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ABG analysis in four steps

    ABG analysis in four steps
             By Dr. Shami Bhagat (SKIMS)

      ABG analysis is a diagnostic test
that helps you assess the effectiveness
of your patient’s ventilation
and acid-base balance. The results
also help you monitor your patient’s
response to treatment. ABG
analysis provides several test results,
but only three are essential
for evaluating acid-base balance:
pH, PaCO2, and HCO3

-. Memorize these normal values for adults:
• pH: 7.35 to 7.45
• PaCO2: 35 to 45 mm Hg
• HCO3 -: 22 to 26 mEq/L.

Remember, the key to interpreting
ABG values at the bedside is
consistency. Follow these four simple steps every time:

• Step 1. List the results for the
three essential values: pH, PaCO2,
and HCO3-.
 pH =7.15
PaCO2 = 30mmHg
HCO3-. = 10meq/l

• Step 2. Compare them with
normal values. If a result indicates
excessive acid, write an A next to it. If a result indicates
excessive base, write a
B next to it. And if a result indicates
a normal balance, write
an N next to it. The pH will
tell you whether the patient
has acidosis or alkalosis.

pH =7.15                     A
PaCO2 = 30mmHg    B
HCO3-. = 10meq/l    A

• Step 3. If you’ve written the same
letter for two or three results, circle them. If you circle pH and
PaCO2, your patient has a respiratory
disorder. If you circle pH
and HCO3
-, your patient has a
metabolic disorder. If you circle
all three results, your patient has
a combined respiratory and
metabolic acid-base disturbance.
pH =7.15                     (A)
PaCO2 = 30mmHg    B
HCO3-. = 10meq/l    (A)
Here, pH =7.15          (A)
HCO3-. = 10meq/l    (A)

• Step 4. To check for compensation,
look at the result you didn’t
circle. If it has moved from
the normal value in the opposite
direction of those circled,
compensation is occurring. If
the value remains in the normal
range, no compensation has occurred.


Once compensation is
complete, the pH will return to

Keep in mind that several factors
can make ABG results inaccurate:
• using improper technique to
draw the arterial blood sample
• drawing venous blood instead of
arterial blood
• drawing an ABG sample within
20 minutes of a procedure, such
as suctioning or administering
respiratory treatment
• allowing air bubbles in the
• delaying transport of the sample
to the lab.

Case history 1
Mary Barker, 34, comes to the
emergency department (ED) with
acute shortness of breath and pain
on her right side. She smokes one
pack of cigarettes a day and recently
started taking birth control
pills. Her blood pressure is 140/
80 mm Hg; her pulse is 110
beats/minute; and her respiratory
rate is 44 breaths/minute. Her
ABG values are as follows:
• pH: 7.50
• PaCO2: 29 mm Hg
• Partial pressure of arterial oxygen
(PaO2): 64 mm Hg
• HCO3
-: 24 mm Hg
• Oxygen saturation (SaO2): 86%.
Interpretation: These ABG values
reveal respiratory alkalosis without
compensation. The patient’s pH
and PaCO2 are alkalotic, and her
- is normal, indicating no
compensation. You would administer
oxygen (O2) therapy, as ordered,
to increase SaO2 to more
than 95%; encourage the patient to
breathe slowly and regularly to decrease
CO2 loss; administer an analgesic,
as ordered, to ease pain; and
support her emotionally to decrease
anxiety. Based on the clues, the
probable underlying cause is pulmonary
Case history 2
John Stewart, 22, is brought to the
ED for an overdose of a tricyclic
antidepressant. He’s unconscious
and has a respiratory rate of 5 to 8
breaths/minute. His ABG values are
as follows:
• pH: 7.25
• PaCO2: 61 mm Hg
• PaO2: 76 mm Hg
• HCO3
-: 26 mm Hg
• SaO2: 89%.
Interpretation: These ABG values
reveal respiratory acidosis without
compensation. The patient’s pH and
PaCO2 are acidotic, and his HCO3
- is
normal, indicating no compensation.
You would administer O2, as ordered.
The patient may be intubated
to protect his airway and placed on
a mechanical ventilator. You would
also treat the underlying cause by
performing gastric lavage and administering
activated charcoal. This
patient’s condition may progress to
metabolic acidosis. If so, you would
give sodium bicarbonate to reverse
the acidosis.

Case history 3
Steve Burr, 38, has type 1 diabetes.
He hasn’t been feeling well
for the last 3 days and hasn’t eaten
or injected his insulin. He’s
confused and lethargic. His respiratory
rate is 32 breaths/minute,
and his breath has a fruity odor.
His serum glucose level is 620
mg/dL. While receiving 40% O2,
his ABG values are:
• pH: 7.15
• PaCO2: 30 mm Hg
• PaO2: 130 mm Hg
• HCO3
-: 10 mm Hg
• SaO2: 94%.
Interpretation: These ABG values
reveal metabolic acidosis with
partial respiratory compensation.
The patient’s pH and HCO3
- indicate
acidosis. His PaCO2 is lower
than normal, reflecting the lungs’
attempt to compensate. Because
pH is abnormal, you know compensation
isn’t complete.

