2. ABG Analysis
• An arterial blood gas (ABG) test measures the
acidity (pH) and the levels of oxygen and
carbon dioxide in the blood from an artery.
This test is used to check how well your lungs
are able to move oxygen into the blood and
remove carbon dioxide from the blood.
• An arterial blood gas measurement provides
valuable information about the blood pH and
the partial pressures of arterial carbon
dioxide (PaCO2) and oxygen (PaO2).
• Most analysers calculate serum bicarbonate
-) and the base excess.
4. 1. Know the Normal Values
Variable Normal Range
pH 7.35 - 7.45
11. The 6 Easy Steps to ABG Analysis:
1. Is the pH normal?
2. Is the CO2 normal?
3. Is the HCO3 normal?
4. Match the CO2 or the HCO3 with the pH.
5. Does the CO2 or the HCO3 got the opposite
Direction of the pH?
6. Are the pO2 and the O2 saturation normal?
12. Step1:Analyze the pH
• Step 1: Analyze the pH
• The firt step in analyzing the ABG is to look for
the pH, Normal blood pH is 7.4, plus or minus
0.05, forming the range 7.35 to 7.45
• < 7.35, it is acidic
• > 7.45,it is alkalotic
13. Step 2: Analyze the CO2
• Examine the pCO2. Normal pCo2 levels are
• <35 is alkalotic
• > 45 is acidic.
14. Step 3: Analyze the HCO3
• Examine the HCO3 level. Normal HCO3 level is
• <22 HCO3 acidic
• >26 HCO3 alkalotic
15. Anion Gap
• In plasma, the sum of the cations (sodium plus
potassium) is normally greater than that of the
anions (chloride plus bicarbonate) by
approximately 14 mmol/L. This is known as the
anion gap. The normal reference range for the
anion gap is
in the equation and
is most often due to compensation for
gastrointestinal bicarbonate loss (eg,
16. Anion – a negatively charge atom
• common anions known are
Most of the times the atoms
of these elements combine with hydrogen
or oxygen to form anion. However there are
certain exceptions to this general rule.
17. Step 4: Match the CO2 or the HCO3
with the pH
• Next match either the pCO2 or the with the
pH to determine the acid-base disorder. For
example, if the
then the acid-base disturbance is being
caused by the respiratory system.
• -This is called RESPIRATORY ACIDOSIS
• If the
the acid-base disturbance is being caused by the
metabolic (or renal) system. So this case is
18. Step 5: Does the CO2 or HCO3 go the
opposite direction of the pH?
• If YES – there is compensation by that system.
• EX. The pH is acidotic, the CO2 is acidotic, and
the HCO3 is alkalotic.
• - The CO2 matches the pH making the Primary
acid-base disorder RESPIRATORY ACIDOSIS.
• The HCO3 is opposite of the pH and would be
evidence of compensation from the metabolic
19. Use HC03 to verify metabolic effect
Normal HCO3- is 22-26
- Compensation may not always be
- Complete compensation returns the pH
balance to normal.
- There are times when the imbalance is
too large for compensation to restore
the pH to normal.
- This is called partial compensation.
21. How it compensate the pH?
• Respiratory sytem balances the pH by the
increase/decrease of Resp. Rate to control the
• The renal system balances pH by producing HCO3
or by eliminating hydrogen ions (H+).
• Ex. Patient is shock– will undergo anaerobic
metabolism, which produces lactic acid. The
production of lactic acid will bind or use up
available HCO3 & will be manifested by a
decreased in the HCO3 level. Therefore, the HCO3
level is an indicator of metabolic acid-base
Compensation – is the attempt of the body to
maintain homeostasis by correcting the pH.
- Dissolved CO2 produced by cellular processes &
exhaled by the lungs – balances the pH
- HCO3 (Bicarbonate) – produced by the kidneys.
- Kidney help control the pH by eliminating (H+)
ions from the components of carbonic acid
(H2CO3)=from Water (H2O)combined with CO2.
