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IMAD CRITICAL CARE NURSING
Lectures Objectives:
Upon completion of this lecture the student will be able to:
 Discuss the purpose of ABG testing
 Describe what an ABG is measuring
 List normal ABG values
 Explain the steps to completing a Modified Allen’s Test
 Differentiate between acidosis and alkalosis
 Have a better understanding of ABG interpretation
What is an ABG?
An arterial blood gas (ABG) is a blood test taken from an artery,
that measures the amount of oxygen and carbon dioxide that is
found in the blood. The purpose of this measurement is to
determine the lungs effectiveness in moving oxygen and carbon
dioxide into and out of the bloodstream.
What does an ABG measure?
 Partial pressure of oxygen (Pao2) – This indicates the ability
of the lungs to move oxygen into the bloodstream.
 Partial pressure of carbon dioxide (PaCo2) – This indicates
the ability of the lungs to retrieve carbon dioxide out of the
bloodstream.
 pH – The pH is the measurement of hydrogen ions (H+)
found in the bloodstream which indicates the acid or base
balance of the blood.
 Bicarbonate (HCo3) – This is the most important buffer
found in the bloodstream, it assists in returning the body
from an acid state back to a normal range.
 Oxygen Content (o2CT) and Oxygen Saturation (o2 Sat) –
Like the Pao2, these values provide information about the
oxygen content found in the bloodstream.
When Should an ABG be Ordered?
There are 4 major reasons to draw an ABG and they are:
 Assessment of oxygenation capacity – determine cause of
pleuritic chest pain or rule out Pulmonary Embolism
 Assessment of oxygen pressure to guide therapy – prevention
of vision problems in premature infants and monitoring risk
of pleural disruption (Pneumothorax) in such disease
processes as ARDS
 Assessment of respiratory adequacy – oxygen and carbon
dioxide measurement to assist with assessment of ventilation
rate, depth and pressure
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IMAD CRITICAL CARE NURSING

Assessment of acid-base balance – disease identification and
determination of metabolic status
Performing a Modified Allen’s Test:
Choose the best site (radial preferred, but brachial and femoral can
be used)
If the radial artery is chosen a Modified Allen’s Test must be
performed before any attempt at artery puncture is attempted. The
purpose of a Modified Allen’s Test is to determine sufficient
collateral circulation from the ulnar nerve (which sits beside the
radial artery), in the event that a thrombus in the radial artery
should occur. To perform a Modified Allen’s Test; follow this
procedure:
 Place the patients arm on a flat surface, palm of hand facing
up and wrist supported on a rolled towel.
 Compress both the radial and ulnar arteries using the index
and middle finger of one hand for several seconds (usually 10
to 15)
 Ask the patient to clench and unclench their fist until
blanching of the skin occurs
 When blanching is noted, release the pressure of the ulnar
artery only and assess the hands ability to return to normal
color (this should occur within 5 to 15 secs.)
 If color returns a positive Modified Allen’s test has occurred
which means that the ulnar artery can supply blood to the
hand should any occlusion of the radial artery occur.
Normal Values
Arterial
Mixed Venous
pH 7.35-7.45
pH 7.34-7.37
PaC02 35-45 mmHg
PvC02 44-46 mmHg
PaO2 80-100 mmHg
Pv02 38-42 mmHg
HC03 22-26 mEq/Liter
HC03- 24-30 mEq/Liter
SaO2 > 95%
Sv02 60-80%
 The level of the C02 is controlled by breathing so any
derangements of carbon dioxide are considered to be a
respiratory problem.
 The level of Bicarb in the blood is controlled by the renal
system so any derangements of Bicarb are considered to be
a metabolic problem.
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IMAD CRITICAL CARE NURSING
A Step by Step Approach to Interpreting ABG’s:
Step 1: Determine primary abnormality
Determine Acidosis versus alkalosis:
 pH <7.35: Acidosis
 pH >7.45: Alkalosis
Step 2: Determine is it is Respiratory or Metabolic:
 For Metabolic disorders remember the Ph changes in the
same direction as the Bicarb or Co2.
 For Respiratory disorders remember the Ph changes in the
opposite direction as the Bicarb or Co2.
For Example:
 Metabolic Acidosis: Serum pH/Serum Bicarb/Co2 all
decreased
 Metabolic Alkalosis: Serum pH/Serum Bicarb/Co2 all
increased
 Respiratory Acidosis: Serum pH decreased - Serum
Bicarb/Co2 increased
 Respiratory Alkalosis: Serum pH increased - Serum
Bicarb/Co2 decreased
Step 3: Determine if the Acidosis/Alkalosis is Partially or
Completely Compensated:
Complete Compensation is simple if you remember one key
concept. The body only “JUST” compensates back to normal range
and then it stops; so if the pH is in normal range, determine which
end of normal and that will tell you what the original problem was.
For Example:
Compensated ABG with pH 7.36 (the patient had an acidosis)
Compensated ABG with pH 7.44 (the patient had an alkalosis)
Partial Compensation works the same way only the pH is not yet
within normal limits.
Examples for Practice: (answers below)
1. pH 7.51, pCO2 40, HCO3- 31
a. Normal
b. Uncompensated metabolic alkalosis
c. Partially compensated respiratory acidosis
d. Uncompensated respiratory alkalosis
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IMAD CRITICAL CARE NURSING
2. pH 7.33, pCO2 29, HCO3- 16 (everything is decreased)
a. Uncompensated respiratory alkalosis
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
3. pH 7.40, pCO2 40, HCO3- 24
a. Normal
b. Uncompensated metabolic acidosis
c. Partially compensated respiratory acidosis
d. Partially compensated metabolic acidosis
1. B – pH is high, Bicarb is high, Co2 is normal and not attempting
to correct the problem so this metabolic alkalosis is
uncompensated.
2. D – pH is low, Bicarb is low, Co2 is low and attempting to
correct the problem (but has not completely helped) so this is
partially compensated metabolic acidosis.
3. A – pH, Bicarb and Co2 are within normal ranges so this is a
normal ABG
Acid-Base Imbalance
Tight controls of the bodies Hydrogen [H+] ion concentration
must be maintained so the body can function normally. Even slight
changes can significantly alter the biologic processes of the cells
and tissue. Pathophysiologic changes in the concentration of
hydrogen ion in the blood, lead to what is commonly referred to as
acid-base imbalance. Acid-base imbalance can occur in one of four
ways, metabolic acidosis or alkalosis and respiratory acidosis or
alkalosis. The pH can either be high (alkalosis) or low (acidosis).
This condition can be caused by a metabolic or respiratory
problem. Monitoring ABG’s is the most effective way to determine
the degree of acid-base imbalance.
Respiratory Acidosis – This occurs when ventilation is
depressed and carbon dioxide is retained (hypercapnia) increasing
hydrogen and producing acidosis. Common causes of respiratory
acidosis include:
 Depression of the Respiratory Center (brain stem, trauma,
over sedation)
 Respiratory Muscle Paralysis
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IMAD CRITICAL CARE NURSING


