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-.

e.g.

 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)

So, its METABOLIC ACIDOSIS

• 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.

#METABOLIC ACIDOSIS WITH COMPENSATION (3^RD Component In Opposite Direction)

Once compensation is

complete, the pH will return to

normal.

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

sample

• 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

HCO3

- 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

embolism.

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.

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Causes of acid-base imbalances at a glance

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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.

     OR

Acid-Base Disorders

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