24. Is there appropriate compensation? Is
it chronic or acute?
Acute: for every 10 increase in pCO2 -> HCO3 increases by 1
and there is a decrease of 0.08 in pH MEMORIZE
Chronic: for every 10 increase in pCO2 -> HCO3 increases by
4 and there is a decrease of 0.03 in pH
Acute: for every 10 decrease in pCO2 -> HCO3 decreases by
2 and there is a increase of 0.08 in PH MEMORIZE
Chronic: for every 10 decrease in pCO2 -> HCO3 decreases
by 5 and there is a increase of 0.03 in PH
25. Is there appropriate compensation? Is
it acute or chronic ?
Winter’s formula: pCO2 = 1.5[HCO3] + 8 ± 2
If serum pCO2 > expected pCO2 -> additional
Another useful tool in estimating the PCO2 in
metabolic acidosis is the recognition that the pCO2 is
always approximately equal to the last 2 digits of the
For every 10 increase in HCO3 -> pCO2 increases by 6
26. Step 6: Analyze the pO2 and the O2
• Finally, evaluate the PaO2 and O2 sat If they
are below normal there is evidence of
• Normal Values (At sea level): Range:
pCO2 35-45 mmHg
O2 Saturation 95-100%
HCO3- 22-26 mEq/L
Base Excess + or -2
27. Step 6: Analyze the pO2 and the O2
• The higher the PaO2, the more oxygen is
bound to hemoglobin and the higher the
saturation (until the limit of 100% saturation is
33. ABG Analysis
ABG Value Which step? Rationale
pH 7.48 Step 1
PaCO2 47 Step 3
High PaC02and High pH
indicates metabolic cause
of alkalosis. Respiratory
acid is compensating for
38. Causes of metabolic acidosis
• Normal anion gap (due to loss of bicarbonate or
ingestion hydrogen ions):
• Renal tubular acidosis.(is a disease that occurs when
the kidneys fail to excrete acids into the urine, which
causes a person's blood to remain too acidic.)
• Addison's disease.-a disease characterized by
progressive anemia, low blood pressure, great
weakness, and bronze discoloration of the skin. It is
caused by inadequate secretion of hormones by the
• Pancreatic fistulae.
• Drugs or toxins: acetazolamide, ammonium chloride.
39. Metabolic alkalosis
Calculate the urinary chloride to differentiate saline responsive vs saline
Must be off diuretics in order to interpret urine chloride
Saline responsive UCL<10 Saline-resistant UCL >10
Vomiting If hypertensive: Cushings, Conn’s -is a condition
associated with the development of high blood
pressure and low potassium levels in the blood.
-Renal failure with alkali administration
NG suction If not hypertensive: severe hypokalemia,
Over-diuresis Exogenous corticosteroid administration
40. Causes of respiratory acidosis
• Acute:Depression of the central respiratory
centre by cerebrovascular disease or drugs.
• Inability to ventilate adequately due to
neuromuscular disease - eg, myasthenia
gravis, amyotrophic lateral sclerosis, Guillain-
Barré syndrome, muscular dystrophy.
• Airway obstruction related
to asthma or exacerbation of chronic
obstructive pulmonary disease (COPD).
41. Causes of respiratory acidosis
• Chronic:Chronic respiratory acidosis may be
secondary to many disorders -
eg, COPD,obesity hypoventilation syndrome
(Pickwickian syndrome), neuromuscular
disorders and restrictive ventilatory defects
such as interstitial fibrosis or thoracic
42. Respiratory Alkalosis
• Respiratory alkalosis results from
hyperventilation - eg, anxiety, stroke,
meningitis, altitude, pregnancy (see the
separate article on Hyperventilation).
44. Case No. 1 • Problems:
• PaCO2 is low.