Disorders of the Chest Wall (kyphoscoliosis, pickwickian
syndrome, flail chest)
Disorders of Lung Parenchyma (pulmonary edema,
pneumonia, asthma, bronchitis)

Respiratory Alkalosis – This occurs when there is alveolar
hyperventilation and excessive reduction of carbon dioxide
(hypocapnia). Stimulation of ventilation is often caused by
hypoxemia, which in turn may be caused by any of the following:
Pulmonary Disease
 Congestive Heart Failure
 High Altitude
 Hypermetabolic States (anemia, fever)
 Salicylate Intoxication
 Hysteria
 Cirrhosis
 Gram Negative Sepsis
Metabolic Acidosis – This occurs when there is an increase in
noncarbonic acid or a decrease (loss) of bicarbonate. This can
occur quickly in such cases of lactic acidosis or more slowly in
cases of Diabetic Ketoacidosis. Other causes of Metabolic Acidosis
include:
 Vomiting (bicarb loss)
 Diarrhea (bicarb loss)
 Renal Failure (bicarb loss)
 Ingestions (ammonia, chloride, salicylates) – (Increased
Noncarbonic Acid)
Metabolic Alkalosis – This occurs when bicarbonate is
increased but more commonly occurs when there is an excessive
loss of metabolic acid. Causes of Metabolic Alkalosis include:
 Prolonged Vomiting
 Gastrointestinal Suctioning
 Excessive Bicarbonate Intake
 Hyperaldosteronism
 Diuretic Therapy
Maintenance of Acid-Base Balance
Normally pH remains relatively constant both outside and inside
the cells. Alterations in the acid-base balance are resisted by
extracellular and intracellular chemical buffers and by respiratory
and renal regulation. While the kidneys and blood buffers attempt
to correct metabolic disorders, the lungs attempt to correct
respiratory disorders.

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IMAD CRITICAL CARE NURSING
The Role of the Lungs:
A normal adult produces about 300 liters of CO2 daily from the
metabolism of foodstuffs. In the blood, CO2 reacts with water to
form carbonic acid, which dissociates to H+ and HCO3-. In the lung
capillaries they are converted back to CO2 and water and the CO2 is
expired. As a secondary respiratory compensation, the lungs react
to metabolic acidosis and alkalosis. Metabolic acidosis stimulates
breathing causing hyperventilation while metabolic alkalosis
suppresses it. These are attempts to correct pH by changing the
concentration of carbon dioxide and carbonic acid in the blood.
The Role of the Bodies Buffering System:
Oxidation of proteins and amino acids produces strong acids, like
sulfuric, hydrochloric, and phosphoric acids, in the normal
metabolism. These and other non-carbonic (non-volatile) acids are
buffered in the body and must then be excreted by the kidneys. The most
important extracellular buffer is bicarbonate, which usually buffers these nonvolatile acids. The kidneys regenerate the bicarbonate used in buffering by
excreting hydrogen ions in the urine as ammonium and titratable acids. Other
major chemical pH buffers in the body are inorganic phosphate and plasma
proteins in the extracellular fluid, cell proteins, organic phosphates and
bicarbonate in the intracellular fluid, and mineral phosphates and mineral
carbonates in bone.
The Role of the Kidneys:
The kidneys have two important roles in the maintaining of the acid-base
balance: to reabsorb bicarbonate from and to excrete hydrogen ions into
urine. 4500 mmol of bicarbonate are filtered into the primary filtrate of urine
daily, but only 2 mmol of bicarb are finally excreted. 70-80% of bicarbonate is
reabsorbed in the first part of proximal tubule, 10-20% in the loop of Henle
and 5-10% in the distal tubules and collecting duct.
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IMAD CRITICAL CARE NURSING
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