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INTERPRETING ABGS – AS EASY AS 1-2-3

CHRIS WEIR, RRT, CPFTNEONATAL RESPIRATORY THERAPIST

WOMEN’S AND CHILDREN’S HOSPITAL AT CENTENNIAL

Basic Concepts

� H+ ion regulation

� The more hydrogen ions, the lower the pH and the more acidic the solution

� The less hydrogen ions, the higher the pH and the more alkaline the solution

� Acid – a substance that can contribute free H+ ions to a solution. A strong acid will contribute the free H+ ion more readily than a weak acid.

� Base – a substance that can combine with/accept a free H+ ion to remove it from a solution.

pH

� Refers to the concentration of the free hydrogen ions in the blood produced by acid and base reactions. Thus, pH tells you about the acidity or alkalinity of the blood.

� Reflects the ratio of HCO3 to CO2

� pH will change if there is a change in the HCO3 not balanced by the CO2

� Normal range: 7.35 - 7.45

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pH

� pH < 7.35 = acidosis (overabundance of acid)

� pH > 7.45 = alkalosis (overabundance of base)

PaCO2

� Partial pressure of CO2 in the blood

� Respiratory component of the blood gas

� Indication of alveolar ventilation

� Normal range: 35 - 45 mmHg

PaCO2

� > 45 mmHg = acidity

CO2 being retained by lungs

� < 35 mmHg = alkalinity

CO2 being blown off by lungs

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PaO2

� Refers to the partial pressure of O2 in the blood

� Normal values: Term 50-80 mmHg (on RA)

PaO2

� Hypoxemia defined as PaO2 < 50 mmHg

� Hyperoxemia defined as PaO2 > 80 mmHg

HCO3

� Chief base found in the blood.

� Normal range: 20 - 26 mEq/l

� Renal component of the blood gas

� Decreased amounts of HCO3 = acidosis

� Increased amounts of HCO3 = alkalosis

� May also combine with other ions (potassium, calcium, magnesium) to form additional alkaline substances.

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Base Deficit/Excess

� Normal range: -4 to +4 mEq/l

� Reflects the concentration of buffer or base available in the blood

� A deficit reflects an excess of acid or a diminished amount of base available. An excess reflects an excess of base in the blood and a deficit in the amount of acid in the blood.

Base Deficit/Excess

� Base deficit >>>> metabolic acidosis

� Base excess >>>> metabolic alkalosis

Normal Blood Gas Values

� Arterial

� pH: 7.35-7.45

� pCO2: 35-45

� p02: 50-80

� HCO3: 20-26

� Base: -4 to 4

� First 48 hours of life:

� pH: 7.30 – 7.45

� HCO3: 19 - 22

� Capillary

� pH: 7.35-7.45

� pCO2: 35-50

� pO2: not reliable

� HCO3: 20-26

� Base: -4 to 4

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STEP 1:

� Look at each component separately:

� pH

� pCO2

� pO2

� HCO3

� Base

STEP 2:

� Acidosis or alkalosis?

� If the pH is normal, you either have a normal blood gas or a compensated blood gas.

�Normal for a baby to be a little more acidotic in the first 48 hours of life.

STEP 3:

� Is this a respiratory or metabolic problem?

� Acidosis = Increased pCO2 and decreased HCO3

� Alkalosis = Decreased pCO2 and increased HCO3

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System At Fault:

� Primary abnormality in the pCO2 is respiratory in

origin

� Primary abnormality in the HCO3 is metabolic in

origin

STEP 4:

� Hypoxemia or Hyperoxemia?

STEP 5:

� Is there any compensation?

� To what degree?

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TO DETERMINE COMPENSATION:

� Look at the system not at fault

� Absent compensation - system not at fault WNL

� Partial compensation - opposite system not WNL & pH not WNL

� Complete compensation - opposite system not WNL & pH WNL

Causes of Blood Gas Abnormalities:

� Respiratory Acidosis

� Respiratory Alkalosis

� Metabolic Acidosis

� Metabolic Alkalosis

� Mixed Acidosis

� Mixed Alkalosis

Respiratory Acidosis:

� Results from the formation of excess

carbonic acid because of increased PCO2

� Caused by insufficient alveolar ventilation

� Blood gas findings: decreased pH,

increased pCO2, normal HCO3

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Respiratory Acidosis:

� Hypoventilation

� Sedation

� PPH

� HMD

� Upper Airway Not Patent

� Pulmonary Hypoplasia

� Recurrent Apnea

� Central Depression

� Pneumothorax

How the body compensates

for respiratory acidosis:

� Over 3-4 days, the kidneys increase the rate of H+

secretion and bicarbonate reabsorption.

� Blood gas findings: low normal pH, increased

pCO2 and bicarbonate

Respiratory Alkalosis:

� Results from alveolar hyperventilation leading to a

deficiency of carbonic acid

� Caused by hyperventilation, usually iatrogenic

� Blood gas findings: increased pH, decreased pCO2, normal HCO3

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How the body compensates

for respiratory alkalosis:

� The kidneys decrease H+ secretion by retaining

chloride and excreting fewer acid salts.

Bicarbonate reabsorption is also decreased.

� Blood gas findings: pH high normal, low pCO2

and bicarbonate levels

Metabolic Acidosis:

� A deficiency in the concentration of bicarbonate in the ECF.