• pH is on the high
side of normal,
• Also, PaO2 is low,
probably due to
displacing air in
affected by the
Mrs. Puffer is a 35-year-old single mother, just
getting off the night shift. She reports to the ED
in the early morning with shortness of breath.
She has cyanosis of the lips. She has had a
productive cough for 2 weeks. Her temperature is
39 degrees celsius, blood pressure 110/76, heart
rate 108, respirations 32, rapid and shallow.
Breath sounds are diminished in both bases, with
coarse rhonchi in the upper lobes. Chest X-ray
indicates bilateral pneumonia.
ABG results are:
45. Continuation of Case No.1
• Mrs. Puffer most likely has ARDS (Acute respiratory
distress syndrome) along with her pneumonia.
• The alkalosis need not be treated directly. Mrs. Puffer is
hyperventilating to increase oxygenation, which is
• High FiO2 (The fraction of inspired O2) can help, but if she
has interstitial lung fluid, she may
, or a BiPAP to raise
her PaO2. Expect orders for antibiotics, and possibly
steroidal anti-inflammatory agents.
• Chest physiotherapy and vigorous coughing or suctioning
will help the patient clear her airways of excess mucous
and increase the number of functioning alveoli.
46. Continuation of case no.1
BiPAP to raise her PaO2. A non-invasive clinical mngt.
• Bilevel Positive Airway
Pressure, and is very similar
in function and design to a
CPAP machine (continuous
positive airway pressure).
Similar to a CPAP machine,
47. Case No 2
• pH is high,
• PaCO2 is low
• respiratory alkalosis.
• If he is hyperventilating from
an anxiety attack, the
simplest solution is to have
him breathe into a paper
bag. He will rebreathe some
exhaled CO2.This will
increase PaCO2 and trigger
his normal respiratory drive
to take over breathing
• * Please note this will not
work on a person with
chronic CO2 retention, such
as a COPD patient. These
people develop a hypoxic
drive, and do not respond to
Mr. Worried is a 52-year-old widow. He is
retired and living alone. He enters the ED
complaining of shortness of breath and
tingling in fingers. His breathing is shallow
and rapid. He denies diabetes; blood sugar is
normal. There are no EKG changes. He has
no significant respiratory or cardiac history.
He takes several antianxiety medications.
He says he has had anxiety attacks before.
While being worked up for chest pain an
ABG is done:
ABG results are:
48. Case No. 3
• The pH is acidotic,
• PaCO2 is 25 (low) which should
• This is a
for the metabolic
• The underlying problem is, of
course, a metabolic acidosis.
• Insulin, so the body can use the
sugar in the blood and stop making
ketones, which are an acidic by-
product of protein metabolism.
• In the mean time, pH should be
maintained near normal so that
oxygenation is not compromised
You are the critical care nurse about
to receive Mr. Sweet, a 24-year-old
DKA (diabetic ketoacidosis) patient
from the ED. The medical diagnosis
tells you to expect acidosis. In report
you learn that his blood glucose on
arrival was 780. He has been started
on an insulin drip and has received
one amp of bicarb. You will be doing
finger stick blood sugars every hour.
ABG results are:
You need to memorize these and know it by heart . Then quickly go over the changes
Then summarize : The easiest one is that for acute situations for every change of 10 in the PCO2 there is should be a change of 0.08 in PH and in chronic situation there should be a change of 0.03 .
- If there is a different change then know that there is most likely a mixed disorder
Metabolic acidosis is the disorder you will mostly encounter in the hospital.
You must memorize Winter’s formula
Winter’s formula calculates the expected pCO2 in the setting of metabolic acidosis.
If the serum pCO2 > expected pCO2 then there is additional respiratory acidosis in which the etiology needs to also be determined.
For metabolic alkalosis , check urine cholride (must be off diuretics)
Urine chloride < 10 implies responsivenss to saline : extracelluar fluid volume depletion
Urine chloride >10 implies resistance to sailne : severe poatssium depletion , mineralcorticoid excees syndrome Etc