ABG values only
Try interpreting this set of ABG values
without a clinical scenario:
• pH: 7.49
• PaCO2: 40 mm Hg
• PaO2: 85 mm Hg
• HCO3
-: 29 mm Hg
• SaO2: 90%
Interpretation: These values reveal
uncompensated metabolic alkalosis.
The pH and HCO3
- indicate
alkalosis. PaCO2 is normal, indicating
no compensation.

Now, interpret these values:
• pH: 7.25
• PaCO2: 56 mm Hg
• PaO2: 80 mm Hg
• HCO3
-: 15 mm Hg
• SaO2 : 93%
Interpretation: These values reveal
mixed acidosis. The pH, HCO3
and PaCO2 all indicate acidosis.

Causes of acid-base imbalances at a glance
Listed below are specific causes of the four acid-base disorders.

1. Respiratory acidosis:

The primary problem is alveolar hypoventilation (increased partial pressure of arterial carbon dioxide [PaCO2]), which may result from:

• acute pulmonary edema
• central nervous system depression
• chronic respiratory disease
• disorders of respiratory muscles and chest wall
• inadequate mechanical ventilation
• oversedation
• severe pulmonary infections.
2. Respiratory alkalosis
The primary problem is alveolar hyperventilation (decreased PaCO2), which may result from:

• anxiety
• early sepsis
• excessive mechanical ventilation
• exercise
• fear
• heart failure
• hypermetabolic states such as fever
• hypoxemia
• liver failure
• pain.

3. Metabolic acidosis

The primary problems are increased acid and decreased bicarbonate (HCO3-).

Increased acid results from:

• anaerobic metabolism
• hyperalimentation
• ketoacidosis
• renal failure
• salicylate intoxication
• severe sepsis
• starvation.

Decreased HCO3- results from:

• anhydrase inhibitors such as acetazolamide
• diarrhea
• hyperkalemia
• intestinal fistulas.

4. Metabolic alkalosis

The primary problems are increased HCO3- and decreased acid. Increased HCO3-
results from:

• excessive ingestion of antacids
• excessive use of bicarbonate
• lactate administration in dialysis.

Decreased acid results from:
• hyperaldosteronism
• hypokalemia
• hypochloremia
• loop or thiazide diuretics
• nasogastric suction
• steroids
• vomiting.

  1. Get a blood gas. Put it in ice, send it to the lab.
  2. Look at pH: Less than 7.4? Acidosis. Greater than 7.4? Alkalosis.
  3. Look at pCO2 and bicarb. Do they move in the same direction as the pH, or the opposite direction? (Nl pCO2 = 40, nl bicarb = 24.) If they move in the saME direction, it’s primary MEtabolic. If they move in a diffeREnt direction, it’s primary REspiratory.
  4. Is the non-primary system compenstating appropriately? (Calculate that with those annoying equations I can never remember.)
  5. Is there an anion gap? (Na – Cl – bicarb > 12) If no, you’re done.
  6. If yes, take the anion gap – 12. Add that to the bicarb level. If it’s greater than 26, you’ve got a metabolic alkalosis as well. If it’s less than 22, you’ve got a non-anion gap metabolic acidosis, too. Classic pimping: You can have 3 disorders co-existing, but not 4. (Your lungs can either be making respiratory acidosis or alkalosis, not both.)
And on to the mnemonics for the causes:
Anion Gap Metabolic Acidosis: MUDPILERS
  • Methanol
  • Uremia
  • DKA/Alcoholic KA
  • Paraldehyde
  • Isoniazid
  • Lactic Acidosis
  • Etoh/Ethylene Glycol
  • Rhabdo/Renal Failure
  • Salicylates
Non-Anion Gap Acidosis: HARDUPS
  • Hyperalimentation
  • Acetazolamide
  • Renal Tubular Acidosis
  • Diarrhea
  • Uretero-Pelvic Shunt
  • Post-Hypocapnia
  • Spironolactone
Acute Respiratory Acidosis (Chronic Respiratory Acidosis = COPD/restrictive lung dz): any hypoventilation state
  • CNS Depression (drugs/CVA)
  • Airway Obstruction
  • Pneumonia
  • Pulmonary Edema
  • Hemo/Pneumothorax
  • Myopathy
Metabolic Alkalosis: CLEVER PD
  • Contraction
  • Licorice*
  • Endo: Conn’s/Cushing’s/Bartter’s)*
  • Vomiting
  • Excess Alkali*
  • Refeeding Alkalosis*
  • Post-hypercapnia
  • Diuretics
* = Associated with High Urine Cl levels
Respiratory Alkalosis: CHAMPS (think speed up breathing)
  • CNS disease
  • Hypoxia
  • Anxiety
  • Mech Ventilators
  • Progesterone
  • Salicylates/Sepsis

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