABG Practice
• pH 7.35
• PaCO2 65
• HCO3 31
• Interpretation?
Answer: C. Compensated Respiratory Acidosis
PaCO2 is high
HCO3 is high
A. Normal

ABG Practice
• pH 7.26
• PaCO2 45
• HCO3 16
• Interpretation?
Answer: F. Metabolic Acidosis
PaCO2 is normal
HCO3 is low
A. Normal

47
ABG Practice
• pH 7.49
• PaCO2 42
• HCO3 29
• Interpretation?
Answer: H. Metabolic Alkalosis
PaCO2 is normal
HCO3 is high
A. Normal

ABG Practice
• pH 7.3
• PaCO2 59
• HCO3 24
• Interpretation?
PaCO2 is high
HCO3 is normal
A. Normal

ABG Practice
• pH 7.44
• PaCO2 53
• HCO3 31
• Interpretation?
Answer: I. Compensated Metabolic Alkalosis
PaCO2 is high
HCO3 is high
A. Normal

ABG Practice
• pH 7.39
• PaCO2 42
• HCO3 23
• Interpretation?
Answer: A. Normal
A. Normal

ABG Practice
• pH 7.31
• PaCO2 36
• HCO3 18
• Interpretation?
Answer: F. Metabolic Acidosis
PaCO2 is normal
HCO3 is low
A. Normal

ABG Practice
• pH 7.17
• PaCO2 89
• HCO3 22
• Interpretation?
PaCO2 is high
HCO3 is normal
A. Normal

ABG Practice
• pH 7.43
• PaCO2 38
• HCO3 24
• Interpretation?
Answer: A. Normal
A. Normal

Abc acid base

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