� Caused by any systemic disease that increases acid production or retention, or problems leading to excessive base losses. Examples –hypoxia leading to lactic acid production, renal disease, and loss of base secondary to diarrhea.

� Blood gas findings: decreased pH, decreased HCO3, normal pCO2

Metabolic Acidosis:

� Hyperalimentation

� Hypoperfusion / hypovolemia

� PDA

� Renal Tubular Acidosis

� Cold Stress

� Renal Failure

� Inborn Error of Metabolism

� Sepsis

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How the body compensates

for metabolic acidosis:

� Healthy lungs will blow off additional CO2 through

hyperventilation. If renal disease is not a problem,

the kidneys will respond by increasing the excretion of acid salts and the reabsorption of

HCO3.

� Blood gas findings: pH low normal, pCO2 and

HCO3 low

Metabolic Alkalosis:

� Results from an excess concentration of HCO3 in the ECF

� Caused by problems leading to increased loss of acid. Examples –severe vomiting, gastric suction, and increased retention or intake of bases, such as occurs with excessive NaHCO3 administration. Also, diuretic therapy, chronic R Acidosis compensation, hypokalemia/hypochloremia.

� Blood gas findings: high pH, high HCO3, normal pCO2

How the body compensates

for metabolic alkalosis:

� The lungs compensate by retaining carbon

dioxide through hypoventilation.

� Blood gas findings: pH high normal, HCO3 and

pCO2 high

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Interventions For

Each Interpretation:

Respiratory Acidosis:

� In ventilated infants, increase the tidal volume.

� Unventilated infants may require more ventilatory support depending on their WOB and degree of

acidosis.

� Correct the cause.

Respiratory Alkalosis:

� Caused by alveolar hyperventilation - WEAN !

� Decrease the TV

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Metabolic Acidosis:

� Identify and treat the cause

� May want to consider volume expansion or bicarbonate administration depending on the

specific clinical situation

� Does the baby need additional buffer added to

the IV fluids/TPN?

Metabolic Alkalosis:

� Identify and treat the cause

� May also want to remove acetate from IV fluids, reduce diuretic doses, treat hyponatremia,

hypokalemia, and hypochloremia

Blood Gas Sampling

� Infection control

� Bleeding disorders

� Steady state

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Error in Blood Gas Measurement

� Temperature

� Hemoglobin

� Dilution

� Air bubbles

Arterial Sampling

� Sites

� Peripheral

� Central

Capillary Sampling

� Technique

� Values

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Practice Problem #1

� A 31 week old infant is one hour old. CXR shows

diffuse atelectasis with air bronchograms.

� CBG – 7.29/59/42/26

Answer – Problem #1

� Acidosis

� pCO2 is high indicating a respiratory problem leading to the acidosis

� Capillary specimen

� No compensation

� Uncompensated respiratory acidosis

� Treatment: NCPAP or MV

Practice Problem #2

� A 33 week infant is receiving mechanical

ventilation for severe TTN. Settings: IMV 25, PIP 18,

PEEP 4, .30.

� ABG: 7.49/26/95/22

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Answer – Problem #2

� Alkalemia

� The PaCO2 is low indicating a respiratory alkalosis

� Pa02 is high

� No compensation

� Uncompensated Respiratory Alkalosis

� Treatment: wean the PIP or rate along with Fi02

Practice Problem #3

� 26 week infant has been on the ventilator for 2

weeks for RDS. PIE is present.

� CBG: 7.37/55/65/29

Answer - #3

� pH normal

� pCO2 is high indicating a respiratory problem, which could lead to acidosis

� Capillary specimen

� Compensation present – pH normal with abnormal HCO3 and pC02. pH closer to acidosis

� Compensated respiratory acidosis

� Treatment: no action needed. Further increases in the pCO2 could result in decompensation.

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Practice Problem #4

� 26 week old infant on the ventilator for RDS.

Settings: IMV 30, 19/5, and .40. Infant has lost 30

gms in the past 12 hours with a Na of 148.

� ABG: 7.29/53/55/17

Answer - #4

� Acidemia

� The PaCO2 is high indicating a respiratory acidosis and the HCO3 is low indicating a metabolic acidosis.

� Oxygen level adequate.

� No compensation – pH not normal

� Uncompensated mixed acidosis.

� Treatment: increase alveolar ventilation and consider giving volume to correct the hypovolemia

Practice Problem #5

� Term infant with tight nuchal cord. Infant pale,

grunting, with cap refill of 8 seconds.

� ABG: 7.15/40/75/15/-15

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Answer - #5

� Acidosis

� Metabolic in origin – decreased HCO3 with normal pCO2

� Oxygen level adequate

� No compensation – pCO2 normal

� Uncompensated Metabolic Acidosis

� Treatment: consider volume or HCO3 depending on respiratory assessment.

References

Gleson, C., & Devaskar, S. (2011). Avery’s Diseases

of the Newborn (9th Ed.). Philadelphia: W.B.

Saunders. ISBN: 978-1437701340.

Gomella, T.L. (2009). Neonatology: Management,

Procedures, On-Call Problems, Diseases and Drugs

(6th Edition). Norwalk, Conn.: Appleton & Lang. ISBN: 9780071544313

Karlsen, K.A. (2012). The S.T.A.B.L.E. Program.

Park City: The S.T.A. B.L.E. Program.