INTERPRETING ABGS – AS
EASY AS 1-2-3
CHRIS WEIR, RRT, CPFT
NEONATAL 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
1
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
2
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.
3
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
Capillary
pH: 7.35-7.45
pH: 7.35-7.45
pCO2: 35-45
pCO2: 35-50
p02: 50-80
pO2: not reliable
HCO3: 20-26
HCO3: 20-26
Base: -4 to 4
Base: -4 to 4
First 48 hours of life:
pH: 7.30 – 7.45
HCO3:
19 - 22
4
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
5
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?
6
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
7
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
8
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
9
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
10
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
11
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
12
Error in Blood Gas
Measurement
Temperature
Hemoglobin
Dilution
Air bubbles
Arterial Sampling
Sites
Peripheral
Central
Capillary Sampling
Technique
Values
13
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
14
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.
15
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
16
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.
17